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Formation of polymer gels, films, and particles via initiated chemical vapor deposition onto liquid substrates

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Formation of polymer gels, films, and particles via initiated chemical vapor deposition onto liquid substrates

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Content 1

Formation of Polymer Gels, Films, and Particles
via Initiated Chemical Vapor Deposition onto
Liquid Substrates

Robert J. Frank-Finney

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
(CHEMICAL ENGINEERING)

August, 2016

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 is a
technique used to deposit functional polymer coatings typically onto solid substrates. Our
group is the first to introduce low vapor pressure liquids, such as ionic liquids and
silicone oils, as substrates into the iCVD process. The use of liquid substrates adds
additional complexity in the form of solubility and surface tension effects. Precursors that
are soluble in the liquid have the ability to absorb into the liquid as well as adsorb to the
surface and create two distinct locations of polymerization. The polymerization that
occurs at the surface can result in polymer films or particles depending on the surface
tension interactions between the polymer and liquid. Section 1 introduces the iCVD
process and provides background on past works that studied vacuum depositions with
liquid substrates as well as the applications and current methods of the polymer materials
made using liquid substrates: polymer-ionic liquid gels, free standing films, and
nanoparticles. Section 2 investigates the polymerization that occurs within an ionic liquid
layer, focusing on the difference between polymerization within the layer and at the
surface, the effect of polymer accumulation within the liquid layer, and the formation of
gels. Section 3 demonstrates the different polymer morphologies formed at a variety of
liquid surfaces and uses these properties to develop a method of forming shaped free
standing films using a two-liquid system. Section 4 studies the growth mechanism of
polymer nanoparticles formed at the liquid surface, the understanding of which can be
used to synthesize of nanoparticles of a controlled size and to form polymer core-shell
particles.

4

Acknowledgments

I would like to thank my advisor, Dr. Malancha Gupta, for all of the support and
mentorship she has provided over the course of my graduate career. I also thank the rest
of the Gupta lab group for their input and assistance, especially the co-authors of my
work, Patrick Haller and Laura Bradley. I would like to recognize the members of my
qualifying and dissertation committees for their guidance as well as Richard Miltner, Dr.
Giles Dillingham, and Eric Oseas for their professional mentorship. Finally I would like
to thank my family for their support in all my endeavors.

5

Table of Contents

Executive Summary ........................................................................................................................ 3
Table of Contents ............................................................................................................................ 5
List of Figures ................................................................................................................................. 7
List of Tables .................................................................................................................................. 9

1. Introduction ............................................................................................................................. 10
1.1 Initiated Chemical Vapor Deposition ............................................................................................... 11
1.2 Deposition onto Liquid Substrates .................................................................................................... 12
1.3 Polymer-Ionic Liquid Gels................................................................................................................ 13
1.4 Free-Standing Polymer Films ........................................................................................................... 14
1.5 Polymer Nanoparticles ...................................................................................................................... 16
1.6 References ......................................................................................................................................... 18

2. Formation of Polymer-Ionic Liquid Gels Using Vapor Phase Precursors ........................ 25
2.1 Abstract ............................................................................................................................................. 26
2.2 Introduction ....................................................................................................................................... 26
2.3 Experimental ..................................................................................................................................... 28
2.4 Results and Discussion ..................................................................................................................... 30
2.5 Conclusions ....................................................................................................................................... 39
2.6 Acknowledgments ............................................................................................................................. 40
2.7 References ......................................................................................................................................... 40

3. Ultrathin Free-Standing Polymer Films Deposited onto Patterned Ionic Liquids
and Silicone Oil ........................................................................................................................... 45
3.1 Abstract ............................................................................................................................................. 46
3.2 Introduction ....................................................................................................................................... 46
3.3 Experimental ..................................................................................................................................... 48
3.4 Results and Discussion ..................................................................................................................... 50
3.5 Conclusions ....................................................................................................................................... 59
6

3.6 Acknowledgments ............................................................................................................................. 60
3.7 References ......................................................................................................................................... 60

