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Fabrication of polymer films on liquid substrates via initiated chemical vapor deposition: controlling morphology and composition

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Fabrication of polymer films on liquid substrates via initiated chemical vapor deposition: controlling morphology and composition

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Fabrication of Polymer Films on Liquid Substrates
via Initiated Chemical Vapor Deposition:
Controlling Morphology and Composition

by

Laura C. Bradley

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY IN
CHEMICAL ENGINEERING
August 2015
2

Committee Members
Dr. Malancha Gupta (Chair)
Dr. Noah Malmstadt
Dr. Barry Thompson

3

Executive Summary
The initiated chemical vapor deposition (iCVD) process is a vapor phase method used to
deposit functional polymer coatings. The technique is typically used to coat solid substrates such
as silicon, fibers, and microfluidic devices. We were recently the first group to introduce low
vapor pressure liquids, including ionic liquids and silicone oil, into the iCVD process. We found
that the deposition onto liquids can result in the formation of either polymer films or polymer
particles at the liquid—vapor interface depending on the spreading coefficient of the polymer on
the liquid. In addition, precursors that are soluble in the liquid substrate can absorb into the bulk
and polymerize resulting in the formation of polymer—liquid gels, layered polymer composite
films, and polymer shells. The overall goal of this thesis is to elucidate the growth mechanism
of polymer films on liquid substrates and understand how process parameters can be
controlled to tailor the morphology and composition of functional polymer materials.
Chapter 1 provides background information on the iCVD technique and the vapor phase
deposition of materials onto liquids using other processes. This thesis combines these two fields
to examine the growth mechanism of polymers on liquid substrates via chemical vapor
deposition. This chapter also reviews ionic liquids, free-radical polymerization within ionic
liquid solvents, and polymerization of ionic liquid monomers. Chapter 2 discusses the
morphology of cross-linked polymer films formed at liquid—vapor interfaces, focusing
specifically on the fabrication of microstructured films. Chapter 3 examines the polymerization
of soluble precursors within the bulk liquid by comparing the concentration and molecular
weight of polymer formed at the liquid surface and within the bulk liquid. Chapter 4 studies the
mechanism of transport of soluble precursors into the bulk liquid which were found to absorb
into the liquid or diffuse through films formed at the liquid surface. Chapter 5 demonstrates the
copolymerization of a reactive ionic liquid substrate with vapor phase precursors in order to
covalently incorporate the ionic liquid functionality into composite films. Chapter 6 reviews a
method we developed to encapsulate liquid droplets in robust polymers sells. Lastly, Chapter 7
summarizes the conclusions and discusses future directions of this work.

4

Acknowledgements
I would first like to thank my PhD advisor, Dr. Malancha Gupta, for all of her input,
advice, and mentorship throughout my graduate work. I truly appreciate all the time she set aside
to help me with research and career development as well as all the fun adventures! I would also
like to thank my thesis committee Dr. Noah Malmstadt and Dr. Barry Thompson as well as my
qualifying committee which included Dr. Katherine Shing and Dr. Pin Wang for their time and
input on my thesis.
My graduate experience would not have been the same without the friendship of all my
labmates. They made my everyday life as a graduate student fun and enjoyable! I would also like
to thank my family for all their support. My Mom and Dad have always encouraged me to
challenge myself and work hard to reach my goals. I could not have been successful in graduate
school without those life lessons. My brothers are my role models, and I have looked up to Ryan
as my example of work ethic and to Collin who reminds me to stay young at heart. I also want to
thank Earl for his supportive friendship.

5

Table of Contents
LIST OF FIGURES 7
LIST OF TABLES 11

CHAPTER 1 INTRODUCTION 12
1.1 Initiated Chemical Vapor Deposition (iCVD) 13
1.2 Deposition onto Liquids 17
1.3 Ionic Liquids 19
1.4 Free-Radical Polymerization within Ionic liquids 22
1.5 Polymerized Ionic Liquid Monomers 25
1.6 References 27

CHAPTER 2 MICROSTRUCTURED POLYMER FILMS FORMED ON LIQUID
SUBSTRATES VIA INITIATED CHEMICAL VAPOR DEPOSITION 34
2.1 Abstract 35
2.2 Introduction 35
2.3 Experimental 35
2.4 Results and Discussion 37
2.5 Conclusions 48
2.6 Acknowledgments 48
2.7 References 49

CHAPTER 3 COMPARISON OF POLYMERIZATION AT THE SURFACE AND
WITHIN THE BULK LIQUID 53
3.1 Abstract 54
3.2 Introduction 54
3.3 Experimental 56
3.4 Results and Discussion 58
3.5 Conclusions 67
3.6 Acknowledgments 67
3.7 References 68

CHAPTER 4 TRANSPORT OF SOLUBLE PRECURSORS INTO THE BULK LIQUID 72
4.1 Abstract 73
4.2 Introduction 73
4.3 Experimental 75
4.4 Results and Discussion 78
4.5 Conclusions 88
4.6 Acknowledgments 89
4.7 References 89
6

CHAPTER 5 COPOLYMERIZATION OF A REACTIVE IONIC LIQUID VIA
INITIATED CHEMICAL VAPOR DEPOSITION 93
5.1 Abstract 94
5.2 Introduction 94
5.3 Experimental 96
5.4 Results and Discussion 99
5.5 Conclusions 108
5.6 Acknowledgments 109
5.7 References 109

CHAPTER 6 ENCAPSULATION OF IONIC LIQUID DROPLETS IN POLYMER
SHELLS VIA VAPOR PHASE POLYMERIZATION 112
6.1 Abstract 113
6.2 Introduction 113
6.3 Experimental 115
6.4 Results and Discussion 116
5 Conclusions 123
6.6 Acknowledgments 124
6.7 References 124

CHAPTER 7 CONCLUSIONS AND FUTURE RESEARCH 127
7.1 Conclusions 128
7.2 Future Research 129

7

List of Figures
Figure 1-1 a) Schematic and b) image of the iCVD set-up............................................................14

Figure 1-2 Schematic of a free-radical reaction mechanism. ........................................................14

Figure 1-3 Reaction mechanism of free-radical polymerization via iCVD. ..................................15

Figure 1-4 a) PHEMA film on [bmim][PF
6
] that completely encapsulates the liquid
droplet. b) A hole was punctured in the PHEMA film. .................................................................19

Figure 1-5 Schematic of polymerization events using liquid substrates........................................19

Figure 2-1 SEM images of a) bottom (liquid) side, b) top (vapor) side, and c) cross-
section of PEGDA microstructured films formed on 5 cSt silicone oil by a 90 minute
reaction at a deposition rate of 10 nm/min. d) Zoomed-in image of the outlined region in
part c. e) FTIR spectra of reference silicone oil and reference PEGDA deposited on a
silicon wafer compared to PEGDA film removed from the surface of silicone oil. ......................41

Figure 2-2 SEM images of bottom sides of PEGDA films formed on 5 cSt silicone oil by
10, 20, 40, and 90 minute reactions. The inserts are cross-sectional images of the
complete films made by 40 and 90 minute reactions. ....................................................................43

Figure 2-3 Cross-sections of films formed by subsequent P(PFDA-co-EGDA) reactions
on a) PEGDA microstructures was removed from the liquid surface and mounted on
carbon tape b) PEGDA microstructures that remained on silicone oil (5cSt). ..............................44

Figure 2-4 SEM images of the bottom sides and cross-sections of PEGDA films as a
function of silicone oil viscosity for a) 10 nm/min and b) 50 nm/min deposition rates. ...............45

Figure 2-5 a) RMS surface roughness measured by AFM of the bottom sides of PEGDA
films as a function of silicone oil viscosity and polymer deposition rate. AFM tapping
mode topographic images (5x5 m frames) of films formed at a 50 nm/min deposition
rate on b) 5 cSt and c) 500 cSt silicone oil. ...................................................................................46

Figure 2-6 SEM images of copolymer P(HEMA-co-EGDA) and P(VP-co-EGDA) films. ..........47

Figure 3-1 Schematic of the iCVD reactor. ...................................................................................58

8

Figure 3-2 a) Storage (G’) and loss (G”) modulus at 20 °C as a function of frequency for
the 4 and 9 wt.% samples made at 80 mTorr pressure. b) The PHEMA-[emim][BF
4
] gels
are robust enough to be handled with tweezers after being removed from the silicon
wafer. c) SEM cross-sectional image of a 43 wt.% PHEMA-[emim][BF
4
] gel on a silicon
wafer. .............................................................................................................................................60

Figure 3-3 FTIR spectra of the PHEMA-[emim][BF
4
] gel compared to reference PHEMA
and reference [emim][BF
4
]. The dashed lines indicate the locations of the carbonyl
stretching of PHEMA and the aromatic C-H symmetric stretching of [emim][BF
4
]. ...................61

Figure 3-4 GPC chromatographs of a) a 5 min deposition of PHEMA onto [emim][BF
4
]
at 80 mTorr, b) reference [emim][BF
4
], and c) reference PHEMA deposited onto a silicon
wafer at 80 mTorr using the 2 kDa-2 MDa column.......................................................................62

Figure 3-5 GPC chromatographs of PHEMA formed within the [emim][BF
4
] layer at 80
mTorr for increasing deposition times using the 300 kDa-20 MDa column. ................................64

Figure 3-6 GPC chromatographs of a) a 5 min deposition of PHEMA onto [emim][BF
4
]
at 120 mTorr and b) reference PHEMA deposited onto a silicon wafer at 120 mTorr using
the 2 kDa-2 MDa column. c) GPC chromatographs of 5 and 36 min depositions of
PHEMA formed within the [emim][BF
4
] layer at 120 mTorr using the 300 kDa-20 MDa
column............................................................................................................................................65

Figure 4-1 FTIR analysis of the solubility of EGDA, PEGDA, PFDA, and PPFDA in
[emim][BF
4
]. The dashed line represents the peak associated with carbonyl stretching. ..............79

Figure 4-2 Schematic of the fabrication of layered polymer films on IL substrates made
by the sequential or simultaneous depositions of EGDA and PFDA. ...........................................82

Figure 4-3 XPS C1s spectra of the top and bottom sides of the three layered polymer
films compared to the C1s spectra of reference polymer films deposited onto a silicon
wafer. .............................................................................................................................................83

Figure 4-4 C1s spectra of the bottom side of the film made by a short simultaneous
deposition (~30 seconds) of EGDA and PFDA. ............................................................................87

Figure 5-1 Comparison of the NMR spectra of [EVIm][TFSI] monomer and
poly([EVIm][TFSI]) formed by the introduction of TBPO initiator. ..........................................100

9

Figure 5-2 Schematic of the iCVD process for the copolymerization of [EVIm][TFSI]
with EGDA delivered through the vapor phase. ..........................................................................100

Figure 5-3 FTIR spectra of (a) the PIL copolymer films from 40 minute reactions
compared to reference PEGDA and poly([EVIm][TFSI]) and (b) the carbonyl peak of
PIL copolymer films from varying reaction times compared to reference PEGDA....................103

Figure 5-4 (a) Atomic concentrations of fluorine, nitrogen, and sulfur measured by XPS
and (b) the corresponding concentration of [EVIm][TFSI] on the bottom side of the films
as a function of reaction time at a pressure of 50 mTorr. ............................................................105

Figure 5-5 (a) Total film thickness as a function of reaction time measured by SEM
cross-sectional images. Schematics depict the thickness and composition of the PEGDA
and copolymer layers as determined by SEM and XPS analysis. (b) SEM cross-sectional
images of PIL copolymer films made by 5 and 90 minute reactions. (c) Concentration of
[EVIm][TFSI] in the bulk films measured by FTIR compared to the estimated
concentration calculated from the structrues shown in part a. .....................................................106

Figure 5-6 (a,b) PIL copolymer films formed on wire mesh supports. (c) Supported films
can withstand being folded to a bend with 1.5 mm diameter. (d) Cross-sectional image of
the bend in part c. .........................................................................................................................108

Figure 6-1 a) Schematic of our process for encapsulating ionic liquids in polymer shells
via iCVD. b) IL marbles sitting on the surface of a water bath that were coated on a bed
of loose PTFE (left) and a bare Petri dish (right). The polymer shell to the left retains the
yellow IL whereas the marble to the right had a hole torn in the polymer coating and the
IL leaked into the water bath. ......................................................................................................116

Figure 6-2 Coated marbles of decreasing volume. ......................................................................118

Figure 6-3 a) Uncoated marble and marble coated with P(PFDA-co-EGDA) sitting on a
glass slide 0.5 inches above the water surface. b) Coated marble remains intact after
being dropped. c) Uncoated marble breaks when dropped and the PTFE scatters on the
surface of the water. .....................................................................................................................119

Figure 6-4 a) FTIR spectra of a P(PFDA-co-EGDA) coating deposited onto an
[emim][BF
4
] droplet placed directly onto a silicon wafer and covered with a layer of
PTFE particles, a reference P(PFDA-co-EGDA) film deposited onto a bare silicon wafer,
the bulk [emim][BF
4
] after polymer deposition, and reference [emim][BF
4
]. b) XPS
spectra of the top and bottom surfaces of the P(PFDA-co-EGDA) film. ....................................120
10

Figure 6-5 Schematic and corresponding images of P(PFDA-co-EGDA) deposited onto
IL droplets on a silicon wafer a,c) without PTFE and b,d) with PTFE at the IL surface. e)
The polymer coating lifted off the droplet shown in c rolled up on itself and f) the
polymer coating lifted off the droplet shown in d held its shape demonstrating that the
integration of PTFE increases the rigidity of the polymer shell. .................................................121

Figure 6-6 SEM images of the a) top side, b) bottom side, and c) cross-section of the
P(PFDA-co EGDA) polymer film with PTFE incorporation. .....................................................122

Figure 6-7 Comparison of uncoated and coated marbles stacked in pyramid. ............................122

Figure 6-8 A P(PFDA-co-EGDA) polymer shell a) encapsulating IL, b) after the IL is
removed with a syringe, and c) injected with dyed red water. ....................................................123

11

List of Tables

Table 3-1 The effect of deposition time on the total PHEMA weight percent in the sample
as measured by NMR. ....................................................................................................................59

Table 4-1 The top and bottom atomic compositions of homopolymer PEGDA and
PPFDA films deposited onto [emim][BF
4
] compared to the measured atomic
compositions of reference polymer films deposited onto silicon wafers. ......................................80

Table 4-2 XPS survey spectra of the top and bottom sides of the layered films made by
the sequential or simultaneous depositions of EGDA and PFDA. ................................................82

Table 4-3 Effect of the PPFDA and PEGDA deposition thicknesses on the composition of
the bottom side of films made by the sequential deposition of PPFDA followed by
PEGDA. .........................................................................................................................................84

Table 4-4 XPS survey spectra of the bottom sides of the layered polymer films made by
the three cases after a 24 hour soak in a methanol bath. The lack of nitrogen in the films
indicates that the IL was removed. ................................................................................................88

Table 5-1 Atomic compositions measured by XPS of the top and bottom sides of the PIL
copolymer films from 40 minute reactions compared to reference PEGDA and
poly([EVIm][TFSI]). ...................................................................................................................101
12

Chapter 1 Introduction

13

1.1 Initiated Chemical Vapor Deposition (iCVD)
This thesis studies the deposition of polymers onto low vapor pressure liquid substrates
using an initiated chemical vapor deposition (iCVD) process. The iCVD technique is a one-step,
solventless process
1,2
that can be used to deposit a wide range of functional acrylate and vinyl
polymers including poly(2-hydroxyethyl methacrylate),
3
poly(4-vinylpyridine),
4
poly(ethylene
glycol diacrylate)
5
and poly(1H,1H,2H,2H-perfluorodecyl acrylate).
6
The iCVD technique is
typically used to deposit coatings onto solid substrates including silicon,
7
polydimethylsiloxane,
8

and alumina membranes.
9
It has been shown to be able to deposit conformal coatings onto
porous substrates as well as those with high aspect ratios.
10,11
Ma et al. made superhydrophobic
fabrics by depositing the fluoropolymer poly(perfluoroalkyl ethyl methacrylate) onto
poly(caprolactone) fibers.
12
Kwong et al. has fabricated paper-based microfluidics devices by
patterning polymers onto cellulose.
13,14
The iCVD technique has also been used to develop novel
materials such as ultra-thin, free-standing membranes by depositing polymer onto a sacrificial
layer of poly(acrylic acid) that was spin coated onto a silicon wafer.
15

Figure 1-1 shows a schematic and image of the iCVD set-up. In the iCVD process,
monomer and initiator are flown in the vapor phase into a vacuum chamber at approximately 0.1
Torr. The initiator source jar is typically kept at room temperature and the flow rate is monitored
using a mass flow controller, while the flow rates of the monomers are controlled by varying the
temperature of the monomer source jars. Inside the chamber, a resistively heated filament array
(typically set between 200-250C) decomposes the initiator molecules into radicals. The
monomer molecules and initiator radicals diffuse to the surface of the substrate on a cooled stage
controlled by a recirculating water bath (typically set between 20-60C) where polymerization
occurs via a free-radical mechanism (Figure 1-2). The majority of polymerization is expected to
14

occur on the surface of the substrate because the cooled temperature promotes high
concentrations of monomer on the sample surface. The free-radical polymerization via iCVD
has been shown to be analogous to solution phase methods.
16
The reactor pressure, substrate
temperature, monomer and initiator flow rates, and filament temperature can all be controlled to
tailor the deposition rate, molecular weight, and composition of polymer coatings. The
advantages of the iCVD technique over other polymerization methods are that it is solventless,
low energy, and has in situ control. The solventless nature significantly reduces organic waste
and enables conformal coatings to be deposited onto substrates with complex geometries and
high-aspect-ratios. In addition, it eliminates issues related to solubility to synthesize functional
copolymer and cross-linked coatings. The cooled substrate temperature enables the functionality
of the monomer precursors to be retained in the polymer films, and the in situ control of the film
thickness and composition is due to the ability to vary the process parameters throughout the
deposition.
Figure 1-1 a) Schematic and b) image of the iCVD set-up.
Figure 1-2 Schematic of a free-radical reaction mechanism.
15

A kinetic model for polymerization in the iCVD process was developed by Lao et al. to
describe the initiation, propagation, and terminations events (Figure 1-3).
17
Polymers produced
by iCVD are identical to polymers made in solution phase processes, therefore Lau developed
the iCVD model based on solution phase kinetics. Lau studied the rate of polymerization of butyl
acrylate by analyzing the monomer concentration, the polymer deposition rate, and the polymer
molecular weight.
17
The iCVD technique is typically operated under mass transfer regime
because decreasing the substrate temperature increases the polymer deposition rate due to
increased monomer flux to the surface of the sample.
18
If the polymerization was kinetically
limited, the polymer deposition rate would decrease with increasing substrate temperature based
on the Arrhenius relationship for the kinetic rate constant, which is not observed.