4. Two Stage Growth of Polymer Nanoparticles at the Liquid-Vapor Interface by
Vapor Phase Polymerization ...................................................................................................... 63
4.1 Abstract ............................................................................................................................................. 64
4.2 Introduction ....................................................................................................................................... 65
4.3 Experimental ..................................................................................................................................... 67
4.4 Results and Discussion ..................................................................................................................... 69
4.5 Conclusions ....................................................................................................................................... 78
4.6 Acknowledgments ............................................................................................................................. 79
4.7 References ......................................................................................................................................... 80

7

List of Figures

Figure 2-1. Schematic of the iCVD reactor. .............................................................................................. 31

Figure 2-2. 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][BF4] 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. ............................................. 32

Figure 2-3. FTIR spectra of the PHEMA-[emim][BF4] gel compared to reference PHEMA and
reference [emim][BF4]. The dashed lines indicate the locations of the carbonyl stretching of
PHEMA and the aromatic C-H symmetric stretching of [emim][BF4]. ..................................................... 33

Figure 2-4. GPC chromatographs of a) a 5 minute deposition of PHEMA onto [emim][BF4] at 80
mTorr, b) reference [emim][BF4], and c) reference PHEMA deposited onto a silicon wafer at 80
mTorr using the 2 kDa-2 MDa column. ...................................................................................................... 35

Figure 2-5. GPC chromatographs of PHEMA formed within the [emim][BF4] layer at 80 mTorr
for increasing deposition times using the 300 kDa-20 MDa column. ......................................................... 36

Figure 2-6. GPC chromatographs of a) a 5 minute deposition of PHEMA onto [emim][BF4] 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 minute depositions of PHEMA formed
within the [emim][BF
4
] layer at 120 mTorr using the 300 kDa – 20 MDa column. ................................... 38

Figure 3-1. Images of 15 minutes of deposition of PHEMA onto A) [bmim][PF6], B)
[bmim][BF4], C) [emim][BF4], and D) silicone oil. E-H) The droplets were subjected to a
continuous stream of air to show that a continuous skin of PHEMA formed on the ILs but only
particles of PHEMA formed on the silicone oil. ......................................................................................... 50

Figure 3-2. FTIR spectra of A) a PHEMA film deposited onto a wafer, B) a PHEMA skin formed
on [bmim][PF6], C) a PHEMA skin formed on [bmim][BF4], D) a PHEMA skin formed on
[emim][BF4], and E) PHEMA particles deposited onto silicone oil. ......................................................... 52

Figure 3-3. Images of 60 minutes of deposition of PHEMA onto a [bmim][PF6] droplet that was
placed on A) a silicon wafer and C) a silicon wafer covered with a layer of silicone oil. B,D) The
substrate was titled at a 15 degree angle after the deposition. .................................................................... 53

Figure 3-4. Contact angle goniometer images of A) IL droplets on silicon wafers, B) IL droplets
on a layer of silicone oil, and C) the same droplets after 60 minutes of deposition of PHEMA. ............... 54

8

Figure 3-5. The fabrication method for making shaped polymer films. .................................................... 55

Figure 3-6. Images and corresponding FTIR spectra of free-standing shaped films of A, B)
PHEMA formed on [bmim][BF4] and C, D) PNIPAAm formed on [bmim][PF6]. The films were
removed from the template and placed in a bath of silicone oil for imaging. ............................................. 56

Figure 3-7. Images of 30 minutes of deposition of PPFDA onto droplets of A) [bmim][PF6], B)
[bmim][BF4], C) [emim][BF4], and D) silicone oil placed on a silicon wafer. E) Images of
PPFDA deposited onto a droplet of [bmim][PF6] placed on a silicon wafer covered with a layer
of silicone oil and F) tilted at a 15 degree angle. ........................................................................................ 58

Figure 4-1. A) SEM image and B) DLS histogram of PnBA particles from a 5 minute deposition
at a deposition rate of 30 nm/min onto 100 cst silicone oil. ....................................................................... 70

Figure 4-2. A) Overlaid histograms of the particle size distributions on silicone oil viscosities of
100, 500, and 1000 cst at a deposition rate of 30 nm/min and a deposition time of 5 minutes. B)
Table of the average radius and size distribution for each viscosity. .......................................................... 71

Figure 4-3. Overlaid histograms of the particle size distribution for a range of deposition times
from 0.5 to 30 minutes on a 1000, 500, and 100 cst silicone oil substrate at a deposition rate of 30
nm/min. ....................................................................................................................................................... 72

Figure 4-4. Particle size as a function of deposition time at a deposition rate of 30 nm/min using a
silicone oil viscosity of a) 1000, b) 500, and c) 100 cst. ............................................................................. 73

Figure 4-5. Particle size as a function of deposition time for a range of deposition rates with a
silicone oil viscosity of 1000 cst. ................................................................................................................ 76

Figure 4-6. Schematic of our core-shell deposition process with PnBA as the core and PoNBMA
as the shell. The shell layer can be exposed to UV light and then dissolved. ............................................. 78

9

List of Tables

Table 2-1. The effect of deposition time on the total PHEMA weight percent in the sample as
measured by NMR. ..................................................................................................................................... 31

Table 4-1. Particle size at each viscosity for a range of deposition rates with deposition time
controlled to a constant deposition thickness of 300 nm. ........................................................................... 74

Table 4-2. Particle size for depositions where the deposition rate was initially the same but then
varied during the deposition. ....................................................................................................................... 76

Table 4-3. Particle size for depositions where the order of the deposition rates was reversed. ................. 77

10

1. Introduction

11

1.1 Initiated Chemical Vapor Deposition

The initiated chemical vapor deposition (iCVD) technique is a one-step,
solventless free radical polymerization process that can be used to make a wide range of
functional polymer films such as poly(2-hydroxyethyl methacrylate) (PHEMA),
1
poly(4-
vinylpyridine) (P4VP),
2
and poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA).
3
The
iCVD process is typically used to deposit polymer coatings onto solid substrates such as
silicon wafers,
4
membranes,
5
wires,
6
carbon nanotubes,
7
and fibers.
8
In the iCVD process,
monomer and initiator molecules are flown into a reactor chamber generally maintained
at a pressure between 50 and 500 mTorr. The initiator molecules are broken into free
radicals by a heated filament array (typically set at 200-250°C). The initiating radicals
and monomer molecules diffuse to the substrate which has its temperature maintained by
a backside chiller (typically set at 20-60°C) and polymerization occurs via a free-radical
mechanism on the surface of the substrate. The deposition rate and molecular weight of
the polymer are dependent on the monomer concentration at the surface which is
governed by the ratio of the monomer partial pressure (P
M
) to the saturation pressure
(P
Sat
). If F
M
is the flow rate of monomer into the reactor, F
total
is the total flow rate of all
species into the reactor (monomer and initiator), and P
total
is the total pressure of the
reactor, then the partial pressure of the monomer is P
M
=F
M
(P
total
/F
total
). The saturation
pressure at a given stage temperature (T
stage
) can be approximated using the Clausius-
Clapeyron equation: P
Sat
=Aexp(-ΔH
Vap
/RT
stage
). Therefore we can increase P
M
/P
Sat
by
increasing the flowrate of the monomer, increasing the total pressure, or decreasing the
stage temperature. Since the precursors are in the vapor phase, this is not a line of sight
process making it an ideal process for conformal coatings on complex substrates.
12

1.2 Deposition onto Liquid Substrates

Low vapor pressure liquids such as ILs and silicone oils have recently been used
as substrates in inorganic vacuum deposition processes. The resulting morphology of the
deposited material varies with precursor and the composition of the liquid. For example,
both smooth silver and chromium films
9
and gold nanoparticles
10
have been
demonstrated being deposited onto ILs. Similar variation in morphology has been seen in
the deposition of inorganic materials onto silicone oil in the form of gold,
11
nickel,
12

aluminum,
13
and iron
14
films and silver
15
and copper
16
nanoparticle clusters. Torimoto et
al. found that sputtering gold onto the surface of ILs resulted in the formation of gold
nanoparticles where the size of the nanoparticles depended on the IL used and the
concentration of the nanoparticles depended on the duration of the sputtering process.
10