Rates of polymerization are known to be functions of the precursor concentrations. The
molar ratio of the initiator to the monomer in the iCVD process is typically much greater than
that used in solution phase methods and it is assumed that the initiator is in excess and not rate
limiting.
19
Studies have also evaluated the effect of the filament temperature on polymer
deposition rates to show that at low filament temperatures (below ~220 C) the kinetics are
limited by the decomposition of the initiator while at higher filament temperatures (above
Figure 1-3 Reaction mechanism of free-radical polymerization via iCVD.
19

16

~220 C) the deposition rate is not sensitive to the filament temperature indicating the kinetics are
limited by the mass transfer of radicals to the substrate surface.
20
Furthermore, negative
activation energies for deposition rate as a function of substrate temperature demonstrate that the
polymer deposition is limited by precursor adsorption to the sample surface.
21
Therefore, the rate
of polymerization in the iCVD process depends primarily on the monomer concentration as the
surface of the sample which can be modeled using the Brunauer-Emmett-Teller (BET) isotherm
showing that the monomer concentration is a function of the ratio of the monomer partial
pressure (P
M
) to the monomer saturation pressure (P
SAT
).
19
The P
M
/P
SAT
ratio is typically
controlled between 0.1 and 0.7 and always maintained less than 1.0 to prevent condensation. The
BET isotherm is used to model monomer adsorption instead of the Langmuir isotherm because
multilayer adsorption occurs for reaction conditions with P
M
/P
SAT
less than ~0.4. A higher
P
M
/P
SAT
corresponds to a higher monomer concentration at the sample surface and a higher
deposition rate. The monomer partial pressure (𝑃 𝑀 ) depends on the monomer flow rate (𝐹 𝑀 ), the
total flow rate of monomer and initiator (𝐹 𝑇𝑜𝑡𝑎𝑙 ), and the reactor pressure (𝑃 ).
𝑷 𝑴 =
𝑭 𝑴 𝑭 𝑻𝒐𝒕𝒂𝒍 𝑷
The monomer saturation pressure is determined by the Clausius-Clapeyron equation where 𝐴 is
the pre-exponential factor, 𝐻 is the heat of vaporization in J/mol, 𝑅 is the gas constant in J/mol-
K and 𝑇 is the substrate temperature in degrees Kevlin.
𝑷 𝑺𝑨𝑻 = 𝑨 𝑬𝒙𝒑 [
− 𝑯 𝑹 𝑻 ] ∗ 𝟏𝟎𝟎𝟎
The monomer concentration at the surface of solid samples can be measured using a quartz
crystal microbalance (QCM). The QCM technique measures the change in the resonance
frequency of a crystal as molecules adsorb to the surface in order to calculate the mass of
17

adsorption using the Sauerbrey equation.
22
The resonance frequency is observed to decrease with
increasing mass uptake. A linear relationship between P
M
/P
SAT
and adsorbed monomer volume
for P
M
/P
SAT
<0.3 has been reported for 1H,1H,2H,2H-perfluorodecyl acrylate
6
and ethyl
acrylate.
19
Higher P
M
/P
SAT
conditions result in the higher deposition rates which produces
polymers with higher molecular weights. In the iCVD process, P
M
/P
SAT
can be tuned by varying
the substrate temperature, reactor pressure, and precursor flow rates which provides multiple
ways to control the polymer deposition rate, molecular weight, and composition to tailor coatings
for specific applications.
1.2 Deposition onto Liquids
Vapor deposition onto liquid substrates was first studied using the sputter deposition of
inorganic materials.
23
The initial works in the field by Ye et al. studied the nucleation, growth
and aggregation of silver clusters and rough films deposited onto a silicone oil surface.
24-26
Silicone oil was found to be a poor stabilizer leading to clustering; therefore Wagener et al.
added a surfactant to the silicone oil substrate to demonstrate the first fabrication of discrete
nanoparticles on liquid surfaces.
27
In 2006, the pioneering work of Torimoto et al. synthesized
gold nanoparticles by sputter deposition onto ionic liquids without any additional stabilizer.
28

They found that the size of the particles depended on the properties of the ionic liquid. A second
significant work at this time was the fabrication of lunar liquid mirrors by the deposition of silver
films onto ionic liquids.
29
These two works changed the trajectory of the field to focus on
exploiting the unique chemical and physical properties of ionic liquids as substrates for materials
synthesis at liquid—vapor interfaces. Since 2007 there have been numerous studies that
investigate the growth mechanism of inorganic particles, aggregates, and films on ionic liquids.
Since Ye’s first publication, there has been a growing interest in the nucleation, diffusion, and
18

aggregation of metal species deposited onto liquid substrates.
30-36
The ability to fabricate
inorganic nanoparticles and films has potential applications in biosensors, catalysts, optics, and
electronics. In 2010, Wander et al. proposed three potential growth mechanisms.
37
The
mechanism of growth is still widely debated in literature as well as which parameters, such as
deposition conditions, surface chemistry, liquid viscosity, or surface tension, dominate the
growth. Several works cite the need for the design of in situ experiments to convincingly
elucidate the growth mechanism.
The study of organic deposition onto liquid substrates is a relatively new field compared
to inorganic deposition. Binh-Khiem and coworkers was the first group to study organic
deposition onto liquid substrates in 2008. They deposited parylene onto silicone oil droplets to
fabricate varifocal lenses that respond to an electrical voltage.
38
Binh-Kheim went on to study
the tensile stress in parylene films deposited onto liquid substrates
39
and fabricated reflective
displays on ionic liquids.
40
Two years after Bihn-Kheim’s first publication, Charmet et al.
published an article studying the fundamentals of a process they had patented in 2006
41
that
deposits parylene onto liquid substrates.
42
The potential to fabricate liquid encapsulating
microdevices is their motivation for the studying the deposition of parylene films onto liquids.
Our research group was the first to demonstrate the ability to deposit polymers onto
low vapor pressure liquids via iCVD. This work started in 2011, at which time there was no
systematic studies on the growth of polymer at liquid-vapor interfaces. The iCVD method is an
excellent tool for investigating the growth mechanism because several process parameters can
be varied to control the growth rate, molecular weight, and composition of the polymers. We
first studied the surface and bulk polymerization of 2-hydroxyethly methacrylate (HEMA) and
1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) onto the ionic liquid 1-butyl-3-
19

methylimidazolium hexafluorophosphate ([bmim][PF
6
]).
43
PFDA was found to form polymer
films on the surface of [bmim][PF
6
] because the monomer is insoluble in [bmim][PF
6
], while the
HEMA monomer polymerized at the liquid surface as well as within the bulk liquid because
HEMA is soluble in [bmim][PF
6
]. The polymer films that form at the liquid surface completely
encapsulate the liquid droplet (Figure 1-4). The ability for polymerization to occur at both the
surface and within the bulk liquid complicates the process since the two events occur
simultaneously (Figure 1-5). We have also demonstrated the ability to encapsulate liquid
droplets within polymer shells
44
and fabricate shaped, free-standing polymer films.
45

1.3 Ionic Liquids
Ionic liquids (ILs) is one of the major classes of liquids we study as substrates in the
iCVD process. ILs have attracted a lot of attention as “green” solvents for a wide range of
Figure 1-4 a) PHEMA film on [bmim][PF
6
] that completely encapsulates the liquid droplet.
b) A hole was punctured in the PHEMA film.
43

Figure 1-5 Schematic of polymerization events using liquid substrates.
43

20

applications such as chemical synthesis, gas sorbents, thermal energy storage, and proton
conducting membranes. ILs are room temperature liquid salts that are non-volatile, non-
flammable, and can be easily recycled. ILs are liquid at room-temperature and not solid salts due
to the large nature of one or both of the ions and the asymmetry of the cation which lowers their
melting point. The kinetics and thermodynamics of chemical reactions in ILs are different than in
common solvents,
46
and these properties have been employed for organic synthesis. Carmichael
et al. showed that several different ILs can carry out the carbon-carbon bond forming Heck
reaction.
47
Neutral ILs are excellent solvents for improving both the rate and selectivity of Diels-
Alder reactions.
48,49
Dubois et al. used ILs as micro-reactors to carry out tetrahydroquinoline
synthesis.
50
Specific ILs can also be used as catalysts in homogeneous hydrogenation reactions,
51

and it has been shown that ILs are easier to separate from the reaction products than commonly
used transition-metal complexes.
ILs have been recently studied for use in biofuel systems due to their ability to dissolve
cellulose. Swatloski and coworkers studied the solubility of cellulose in a series of ILs
containing the cation 1-butyl-3-methylimidazolium through heating, microwave, and sonicating
methods. Their results showed 1-butyl-3-methylimidazolium chloride had the highest cellulose
solubility of 25 wt. %.
52
The chloride anion is a hydrogen-bond acceptor and is able to disrupt
the hydrogen bonds in the cellulose structure. Zhang et al. showed that 1-allyl-3-
methylimidazolium chloride readily dissolves cellulose at 60 C.
53
Cellulose can be precipitated
from the IL solution using water, ethanol, or acetone and the degree of regeneration can be
tailored to the next step in biofuel fabrication.
54,55
More recently 97% cellulose conversion was
achieved by Zhang and coworkers using 1-ethyl-3-methylimidazolium chloride and water.
56

21

ILs have also been studied as thermal energy storage fluids for solar energy applications.
Van Valkenburg’s study on the physical properties of 1-methyl-3-ethylimidazolium
tetrafluoroborate, 1-methyl-3-butylimidazolium tetrafluoroborate, and 1,2-dimethyl-3-
propylimidazolium bis(trifluorosulfonyl)imide concluded that the stability over a wide
temperature range, negligible vapor pressure, and ability to store large amounts of heat make ILs
superior to commercially available energy storage fluids.
57
Molten salts currently used for energy
storage have a high melting point at 220°C. These high melting temperatures increase operating
costs because heat must be provided to prevent freezing during low sunlight hours. ILs are
potential heat transfer fluids owing to their low melting point (typically less than 100°C) and
high thermal stability.
58

The solubility of carbon dioxide (CO
2
) in imidazolium based ILs has attracted a lot of
attention because ILs do not dissolve CO
2
enabling it to be recovered as a pure product.
59

Blanchard and coworkers reported that 1-butyl-3-methylimidazolium hexafluorophosphate has a
high solubility of 0.6 mole fraction CO
2
at 8 MPa. The IL and CO
2
phases are not miscible
resulting in a heterogeneous mixture where each phase is nearly pure IL or pure CO
2
. The
mechanism of CO
2
solubility in ILs is debated between researchers. It has been proposed that
CO
2
occupies small void spaces without altering the structure of the IL
60
or that CO
2
experiences
a Lewis acid-base interaction with the anion in the IL.
61
Researchers have used CO
2
to extract
chemical synthesis products from IL solutions.
59,62,63
When a solution of IL and organic
compounds is pressurized with CO
2
, the liquid separates into two phases by density enabling the
organic compounds to be recovered and the IL to be recycled.
The unique chemical properties of ILs have been combined with the mechanical
properties of polymers to make polymer—IL composites for proton conducting membranes,
22

actuators, and transistors. Polymer—IL composites are typically fabricated either by co-solvent
evaporation or solution phase polymerization using IL solvents. Noda et al. studied the formation
of composites made of poly(2-hydroxyethyl methacrylate) (PHEMA) and 1-ethyl-3-
methylimidazolium tetrafluoroborate ([emim][BF
4
]) by in-situ polymerization. The conductivity
of the resulting PHEMA-[emim][BF
4
] gel was within one order of magnitude of the conductivity
of pure [emim][BF
4
]. The goal for fabricating polymer—IL composites as proton conducting
membranes is to achieve the same conductivity as the pure IL while incorporating enough
polymer to improve the mechanical strength. Ye et al. fabricated a solid polymer—IL composite
using a triblock copolymer in [bmim][PF
6
] with 5 wt% polymer and an ionic conductivity nearly
identical to the pure IL.
64
Efforts to make polymer—IL composites by solution phase
polymerization in ILs have resulted in high molecular weight polymers.
65,66
Increasing the
polymer molecular weight within the composite is advantageous because it decreases the amount
of polymer needed to form a solid while increasing the conductivity.
67-70

1.4 Free-Radical Polymerization within Ionic liquids
Free-radical polymerization within ILs has been shown to produce high molecular weight
polymers compared to traditional organic solvents.
71,72
The higher molecular weights formed
within ILs is attributed to increased propagation rates and decrease termination rates.
73
The
molecular weight of poly(methyl methacrylate) (PMMA) polymerized within IL solvents has
been reported on the order of 10
6
Da which is orders of magnitude higher than MMA
polymerized in organic solvents (10
4
Da).
74,75
Research shows that the propagation rates increase
with increasing concentration of IL. García and coworkers studied the polymerization of MMA
and glycidyl methacrylate (GMA) in a series of ILs and also found that the propagation rate
23

constant increased with increasing concentration of IL.
76
At low monomer concentrations for
MAA and GMA in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF
6
]), the
propagation rate constants for each monomer were similar due to the molecular environment
dominated by the IL.
In addition to increasing the propagation rate constant (k
p
), ILs have been also shown to
decrease the termination rate constant (k
t
) leading to an overall decrease in the ratio of k
p
/k
t
and a
significant increase in the molecular weight. Harrisson et al. studied the change in both the
propagation and termination rate constant for MMA in [bmim][PF
6
].
77
Harrisson reports that the
termination rate constant decreased by an order of magnitude with increasing concentration of IL
due to increased viscosity. Barth et al. reported that the termination rate coefficient decreased by
a factor of ten for MMA polymerization in 85 %v/v [bmim][BF
4
] compared to pure MMA
monomer.
78
Schmidt-Naake systematicaly studied the effect of liquid viscosity on both the
propagation and termination rates reporting a five-fold decrease in the termination rate constant
when the viscosity of the liquid increased from 1 to 4 mPa-s.
79
It is hypothesized that the high
viscosity of ILs slows diffusion thereby decreasing the rate of primary termination.
The polarity of ILs has been attributed to decreasing the activation energy of the
transition state during free-radical polymerization. García also showed that more polar ILs are
able to polarize the transition state structure leading to larger increases in the propagation rate
constant.
69
Haddelton and coworkers reported that the activation energy of MMA polymerization
in [bmim][PF
6
] decreased from 22.1 to 20.4 kJ/mol with increasing IL concentration from 0 to
50 % v/v, respectively.
70
Beuermann and coworkers recently compared the Kamlet-Taft
parameters for the radical polymerization of MMA in ILs and organic solvents and found that the
enhancement in the propagation rate constant was related to the dipolarity/polarizbility and
24

electron pair accepting ability of the solvent.
80
Beuermann also reported that the ability for ILs to
donate electron pairs had significantly less of an effect on the propagation rates. Andrzejewska et
al. reported a systematic study on polymerization in a series of ILs with varying polarity and
found that monomer/IL systems with stronger interactions resulted in higher rates of
polymerization.
81
More specifically, they reported that the polarity of the tetrafluoroborate anion
had a larger effect on kinetic enhancement than the 1-ethyl-3-methylimidazolium cation.
The mechanism of the enhanced polymerization kinetics within IL solvents is widely
disputed. A common theory is for the “protected radical” in which IL molecules stabilize the
radical such that it does not react thereby decreasing primary termination. The protected radical
mechanism is supported by the formation of high molecular weights despite similar
decomposition rates and efficiency of initiators in ILs and organic solvents.
82
Irvine and
coworkers produced the first systematic study on the polymerization of MMA in [bmim][PF
6
] to
evaluate the mechanism of the enhance kinetics.
67
Irvine argued that the proposed “protected
radical” cannot be the only acting mechanism as one would expect that by this mechanism alone
both the propagation and termination rate constants would also decrease and therefore does not
explain the high molecular weight polymers. Irvine proposed that the presence of small monomer
domains within the IL coupled with the protected radical mechanism to explain the enhanced
kinetics. In this theory, the protected radical would be adjacent to a monomer domain which
would facilitate propagation with limited termination resulting in high molecular weight
polymers.

25

1.5 Polymerized Ionic Liquid Monomers
The polymerization of IL monomers has created a new class a materials being developed
as polyelectrolytes,
83
separation membranes,
84
and catalytic supports.
85
The development of
polymeric ILs (PILs) was motivated by the need to combine the chemical properties of ILs with
the mechanical properties of polymers.
86
PILs are most commonly made by solution phase free-
radical polymerization of imidazolium cations with vinyl moieties,
87,88
and the PIL product can
be easily isolated by precipitation using an appropriate solvent. Other methods used to fabricate
PILs include atom transfer polymerization,
89
reversible addition-fragmentation transfer
polymerization,
90
and ring-opening methatesis polymerization.
91
The PIL backbone can be made
up of either the cation and/or the anion
86
while unreacted ions not included in the backbone
remain mobile between the polymer chains. Incorporating the cation into the polymer backbone
can allow for the initial anion of the imidazolium IL to be exchanged either before or after
polymerization in order to tune the properties of the PIL.
86,92,93

The electrochemical properties of ILs make them useful electrolytes for fuel cell
membranes.
94,95
PILs have been studied for use as fuel cell membranes to overcome issues
caused by leakage or stability; however, research shows that the conductivity of PILs is several
orders of magnitude lower than the conductivity of unreacted ILs due to the limited mobility of
the ions.
93
For example, Hirao et al. polymerized N-vinylimidazolium tetrafluoroborate and
found that the ionic conductivity decreased five orders of magnitude compared to the unreacted
IL.
96
Several works have shown that the ionic conductivity of PILs depends primarily on the
glass transition temperature which shows that the ion mobility is related to the polymer chain
dynamics.
97
This relationship is even more evident in the synthesis of copolymer PILs which
results in self-assembled nanostructured and physically cross-linked composites with enhanced
26

mechanical strength. Weber and coworkers showed that the ionic conductivity of copolymer
PILs depends on the nanostructure morphology in addition to the composition and synthesis
technique.
98

ILs have been shown to be able to absorb carbon dioxide and sulfur dioxide for capture,
and current research on the capacity of gas absorption in PILs compared to neat ILs is
inconsistent. Some researchers propose that the polymerization of IL cations with long alkyl
side chains creates a large free volume which increases the permeability of all gases compared to
neat ILs that can freely diffuse and reorganize to minimize free volume.
84
Tang et al. showed
that poly[p-vinylbenzyltrimethylammonium tetrafluoroborate] and poly[2(methacryloyloxy)-
ethyltrimethylammonium tetrafluoroborate] had higher CO
2
absorption at 22 C and 592.3
mmHg of 10.22 and 7.99 mol%, respectively, compared to the IL 1-butyl-3-methylimidazolium
hexafluorophosphate that absorbed 1.34 mol%.
99
They also demonstrated that the absorption and
desorption kinetics of PILs is significantly faster than in neat ILs making them more efficient
materials for gas separation. In addition to research supporting PILs as superior gas separation
materials to neat ILs, there are several works demonstrating that certain PILs have decreased
permeability and selectivity compared to neat ILs.
100
Li et al. demonstrated that three
polymerized vinyl-imidazolium ILs demonstrated inferior permeability and selectivity, therefore
the PIL was mixed with ILs in order to improve performance.
101
Patterns identifying which
components make PILs viable gas separations are currently unclear, and continued research is
required to elucidate these trends.
86,88

PILs have also been studied as catalysts supports for heterogeneous reaction systems.
Zhao et al. demonstrated that the polymerization of imidazolium based ILs with poly(acrylic
acid) formed a micro/mesoporous structure that could be used to load copper salts for use as a
27

catalyst in aerobic oxidation of hydrocarbons.
102
Wu et al. demonstrated the functionalization of
carbon nanotubes with PILs which could serve as catalyst supports for fuel cell applications.
103

The PIL layer is uniform and has a high density of reactive sites to anchor and grow metal
nanoparticles. The ionic nature of the PIL also serves to stabilize the carbon nanotubes with good
distribution throughout the medium. While PIL materials can act to support catalyst particles, ILs
have been shown to improve the rate and selectivity for a variety of chemical processes. PILs
have the potential to offer the same advantages for synthesis but can be separated from the
reaction media more easily and effectively. For example, Pinaud et al. fabricated poly(N-
heterocyclic-carbene)s (poly(NHC)s) and their CO
2
adduct (poly(NHC-CO
2
))from IL monomers
to use as catalysts for benzoin condensation and trans-esterification reactions.
104
The
poly(NHC)s and poly(NHC-CO
2
) polymers were soluble with the reactants and could be easily
separated from the solution to be recycled. Kim et al. fabricated a PIL specifically for
nucleophilic displacement reactions.
105
The ability to tune PILs offers opportunity to design and
optimize PIL catalysts for specific reaction systems.