Similarly, Deng et al. deposited C:F onto a silicone oil substrate resulting in clusters that
aggregated to form fractal structures. They found that increasing the viscosity of the
silicone oil decreased the cluster size.
17
Xie et al. described a two-stage growth model
for the deposition of gold and silver onto silicone oil surfaces with the first stage
involving nucleation and growth of molecules and the second involving diffusion along
the liquid surface.
18
Ye et al. proposed that nanoparticles are formed instead of films due
to an adsorbed layer of oil molecules preventing contact between particles.
15
Liquid
substrates have also been used in the vapor-phase deposition of organic parylene.
19
For
example, Binh-Khiem et al. deposited smooth, transparent parylene films onto glycerin,
liquid paraffin, and 1,3,5-trimethyl-1,1,3,5,5-pentaphenyl-trisiloxane silicone oil.
20

13

1.3 Polymer-Ionic Liquid Gels

Polymer-Ionic liquid gels have gained attention due to the material combining the
unique properties of ILs, such as high ionic conductivity,
21
wide electrochemical
window,
22
and good thermal stability,
23,24
with the mechanical strength and flexibility of
polymers. Polymer-IL gels have the potential to be utilized in a range of applications
including fuel cell membranes,
25,26,27
polymer actuators,
28,29,30
and thin film
transistors.
31,32,33
MacFarlane and coworkers used polymer-ionic liquid gels as the
polyelectrolyte membrane sandwiched between flexible electrodes to create
electromechanical actuations when a voltage was applied.
30
Frisbie and coworkers used
these gels as the dielectric gate material in flexible transistors due to the high dielectric
constants, fast polarization response times, and high operating frequencies of ionic
liquids.
31
A voltage is applied to the gel causing the ions to separate to the poles so that
no current can pass through the gel. The low vapor pressure of the ionic liquids also
means that there is minimal solvent evaporation which is typical of other types of gel
electrolytes. Current methods for fabricating polymer-IL gels include solution phase free
radical polymerization
34,35,36
and co-solvent evaporation.
37,38,39
An example of in situ
polymerization of monomer in IL is a study by Watanabe and coworkers using free
radical polymerization of the vinyl monomers 2-hydroxyethyl methacrylate in 1-ethyl-3-
methylimidazolium tetrafluoroborate
34
and methylmethacrylate in 1-ethyl-3-methyl
imidazolium bis(trifluoromethane sulfonyl)imide
35
to form solid gels. One popular type
of cosolvent evaporation is the self-assembly of triblock copolymers where the end
blocks are hydrophobic and the middle block is hydrophilic. The polymer network is
14

formed based on the hydrophilic/hydrophobic phase separation which allows for solid
gels to form at low polymer concentrations. He et al. reported that the self-assembly of
the tri-block copolymer, polystyrene-block-poly(ethylene oxide)-block-polystyrene,
formed solid gels at polymer concentrations as low as 5 wt % in 1-butyl-3-
methylimidazolium hexafluorophosphate.
40
In a similar study, Imaizumi et al. reported
that the tri-block copolymer polystyrene-block-poly(methyl methacrylate)-block-
polystyrene formed solid gels at concentrations as low as 8 wt % in 1-ethyl-3-
methylimidazolium bis(trifluoromethanesulfonyl)amide.
41

1.4 Free-Standing Polymer Films

Free-standing polymer thin films of controllable configuration and thickness have
demonstrated the potential for multi-functionalization and customization that make them
valuable in a broad range of applications. The ability to carry a charge makes conjugated
polymer films useful in lightweight electrochemical devices
42
, while some fabrications
produce a tunable internal multi-layered organization of the film which is applicable to
optoelectronics
43
, magnetic technologies
44
, and sensor arrays
45
. Free-standing films can
be used as scaffolds for the organization of nanoparticles
46
or, in multi-layer
configurations, to sandwich nanoparticles
47
in order to adjust or enhance the functionality
of the film. A stimuli-responsive surface is a highly valuable function of some polymer
and co-polymer films; these surfaces have a variety of triggers: pH-responsive at
physiological conditions
48
, temperature responsive over adjustable pH range
49
, or pH and
temperature responsive independent of the other
50
. The stimuli-responsive surfaces have
15

applications in drug delivery,
48,50
biomedical and tissue engineering,
49
and can be used as
switches in electrical circuits.
51