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34

Chapter 2 Microstructured Polymer Films Formed on Liquid
Substrates via Initiated Chemical Vapor Deposition

Submitted to Langmuir 2015.

35

2.1 Abstract
In this paper, we studied the formation of microstructured films at liquid surfaces via
vapor phase polymerization of cross-linked polymers. The films were comprised of micron-sized
coral-like structures that originate at the liquid—vapor interface and extend vertically. The
growth mechanism of the microstructures was determined to be simultaneous aggregation of the
polymer on the liquid surface and wetting of the liquid on the growing aggregates. We
demonstrate that we can increase the height of the microstructures and increase the surface
roughness of the films by either decreasing the liquid viscosity or decreasing the polymer
deposition rate. Our vapor phase method can be extended to synthesize functional, free-standing
copolymer microstructured thin films for potential applications in tissue engineering, electrolyte
membranes, or separations.
2.2 Introduction
The development of microstructured polymer films is a continuously growing field due to
a wide range of applications in tissue engineering,
1-3
electrolyte membranes,
4-6
and separations.
7-9

Recently, Kawano et al. fabricated polymer scaffolds with varying pore sizes to selectively
control cell differentiation for regenerative tissue engineering.
10
Garcia-Ivars et al. demonstrated
the synthesis of ultrafiltration membranes with either organic or inorganic additives to improve
performance and anti-fouling properties.
11
Similarly, Cheng et al. synthesized microporous
polymers for electrolyte membranes in which they controlled the structure and crystallinity by
varying the polymer composition to maximize the ionic conductivity of the electrolyte.
12

Microstructured polymers are typically made using thermally induced phase-separation,
13-15

phase-inversion,
16-18
spinodal decomposition,
19-21
or lithography.
22-24
These commonly used
solution-phase methods rely on tailoring the chemical and physical properties of multicomponent
36

solutions. In this work, we demonstrate the formation of microstructured polymer films by the
deposition of cross-linked polymers at liquid—vapor interfaces via initiated chemical vapor
deposition (iCVD). In the iCVD process, monomer and initiator precursors are flowed in the
vapor phase into a vacuum chamber where a heated wire filament decomposes the initiator into
radicals. The monomer molecules and initiator radicals adsorb to the substrate surface where
polymerization occurs through a free-radical mechanism.
25,26
The advantages of using the iCVD
method to make microstructured films over solution-phase techniques are that the solventless
process produces high purity polymers and eliminates any issues related to precursor solubility
which thereby allows for the synthesis of functional cross-linked and copolymer films.
The iCVD technique is traditionally used to deposit dense, conformal coatings onto solid
substrates,
27-30
however recent studies have demonstrated the synthesis of microstructured films.
For example, Tao et al. made porous membranes by simultaneous polymerization of cross-linked
copolymers and condensation of an inert porogen species followed by the subsequent removal of
the porogen.
31
Similar to solution-phase methods, the morphology of the films presented by Tao
was shown to depend on the phase separation between the polymer and the condensed porogen.
Seidel et al. produced microstructured membranes with dual-scale porosity by simultaneous
polymerization and deposition of solid monomer.
32,33
This technique required that the substrate
temperature be maintained below the freezing point of at least one monomer precursor to induce
physical monomer deposition which controls the polymer morphology. The methods presented
by Tao and Seidel require conditions outside typical iCVD processing parameters in order to
facilitate either porogen condensation or solid monomer deposition.
We have recently shown that we can deposit polymers via iCVD onto liquid substrates
with low vapor pressures.
34-40
The deposition of linear polymers leads to the formation of either
37

dense polymer films or discrete polymer particles depending on the polymer—liquid surface
tension interactions.
41,42
In this paper, we demonstrate that we can synthesize unique
microstructured films by depositing cross-linked polymers at liquid surfaces under standard
iCVD processing conditions. The chemical cross-links resulted in the formation of films
containing micron-sized coral-like features. The growth mechanism and polymer morphology
were found to be driven by simultaneous aggregation of the polymer at the liquid surface and
wetting of the liquid on the growing aggregates. We demonstrate that we can control the
morphology of the microstructured films by varying the liquid viscosity and polymer deposition
rate. Furthermore, we show that we can tailor the film chemistry by synthesizing copolymer
microstructured films for potential applications in tissue engineering, electrolyte membranes, or
separations.
2.3 Experimental
Silicone oil (5, 100, 500 cSt Sigma-Aldrich; 5000 cSt Dow Corning), ethylene glycol
diacrylate (EGDA) (97% Monomer-Polymer), 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA)
(97% Synquest), 2-hydroxyethyl methacrylate (HEMA) (98% Sigma-Aldrich), 1-vinyl-2-
pyrrolidone (VP) (99% Sigma-Aldrich), tert-butyl peroxide (98% Sigma-Aldrich), and hexane
(98% Sigma-Aldrich) were all used as received. The solubility of the monomers (EGDA, PFDA,
HEMA, and VP) in 5 cSt silicone oil were independently tested by preparing 1:1 v/v mixtures
which were allowed to equilibrate for 48 hours. The composition of the silicone oil phase was
analyzed using Fourier transform infrared (FTIR) spectroscopy on a Thermo-Scientific Nicolet
instrument by collecting spectra between 4000 and 500 cm
-1
with a resolution of 4 cm
-1
. No
solution contained the characteristic carbonyl stretching of the monomers in the silicone oil
phase confirming that all the monomers are insoluble in silicone oil.
38

Polymer depositions were carried out in a custom built deposition chamber (GVD
Corporation, 250 mm diameter, 48 mm height). For all depositions, the substrates were
maintained at 25 C and the nichrome filament array (80% Ni, 20% Cr, Omega Engineering) was
heated to 230 C. The tert-butyl peroxide initiator was maintained at room temperature and
flowed into the reactor chamber at a rate of 1.5 standard cubic centimeters per minute (sccm)
through a mass flow controller. The polymer deposition rates were monitored on a reference
silicon wafer using an in-situ 633 nm helium-neon laser interferometer (Industrial Fiber Optics).
For PEGDA depositions at rates of 10 and 50 nm/min, the EGDA jar temperatures were 30 and
45C, the reactor pressures were 50 and 65 mTorr, and the ratio of the monomer partial pressure
to the monomer saturation pressures (P
M
/P
SAT
) were 0.13 and 0.17, respectively. For the
copolymer depositions, the deposition rate was kept constant at 10 nm/min using a reactor
pressure of 50 mTorr and an EGDA jar temperature of 30 C corresponding to a flowrate of 0.5
sccm. The jar temperatures for PFDA and HEMA were both maintained at 25 C corresponding
to flowrates of 0.5 sccm for both monomers and a P
M
/P
SAT
of 0.18 and 0.06 for PFDA and
HEMA, respectively. The P
M
/P
SAT
of EGDA for the copolymerization with PFDA and HEMA
was 0.11. The jar temperature of VP was maintained at 40 C corresponding to a flowrate of 1.0
sccm and a P
M
/P
SAT
of 0.14, while the EGDA P
M
/P
SAT
was 0.09. The line temperatures for all the
monomers were maintained 15 C above the jar temperature. The liquid substrates were
introduced into the deposition chamber by dispensing 20 L of 1:1 v/v silicone oil—hexane
solutions onto 1.5 cm x 1.5 cm silicon wafers. Mixing silicone oil in hexane enabled a consistent
volume of silicone oil (10 L) to be drop cast for each liquid viscosity (5-5000 cSt). The hexane
was removed from the silicone oil by pumping down the samples for 20 minutes in the
deposition chamber prior to the polymer depositions. After deposition, the polymer film was
39

removed from the liquid surface by pressing a piece of carbon tape on a silicon wafer support to
the top (vapor) side of the polymer film on the liquid substrate. When the tape was lifted, the
polymer film was removed from the liquid surface and the bottom (liquid) side of the film was
exposed. The samples were then soaked in hexane for ten minutes to remove any residual liquid.
The spreading coefficients of the polymer on the liquid substrates were calculated by
measuring the polymer—vapor surface tensions (𝛾 𝑃𝑉
), liquid—vapor surface tensions (𝛾 𝐿𝑉
), and
the advancing contact angles of the liquid on the polymers (𝜃 ) using a goniometer (ramé-hart
Model 290-F1). The liquid—vapor surface tensions were determined by the pendent drop
method using 3 L volumes. The pipette tips used for the pendent drop method were coated with
trichloro(1H,1H,2H,2H-perfluorooctyl)silane (97% Sigma-Aldrich) via physical vapor
deposition in a desiccator for 30 minutes prior to the measurements in order to prevent the liquid
from wetting the outside of the pipettes. The polymer—liquid surface tensions were measured by
the acid-base method on polymer coated glass slides using 5 droplets (5 L) each of deionized
water, glycerol (EMD Chemicals), and diiodomethane (99% Sigma-Aldrich). The advancing
contact angles were measured by the step volume method using an initial drop volume of 3 L
and a step size of 1 L.
The morphology of the films was imaged by scanning electron microscopy (SEM)
(JOEL-7001) using a 5 kV acceleration voltage. Prior to imaging, the samples were sputter
coated for 30 seconds with platinum. The reported film thicknesses were determined from cross-
sectional SEM images by measuring the thickness at three positions spaced 3 m apart for three
independent samples. The composition of the polymer films was analyzed using FTIR
spectroscopy by mounting the films on a silicon wafer and collecting 32 scans between 4000 and
500 cm
-1
with a resolution of 4 cm
-1
. The surface roughness of the films was measured using
40

atomic force microscopy (AFM) (Veeco diInnova DLPCA-200). Images were collected in
tapping mode with a scan rate of 0.5 Hz. The reported root-mean squared (RMS) surface
roughness was calculated using Gwyddion software on 5 m x 5 m areas. The reported values
are an average of three total samples from two independent depositions, and the reported error is
the standard deviation.
2.4 Results and Discussion
To study the growth of microstructured films on liquid substrates, we used the model
system of poly(ethylene glycol diacrylate) (PEGDA) deposited onto silicone oil. The EGDA
monomer is a cross-linker and is not soluble in the liquid as verified by FTIR spectroscopy
therefore polymerization only occurs at the liquid surface. SEM images of films formed by 90
minute depositions at a deposition rate of 10 nm/min (as measured on a reference silicon wafer)
showed that the films contained micron-sized coral-like microstructures that were comprised of
spherical aggregates. The microstructures originate at the bottom (liquid) side and extend
vertically through the film cross-section to the top (vapor) side of the films (Figure 2-1a-c). The
resulting polymer films have void space between the microstructures and within individual
microstructures between the spherical aggregates and therefore the total thickness of the PEGDA
microstructured films formed after 90 minutes (9.1  0.8 m) was significantly greater than the
deposition thickness of the dense PEGDA film formed on a reference silicon wafer (~0.9 m)
(Figure 2-1d). We examined the chemical composition by FTIR spectroscopy which showed
that the spectra of the films removed from the surface of the silicone oil and the film formed on a
reference silicon wafer were identical and displayed the characteristic carbonyl stretching
43
at
1735 cm
-1
. Furthermore, the spectrum of the PEGDA film does not contain the characteristic
signals of silicone oil
44
for the Si-C stretching between 900-800 cm
-1
confirming that the
41

microstructured films are composed of homopolymer PEGDA and that silicone oil is not
integrated into the microstructures (Figure 2-1e).

Figure 2-1 SEM images of a) bottom (liquid) side, b) top (vapor) side, and c)
cross-section of PEGDA microstructured films formed on 5 cSt silicone oil by a
90 minute reaction at a deposition rate of 10 nm/min. d) Zoomed-in image of the
outlined region in part c. e) FTIR spectra of reference silicone oil and reference
PEGDA deposited on a silicon wafer compared to PEGDA film removed from the
surface of silicone oil.

42

We then studied the growth mechanism of the microstructured films by varying the
deposition time of PEGDA onto silicone oil (5 cSt) between 10 and 90 minutes (Figure 2-2).
SEM images of the bottom side at 10 and 20 minutes show that the films are not complete,
containing large gaps which decrease in size between 10 and 40 minutes. Complete
microstructured films that cover the entire liquid surface form for depositions longer than 40
minutes. Between 40 and 90 minutes, the topography of the bottom side did not change as shown
by similar RMS surface roughness measured by AFM of 412  25 and 417  6 nm, respectively,
while the thickness of the microstructured films continued to increase from 3.8  0.5 to 9.1  0.8
m, respectively (Figure 2-2, insets). We have previously demonstrated that the morphology of
linear polymers deposited at liquid—vapor interfaces depends on the spreading coefficient of the
polymer on the liquid (𝑆 = 𝛾 𝐿𝑉
(1 + cos 𝜃 ) − 2𝛾 𝑃𝑉
),
45
in which systems with positive spreading
coefficients form films and systems with negative spreading coefficients form particles.
41
The
calculated spreading coefficient for PEGDA on silicone oil (5 cSt) is approximately -56 mN/m
which predicts the formation of polymer particles; however discrete particles do not form due to
chemical cross-links forming long range polymer networks as observed in Figure 2-2. We
hypothesized that the cross-linked microstructured PEGDA films grow by simultaneous polymer
aggregation at the liquid surface and wetting of the liquid on the growing aggregates. The
presence of gaps in the films at short deposition times demonstrates that the polymer diffuses and
aggregates on the liquid surface as predicted by the negative spreading coefficient. Since cross-
linked polymers have extremely limited mobility,
46
we expect the aggregation occurs by the
diffusion of nucleated chains before they become cross-linked with the larger polymer network.
We confirmed that there was enough polymer deposited in a 10 minute deposition to form a
complete film over the entire liquid surface by observing the formation of smooth, continuous
43

Figure 2-2 SEM images of bottom sides of PEGDA films formed on 5 cSt
silicone oil by 10, 20, 40, and 90 minute reactions. The inserts are cross-sectional
images of the complete films made by 40 and 90 minute reactions.
films on a reference silicon wafer as well as on high viscosity (5000 cSt) silicone oil due to
slower rates of polymer diffusion on the liquid surface.
41
To verify that the silicone oil wets
between the microstructures, we performed depositions of 1H,1H,2H,2H-perfluorodecyl acrylate
(PFDA) cross-linked with EGDA (P(PFDA-co-EGDA)) on a PEGDA microstructured film that
was removed from the liquid surface as well as a microstructured film that remained on the
silicone oil. The iCVD technique characteristically deposits conformal coatings on substrates
with complex geometries, therefore coating the microstructured film that was removed from the
liquid surface with P(PFDA-co-EGDA) resulted in a uniform increase in the width of the
individual microstructures (Figure 2-3a). For the deposition onto the microstructured film that
44

remained on the silicone oil, we observed the formation of a dense layer on the top side (Figure
2-3b). The spreading coefficient of P(PFDA-co-EGDA) is positive (S=18 mN/m) which leads to
the formation of dense films at the liquid surface, therefore the dense layer formed on the top
side of the microstructured film demonstrates that the liquid wets between the microstructures to
the top side, preserving the morphology of the PEGDA microstructures.

Figure 2-3 Cross-sections of films formed by
subsequent P(PFDA-co-EGDA) reactions on
a) PEGDA microstructures was removed from
the liquid surface and mounted on carbon tape
b) PEGDA microstructures that remained on
silicone oil (5cSt).

We examined how we could influence the morphology of the PEGDA microstructured
films by increasing the liquid viscosity which has been shown to slow the diffusion of polymers
at liquid surfaces.
47-49
We performed 60 minute depositions onto 5, 100, 500 and 5000 cSt
silicone oil at a rate of 10 nm/min, and SEM images of the bottom side show that the lateral
feature size decreases with increasing liquid viscosity (Figure 2-4a). This trend was quantified
by measuring a decrease in the RMS surface roughness of the bottom side of the films from 428
 6 nm on 5 cSt silicone oil to 15  7 nm on 5000 cSt silicone oil. The decrease in the RMS
surface roughness is due to slower rates of polymer diffusion resulting in less aggregation on the
liquid surface. This also leads to less void space between the microstructures and therefore a
decrease in the thickness of the microstructured films from 5.4  0.3 to 4.0  0.3 to 2.7  0.3 m
with increasing liquid viscosity from 5 to 100 to 500 cSt, respectively. The films formed on the
high viscosity (5000 cSt) silicone oil were smooth and dense due to very slow diffusion of the
polymer at the liquid surface.
41

45

Figure 2-4 SEM images of the bottom sides and cross-sections of PEGDA films
as a function of silicone oil viscosity for a) 10 nm/min and b) 50 nm/min
deposition rates.
While increasing the liquid viscosity will decrease the diffusion rate of the polymer on
the liquid surface, it will also decrease the wetting rate of the liquid on the polymer.
50
Therefore,
we evaluated the effect of the polymer deposition rate on the film morphology in order to
determine which effect dominates the growth mechanism of the films. Increasing the polymer
deposition rate from 10 to 50 nm/min will result in larger molecular weights of nucleated
chains
51
that will have slower diffusion on the liquid surface.
52-54
We found that for films formed
on 5, 100, and 500 cSt silicone oil, increasing the deposition rate decreased the lateral feature
size on the bottom side and decreased the total film thickness (Figure 2-4b). These qualitative
changes in morphology were reflected in a measured decrease in the RMS surface roughness on
46

the bottom side of the films with increasing deposition rate as well as an observed decrease in the
lateral feature size in the AFM topographical images (Figure 2-5). The surface roughness of all
the films was greater than the surface roughness of polymer deposited onto reference silicon
wafers which was 2  1 and 3  1 nm for deposition rates of 10 and 50 nm/min, respectively.
Since the wetting rate of the liquid on the polymer is constant at the same liquid viscosity, the
significant changes in the film morphology on the same viscosity liquid with increasing
deposition rate demonstrates that the growth of the microstructured films is dominated by the
diffusion and aggregation of polymer at the liquid surface.