There are a variety of methods for fabricating free-standing polymer thin films
including: interface oxypolymerization, self-assembled monolayer (SAM) and
microcontact printing, layer-by-layer assembly (LBL), and spin coating assisted layer-by-
layer assembly (SA-LBL). SAM and microcontact printing patterns a surface into
adsorbing and non-adsorbing regions, adsorption of an initial polymer layer and covalent
attachment of additional layers; this method forms a cross-linked film that requires a
sacrificial layer.
52
The LBL method deposits alternating layers of oppositely charged
polymers using primarily electrostatic forces;
53,54
the film is tightly bound to its substrate
so the substrate plays an active role or remains as a passive support.
53
The SA-LBL is
based on the LBL with the addition of spin coating the layers onto a substrate; this
method produces a highly flat and smooth surface with a versatile interface.
54
The above
methods all involve multiple steps and a substrate or sacrificial layer. Interface
oxypolymerization is a one-pot method that forms a free-standing film at the ionic
liquid/air interface, however the propagation rate is measured in nanometers (nm) per
day.
55
An alternate method of free-standing polymer thin film fabrication is initiated
chemical vapor deposition on an ionic liquid; this is a one pot method that forms a free
standing film with a propagation rate that can be measured in nm/minute.

16

1.5 Polymer Nanoparticles

Polymer nanoparticles is an expanding field that plays a significant role in a wide
variety of applications ranging from photonics
56
to electronics
57
to drug delivery
58,59
.
Most applications require particles of a controlled and uniform size. Thus the method of
fabrication must be able to offer systematic control over the size and distribution of the
manufactured particles while maintaining the functionality of the polymer. There are two
main approaches to the preparation of polymer nanoparticles, dispersion of pre-formed
polymer and the polymerization of monomers.
60

The dispersion of preformed polymer approach has several techniques including
solvent evaporation,
61,62
salting out,
63,64
and dialysis.
65,66
These techniques all involve
dissolving the polymer in an organic solvent and mixing it with a non-solvent so that
upon removal of the organic solvent the polymer will crash out forming nanoparticles.
Solvent evaporation involves dissolving the polymer in an organic solvent and mixing it
with a non-solvent. Upon removal of the organic solvent, the polymer will crash out to
form nanoparticles. In the salting out technique, an aqueous solvent is mixed with the
organic solvent and the addition of salt or sucrose is used to change the solubility of the
aqueous phase and emulsify the polymer solution. The dialysis technique is similar to
solvent evaporation, however instead of mixing the solvent and non-solvent, it uses a
membrane for solvent displacement. The size of the polymer particles in these techniques
is generally based on the rate of solvent evaporation. For example, a solvent evaporation
technique using THF/water solvent systems was used to make polystyrene and
polymethyl methacrylate particles with average particle diameters from 150 – 1000 nm
by varying the initial polymer concentration and solvent mixing ratio.
67
One preformed
17

polymer technique that does not use organic solvents is the rapid expansion of
supercritical solution which dissolves the polymer in a supercritical fluid and rapidly
expands the solution across an orifice or nozzle into ambient air or solution.
68
Meziani et
al. formed poly(heptadecafluorodcylacrylate) (PHDFDA) nanoparticles with an average
diameter of 41±8 nm by dissolving the PHDFDA in supercritical CO
2
and passing the
pressurized solution through a nozzle into ambient water.
69