Figure 2-5 a) RMS surface roughness measured by AFM of the bottom sides of
PEGDA films as a function of silicone oil viscosity and polymer deposition rate.
AFM tapping mode topographic images (5x5 m frames) of films formed at a 50
nm/min deposition rate on b) 5 cSt and c) 500 cSt silicone oil.
To demonstrate the generality of our technique for the synthesis of functional
microstructured films, we examined the morphology of cross-linked copolymer films by
depositing 2-hydroxyethyl methacrylate (HEMA) cross-linked with EGDA (P(HEMA-co-
EGDA)) and 1-vinyl-2-pyrrolidone (VP) cross-linked with EGDA (P(VP-co-EGDA)) onto 500
47

cSt silicone oil. The film morphology was examined for 60 minute depositions at a rate of 10
nm/min. We used FTIR spectroscopy to confirm the incorporation of HEMA into the P(HEMA-
co-EGDA) film by the presence of O-H stretching
55
between 3700-3050 cm
-1
and the
incorporation of VP into the P(VP-co-EGDA) films by the carbonyl stretching of VP
56
centered
at 1684 cm
-1
(Figure 2-6a). The spreading coefficients for both copolymers are negative (-63 and
-58 mN/m for P(HEMA-co-EGDA) and P(VP-co-EGDA), respectively) dictating that it is
energetically favorable for the polymer to aggregate on the liquid surface. The copolymer films
were observed to contain microstructures which resulted in a RMS surface roughness on the
bottom side of 245  15 and 193  4 nm for P(HEMA-co-EGDA) and P(VP-co-EGDA),
respectively (Figure 2-6b). These results demonstrate that the iCVD technique can be used to
synthesize functional microstructured films with tailored compositions by the deposition of
copolymers onto liquid surfaces for potential applications in tissue engineering, electrolyte
membranes, and separations. Furthermore, the mechanical properties of the films could be
optimized by extending the deposition time to increase the total film thickness or by forming the
films on wire supports
39
placed at the liquid surface.

Figure 2-6 SEM images of copolymer P(HEMA-co-EGDA) and P(VP-co-
EGDA) films.
48

2.5 Conclusions
In conclusion, we have demonstrated the formation of microstructured cross-linked
polymer films on liquid substrates via iCVD. The films were comprised of micron-sized coral-
like structures that originate at the liquid interface and extend vertically. We determined that the
microstructures form by simultaneous diffusion and aggregation of polymer at the liquid surface
and wetting of the liquid on the growing aggregates. The growth of the microstructures was
dominated by the diffusion and aggregation of the polymer at the liquid surface which could be
influenced by varying the liquid viscosity or the polymer deposition rate. Increasing the liquid
viscosity and increasing the polymer deposition rate were found to decrease the thickness of the
microstructured film and the surface roughness of the bottom side due to decreased aggregation
of the polymer on the liquid surface. This work demonstrates that microstructured films form
only in cross-linked systems with negative spreading coefficients and that the morphology of the
films can be controlled to fabricate functional polymer materials for potential applications in
tissue engineering, electrolyte membranes, and separations. Furthermore, the liquid substrate can
allow for easy fabrication of free-standing thin films and our synthesis method operates under
standard iCVD processing conditions which can allow for a wide range of functionalities, such
as thermo-responsive or pH-responsive, to be incorporated into copolymer microstructured films.

2.6 Acknowledgments
Acknowledgment is made to the Donors of the American Chemical Society Petroleum
Research Fund for partial support of this research. L.C.B. is supported by a National Science
Foundation Graduate Research Fellowship under Grant DGE- 0937362.

49

2.7 References

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53

Chapter 3 Comparison of Polymerization at the Surface and
within the Bulk Liquid

Publication Citation: R. J. Frank-Finney,
†
L. C. Bradley,
†
M. Gupta. Formation of Polymer—Ionic
Liquid Gels Using Vapor Phase Precursors. Macromolecules 2013, 46, 6852−6857.

†
Authors contributed equally to this work.

54

3.1 Abstract
We studied the polymerization via deposition of vapor phase precursors onto thin layers
of IL to compare the polymerization at the surface and within the bulk liquid. The solubility of 2-
hydroxyethyl methacrylate (HEMA) in 1-ethyl-3-methylimidazolium tetrafluoroborate
([emim][BF
4
]) enabled polymerization at both the IL-vapor interface and within the IL layer. At
short deposition times, there were two distinct molecular weights reflecting polymerization at the
IL-vapor interface and within the IL layer, while at longer deposition times the molecular weight
distribution within the IL layer broadened. The polymer chains within the IL were orders of
magnitude larger than the polymer chains at the IL-vapor interface and increasing the reactor
pressure was shown to increase the molecular weight. Our ability to form high molecular weight
polymer chains allows for the formation of gels for utilization as fuel-cell membranes and thin
film transistors.
3.2 Introduction
The initiated chemical vapor deposition (iCVD) technique is a solventless polymerization
process that is typically used to deposit coatings onto solid substrates. Monomer and initiator
molecules are flown into a vacuum chamber and a heated filament array decomposes the initiator
into radicals. Polymerization occurs on the surface of the cooled substrate via a free-radical
mechanism.
1,2
The iCVD process can be used to deposit a wide variety of functional coatings
including hydrophilic,
3
hydrophobic,
4
temperature-responsive,
5
and light-responsive
6
polymers.
The iCVD process does not require solvents and can therefore be used to conformally coat a
variety of complex substrates such as fibers,
7
membranes,
8
and wires.
9
We have recently
introduced low vapor pressure liquids such as ionic liquids (ILs) and silicone oils as substrates in
the iCVD process.
10-12
The introduction of liquid substrates adds complexity because
55

polymerization can now occur at both the liquid-vapor interface as well as within the liquid for
cases in which the monomer is soluble in the liquid.
12

We use iCVD to deposit polymer onto thin layers of IL to compare the polymerization at
the surface and within the bulk liquid. We examine the polymer concentration and molecular
weight distribution at the IL-vapor interface and within the IL as a function of deposition time
and reactor pressure. The polymerization kinetics within the ILs are expected to be different than
those at the IL-vapor interface because recent studies have shown that using ILs as solvents in
solution-phase polymerization results in higher polymerization rates and higher molecular weight
polymers compared with organic solvents.
13,14
For example, Harrison et al. studied the
polymerization of methyl methacrylate (MMA) in 1-butyl-3-methylimidazolium
hexafluorophosphate and found that increasing the volume fraction of the IL increased the
propagation rate constant and decreased the termination rate constant leading to higher molecular
weights.
15
Hong et al. performed free-radical polymerization of MMA and styrene in several
different ILs and found that the resulting polymer molecular weights were up to ten times larger
than polymer synthesized in benzene.
16
In our study, we demonstrate that we can tune the
molecular weight and polymer concentration of our gels by varying process parameters. The
results from our study can be used to fabricate polymer-IL gels
17
that combine the unique
properties of ILs, such as high ionic conductivity,
18,19
wide electrochemical window,
20,21
and
good thermal stability,
22,23
with the mechanical strength and flexibility of polymers. Current
methods for fabricating polymer-IL gels include solution-phase free-radical polymerization
24-26

and cosolvent evaporation.
27-29
For example, He et al. used cosolvent evaporation to self-
assemble the triblock copolymer polystyrene-block-poly(ethylene oxide)-block-polystyrene in 1-
butyl-3-methylimidazolium hexafluorophosphate to form gels with polymer concentrations as
56

low as 5 wt.%.
30
Shibayama and coworkers fabricated gels with similarly low polymer
concentrations (3-6 wt. %) by mixing tetra-arm poly(ethylene glycol) in 1-ethyl-3-
methylimidazolium bis(trifluoromethanesulfonyl)amide.
31
It is desirable to develop polymer-IL
gels with low polymer concentrations to optimize the properties gained from the IL such as
conductivity,
32
which can be done by increasing the molecular weight of the polymer to decrease
the polymer concentration needed to form a gel.
33-36
For example, Li et al. demonstrated that the
critical concentration for poly(vinyl chloride) in bis(2-ethylhexyl) phthalate to form a gel was
inversely proportional to the molecular weight of the polymer.
37
We demonstrate that our method
can be used to controllably tune the polymer concentration and molecular weight distribution to
produce polymer-IL gels that have potential applications as fuel-cell membranes,
38-40
polymer
actuators,
41-43
and thin-film transistors.
44-46

3.3 Experimental
1-Ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF
4
]) (97% Aldrich), 2-
hydroxyethyl methacrylate (HEMA) (98% Aldrich), and tert-butyl peroxide (TBPO) (98%
Aldrich) were used without further purification. All depositions were performed in a custom-
built reaction chamber (GVD Corporation, 250 mm diameter, 48 mm height) with a substrate
temperature maintained at 45 C using a recirculating chiller. The HEMA monomer and TBPO
initiator flow rates were held constant at 1 and 2 standard cubic centimeters per minute
respectively, and the polymer thickness deposited onto a reference silicon wafer was monitored
using an in situ interferometer. A nichrome filament array (80% Ni, 20% Cr, Omega
Engineering) was held 32 mm above the substrate and resistively heated to 225 °C. Polymer was
deposited onto ~3 m thick layers of IL that were spin-coated (30 seconds at 750 rpm, 100 μL of
57

1:3 [emim][BF
4
]/methanol solution) onto 1.5 cm x 1.5 cm silicon wafers that were roughened
with a scouring pad to prevent the IL from dewetting.
Fourier transform infrared (FTIR) spectroscopy (Thermo Nicolet iS10) was used to
confirm the presence of both PHEMA and [emim][BF
4
] in the polymer gels. Gel permeation
chromatography (GPC) was used to measure the molecular weight distribution of the polymer
using a HPLC pump (Agilent 1200 series) combined with a refractive index detector (Wyatt
Optilab rEX). Shodex columns KD-806 (2 kDa-2 MDa) and LF-804 (300 kDa-20 MDa) were
used separately in the setup. Dimethylformamide (DMF) was used as the eluent with a flow rate
of 0.5 mL/min. Weight-average molecular weights and polydispersity indices (PDI) were
determined using calibration curves based on polystyrene standards (Agilent). A DynaPro Titan
dynamic light scattering system with temperature-controlled microsampler (Wyatt) was used to
confirm the order of magnitude of the polymer molecular weights using DMF as the solvent. For
each deposition, the polymer thickness on a reference wafer was used to estimate surface
polymerization and the average of five samples was reported for the total polymer weight percent
measured using
1
H nuclear magnetic resonance (NMR) spectroscopy by comparing the integrals
of the methylene proton peaks for PHEMA at 4.05 ppm and [emim][BF
4
] at 4.25 ppm. The NMR
experiments were performed on a Varian Mercury 400 instrument using deuterated methanol
(99.8%, Cambridge Isotope Laboratories) as the solvent. The samples were scanned from 0 to 10
ppm with a total of 60 scans per sample. Dynamic mechanical analysis (DMA) was used to
determine the viscoelastic behavior of the samples by measuring the oscillatory shear moduli
across a range of frequencies from 0.1 to 200 Hz using a MCR Rheometer (Anton Paar) with a
parallel-plate configuration (50 mm diameter plate). The thickness of the gels was measured by
cracking the sample and taking cross-sectional images using a JEOL-7001 low-vacuum scanning
58

electron microscope (SEM). A thin layer of gold was sputtered onto the cross-section of the
sample before imaging. To determine the conductivity of the gels, they were transferred to a
Teflon substrate and the sheet resistivity was measured using a four-point probe with a linear
configuration and 1 mm spacing between probes (Lucas Laboratories S-302-4).
A quartz crystal microbalance (QCM) (Sycon Instruments) with a 6 MHz gold-plated
crystal was used to measure the monomer adsorption at the IL-vapor interface and monomer
absorption into the IL layer. The experiments were performed under the same pressure,
temperature, and flow rates used in the polymer depositions, however the initiator was replaced
with nitrogen in order to measure only the mass of the HEMA monomer. We estimated the
monomer concentration at the IL-vapor interface by measuring HEMA adsorption onto a bare
crystal. To determine the monomer concentration within the IL layer, we measured the total mass
of HEMA on a QCM crystal that contained a thin layer (<1 kÅ) of IL and subtracted the
respective surface adsorption we previously measured on the bare crystal. The experiments were
allowed to reach equilibrium which was identified by a plateau in the adsorbed or absorbed
volume (~20 min), and the reported values are an average of five trials.
3.4 Results and Discussion

Figure 3-1 Schematic of the iCVD reactor.
We deposited polymer via iCVD onto thin layers of IL that were spin-coated onto silicon
wafers. A schematic of our process is shown in Figure 3-1. We used HEMA and [emim][BF
4
] as
59

our model system because HEMA monomer is soluble in [emim][BF
4
]
24
allowing for
polymerization at both the IL-vapor interface and within the IL layer. The PHEMA polymer is
also soluble in [emim][BF
4
]
47
due to the ability for the BF
4
anion to form hydrogen bonds with
hydroxyl groups.
48,49
We deposited polymer at a rate of 4 nm/min as measured on a reference
silicon wafer at a reactor pressure of 80 mTorr and varied the deposition time from 5 to 100 min.
We found that the total polymer concentration in the samples, measured using NMR
spectroscopy, increased from 4 1 wt% for the 5 min deposition to 60 4 wt.% for the 100 min
deposition (Table 3-1). In the iCVD technique, process parameters such as pressure, substrate
temperature, and monomer and initiator flow rates have a large effect on monomer adsorption,
50

whereas varying the surface chemistry does not have a significant impact on adsorption. Because
our depositions onto the reference silicon wafer and IL were conducted under the same
processing conditions, we assumed that the monomer adsorption on both surfaces was similar
and therefore the mass of the polymer deposited at the IL-vapor interface was the same as the
mass deposited onto the reference silicon wafer. Using this assumption, we estimated that the
contribution from surface polymerization at a deposition rate of 4 nm/min ranges from ~1 wt%
with respect to the IL layer for the 5 min deposition to ~11 wt% for the 100 min deposition.
Because the total polymer concentration in the samples is much greater than the concentration
estimated from surface polymerization, the majority of the polymer must have been polymerized
Table 3-1 The effect of deposition time on the total PHEMA weight percent in
the sample as measured by NMR.

Deposition Time (min) 5 10 14 29 52 100
Total PHEMA Weight Percent in
Sample
4  1 9  1 19  4 24  5 45  7 60  4
60

within the IL layer. DMA analysis showed that the transition from a viscous liquid to a solid-like
gel occurred between 4 and 9 wt% (Figure 3-2a). For the 4 wt% sample, the loss modulus (G”)
was greater than the storage modulus (G’) across all frequencies displaying the behavior of a
viscous liquid, while the analysis of the 9 wt% sample showed that G’ was greater than G”
across all frequencies exhibiting the behavior of a solid-like gel. An example of a PHEMA-
[emim][BF
4
] gel is shown in Figure 3-2b. The gel was removed from the silicon wafer after
deposition and is flexible and robust enough to be handled with tweezers. We determined the
correlation between the polymer concentration and the thickness of the gels using cross-sectional

Figure 3-2 a) Storage (G’) and loss (G”) modulus at 20 °C as a function of
frequency for the 4 and 9 wt.% samples made at 80 mTorr pressure. b) The
PHEMA-[emim][BF
4
] gels are robust enough to be handled with tweezers after
being removed from the silicon wafer. c) SEM cross-sectional image of a 43 wt.%
PHEMA-[emim][BF
4
] gel on a silicon wafer.

61

SEM images (Figure 3-2c). The thicknesses of the gels were 31, 14 4, and 20 4 m for
polymer concentrations of 141, 294, and 433 wt%, respectively. The large thickness increase
from 3 to 20 m is possibly due to the formation of pores, which will be further investigated.
FTIR was used to confirm that PHEMA and [emim][BF
4
] were both present in the gels as
identified by the carbonyl stretching of PHEMA
51
at 1700 cm
-1
and the aromatic C-H symmetric
stretching of [emim][BF
4
]
52
between 3200-3300 cm
-1
(Figure 3-3). The conductivities of the 29
and 43 wt% gels were measured to be 1.0x10
-2
and 1.3x10
-3
S/cm at 25

°C, respectively, which
are within one order of magnitude of the conductivity of the pure IL (2.2x10
-2
S/cm) and are
similar to the conductivities of PHEMA-[emim][BF
4
] gels made by solution-phase
polymerization.
24

Figure 3-3 FTIR spectra of the PHEMA-[emim][BF
4
] gel compared to reference
PHEMA and reference [emim][BF
4
]. The dashed lines indicate the locations of
the carbonyl stretching of PHEMA and the aromatic C-H symmetric stretching of
[emim][BF
4
].

We predicted that the molecular weight of the polymer formed within the IL layer would
be larger than the molecular weight of the polymer formed at the IL-vapor interface. The GPC
chromatograph in Figure 3-4a represents the molecular weight distribution for the 5 min
62

deposition using a 2 kDa-2 MDa column and shows two distinct peaks at 5 and 11 mL elution
volumes. The sharp peak at 11 mL is at the low molecular weight limit of the column and
represents the IL. The tail on the left side of the IL peak is not present in the chromatograph of
pure [emim][BF
4
] (Figure 3-4b) and appears at the same elution volume as PHEMA deposited
onto a reference silicon wafer that has a molecular weight of 1.8x10
4
Da (1.7 PDI) (Figure 3-
4c); therefore, this tail most likely represents low molecular weight polymer formed at the IL-
vapor interface. The similarity in the molecular weights at the IL-vapor interface and on the
silicon wafer suggests that the rates of polymerization at the two surfaces are similar and

Figure 3-4 GPC chromatographs of a) a 5 min deposition of PHEMA onto
[emim][BF
4
] at 80 mTorr, b) reference [emim][BF
4
], and c) reference PHEMA
deposited onto a silicon wafer at 80 mTorr using the 2 kDa-2 MDa column.
63

supports our previous assumption that the mass of polymer deposited on both surfaces is the
same. The peak at 5 mL represents high molecular weight polymer formed within the IL layer.
Because this peak is at the high molecular weight limit of the column, we also analyzed the
sample using a 300 kDa-20 MDa column and found that the molecular weight was 1.2x10
7
Da
(1.8 PDI). Dynamic light scattering was used to confirm these high molecular weights. Lau and
coworkers have reported molecular weights as large as 8.2x10
5
Da for PHEMA formed on solid
substrates via iCVD by maximizing the monomer surface concentration.
53
The polymer formed
within the IL layer is larger possibly due to increased propagation rates and decreased
termination rates.
15,54
High molecular weights on the order of 10
6
Da have also been reported for
the free-radical polymerization of poly(methyl methacrylate) in IL solvents.
55,56

As the deposition time increases from 5 to 100 min, the peak representing the polymer
formed within the IL layer increases and broadens (Figure 3-5). The chromatographs have been
normalized to the IL peak because the volume of IL was kept constant; therefore the increase in
peak area reflects an increase in the concentration of the polymer formed within the IL layer with
increasing time, which is consistent with our NMR data (Table 3.1). The broadening of the peak
toward higher elution volumes and the evolution of a tail that extends above the baseline reflects
the formation of lower molecular weight chains within the IL layer as the deposition time
increases, indicating that the polymerization kinetics are not constant with increasing polymer
concentration. We hypothesized that the shift toward forming lower molecular weight polymer
chains within the IL layer is due to the increased polymer concentration causing an increase in
viscosity which may change the solubility of the monomer within the IL layer or change the
polymerization rate constants. We used QCM to measure the effect of polymer accumulation on
monomer solubility by performing a 14 min PHEMA deposition onto a QCM crystal that
64