The polymerization of monomers approach has several techniques including
emulsion,
70,71
surfactant-free emulsion,
72,73
and controlled/living radical
polymerization.
74,75,76
These systems are all similar in that they use radical
polymerization in an emulsified system and so they all contain an aqueous phase, a water
soluble initiator, and monomer. Conventional emulsion polymerization uses a surfactant
as a stabilizer. Initiator and surfactant concentration are common parameters used to
control particle size.
77
For example, Muñoz-Bonilla et al. used emulsion polymerization
to make polymethyl methacrylate nanoparticles of controlled average particle diameter
from 133 - 462 nm with coefficients of variation (C.V.) from 0.01 – 0.14 by altering the
surfactant concentration.
78
Surfactant free emulsion uses ionizable initiators or ionic co-
monomers as stabilizers instead of surfactants. This has the advantage of avoiding the
need for the difficult removal of stabilizing surfactants, however the tradeoff is that it is
difficult to prepare monodisperse and precisely controlled particle sizes which could limit
their utility.
79
Controlled/living radical polymerization techniques, such as atom transfer
radical polymerization (ATRP)
74,75
and reversible addition and fragmentation transfer
chain polymerization (RAFT),
76
use an emulsion system but adds a control agent to limit
the polymerization and thereby control the molar mass and distribution which results in
18

controlled particle sizes.
80
For example, Oh et al. polymerized oligo(ethylene glycol)
monomethyl ether methacrylate via ATRP using a miniemulsion system to synthesize
particles with an average diameter 205 nm with a C.V. of 0.03.
81
A common problem
with controlled/living radical polymerization is the presence of a residual control agent in
the product.
60

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25

2. Formation of Polymer-Ionic Liquid Gels Using Vapor Phase
Precursors

Citation: R. J. Frank-Finney, L. C. Bradley, M. Gupta. Macromolecules 2013, 46, 6852-6857.

26

2.1 Abstract

We studied a new method for preparing polymer-ionic liquid (IL) gels via deposition of
vapor phase precursors onto thin layers of IL. The solubility of 2-hydroxyethyl methacrylate in
1-ethyl-3-methylimidazolium tetrafluoroborate enabled polymerization at both the IL-vapor
interface and within the IL layer. We observed a transition from a viscous liquid to a gel with
increasing polymer concentration. 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.

2.2 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.
82,83
The iCVD process can be used to deposit a wide variety of functional coatings
including hydrophilic,
84
hydrophobic,
85
temperature-responsive,
86
and light-responsive
87

polymers. The iCVD process does not require solvents and can therefore be used to conformally
coat a variety of complex substrates such as fibers,
88
membranes,
89
and wires.
90
We have
27

recently introduced low vapor pressure liquids such as ionic liquids (ILs) and silicone oils as
substrates in the iCVD process.
91,92,93
The introduction of liquid substrates adds complexity
because 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.
93

We use iCVD to deposit polymer onto thin layers of IL to form polymer-IL gels. 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 at the IL-vapor interface as recent studies
have shown that using ILs as solvents in solution phase radical polymerization results in higher
polymerization rates and higher molecular weight polymers compared to organic solvents.
94,95
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.
96
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.
97
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
98
that combine the unique properties of ILs, such as high ionic conductivity,
99,100
wide
electrochemical window,
101,102
and good thermal stability,
103,104
with the mechanical strength and
flexibility of polymers. Current methods for fabricating polymer-IL gels include solution-phase
free-radical polymerization
105,106,107
and cosolvent evaporation.
108,109,110
For example, He et al.
used cosolvent evaporation to self-assemble the triblock copolymer polystyrene-block-
28

poly(ethylene oxide)-block-polystyrene in 1-butyl-3-methylimidazolium hexafluorophosphate to
form gels with polymer concentrations as low as 5 wt.%.
111
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.
112
It is desirable to
develop polymer-IL gels with low polymer concentrations to optimize the properties gained from
the IL such as conductivity,
113
which can be done by increasing the molecular weight of the
polymer to decrease the polymer concentration needed to form a gel.
114,115,116,117
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.
118
We
demonstrate that our method can be used to controllably tune the polymer concentration and
molecular weight distribution to produce polymer-IL gels which have potential applications as
fuel-cell membranes,
119,120,121
polymer actuators,
122,123,124
and thin film transistors.
125,126,127

2.3 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
29

Engineering) was held 32 mm above the substrate and resistively heated to 225 °C. Polymer was
deposited onto ~3 μm thick layers of IL that were spin coated (30 seconds at 750 rpm, 100 μL of
1:3 [emim][BF
4
]:methanol solution) onto 1.5 cm x 1.5 cm silicon wafers which 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 LF-804 (2 kDa-2 MDa) and KD-806 (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 5 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, Inc) 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
30

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
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 Labs 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. In order 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 minutes), and the reported values are an average of 5 trials.