Figure 3-5 GPC chromatographs of PHEMA formed within the [emim][BF
4
]
layer at 80 mTorr for increasing deposition times using the 300 kDa-20 MDa
column.

contained a thin layer of IL and then subsequently comparing the absorption of HEMA monomer
into this polymer-IL sample to the absorption of HEMA monomer into a pure IL layer. The
equilibrium concentration of HEMA within the polymer-IL sample was 1 1 wt% which was less
than the equilibrium concentration within the pure IL (4 1 wt%) demonstrating that the
solubility of the HEMA monomer decreases with increasing polymer concentration which could
explain the formation of lower molecular weight chains with increasing time. The broadening of
the molecular weight distribution with increasing polymer concentration has also been observed
by Schmidt-Naake and coworkers who showed that the molecular weight of poly(methyl
methacrylate) polymerized in 1-ethyl-3-methyl-imidazolium ethylsulfate decreased with
increasing reaction time, which they attributed to an increase in viscosity, leading to a decrease
in the propagation rate constant.
57
It is likely that the formation of lower molecular weight
polymers with increasing deposition time is due to both a decrease in the monomer concentration
in the IL layer and a decrease in the propagation rate.
65

We also studied the effect of increasing the reactor pressure on the molecular weight of
the polymer. In the iCVD process, increasing the reactor pressure increases the monomer
concentration due to greater adsorption resulting in higher molecular weight chains.
4
Our QCM
measurements showed that increasing the reactor pressure from 80 to 120 mTorr increased the
monomer concentration at the IL-vapor interface from 1.9 0.1 to 2.2 0.1 μg/cm
2
and increased
the monomer concentration within the IL layer from 4 1 to 8 2 wt%. GPC analysis of a 5 min

Figure 3-6 GPC chromatographs of a) a 5 min deposition of PHEMA onto
[emim][BF
4
] at 120 mTorr and b) reference PHEMA deposited onto a silicon
wafer at 120 mTorr using the 2 kDa-2 MDa column. c) GPC chromatographs of 5
and 36 min depositions of PHEMA formed within the [emim][BF
4
] layer at 120
mTorr using the 300 kDa-20 MDa column.
66

deposition at 120 mTorr using the 2 kDa-2 MDa column (Figure 3-6a) shows a polymer tail to
the left of the IL peak at 11 mL that matches the elution volumes of the polymer deposited onto a
reference silicon wafer that has a molecular weight of 2.7x10
4
Da (2.2 PDI) (Figure 3-6b). This
tail therefore represents polymer formed at the IL-vapor interface. The high molecular weight
polymer formed within the IL layer was measured to be 1.7x10
7
Da (2.4 PDI) using the 300 kDa-
20 MDa column, which confirms that increasing the reactor pressure leads to the formation of
higher molecular weights at both the IL-vapor interface and within the IL layer due to higher
monomer concentrations. When the deposition time was increased from 5 to 36 min at 120
mTorr, the evolution of a broad molecular weight distribution within the IL layer was observed
similar to that seen at 80 mTorr which indicates that the formation of smaller molecular weight
chains with increasing deposition time is independent of reactor pressure (Figure 3-6c).
The polymerization of HEMA within the [emim][BF
4
] layer can result in the formation of
a gel depending on the polymer concentration and the molecular weight. We examined the
polymer concentration required to transition from a viscous liquid to a gel at 80 and 120 mTorr
by measuring the loss and storage moduli using DMA. At 80 mTorr, the transition to a gel
occurred between 4 1 and 9 1 wt% corresponding to 5 and 10 min deposition times,
respectively. When the reactor pressure was increased to 120 mTorr, the transition to a gel
occurred between 2 1 and 5 1 wt% corresponding to 2 and 5 min deposition times, respectively.
The lower amount of polymer needed at a higher pressure is consistent with literature that reports
that the critical polymer concentration needed to form a gel is inversely proportional to the
molecular weight of the polymer.
37

67

3.5 Conclusions
We have demonstrated that the polymerization HEMA at the surface and within the bulk
liquid of thin layers of [emim][BF
4
] via iCVD results in different polymer concentrations and
molecular weight. Polymerization occurs at the IL-vapor interface as well as within the IL layer
due to the solubility of HEMA in the IL. We found that increasing the deposition time increased
the polymer concentration, leading to a transition from a viscous liquid to a gel. For short
deposition times, we observed the formation of two distinct molecular weights. The molecular
weight of the shorter chains were similar to the molecular weight of polymer deposited onto a
reference silicon wafer and therefore represents polymerization at the IL-vapor interface, while
the longer chains were formed within the IL layer because polymerization in IL solutions have
been observed to have higher propagation rates and lower termination rates. We found that the
molecular weight distribution broadened with increasing deposition time, reflecting the
formation of lower molecular weight chains within the IL layer. We attributed this decrease in
the molecular weight to a decrease in the monomer solubility and a decrease in the propagation
rate. We also showed that increasing the reactor pressure increased the molecular weight of the
polymer chains, which decreased the polymer concentration required to form a gel. The ability to
control the polymer concentration and molecular weight by varying the deposition time and
reactor pressure allows for the properties of polymer-IL gels to be tuned for applications such as
fuel-cell membranes and thin-film transistors.
3.6 Acknowledgments
We acknowledge the Donors of the American Chemical Society Petroleum Research
Fund for partial support of this research. L.C.B. is supported by a fellowship from the Chevron
68

Corporation (USC-CVX UPP). We thank Dr. Shuxing Li and the USC NanoBiophysics Core
Facility for assistance with gel permeation chromatography.
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72

Chapter 4 Transport of Soluble Precursors into the Bulk
Liquid

Publication Citation: L. C. Bradley, M. Gupta. Formation of Heterogeneous Polymer Films via
Simultaneous or Sequential Depositions of Soluble and Insoluble Monomers onto Ionic Liquids.
Langmuir 2013, 49, 10448-10454.
73

4.1 Abstract
In this work, we studied the formation of heterogeneous polymer films on ionic liquid
(IL) substrates via the simultaneous or sequential depositions of monomers that are either soluble
or insoluble in the liquid. We found that the insoluble monomer, 1H,1H,2H,2H-perfluorodecyl
acrylate (PFDA), only polymerizes at the IL surface, while the soluble monomer, ethylene glycol
diacrylate (EGDA), can polymerize at both the IL surface and within the bulk liquid. The
polymer chains that form within the bulk liquid entrap IL as they integrate into the polymer film
formed at the IL surface, resulting in heterogeneous films that contain IL on the bottom side.
Varying the order in which the soluble and insoluble monomers were introduced into the system
led to different film structures. When the insoluble monomer was introduced first, a film formed
at the surface and the soluble monomer then diffused through this film and polymerized within
the bulk, leading to a sandwich structure. When the soluble monomer was introduced first, a
layered film was formed whose structure followed the order in which the monomers were
introduced. When the two monomers were introduced simultaneously, the soluble monomer
polymerized in the bulk while a copolymer film formed at the surface. This study provides an
understanding of how to control the composition of layered polymer films deposited onto IL
substrates in order to develop new composite materials for separation and electrochemical
applications.
4.2 Introduction
Multilayered polymer films are useful for separation,
1,2
optical,
3
and electrochemical
applications.
4
Methods for fabricating layered polymer films include solution casting,
5-7
self-
assembly,
8-10
and layer-by-layer deposition.
11,12
Several methods have been developed to make
74

free-standing multilayer films including using sacrificial layers and different substrate materials.
For example, Jiang and coworkers fabricated free-standing layered polymer films that contained
gold nanoparticles using a cellulose acetate sacrificial layer that was dissolved in acetone.
13
Ono
et al. demonstrated the ability to make self-standing polyelectrolyte multilayer membranes by the
decomposition of a pH-responsive bottom layer.
14
Lutkenhaus et al. was able to remove
poly(ethylene oxide)/poly(acrylic acid) multilayers with tweezers from a hydrophobic Teflon
substrate due to weak adherence,
15
and Ma et al. showed that poly(acrylic acid)/poly(allylamine
hydrochloride) multilayer films can be removed from silicon by immersing the samples in an
exfoliating solution which disrupts the electrostatic forces between the polymer film and
silicon.
16

In this work, we demonstrate that free-standing layered polymer films can be made by
polymerizing functional monomers on ionic liquid (IL) substrates using initiated chemical vapor
deposition (iCVD).
17-21
The negligible vapor pressure of ILs enables them to be used in the
iCVD process.
22
The use of IL substrates allows the polymer films to be easily removed after
deposition
23
and, more importantly, the IL can be incorporated into the films for additional
functionality. The ability to incorporate ILs into the layered polymer films is useful for
fabricating polymer-IL composites
24-26
that have potential applications as separation media due
to the solubility of gases in ILs,
27,28
as well as fuel cell membranes
29-31
and polymer actuators
32-34
due to the thermal stability
35-37
and high conductivity of ILs.
38-40

In the iCVD process, monomer and initiator vapors are flown into a vacuum chamber,
and a heated filament array decomposes the initiator molecules into radicals to start the
polymerization process.
41,42
The iCVD process has been used to deposit a wide variety of
functional polymers including hydrophobic,
43
hydrophilic,
44
light-responsive,
45
and thermally-
75

responsive
46
coatings. Since the iCVD technique is a dry process, it can be used to deposit
conformal coatings onto complex structures such as trenches,
47
membranes,
48
fibers,
49
and
microfluidic devices.
50
In this paper, we studied the formation of layered polymer films on IL
substrates via simultaneous or sequential iCVD depositions of monomers that are either soluble
or insoluble in the liquid. We performed simultaneous depositions by flowing both the soluble
and insoluble monomers into the vacuum chamber at the same time, while sequential depositions
were performed by introducing the monomers consecutively. The results of our study
demonstrate that the structure of the films can be tuned by varying the solubility of the
monomers in the IL, the deposition thickness, and the order in which the monomers are
introduced into the reactor. The insights gained from our study provide an understanding of how
to control the composition of polymer shells
51
and free-standing polymer films
23
deposited via
vapor phase deposition and will enable the formation of new composite materials for separation
and electrochemical applications.
4.3 Experimental
1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF
4
]) (97% Sigma-Alrich),
1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (97% Synquest), ethylene glycol diacrylate
(EGDA) (97% Monomer-Polymer), and tert-butyl peroxide (TBPO) (97% Sigma-Alrich) were
used as received. All polymer depositions were carried out in a custom-built reactor chamber
(GVD Corp, 250mm diameter, 48 mm height). The substrate temperature was maintained at
28C by a recirculating water bath, and a nichrome filament array (80% Ni, 20% Cr, Omega
Engineering) was placed 32 mm above the substrate and resistively heated to 230C. The TBPO
initiator was kept at 25 C while the EGDA and PFDA monomers were heated to 35 and 55 C,
respectively, and the reactor pressure for all depositions was kept constant at 35 mTorr. The
76

reported deposition thicknesses were measured on a reference silicon wafer using an in-situ 633
nm helium-neon laser interferometer (Industrial Fiber Optics), which was determined to have an
error of 20 nm using profilometry (Ambios Technology XP-2 stylus profilometer). The
deposition rates under the prescribed conditions were approximately 20 and 10 nm/min for
PPFDA and PEGDA, respectively. All polymer films were deposited onto 15 L droplets of
[emim][BF
4
] on a silicon wafer. After deposition, the polymer films were removed from the
surface of the IL by first detaching the films along the edge of the IL droplet using tweezers. In
order to analyze the composition of the bottom (IL) side using X-ray photoelectron spectroscopy
(XPS), a piece of carbon tape was placed on a small silicon wafer and then pressed on the top
(vapor side) of the film to lift it off the droplet. The silicon wafer with the film adhered to the
carbon tape was then placed in a methanol bath for 30 seconds to remove excess IL. In order to
analyze the composition of the top side, the films were floated off the surface of the IL droplet in
a methanol bath for 30 seconds and then floated onto a silicon wafer with carbon tape. The
PPFDA homopolymer films fractured when they were mounted on the carbon tape for XPS
analysis, therefore copper/nickel tape was used instead of carbon tape to avoid any signal from
the underlying tape influencing the carbon/oxygen/fluorine atomic ratios detected in the polymer
films. To test the composition at the silicon-polymer interface for the sequential depositions onto
silicon wafers, the films were removed from the silicon wafers using a razor blade and placed on
copper/nickel tape for XPS analysis.
To test the solubility of EGDA and PFDA in [emim][BF
4
], we first mixed 1 mL of each
monomer separately in 1 mL of IL and allowed the solutions to equilibrate for 48 hours. To test
the solubility of PEGDA and PPFDA in [emim][BF
4
], we separately mixed 0.01 grams of
polymer that was deposited onto a reference silicon wafer in 0.5 mL of IL and allowed the
77

solutions to equilibrate for 48 hours. Fourier transform infrared spectroscopy (FTIR) (Thermo
Nicolet iS10) was used to identify the composition of the solutions to determine if the monomers
and polymers were soluble in the IL. In the FTIR spectra, the monomers and polymers were
characterized by the carbonyl stretching vibration
43,52
at 1730 cm
−1
, and [emim][BF
4
] was
identified by the C−H symmetric stretching vibrations
53
between 3050 and 3250 cm
−1
. The
EGDA and [emim][BF
4
] were mixed into one homogeneous phase and the FTIR contained the
characteristic peaks for both EGDA and [emim][BF
4
]. The PFDA and [emim][BF
4
] mixture
phase separated, and the FTIR of the [emim][BF
4
] phase did not contain the characteristic
carbonyl signal for PFDA. Neither PEGDA nor PPFDA were soluble in [emim][BF
4
]. Similar to
the PFDA monomer, PPFDA is insoluble in [emim][BF
4
]. Although the EGDA monomer is
soluble in [emim][BF
4
], PEGDA is not soluble because it is cross-linked.
To study the composition of the homopolymer films, we performed 400 nm PEGDA and
1 m PPFDA depositions onto [emim][BF
4
]. We performed a thicker PPFDA deposition because
these films are more fragile than PEGDA films. The experiments to test bulk polymerization of
EGDA in [emim][BF
4
] were performed by first flowing EGDA into the reactor at 50 mTorr,
pumping down the reactor to base pressure for 5 minutes, and then flowing in TBPO initiator
with the filament heated for 20 minutes at 50 mTorr. In the sequential depositions, the reactor
was pumped down to base pressure for 10 minutes before the second monomer was introduced.
The deposition thicknesses, measured on a reference silicon wafer, for the sequential depositions
were 400 nm for each monomer, while the simultaneous deposition thickness was 800 nm.
XPS experiments to identify the composition of the polymer films were performed on a
Surface Science MProbe Instrument with a monochromatic Al K  X-ray source using an 800 m
diameter spot size. Survey spectra were taken from 0 to 1000 ev with a step size of 1 eV and a
78

total of 5 scans, and high resolution C1s spectra were taken between 280 and 294 eV with a step
size of 0.065 eV and a total of 50 scans. For XPS analysis, the polymer films were mounted on a
silicon wafer using either carbon tape or copper/nickel tape. Three samples were analyzed for
each experiment, and the sample with the median atomic percentages was reported. XPS is
typically quoted as having an accuracy of ~10% for atomic concentrations,
54
however good
precision is reported between similar samples. For all experiments, the reproducibility in the
atomic concentrations for similar samples was within 2%. XPS survey and C1s spectra were also
collected for polymer deposited onto silicon wafers to serve as reference films.
4.4 Results and Discussion
In order to determine the effect of monomer solubility on the structure of films deposited
onto IL substrates, we first compared the structure of homopolymer films made of soluble and
insoluble monomers. We used [emim][BF
4
] as the IL, PFDA as the insoluble monomer, and
EGDA as the soluble monomer. FTIR was used to test the solubility of the monomers and
polymers by separately mixing each with [emim][BF
4
]. The analysis confirmed that EGDA is
soluble in [emim][BF
4
], whereas the solubility of PFDA, PEGDA, and PPFDA is negligible
(Figure 4-1). In addition to FTIR analysis, solubility parameters may be useful for predicting the
solubility of monomers within the IL. For example, the large difference in the solubility
parameters of PFDA
55
(15.08 MPa
0.5
) and [emim][BF
4
]
56
(26.11 MPa
0.5
) agrees with our FTIR
analysis that shows that PFDA is insoluble in [emim][BF
4
]. Monomers with solubility
parameters comparable to the IL may be soluble if the proportions of contributions from intra-
molecular and inter-molecular interactions are similar.
57-59
We compared the structure of
homopolymer films by depositing polymer films onto droplets of [emim][BF
4
], removing the
films, and then analyzing the compositions of the top (vapor side) and bottom (IL side) using
79

Figure 4-1 FTIR analysis of the solubility of EGDA, PEGDA, PFDA, and
PPFDA in [emim][BF
4
]. The dashed line represents the peak associated with
carbonyl stretching.

XPS since this technique only probes the top 5 nm of the sample (Table 4-1). The survey spectra
of the films deposited onto the ILs were compared to the survey spectra of reference polymer
films that were deposited onto silicon wafers, and the presence of [emim][BF
4
] in the films was
identified by nitrogen in the survey spectra. Our analysis showed that the composition of the top
and bottom sides of the PPFDA film was similar to the reference spectra. There was no
[emim][BF
4
] incorporation into the film which was expected since the solubility of PFDA limits
polymerization to only the IL surface. In contrast, the top side of the PEGDA film was similar to
80

Table 4-1 The top and bottom atomic compositions of homopolymer PEGDA and
PPFDA films deposited onto [emim][BF
4
] compared to the measured atomic
compositions of reference polymer films deposited onto silicon wafers.

the reference film, while the bottom side showed PEGDA with [emim][BF
4
] incorporation. In
this case, polymerization occurs both at the IL surface and within the bulk IL since the monomer
and initiator can absorb.
22
The PEGDA polymer that forms within the bulk can become
integrated into the PEGDA film that forms at the IL surface. The IL incorporation is likely
caused by the entrapment of IL between the polymer chains formed in the bulk as they get
integrated into the film. We verified that EGDA can polymerize within the bulk by first
saturating [emim][BF
4
] droplets with monomer, pumping down the reactor to remove monomer
that was adsorbed on the IL surface, and then introducing initiator radicals. After the introduction
of the initiator radicals, polymer pieces were visually observed in the IL droplets confirming
polymerization of the absorbed EGDA. The analysis of the homopolymer depositions showed
that if the monomer is not soluble polymerization occurs only at the IL surface and there is no IL
incorporated into the film, whereas if the monomer is soluble polymerization occurs at both the
surface and within the bulk leading to IL incorporation into the film.
We then studied the formation of multilayered polymer films by sequential or
simultaneous depositions of soluble and insoluble monomers in order to understand how varying
81

the order of the monomer precursors affects the structure of the films (Figure 4-2). We first
examined the sequential depositions by comparing the deposition of PEGDA followed by
PPFDA to the deposition of PPFDA followed by PEGDA (Table 4-2, Figure 4-3). The atomic
composition of the samples was characterized using XPS survey spectra and high resolution C1s
spectra in order to differentiate PPFDA and PEGDA. In the C1s spectra, PEGDA was identified
by the characteristic alkyl, ester, and carbonyl signals between 282 and 289 eV,
60
and PPFDA
was identified by the presence of signal between 289 and 293 eV which represents C-F
2
and C-F
3

groups.
43
The sequential deposition of PEGDA followed by PPFDA formed a layered film whose
structure followed the order in which the monomers were introduced. The bottom layer was
composed of PEGDA and [emim][BF
4
] and the top layer was composed of PPFDA. The
sequential deposition of PPFDA followed by PEGDA resulted in a sandwich structure with
PEGDA on the top, PPFDA in the middle, and PEGDA with [emim][BF
4
] incorporation on the
bottom. Since a PPFDA homopolymer film was deposited onto the surface of the IL first, the
EGDA monomer and initiator radicals must have diffused through this PPFDA film and
polymerized within the bulk IL to form the bottom layer. The iCVD technique characteristically
deposits pinhole free, non-porous polymer films,
18
therefore we expect that EGDA diffused
through the PPFDA film by the solution-diffusion mechanism which is the most widely accepted
model for transport in non-porous polymer films.
61
The XPS C1s spectra of the bottom layer
contained both PEGDA and PPFDA signals, indicating that the thickness of the PEGDA bottom
layer was less than 5 nm. The IL was expected to be incorporated into the bottom side of the
films in both sequential depositions by the same mechanism proposed for the homopolymer
PEGDA case. We studied whether the second monomer could polymerize within films made by
polymerization of the first monomer by performing sequential depositions onto silicon wafers
82

Figure 4-2 Schematic of the fabrication of layered polymer films on IL substrates
made by the sequential or simultaneous depositions of EGDA and PFDA.

Table 4-2 XPS survey spectra of the top and bottom sides of the layered films
made by the sequential or simultaneous depositions of EGDA and PFDA.

83

Figure 4-3 XPS C1s spectra of the top and bottom sides of the three layered
polymer films compared to the C1s spectra of reference polymer films deposited
onto a silicon wafer.

and then analyzing the composition at the silicon-polymer interface using XPS. To test whether
EGDA polymerized within PPFDA films, we deposited 200 nm of PPFDA followed by 200 nm
of PEGDA onto a silicon wafer and found that the atomic concentrations at the silicon-polymer
interface were characteristic of homopolymer PPFDA indicating that the EGDA monomer did
not polymerize within the PPFDA film. Similarly, we tested whether PFDA polymerized within
PEGDA films by depositing 200 nm of PEGDA followed by 200 nm of PPFDA onto a silicon
wafer and found that the atomic concentrations at the silicon-polymer interface were
characteristic of homopolymer PEGDA indicating that PFDA did not polymerize within the
84

PEGDA film. These results show that under our deposition conditions, the second monomer does
not polymerize within the films of the first monomer.
In order to understand how we can control the heterogeneous structure of the sandwich
films, we systematically studied the effect of increasing the PPFDA deposition thickness on the
diffusion and subsequent polymerization of the EGDA monomer. The PPFDA deposition
thickness was varied from 50 to 800 nm to form the PPFDA film on the IL surface and then we
performed a subsequent 400 nm PEGDA deposition (Table 4-3). All the thickness values
reported in this study are the deposition thicknesses measured on reference silicon wafers which
were monitored using in-situ interferometry. For the 50 nm PPFDA sample, the atomic
percentages of the bottom side showed the formation of a complete PEGDA bottom layer with
IL incorporation. We define a complete PEGDA bottom layer as having a thickness greater than
5 nm, which can be assumed by the lack of PPFDA as indicated by the low atomic percentage of
Table 4-3 Effect of the PPFDA and PEGDA deposition thicknesses on the
composition of the bottom side of films made by the sequential deposition of
PPFDA followed by PEGDA.

85

fluorine in the survey spectra which is associated with the IL. As the PPFDA deposition
thickness increased from 50 nm to 100 nm to 200 nm, the carbon atomic percentages on the
bottom side kept decreasing, while the fluorine atomic percentage increased, which indicates that
there is less PEGDA on the bottom side of the films due to the increasing resistance of the
PPFDA film. The solution-diffusion mechanism for transport through nonporous polymers
consists of three steps: 1) absorption into the polymer film at the feed side, 2) diffusion across
the polymer film, and 3) desorption on the permeate side.
61
It is assumed that the diffusion
through the polymer film is the rate limiting step and that the rate of diffusion is given by Fick’s
law in which the flux of precursors through the polymer film is inversely proportional to the film
thickness resulting in thinner PPFDA films having higher precursor concentrations at the
polymer-IL interface leading to higher rates of polymerization. We do not expect that the
precursor concentration within the polymer films is in equilibrium because the IL droplet acts as
a sink for precursor absorption. Using Fick’s second law for time dependent diffusion and the
diffusivity of organic gases in ILs reported by Morgan and coworkers,
62
we estimated that it
takes several hours for the precursor concentration in the IL droplet to reach equilibrium which is
greater than the time of our depositions. The experimental data does not show a significant
difference in the compositions of the bottom side of the 200, 400, and 800 nm PPFDA samples
likely due to a low flux of precursors through these films. Although the total film thickness
increased during the subsequent deposition of PEGDA, our results give us insight into the
resistance of the initial PPFDA film on the diffusion of EGDA and initiator radicals into the IL.
We also investigated the effect of increasing the PEGDA deposition thickness using the 50 nm
PPFDA sample and found that a complete PEGDA bottom layer formed between PEGDA
deposition thicknesses of 200 and 400 nm, which corresponds to 20 and 40 minutes (Table 4-3).
86

The long time required to form the complete PEGDA bottom layer enables the composition of
the bottom side to be easily tuned by varying the deposition thickness of PEGDA in addition to
varying the deposition thickness of PPFDA. The layered structures that we observed were due to
process conditions and not the rearrangement of the polymer chains. If the polymer chains were
forming the structures thermodynamically, the composition of the bottom side would be
independent of our process conditions. Instead, we observe that the order in which the monomers
are introduced into the reactor and the deposition thickness changes the composition of the
bottom side of the structures.
We then studied the simultaneous deposition of EGDA and PFDA in order to understand
how introducing both monomers into the system at the same time can affect the structure of the
films. The bottom layer was composed of PEGDA and [emim][BF
4
] and the top layer was
composed of a copolymer of PFDA cross-linked with EGDA (P(PFDA-co-EGDA)) (Table 4-2,
Figure 4-3). The copolymer layer was identified by the characteristic signals for both PPFDA
and PEGDA in the C1s spectra and atomic percentages for carbon, oxygen, and fluorine that are
between the values for the PEGDA and PPFDA homopolymers. Unlike the sequential deposition
where there was an initial PPFDA film at the IL surface before the EGDA monomer was
introduced into the reactor, in the simultaneous deposition the EGDA and PFDA monomers flow
into the reactor at the same time which enables the EGDA monomer to absorb into the bulk IL
and polymerize while a copolymer film is formed at the surface of the IL. The PEGDA that
forms within the bulk gets integrated into the film formed at the surface, trapping IL and leading
to a heterogeneous structure. We studied how the PEGDA bottom layer was formed by stopping
the simultaneous deposition the moment a film could be seen on the surface of the IL (~30
seconds). The XPS survey scan of the bottom side showed carbon, oxygen, fluorine, and nitrogen
87

atomic percentages of 71.6, 22.4, 5.2, and 0.9, respectively, which are characteristic of PEGDA
with IL incorporation, and the XPS C1s scan was used to confirm that the fluorine content in the
survey scan was due to the presence of [emim][BF
4
] and not PPFDA (Figure 4-4). The
formation of a complete PEGDA layer at such a short deposition time indicates that this layer
was formed by monomer and initiator radicals absorbing and polymerizing in the bulk within the
time it took for the copolymer film to form at the IL surface. Although EGDA monomer and
initiator radicals may diffuse through the copolymer film as we observed in the sequential
depositions, this process takes much longer than 30 seconds to form a complete PEGDA film.

Figure 4-4 C1s spectra of the bottom side of the film made by a short
simultaneous deposition (~30 seconds) of EGDA and PFDA.

In all the sequential and simultaneous depositions, we observed that the bottom side was
composed of PEGDA with IL incorporation. We tested whether the IL could be removed by
soaking the samples in a methanol bath for 24 hours. The XPS survey scans for the bottom side
of the samples did not contain nitrogen demonstrating that our process could also be used to
make multilayered polymer films that do not contain IL (Table 4-4).

88

Table 4-4 XPS survey spectra of the bottom sides of the layered polymer films
made by the three cases after a 24 hour soak in a methanol bath. The lack of
nitrogen in the films indicates that the IL was removed.

4.5 Conclusions
In conclusion, we have demonstrated the ability to tune the composition of layered
polymer films deposited onto IL substrates via iCVD by varying the order of soluble and
insoluble monomers. We demonstrated that the insoluble monomer PFDA polymerizes only at
the surface of the IL while the soluble monomer EGDA can polymerize at the surface and within
the bulk IL. The soluble monomer and initiator can absorb into the IL both before and after a
film is formed at the IL surface. We found that when the soluble monomer is introduced into the
reactor first or simultaneously with the insoluble monomer, the soluble monomer absorbs and
polymerizes within the bulk IL and integrates into the bottom layer of the film, entrapping IL in
the process. When the insoluble monomer is introduced first, the initiator and soluble monomer
diffuse through the film and subsequently polymerize in the bulk, contributing to the bottom
layer of the film. The composition of the bottom layer can be tuned by varying the deposition
thickness of the insoluble monomer or the soluble monomer. Our method for fabricating layered
polymer films via iCVD offers an environmentally friendly and easily controllable technique to
89

produce tailored composites for a wide range of applications in separation and electrochemical
processes.
4.6 Acknowledgments
Acknowledgment is made to the Donors of the American Chemical Society Petroleum
Research Fund for partial support of this research. L.C.B. is supported by a fellowship from the
Chevron Corporation (USC-CVX UPP). We thank the Molecular Materials Research Center of
the Beckman Institute at the California Institute of Technology for use of their XPS.
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(56) Weerachanchai, P.; Chen, z.; Leong, S. S. J.; Chang, M. W.; Lee, J.-M. Hildebrand
solubility parameters of ionic liquids: Effects of ionic liquid type, temperature and DMA
fraction in ionic liquid. Chem. Eng. J. 2012, 213, 356–362.
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Based Microfluidic Devices. Anal. Chem. 2003, 75, 6544-6554.
(58) Ueki, T.; Watanabe, M. Polymers in Ionic Liquids: Dawn of Neoteric Solvents and
Innovative Materials. Bull. Chem. Soc. Jpn. 2012, 85, 33-50.
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by Initiated Chemical Vapor Deposition. J. Electrochem. Soc. 2008, 155, G78-G86.
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93

Chapter 5 Copolymerization of a Reactive Ionic Liquid via
Initiated Chemical Vapor Deposition

Publication Citation: L. C. Bradley, M. Gupta. Copolymerization of 1-Ethyl-3-vinylimidazolium
bis(trifluoromethylsulfonyl)imide via Initiated Chemical Vapor Deposition. Macromolecules 2014, 47,
6657-6663.

94

5.1 Abstract
We studied the copolymerization of an ionic liquid (1-ethyl-3-vinylimidazolium
bis(trifluoromethylsulfonyl)imide ([EVIm][TFSI])) with ethylene glycol diacrylate (EGDA) via
initiated chemical vapor deposition to form polymerized ionic liquid (PIL) copolymer films. The
copolymerization was carried out by placing droplets of [EVIm][TFSI] in the reactor and
introducing EGDA and tert-butyl peroxide initiator in the vapor phase. The heterogeneous films
that formed at the surface of the liquid droplets were composed of a homopolymer PEGDA top
layer that formed by the polymerization of adsorbed EGDA at the liquid—vapor interface and a
poly([EVIm][TFSI]-co-EGDA) copolymer bottom layer that formed by the copolymerization of
[EVIm][TFSI] with absorbed EGDA within the liquid. The copolymer layer contained a gradient
composition with decreasing concentration of [EVIm][TFSI]. We showed that the composition
of the copolymer films can be controlled by tuning the reaction time and pressure. In addition,
we demonstrated that the films can be formed on solid supports which could allow these
materials to be used as separation membranes and catalyst supports.
5.2 Introduction
The polymerization of ionic liquid (IL) monomers is an emerging field that has led to the
creation of a new class of materials known as polymerized ILs (PILs) which are being developed
as ion conducting membranes,
1,2
separation membranes,
3,4
and catalyst supports.
5,6
The
development of PILs is motivated by the need to combine the chemical properties of ILs with the
mechanical properties of polymers.
7
PILs are commonly made by solution-phase, free-radical
polymerization of imidazolium cations with vinyl moieties,
8,9
and the PIL product can be isolated
through precipitation. Other methods used to fabricate PILs include atom transfer
95

polymerization,
10
reversible addition—fragmentation transfer polymerization,
11
and ring-opening
metathesis polymerization.
12

The immobilization of PILs onto polymer supports can be used to reduce the total amount
of IL used,
13
facilitate simple and effective recycling,
14
and improve mechanical properties.
15

Several researchers have demonstrated the use of grafting methods to attach PILs to support
materials. For example, Lozano et al. grafted 1-decyl-2-methylimidazolium cations onto a
polymer matrix to synthesize a support for biocatalysts.
13
Hu et al. grafted poly(ethylene glycol)
onto PILs to make membranes that were less brittle than those made of pure homopolymer PIL.
15

Samadi et al. demonstrated the grafting of PIL onto macroporous cellulose for the separation of
carbon dioxide from a gaseous mixture.
16
In addition to grafting methods, copolymerization can
also be used to immobilize PILs within polymer networks. For example, Xiong and co-workers
studied the copolymerization of a phosphorus IL and ethylene glycol dimethacrylate to form
nanoparticles for use as reusable catalysts.
14,17
Nulwala

and co-workers fabricated a PIL block
copolymer for use as a gas separation membrane with increased permeability.
18
In addition,
Winey and co-workers synthesized PIL diblock copolymers with high ionic conductivity for
energy conversion and storage applications.
19

In this paper, we study the copolymerization of 1-ethyl-3-vinylimidazolium
bis(trifluoromethylsulfonyl)imide ([EVIm][TFSI]) with ethylene glycol diacrylate (EGDA) via
initiated chemical vapor deposition (iCVD) to form PIL copolymer films. In the iCVD process,
vapor-phase monomer and initiator precursors are introduced into a vacuum chamber where a
heated filament array decomposes the initiator into radicals. The monomer and initiator radicals
diffuse to the surface of a cooled substrate where polymerization occurs through a free-radical
mechanism.
20,21
The advantages of the iCVD technique over other polymerization methods are
96

that substrates with complex geometries can be coated,
22,23
the functionality of the film can be
tuned by varying the monomer,
24-28
polymers can be deposited at low substrate temperatures,
20,29

and the polymer thickness and growth rate can be controlled in situ.
30,31
The iCVD process is
typically used to deposit coatings onto solid substrates,
32-35
however we were recently the first
group to deposit polymers onto liquid substrates such as ILs and silicone oils.
36,37
These liquids
have extremely low vapor pressures and are therefore stable in our vacuum system. We found
that polymerization can occur at both the liquid—vapor interface and within the liquid for cases
in which the monomer is soluble,
38,39
and we demonstrated that we can make nanoparticles,
40

free-standing films,
37
encapsulated liquid droplets,
41
layered films,
38
and polymer—IL gels.
39
Our previous studies involved nonreactive liquid substrates. In this work, we use a reactive IL as
the substrate which copolymerizes with vapor-phase precursors to form PIL copolymer films at
the liquid surface. We show that process conditions such as reaction time and pressure can be
controlled to tune the composition and thickness of the copolymer films. We also demonstrate
that we can form the films on wire mesh supports which is useful for using these films as
separation membranes,
42-44
solid-state electrolytes,
45-47
and catalyst supports.
5,48,49
5.3 Experimental
1-Ethyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide ([EVIm][TFSI]) (98%
IoLiTec), ethylene glycol diacrylate (EGDA) (97% Monomer-Polymer), and tert-butyl peroxide
(TBPO) (97% Sigma-Aldrich) were all used as received without further purification. All
polymerization reactions were performed in a custom-built reactor chamber (GVD Corp, 250
mm diameter, 48 mm height). Droplets (12 µL) of [EVIm][TFSI] were placed on a glass slide
which was then placed on the reactor stage maintained at 30 C by a recirculating chiller. To test
the homopolymerization of [EVIm][TFSI], TBPO initiator was flown at room temperature into
97

the chamber through a mass flow controller at 1.5 standard cubic centimeters per minute (sccm)
with the filament heated to 230 C for 60 minutes at a reactor pressure of 50 mTorr. After
polymerization, the droplets were washed with ethanol to precipitate the polymer which was then
purified by alternating washes in acetone and ethanol. Nuclear magnetic resonance (NMR)
spectroscopy was used to confirm the polymerization of [EVIm][TFSI] using deuterated acetone
as the solvent and performing 60 scans from 0 to 10 ppm with a 10 second delay time. The
copolymerization of [EVIm][TFSI] was carried out by flowing TBPO and EGDA into the reactor
chamber at 1.5 and 1.0 sccm, respectively. The EGDA flow rate was achieved by heating the
monomer jar to 35 and 40 C for depositions at 30 and 50 mTorr, respectively, and the line
temperature was kept 15 C above the jar temperature. The deposition rate was measured on a
reference silicon wafer using an in situ 633 nm helium—neon laser interferometer (Industrial
Fiber Optics).
After deposition, the polymer films were removed from the liquid surface by detaching
the films at the edges of the droplets using tweezers and washed in an acetone bath for 2 minutes
followed by an ethanol bath for 2 minutes. Fourier transform infrared spectroscopy (FTIR)
(Thermo Nicolet iS10) was used to analyze the bulk composition of the PIL copolymer films.
The concentration of [EVIm][TFSI] in the films was measured by taking the ratio of the FTIR
peak area of the EGDA carbonyl stretching at 1732 cm
-1
to the [EVIm][TFSI] imidazolium
stretching vibrations between 3050 and 3200 cm
-1
. The peak ratio was compared to a calibration
curve made from solutions with varying concentrations of EGDA and [EVIm][TFSI]. The
reported values for the FTIR peak ratio and calculated [EVIm][TFSI] wt % are an average of 10
total samples from two depositions. Each FTIR sample was composed of three films on a silicon
wafer and spectra were collected by performing 200 scans from 400 to 4000 cm
-1
.
98

X-ray photoelectron spectroscopy (XPS) (Surface Science MProbe) was used to measure
the atomic composition of the top (vapor) and bottom (liquid) sides of the PIL copolymer films
using a monochromatic Al K  X-ray source. Survey spectra were collected from 0 to 1000 eV at
a step size of 1eV and a total of five scans. The reported atomic compositions of the films are an
average of four total samples from two depositions. In order to analyze the bottom side, the films
were removed from the liquid droplets by placing a piece of copper tape (Ted Pella, Inc.) on a
silicon wafer and pressing the tape onto the top of films. The removed films were then washed in
an acetone bath for 2 minutes followed by an ethanol bath for 2 minutes. In order to analyze the
top side, the films were removed from the liquid droplets by pressing a bare silicon wafer on the
top of the films. When the silicon wafer was lifted off the droplets, the exposed bottom side of
the films was gently washed with acetone and a piece of copper tape was pressed onto the
bottom side of the films resulting in the top side of the films being exposed for analysis. These
films were then washed for 2 minutes in acetone followed by 2 minutes in ethanol followed by
an additional 24 hours in an acetone bath to remove residual IL trapped between the film and the
tape.
The total film thickness was measured by imaging the cross-section of the films that had
been prepared for XPS analysis using a scanning electron microscope (SEM) (Topcon, Aquila
hybrid SEM). The cross-section of the films was prepared by dipping the films mounted on
copper tape into liquid nitrogen for 30 seconds and then cutting the samples in half using a razor
blade. The reported thicknesses are an average of measurements taken on three different films,
with three locations imaged on each film, and five measurements taken 5 µm apart at each
location.
99

The equilibrium concentration of EGDA absorbed in [EVIm][TFSI] was measured using
a quartz crystal microbalance. The mass of EGDA adsorbed at the liquid surface was estimated
by measuring the mass uptake of EGDA onto a bare crystal. The concentration of EGDA
absorbed in the bulk liquid was calculated by measuring the mass uptake of EGDA onto a crystal
that contained a thin layer of [EVIm][TFSI] and subtracting the previously measured surface
adsorption. The measurements were conducted under the deposition conditions, however the
initiator flow was replaced with nitrogen to measure only the uptake of EGDA into the liquid
because nitrogen does not significantly absorb. For each measurement, the system was allowed
to equilibrate for 20 minutes and the reported values are an average of five trials.
5.4 Results and Discussion
We first studied homopolymerization of [EVIm][TFSI] by placing droplets of the IL on a
glass slide and flowing TBPO initiator into the iCVD chamber with the filament heated for 60
minutes. Rinsing the IL droplets with ethanol led to the precipitation of polymer which
confirmed that radicals delivered through the vapor phase can initiate the polymerization of
[EVIm][TFSI]. The NMR spectrum of the purified poly([EVIm][TFSI]) contains the
characteristic signal for hydrogen on the polymer backbone at 4.2 ppm and does not contain the
vinyl signals for the [EVIm][TFSI] monomer at 5.5, 6.0, or 7.4 ppm confirming a free-radical
polymerization mechanism (Figure 5-1).
50
FTIR spectroscopy also verified the polymerization
of [EVIm][TFSI] by the disappearance of the signal for the vinyl bond
51
at 1660 cm
-1
. Our
previous works have exclusively studied nonreactive liquid substrates, and this is the first
demonstration that ILs can be polymerized in the iCVD process.
100

Figure 5-1 Comparison of the NMR spectra of [EVIm][TFSI] monomer and
poly([EVIm][TFSI]) formed by the introduction of TBPO initiator.