2.4 Results and Discussion

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 2-1. We used HEMA and [emim][BF
4
] as
our model system because HEMA monomer is soluble in [emim][BF
4
],
105
allowing for
31

polymerization at both the IL-vapor interface and within the IL layer. The PHEMA polymer is
also soluble in [emim][BF
4
]
128
due to the ability for the BF
4
anions to form hydrogen bonds with
hydroxyl groups.
129,130

Figure 2-1. Schematic of the iCVD reactor.

We deposited polymer at a rate of 4 nm/min, as measured on a reference silicon wafer, and
varied the deposition time from 5 to 100 min at a reactor pressure of 80 mTorr. 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 2-1). In
the iCVD technique, process parameters such as pressure, substrate temperature, and monomer
and initiator flow rates have a large effect on monomer adsorption,
131
whereas varying the
surface chemistry does not have a significant impact on adsorption. Since our depositions onto
the reference silicon wafer and IL were conducted at the same processing conditions, we
assumed that the monomer adsorption on both surfaces was similar and therefore the mass of the
Table 2-1. 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
32

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 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 2-2a). 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
Figure 2-2. 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][BF4] 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.
33

frequencies exhibiting the behavior of a solid-like gel. An example of a PHEMA-[emim][BF
4
]
gel is shown in Figure 2-2b. 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 SEM images
(Figure 2-2c). The thicknesses of the gels were 3 ± 1, 14 ± 4, and 20 ± 4 μm for polymer
concentrations of 14 ± 1, 29 ± 4, and 43 ± 3 wt %, respectively. The large thickness increase
from 3 to 20 μm 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
132
at 1700 cm
-1
and the aromatic C-H symmetric
stretching of [emim][BF
4
]
133
between 3200-3300 cm
-1
(Figure 2-3). 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.
105

Figure 2-3. FTIR spectra of the PHEMA-[emim][BF4] gel compared to reference PHEMA and
reference [emim][BF4]. The dashed lines indicate the locations of the carbonyl stretching of
PHEMA and the aromatic C-H symmetric stretching of [emim][BF4].
34

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 2-4a represents the molecular weight distribution for the 5 min
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 2-4b) and appears at the same elution volume as PHEMA deposited
onto a reference silicon wafer which has a molecular weight of 1.8x10
4
Da (1.7 PDI) (Figure 2-
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
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.
Since 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.
134
The polymer formed within the
IL layer is larger possibly due to increased propagation rates and decreased termination
rates.
96,135
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.
136,137

35

Figure 2-4. GPC chromatographs of a) a 5 minute deposition of PHEMA onto [emim][BF4] at
80 mTorr, b) reference [emim][BF4], and c) reference PHEMA deposited onto a silicon wafer at
80 mTorr using the 2 kDa-2 MDa column.

As the deposition time increases from 5 to 100 min, the peak representing the polymer
formed within the IL layer increases and broadens (Figure 2-5). The chromatographs have been
normalized to the IL peak since 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 2-1). The broadening of the peak
towards 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
36

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 and/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
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 ± 1wt %), 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 co-workers who showed that the molecular weight of poly(methyl
Figure 2-5. GPC chromatographs of PHEMA formed within the [emim][BF4]
layer at 80 mTorr for increasing deposition times using the 300 kDa-20 MDa
column.