Figure 5-2 Schematic of the iCVD process for the copolymerization of
[EVIm][TFSI] with EGDA delivered through the vapor phase.

101

We then studied the copolymerization of [EVIm][TFSI] by introducing EGDA and
TBPO in the vapor phase (Figure 5-2). Since EGDA and TBPO are soluble in [EVIm][TFSI],
we expect polymerization to occur at both the liquid—vapor interface and within the IL as we
have shown in other soluble systems.
38,39
Reactions for 40 minutes at 50 mTorr resulted in the
formation of PIL copolymer films at the liquid surface which completely encapsulated the IL
droplets. The films were removed from the liquid surface and thoroughly washed. To determine
the structure of the films, the atomic compositions of the top (vapor) side and bottom (liquid)
side of the films were measured using XPS which probes approximately 5 nm of the sample
surface (Table 5-1). The top side of the films was composed of only carbon and oxygen
indicating a top layer of homopolymer PEGDA that was formed by the polymerization of
adsorbed EGDA at the liquid—vapor interface.
38
The high atomic concentration of carbon on the
top side (83 ± 1 %) compared to the PEGDA reference (70 ± 1 %) is due to carbon
contamination from the solvent washes which was confirmed by a measured increase in the

Table 5-1 Atomic compositions measured by XPS of the top and bottom sides of
the PIL copolymer films from 40 minute reactions compared to reference PEGDA
and poly([EVIm][TFSI]).

Atomic Composition
Sample % C % O % F % N % S
PEGDA Reference 70 1 30  1 0  0 0  0 0  0
Poly([EVIm][TFSI]) Reference 31  4 18  1 28  4 13  1 10  1
Top (Vapor) Side 83  1 17  1 0  0 0  0 0 0
Bottom (Liquid) Side 57  3 20  1 13  1 6  1 4  1

102

carbon concentration (75 ± 1 %) when the PEGDA reference was exposed to the same washing
procedure as the top side of the films. The bottom side of the films contained fluorine, nitrogen,
and sulfur which indicates the presence of [EVIm][TFSI] but the concentrations were less than
the reference poly([EVIm][TFSI]) which indicates the presence of both monomers on the bottom
side of the films. This was further confirmed by the carbon and oxygen concentrations which
were between poly([EVIm][TFSI]) and PEGDA. This copolymer bottom layer was formed by
the copolymerization of [EVIm][TFSI] with EGDA within the liquid, as we have previously
shown that soluble precursors can absorb and also diffuse through polymer films at the liquid
surface and then polymerize within the liquid.
38
We estimated the concentration of
[EVIm][TFSI] on the bottom side of the films to be 38  4 wt % from the nitrogen and carbon
concentrations. To estimate the thickness of the copolymer layer, we measured the total
thickness of the films from cross-sectional SEM images to be 1.6 ± 0.1 µm, and we assumed that
the thickness of the top PEGDA layer was similar to the thickness of polymer formed on the
reference silicon wafer (0.7 ± 0.1 µm) because we have previously shown that the molecular
weight of polymer formed at the surface of liquid substrates and on reference silicon are similar
indicating comparable rates of polymerization.
39
We estimated the thickness of the copolymer
layer to be 0.9 ± 0.1 µm which makes up a significant portion of the films formed by 40 minute
reactions.
FTIR analysis of the PIL copolymer films also confirmed the incorporation of both
monomers by the presence of the carbonyl peak at 1732 cm
-1
characteristic of PEGDA
52
and the
imidazolium C−H symmetric stretching vibrations
51
between 3050 and 3200 cm
-1
and C=N
stretching
53
at 1560 cm
-1
characteristic of [EVIm][TFSI]

(Figure 5-3a). We have previously
shown that nonreactive IL integrated into polymer films can be removed by soaking the films in
103

Figure 5-3 FTIR spectra of (a) the PIL copolymer films from 40 minute reactions
compared to reference PEGDA and poly([EVIm][TFSI]) and (b) the carbonyl
peak of PIL copolymer films from varying reaction times compared to reference
PEGDA.

a solvent bath overnight.
38
In contrast, we found that [EVIm][TFSI] could not be removed from
the films even after soaking in an acetone bath for one week further confirming that
[EVIm][TFSI] is copolymerized with EGDA. The copolymerization was also confirmed by the
broadening of the carbonyl peak toward lower wavenumbers in the PIL copolymer films for a
range of reaction times compared to the PEGDA reference (Figure 5-3b) which is likely due to
the presence of hydrogen bonding in the copolymer layer.
54-56
The weak signal for the vinyl
bond in the spectra of the PIL copolymer films was determined to be unreacted [EVIm][TFSI]
104

because trace amounts of the IL monomer were detected in the solvent bath using NMR
spectroscopy. Furthermore, there is no vinyl signal in the FTIR spectrum of the PEGDA
reference, indicating that all the vinyl bonds are reacted, and similarly we expect that all the
vinyl bonds of EGDA are reacted in the PIL copolymer films.
To study the growth of the films as a function of time, we varied the reaction time
between 5 and 90 minutes at a constant reactor pressure of 50 mTorr and deposition rate of 18 
2 nm/min as measured on a reference silicon wafer using in situ interferometry. The composition
of the bottom side of the films measured by XPS survey scans showed that the concentrations of
fluorine, nitrogen, and sulfur decreased between 5 and 20 minutes reflecting a decrease in the
concentration of [EVIm][TFSI] from 79 ± 12 to 60 ± 13 to 37 ± 7 wt % at 5, 10, and 20 minutes,
respectively, whereas the composition did not significantly change for reaction times between 20
and 90 minutes (Figure 5-4). The total thickness of the films was measured to increase linearly
between 5 and 90 minutes at a rate of 23 nm/min (Figure 5-5). By subtracting the growth rate of
the top PEGDA layer (18 nm/min), the growth rate of the copolymer layer was calculated to be 5
nm/min. The constant growth rate of the copolymer layer confirms that polymerization continues
to occur within the liquid over the entire range of reaction times. It is important to note that the
copolymer layer formed in the first 5 minutes had a high growth rate of 182 nm/min, which is
due to a high absorption of precursors into the bulk liquid, while the subsequent slow growth rate
for reaction times longer than 5 minutes (5 nm/min) is due to the lower flux of precursors
through the film already formed at the liquid surface.
38
We combined the XPS and SEM results
to illustrate both the composition and thickness of the PIL copolymer films as a function of time
as shown in Figure 5a. The copolymer layer contained a gradient composition with a decreasing
concentration of IL from ~80 to ~35 wt %. The decrease in the concentration of [EVIm][TFSI]
105

Figure 5-4 (a) Atomic concentrations of fluorine, nitrogen, and sulfur measured
by XPS and (b) the corresponding concentration of [EVIm][TFSI] on the bottom
side of the films as a function of reaction time at a pressure of 50 mTorr.

on the bottom side of the films from 5 to 20 minutes is likely due to the accumulation of
absorbed EGDA monomer in the liquid. We confirmed the presence of unreacted EGDA in the
liquid after 20 minute reactions using NMR to detect the characteristic vinyl signals of the
EGDA monomer at 5.9, 6.2, and 6.4 ppm which are unique from the vinyl signals of
[EVIm][TFSI]. For reactions longer than 20 minutes, the constant composition of the bottom
side and the constant growth rate of the copolymer layer suggest that polymerization reached a
steady-state condition. To further validate the structures in Figure 5a, we measured the bulk
composition of the films using FTIR and found good agreement with our proposed structures
106

(Figure 5-5c). The concentration of [EVIm][TFSI] within the bulk films was measured to
decrease continuously from 75  3 to 26  9 wt % between 5 and 90 minutes. The steady
decrease in the concentration is due to the higher growth rate of the PEGDA layer (18 nm/min)
compared to the copolymer layer (5 nm/min).

Figure 5-5 (a) Total film thickness as a function of reaction time measured by
SEM cross-sectional images. Schematics depict the thickness and composition of
the PEGDA and copolymer layers as determined by SEM and XPS analysis. (b)
SEM cross-sectional images of PIL copolymer films made by 5 and 90 minute
reactions. (c) Concentration of [EVIm][TFSI] in the bulk films measured by FTIR
compared to the estimated concentration calculated from the structrues shown in
part a.
107

In order to increase the concentration of IL on the bottom side of the films, we studied the
effect of decreasing the reactor pressure. In the iCVD process, decreasing the reactor pressure
from 50 to 30 mTorr decreases the concentration of EGDA adsorbed at the liquid—vapor
interface
24,30
as measured by a decrease in the deposition rate measured on a reference silicon
wafer from 18  2 to 6  1 nm/min. Decreasing the pressure also leads to a decrease in the
equilibrium concentration of EGDA absorbed in [EVIm][TFSI] from 23  2 to 6  2 wt % as
measured by a quartz crystal microbalance. We compared films from 40 minute reactions and
found that decreasing the reactor pressure from 50 to 30 mTorr resulted in an increase in the
concentration of [EVIm][TFSI] on the bottom side from 38 ± 4 to 58 ± 12 wt %. The total film
thicknesses were found to be 1.0 ± 0.1 and 1.6 ± 0.1 for 30 and 50 mTorr, respectively. The
thicknesses of the PEGDA top layers were estimated to be 0.2 ± 0.1 and 0.7 ± 0.1 µm for 30 and
50 mTorr, respectively, and the thicknesses of the copolymer layers were calculated to be 0.8 ±
0.1 and 0.9 ± 0.1 µm, respectively. The thickness of the PEGDA layer decreases with decreasing
pressure due to the lower deposition rate at the liquid surface; however, the thickness of the
copolymer layer was similar in both cases likely because there is less resistance to the diffusion
of precursors into the liquid at the lower pressure due to the thinner film at the liquid surface.
Additionally, the concentration of [EVIm][TFSI] in the bulk films as measured by FTIR
increased from 53 ± 7 to 86 ± 1 wt % with decreasing pressure due to both the thinner PEGDA
layer and the higher concentration of [EVIm][TFSI] on the bottom side of the films.
While the PIL copolymer films are strong enough to be removed from the liquid surface
using tweezers, the films do not hold their shape and fold over on themselves. In order to control
the shape of the films for different applications, we formed the films on supports by placing a
piece of wire mesh over the top of the liquid. After the reaction, the wire mesh was removed
108

from the liquid surface and thoroughly washed. The PIL copolymer films formed over the entire
area of the mesh and filled the spaces between the wire grid (Figure 5-6a,b). The polymer films
on the mesh supports were robust enough to be folded to a bend 1.5 mm wide (Figure 5-6c,d).

Figure 5-6 (a,b) PIL copolymer films formed on wire mesh supports. (c)
Supported films can withstand being folded to a bend with 1.5 mm diameter. (d)
Cross-sectional image of the bend in part c.

5.5 Conclusions
We have demonstrated the ability to copolymerize [EVIm][TFSI] with vapor-phase
precursors in the iCVD system to fabricate PIL copolymer films. The films were heterogeneous
composed of a homopolymer PEGDA top layer and a copolymer poly([EVIm][TFSI]-co-EGDA)
bottom layer. The PEGDA layer was formed by the polymerization of adsorbed EGDA at the
liquid—vapor interface whereas the copolymer layer was formed by the copolymerization of
[EVIm][TFSI] with absorbed EGDA within the liquid. We studied the growth of the PIL
copolymer films by varying the reaction time and found that the copolymer layer has a high
growth rate (182 nm/min) and a high concentration of [EVIm][TFSI] on the bottom side (~80 wt
%) for short reactions of 5 minutes. With increasing reaction time between 5 and 20 minutes, the
concentration of [EVIm][TFSI] decreases on the bottom side from ~80 to ~35 wt %, before
reaching a steady-state condition for reaction times longer than 20 minutes. For all reaction times
longer than 5 minutes, the copolymer layer grows at a constant rate of 5 nm/min. The
109

concentration of [EVIm][TFSI] on the bottom side of the films could be increased by decreasing
the reactor pressure. The ability for both the reaction time and pressure to be easily tuned in the
iCVD process enables the relative thicknesses of the PEGDA and copolymer layers to be
controlled and the reaction conditions to be optimized to obtain high concentrations of
[EVIm][TFSI]. Furthermore, the PIL copolymer films can be formed on wire mesh supports to
be shaped for different applications in separations and catalysis.
5.6 Acknowledgments
Acknowledgment is made to the Donors of the American Chemical Society Petroleum
Research Fund for partial support of this research. L.C.B. is supported by a National Science
Foundation Graduate Research Fellowship under Grant DGE-0937362. We thank the Molecular
Materials Research Center of the Beckman Institute at the California Institute of Technology for
use of their XPS.
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112

Chapter 6 Encapsulation of Ionic Liquid Droplets in Polymer
Shells via Vapor Phase Polymerization

Publication Citation: L. C. Bradley, M. Gupta. Encapsulation of Ionic Liquids within Polymer
Shells via Vapor Phase Deposition. Langmuir 2012, 28, 10276-10280.

113

6.1 Abstract
We demonstrate the use of vapor phase deposition to completely encapsulate ionic liquid
(IL) droplets within robust polymer shells. The IL droplets were first rolled into liquid marbles
using poly(tetrafluoro ethylene) (PTFE) particles because the marble structure facilitates
polymerization onto the entire surface area of the IL. Polymer shells composed of 1H,1H,2H,2H-
perfluorodecyl acrylate crosslinked with ethylene glycol diacrylate (P(PFDA-co-EGDA)) were
found to be stronger than the respective homopolymers. Fourier transform infrared spectroscopy
showed that the PTFE particles become incorporated into the polymer shells. The integration of
the particles increased the rigidity of the polymer shells and enabled the pure IL to be recovered
or replaced with other fluids. Our encapsulation technique can be used to form polymer shells
onto dozens of droplets at once and can be extended to encapsulate any low vapor pressure liquid
that is stable under vacuum conditions.
6.2 Introduction
Ionic liquids (ILs) have recently attracted significant interest as environmentally-friendly
alternatives to traditional organic solvents because they are non-volatile, non-flammable, and can
be easily recycled.
1,2
A significant amount of research has been focused on sing ILs in chemical
synthesis,
3
cellulose dissolution,
4
and thermal energy storage.
5
ILs have also been shown to
absorb harmful gases such as carbon dioxide
6
and sulfur dioxide.
7
Immobilization and
encapsulation of ILs is important to implementing ILs at the industrial scale to bypass issues
caused by their high viscosity, and offers a way to increase the surface area to volume ratio for
specific applications, such as gas absorption.
8,9

114

In this chapter, we demonstrate the ability to simultaneously encapsulate dozens of
millimeter-sized IL droplets in robust polymer shells using initiated chemical vapor deposition
(iCVD). The iCVD process is a one-step, solventless polymerization technique in which
monomer and initiator vapors are flown into a vacuum chamber and a heated filament array
decomposes the initiator into radicals.
10,11
The initiating radicals and monomer molecules diffuse
to a cooled stage and polymerization occurs on the surface of the substrate via a free radical
mechanism. ILs have been immobilized onto the surfaces of solid supports such as porous silica
particles,
12,13
sol-gel materials,
14-17
polymer membranes,
18,19
and particles;
20
and they have been
encapsulated in polymer materials using suspension spraying,
9
emulsion polymerization,
21
and
microfluidic processes.
22
The iCVD technique is unique from conventional encapsulation
methods because it enables ILs to be encapsulated within a wide range of polymers including
insoluble fluoropolymers and cross-linked polymers. Also, iCVD does not require the use of
solvents, and therefore the IL is not lost to a solvent phase during encapsulation.
The iCVD technique is typically used to deposit polymer coatings onto solid
substrates;
23,24
however we have recently shown that low vapor pressure liquids such as ILs and
silicone oil can also be introduced into the iCVD process.
25,26
In our first study, we examined
surface versus bulk polymerization of hydroxyethyl methacrylate and 1H,1H,2H,2H-
perfluorodecyl acrylate at different temperatures. In our next study, we compared polymerization
on ILs versus silicone oil surfaces. We found that continuous films formed on the IL whereas
only polymer particles formed on the silicone oil. In our current study, we demonstrate for the
first time that we can completely encapsulate IL droplets within polymer shells. The IL droplets
are first rolled into liquid marbles using micron-sized particles. The IL marbles are then coated
115

on a bed of particles which enables the encapsulated IL droplets to remain intact when they are
removed from the substrate.
6.3 Experimental
1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF
4
]) (97%, Aldrich),
1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (97% Aldrich), ethylene glycol diacrylate
(EGDA) (97%, Monomer-Polymer and Dajac Labs), tert-butyl peroxide (TBPO) (98%, Aldrich),
and poly(tetrafluoroethylene) (PTFE) particles (Aldrich, 35µm) were used without further
purification. Marbles were made by dispensing 2 mm diameter droplets of IL into a Petri dish (5
cm diameter) containing 0.5 grams of PTFE particles. The Petri dish was tilted until the IL
droplet was completely covered with PTFE particles. The marbles were then transferred to
another Petri dish (5 cm diameter) containing 1.5 grams of PTFE uniformly spread across the
bottom. The dish was then placed into the iCVD chamber. All polymer depositions were carried
out in a custom designed reaction chamber (GVD Corp, 250 mm diameter, 48 mm height). A
nichrome filament array (80% Ni, 20% CR, Omega Engineering) was placed 32 mm above the
substrate and was resistively heated to 250°C. The TBPO initiator was maintained at room
temperature and flowed into the reactor at a rate of 1.35 sccm using a mass flow controller
(Model 1479A, MKS) for all polymer depositions. The stage temperature was maintained at
30°C using a recirculating chiller. For all depositions, the PFDA and EGDA monomers were
heated to 50°C and 35°C respectively. The reactor pressure was kept constant at 80 mTorr for the
depositions of PEGDA and P(PFDA-co-EGDA) and 60 mTorr for the deposition of PPFDA.
Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet iS10) was used to analyze the
polymer coatings and the bulk IL. X-ray photoelectron spectroscopy (XPS) experiments were
carried out using a Surface Science Instruments M-Probe spectrometer with a monochromatic Al
116

K  X-ray source. High resolution spectra were taken between 280 and 296 eV binding energies
with a step size of 0.065 eV. Scanning electron microscopy (SEM) ( JEOL-6610) was used to
visualize the polymer shells.
6.4 Results and Discussion
Figure 6-1a shows a schematic of our
fabrication process. To ensure that the entire surface
area of the IL was coated during deposition, we first
rolled 2 mm diameter droplets of IL into liquid
marbles using 35 µm diameter
poly(tetrafluoroethylene) (PTFE) particles.
Aussillous and Quéré were the first to fabricate liquid
marbles in 2001 by rolling water droplets over
hydrophobic silane-treated lycopodium grains.
27
Gao
and McCarthy used this rolling technique to fabricate
IL marbles using hydrophobic oligomeric and
polymeric tetrafluoroethylene particles.
28
For our
study, 1-ethyl-3-methylimidazolium tetrafluoroborate
([emim][BF
4
]) was chosen as the model IL because
its low viscosity (37.7 cP
29
) relative to other
alkylimidazolium-based ILs enabled small marbles to
be made reproducibly and its yellow color allowed
for visualization of the IL.
Figure 6-1 a) Schematic of our process
for encapsulating ionic liquids in polymer
shells via iCVD. b) IL marbles sitting on
the surface of a water bath that were
coated on a bed of loose PTFE (left) and
a bare Petri dish (right). The polymer
shell to the left retains the yellow IL
whereas the marble to the right had a hole
torn in the polymer coating and the IL
leaked into the water bath.
117

We placed our IL marbles in Petri dishes filled with 1.5 grams of loose PTFE particles
and inserted the Petri dishes into the iCVD reactor. A thinner layer of PTFE would increase the
deposition onto the IL, however the bed of loose PTFE tends to shift during the pump down of
the reactor. We use excess PTFE to ensure that the entire surface of the Petri dish remains
covered. We then continuously flowed gaseous monomer and initiator molecules into the iCVD
chamber. The heated filament array inside the chamber cleaved the initiator molecules into free
radicals. These free radicals and the monomer molecules diffused to the surface of the marbles
and polymerized via a free radical mechanism. The PTFE particles on the marble surface created
an air gap between the IL and the surface underneath the marble which enabled initiator and
monomer molecules to diffuse and polymerize on the underside of the IL. The iCVD technique
can uniformly coat surfaces with features on the order of microns;
30
therefore, using 35 µm
diameter PTFE particles ensured that polymerization occurred on the entire surface of the IL and
formed a continuous shell. The IL marbles were coated with three different polymer shells:
poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA), poly(ethylene glycol diacrylate)
(PEGDA), and PFDA cross-linked with EGDA (P(PFDA-co-EGDA)). These compositions were
chosen because PFDA polymerizes only at the surface of the IL since it does not absorb into the
bulk IL and EGDA enhances the mechanical strength of the polymer shell through cross-linking.
It was necessary to coat the IL marbles on a bed of loose PTFE to prevent bridging between the
polymer deposited onto the marble and the Petri dish. Figure 6-1b compares IL marbles sitting
on the surface of a water bath that were coated on a bed of loose PTFE and a bare Petri dish.
When a coated marble was lifted off the bare Petri dish, a hole was torn in the polymer shell
where it was connected to the Petri dish and therefore the yellow IL leaked out when placed on
the water bath. In contrast, the IL marble coated on a bed of loose PTFE particles retained the
118

yellow IL inside the polymer shell when placed on water demonstrating that the polymer shell
was continuous. When the marble was removed from the bed of PTFE after deposition, the
polymer was torn between the loose PTFE particles preserving the continuous polymer shell. We
tried to dye the IL to measure the release rate of IL from the polymer shells but we were unable
to find a dye that was fully miscible with the IL. Instead, we placed ten coated IL marbles on a
piece of cellulose paper to measure the release rate. If the IL leaked, it would show up on the
cellulose paper which readily wicks IL. After 22 days, there was no IL on the cellulose paper
indicating no penetration of the IL through the polymer shells. Figure 6-2 shows that our method
can be scaled down to smaller droplets by dispensing smaller volumes of IL.

The mechanical strength of the three polymer shells was compared by dropping uncoated and
coated marbles (~2 to 3 µm thick coatings on a reference wafer) onto a water bath from a height
of 0.5 inches (Figure 6-3). This drop method enabled us to determine if the polymer shells
remained continuous after impact by observing the retention of the yellow IL inside the polymer
shells. The uncoated marbles and the PPFDA and PEGDA polymer shells broke on impact with
the water bath whereas the P(PFDA-co-EGDA) polymer shell remained intact. We compared the
rigidity of the three polymer compositions by depositing coatings onto [emim][BF
4
] droplets that
were placed directly onto a silicon wafer and covered with a layer of PTFE particles since this
produced large (~5 mm diameter) and relatively flat coatings that could be easily examined. We
Figure 6-2 Coated marbles of decreasing volume.
119

found that the P(PFDA-co-EGDA) polymer coatings were robust enough to be easily lifted off
the IL droplets with tweezers in one continuous piece whereas the PPFDA and PEGDA coatings
ripped when grasped with tweezers. These observations and the drop method results suggest that
the P(PFDA-co-EGDA) shells have increased mechanical strength relative to either the PPFDA
or PEGDA shells.

When the P(PFDA-co-EGDA) coating was lifted off the IL droplet, there was no visible
PTFE at the surface or within the bulk IL which suggested that the PTFE particles became
incorporated into the polymer coating. Fourier transform infrared (FTIR) spectroscopy was used
to verify the integration of the PTFE particles into the polymer coating (Figure 6-4a). The
spectrum of the polymer coating shows the characteristic CF
2
wagging vibrations for PTFE at
504 and 556 cm
-1
which are distinct from the reference P(PFDA-co-EGDA) spectrum in the 450-
600 cm
-1
region verifying the integration of the PTFE into the polymer coating.
31
The polymer
coating also contains IL identified by the presence of the imidazolium ring C-H symmetric
stretching vibrations between 3050 and 3250 cm
-1
.
32
The bulk [emim][BF
4
] was tested to confirm
the absence of PTFE. The spectrum does not have the characteristic PTFE signals and is nearly
identical to the reference [emim][BF
4
] spectrum displaying the representative C-H out of plane
Figure 6-3 a) Uncoated marble and marble coated with P(PFDA-co-EGDA)
sitting on a glass slide 0.5 inches above the water surface. b) Coated marble
remains intact after being dropped. c) Uncoated marble breaks when dropped and
the PTFE scatters on the surface of the water.
120

bending vibration at 521cm
-1
and the C-H symmetric stretching vibrations of the imidazolium
ring.
33
FTIR cannot distinguish the location of the PTFE particles within the film, therefore we
used X-ray photoelectron spectroscopy (XPS) to study the chemical composition of the top and
bottom surfaces of the film since XPS only probes the top 5 nm of the surface (Figure 6-4b).
The top of the sample is defined as the polymer-air interface and the bottom of the sample is the
polymer-IL interface. Neither spectrum contained an overwhelming CF
2
signal at 292 eV which
is characteristic of PTFE.
34
This indicates that the PTFE particles are buried within the polymer
coating. The spectrum of the top surface matched well with the reference spectrum of P(PFDA-
co-EGDA). The intensity of the signal at 292 eV is similar to that in the reference polymer film
indicating that the CF
2
groups in the top of the polymer film are from the P(PFDA-co-EGDA)
Figure 6-4 a) FTIR spectra of a P(PFDA-co-EGDA) coating deposited onto an
[emim][BF
4
] droplet placed directly onto a silicon wafer and covered with a layer of
PTFE particles, a reference P(PFDA-co-EGDA) film deposited onto a bare silicon
wafer, the bulk [emim][BF
4
] after polymer deposition, and reference [emim][BF
4
]. b)
XPS spectra of the top and bottom surfaces of the P(PFDA-co-EGDA) film.

121

polymer and not from the PTFE particles. The spectrum of the bottom surface contained the
three identifying peaks for PEGDA
35
but no identifying peaks for PPFDA. This is likely due to
the fact that only EGDA can absorb into the IL and polymerize within the bulk. The resulting
PEGDA becomes integrated into the bottom portion of the film. The small signal at 292 eV
indicates that some PTFE particles are not completely coated on the bottom side.
To study the effect of the PTFE particles on the rigidity of the polymer coating, we
compared P(PFDA-co-EGDA) coatings deposited onto IL droplets placed on a silicon wafer
without PTFE and with PTFE at the IL surface (Figure 6-5). The smooth film deposited onto the
IL droplet without PTFE rolled up on itself when it was lifted off the IL while the rough film on
the IL with PTFE held its shape after it was removed from the IL demonstrating that the
integration of PTFE increased the rigidity of the polymer shell. The PTFE particles are
Figure 6-5 Schematic and corresponding images of P(PFDA-co-EGDA)
deposited onto IL droplets on a silicon wafer a,c) without PTFE and b,d) with
PTFE at the IL surface. e) The polymer coating lifted off the droplet shown in c
rolled up on itself and f) the polymer coating lifted off the droplet shown in d held
its shape demonstrating that the integration of PTFE increases the rigidity of the
polymer shell.
122

incorporated into the polymer film as shown by the SEM images in Figure 6-6. The reference
polymer was ~3 µm and the thickness of the polymer shell is ~4 µm measured from the cross-
section in Figure 6-6c. The increase in the thickness is most likely due to the presence of IL.

The mechanical strength of the P(PFDA-co-EGDA) polymer shells was tested by
examining their ability to hold their shape under the weight of other coated marbles. When
uncoated and coated IL marbles were stacked in pyramids, the uncoated marbles conformed to
one another because they are malleable while the robust polymer shells of the coated marbles
maintained their shape. We tried using micro-tensile testing
(DEBEN, 5kN) but our films are extremely thin and therefore
the force required for pulling the film is below the threshold
of the instrument. We expect the compression strength would
be very low as well. We were not able to use nanoindentation
because the technique requires smooth films and our coatings
are rough due to the incorporation of PTFE particles.
However, the polymer shells are robust enough for our
intended applications because they can be dropped and
stacked as we show in Figure 6-3 and Figure 6-7. The
Figure 6-6 SEM images of the a) top side, b) bottom side, and c) cross-section of
the P(PFDA-co EGDA) polymer film with PTFE incorporation.
Figure 6-7 Comparison of
uncoated and coated marbles
stacked in pyramid.

123

polymer shells are also strong enough for the IL to be replaced with dyed red water (Figure 6-8).
A small hole was made in the polymer shell, the IL was removed using a syringe, and dyed red
water was injected into the polymer shell through the same hole. Figure 6-8b illustrates that the
polymer shell did not retain its shape when the IL was removed; however, the polymer shell
regained its shape when it was filled with the dyed red water (Figure 6-8c). This showed that the
polymer shells can also contain high vapor pressure liquids that cannot be introduced into the
iCVD vacuum chamber by easily exchanging the interior fluid using a syringe.

5 Conclusions
This work demonstrated the encapsulation of IL droplets within robust polymer shells via
iCVD. The P(PFDA-co-EGDA) polymer shells were stronger than the respective homopolymer
shells. The incorporation of the PTFE particles into the polymer shell eliminated PTFE
contamination of the interior fluid and allowed the coated IL to be treated as a two phase product
consisting of an exterior polymer shell and interior IL that can be easily replaced if necessary.
Encapsulating ILs in solid polymer shells may reduce the effect of their high viscosity which is a
challenge in their implementation into industrial processes. For example, the immobilized IL
droplets can be stacked and used in gas separation processes to allow the desired species to
diffuse into the IL phase while the remaining species can easily flow between the spherical
Figure 6-8 A P(PFDA-co-EGDA) polymer shell a) encapsulating IL, b) after the
IL is removed with a syringe, and c) injected with dyed red water.

124

droplets. It has been shown that small gas molecules such as carbon dioxide and oxygen can
diffuse through dense polymer films.
36-38
Future studies will focus on optimizing the thickness
and chemical functionality of the shell for maximizing gas absorption.
Our encapsulation process can be used to encapsulate low vapor pressure liquids that are
stable under iCVD conditions such as ILs, silicone oil, and glycerol. Our technique can be easily
extended to fabricate stimuli-responsive polymer shells by using precursors such as N-
isopropylacrylamide
39
or o-nitrobenzyl methacrylate.
40

6.6 Acknowledgments
LCB was supported by a fellowship from the Chevron Corporation (USC-CVX UPP). We thank
the Molecular Materials Research Center of the Beckman Institute of the California Institute of
Technology for use of their XPS.

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127

Chapter 7 Conclusions and Future Research

128

7.1 Conclusions
This thesis studied the formation of polymer films on liquid substrates via iCVD. Using
liquid substrates introduces surface tension and solubility effects that are not relevant for solid
substrates. This work aimed to understand how these effects can be controlled to tune the
morphology and composition of polymer films.
The morphology of polymers deposited at liquid—vapor interfaces was shown to depend
on the surface tension interactions between the polymer and the liquid which can be quantified
by the polymer spreading coefficient. For systems with negative spreading coefficients, it is
energetically favorable for the polymer to aggregate. In linear polymer systems we observed the
formation of discrete polymer particles, whereas in cross-linked polymer systems we observed
the formation of microstructured films due to the chemical cross-linked forming long range
networks. For systems with positive spreading coefficients, it is energetically favorable for the
polymer to spread on the surface of the liquid which resulted in continuous, dense films at the
liquid surface for both linear and cross-linked polymers.
The most significant variable influencing the composition of polymer films is the
precursor solubility in the liquid. For systems in which the monomer is insoluble polymerization
only occurs at the liquid—vapor interface, whereas when the monomer is soluble in the liquid
polymerization can also occur within the bulk. We have shown that soluble monomers can
polymerize within the bulk by monomer absorption into liquid or diffusion through polymer
films at the liquid surface, and varying the order in which the soluble and insoluble monomers
are introduced into the system leads to different film structures. The iCVD technique also
enables process parameters such as reaction time and pressure to be easily varied to control the
amount and molecular weight of polymer formed in the bulk to make polymer—ionic liquid gels.
129

The molecular weight of polymer formed in the bulk ionic liquid was orders of magnitude larger
than polymer formed at the liquid—vapor interface and therefore we observed a transition of thin
liquid layers from a viscous liquid to a gel at low polymer concentrations. Reactive ionic liquids
can also be introduced into the iCVD process and copolymerized with vapor phase precursors to
covalently incorporate the ionic liquid functionality into composite films. The films contained a
gradient composition due to transport of vapor phase precursors into the bulk liquid.
In summary, the deposition of polymers onto liquid substrates leads to unique polymer
morphologies and structures compared to solid substrates. Polymer diffusion and aggregation at
the liquid surface leads to unique polymer morphologies and the presence of polymerization
within the bulk liquid can produce unique layered polymer composite and polymer—liquid gels.
The morphology and composition of polymer films formed on liquid substrates are highly
controllable through the tuning of process conditions such as polymer—liquid combinations,
reaction time, pressure, and flow rates to fabricate novel functional polymer films.

7.2 Future Research
This work consisted of fundamental, systematic studies on the effect of iCVD processing
conditions on the morphology and composition of polymer films to elucidate the growth
mechanism on liquid substrates. The next step to progress this area of research is to apply our
fundamental understanding to design functional polymer materials. I believe this work could
have significant impact on two areas: 1) proton conducting membranes for fuel cell applications
and 2) carbon dioxide capture.
First, we demonstrated in Chapter 3 that we could accumulate polymer within thin liquid
layers to form polymer—ionic liquid gels. The overall goal is to maximize the properties of the
130

ionic liquid, specifically to obtain conductivity as close as possible to that of the pure ionic
liquid. Without any optimization, we showed that the conductivity of sold-like gels containing
~29 wt% polymer was on the order of the magnitude as the pure ionic liquid. In this study, we
only evaluated one polymer—liquid system (PHEMA in [emim][BF
4
]), therefore there is a lot of
room to expand the chemistries to included ionic liquids with inherently higher conductivity. In
addition, the polymer chemistries should be expanded to identify polymer—liquid systems that
can form solid-like gels at polymer concentrations lower than ~5wt%, which is the lowest value
reported in literature (by us and other groups). Second, I propose applying our encapsulated ionic
liquids and our polymerized ionic liquid composite films for carbon capture technologies. In
addition to tailoring the IL and polymer compositions, further studies on how to manipulate
liquid volumes to control the size, shape, and number of either liquid droplets or liquid films
inside the iCVD reaction chamber could prove beneficial for designing functional polymer—IL
composite for gas separation applications.

Abstract (if available)

Abstract The initiated chemical vapor deposition (iCVD) process is a vapor phase method used to deposit functional polymer coatings. The technique is typically used to coat solid substrates such as silicon, fibers, and microfluidic devices. We were recently the first group to introduce low vapor pressure liquids, including ionic liquids and silicone oil, into the iCVD process. We found that the deposition onto liquids can result in the formation of either polymer films or polymer particles at the liquid—vapor interface depending on the spreading coefficient of the polymer on the liquid. In addition, precursors that are soluble in the liquid substrate can absorb into the bulk and polymerize resulting in the formation of polymer—liquid gels, layered polymer composite films, and polymer shells. The overall goal of this thesis is to elucidate the growth mechanism of polymer films on liquid substrates and understand how process parameters can be controlled to tailor the morphology and composition of functional polymer materials.

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Asset Metadata

Creator Bradley, Laura C. (author)

Core Title Fabrication of polymer films on liquid substrates via initiated chemical vapor deposition: controlling morphology and composition

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 07/17/2015

Defense Date 05/11/2015

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemical vapor deposition,ionic liquids,OAI-PMH Harvest,thin polymer films

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Gupta, Malancha (committee chair), Malmstadt, Noah (committee member), Thompson, Barry C. (committee member)

Creator Email bradley.laurac@gmail.com,laura.bradley@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-599190

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