37

methacrylate) polymerized in 1-ethyl-3-methyl-imidazolium ethylsulfate ([EMIM][EtSO
4
])
decreased with increasing reaction time which they attributed to an increase in viscosity leading
to a decrease in the propagation rate constant.
138
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.
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.
85
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 deposition at 120 mTorr using the 2 kDa-2 MDa column (Figure 2-6a) 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 which has a molecular weight of 2.7x10
4
Da (2.2 PDI)
(Figure 2-6b). 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 2-6c).
38

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,
Figure 2-6. GPC chromatographs of a) a 5 minute deposition of PHEMA onto [emim][BF4] 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 minute depositions of PHEMA formed
within the [emim][BF
4
] layer at 120 mTorr using the 300 kDa – 20 MDa column.

39

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.
118

2.5 Conclusions

We have demonstrated that polymer-IL gels can be formed by polymerizing HEMA in
the presence of thin layers of [emim][BF
4
] via iCVD. 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 weights 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
40

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.

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 fellowship from the Chevron
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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45

3. Ultrathin Free-Standing Polymer Films Deposited onto Patterned
Ionic Liquids and Silicone Oil

Citation: R. J. Frank-Finney, P. D. Haller, M. Gupta. Macromolecules 2012, 45, 165-170.

46

3.1 Abstract

In this paper, we studied the vapor deposition of polymers onto the surfaces of silicone
oil and imidazolium-based ionic liquids (ILs). We found that the deposition of poly(2-
hydroxyethyl methacrylate) (PHEMA) and poly(N-isopropylacrylamide) (PNIPAAm) resulted in
polymer particles on silicone oil whereas continuous polymer skins formed on 1-butyl-3-
methylimidazolium hexafluorophosphate ([bmim][PF
6
]), 1-butyl-3-methylimidazolium
tetrafluoroborate ([bmim][BF
4
]), and 1-ethyl-3-methylimidazolium tetrafluoroborate
([emim][BF
4
]). The silicone oil and ILs were patterned onto a common substrate by exploiting
their different wetting properties. Ultrathin free-standing PHEMA and PNIPAAm films of
different shapes were produced by confining the shape of the IL within a wax barrier,
surrounding it with silicone oil, and then depositing the polymer. The silicone oil prevented the
polymer film from connecting to the underlying substrate and maintained the shape of the
polymer film during deposition. Our process allows for multi-dimensional control over the
resulting free-standing film: the area of the shape can be controlled by patterning the IL and the
thickness of the film can be controlled by adjusting the duration of polymer deposition. The films
are highly pure and do not contain any residual monomer or solvent entrapment which extends
their potential applications to include in vivo biomedical research.

3.2 Introduction

The initiated chemical vapor deposition (iCVD) technique is a one-step, solventless free
radical polymerization process that can be used to deposit a wide range of polymer films such as
poly(2-hydroxyethyl methacrylate) (PHEMA),
139
poly(4-vinylpyridine) (P4VP),
140
and
47

poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA).
141
The iCVD technique is typically used
to deposit polymer coatings onto solid substrates such as silicon wafers,
142
membranes,
143

wires,
144
carbon nanotubes,
145
and fibers.
146
We recently demonstrated the ability to deposit
polymer coatings onto ionic liquids (ILs).
147
ILs are salts that are liquids at ambient temperatures
and they have recently attracted significant interest as environmentally-friendly alternatives to
traditional volatile organic solvents because they are non-volatile, non-flammable, and can be
easily recycled.
148,149
Our previous work examined the deposition of PHEMA and PPFDA in the
presence of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF
6
]) droplets. We
found that polymerization occurred either at the vapor–IL interface and/or within the bulk IL
depending on the solubility of the monomer within the IL and the reaction conditions such as the
duration of deposition and stage temperature.
In this paper, we use iCVD to deposit polymers onto silicone oil for the first time. We
observe different polymer morphologies on the silicone oil as compared to the ILs and we
exploit this difference to fabricate ultrathin free-standing polymer films of different shapes by
combining the silicone oil and ILs onto a common substrate. The generality of our fabrication
method is demonstrated for multiple polymers and a range of imidazolium-based ILs. Our ability
to produce free-standing polymer films is useful for a wide variety of applications in optics,
150

sensing,
151,152
and separations.
153
The fabrication of free-standing polymer films typically
requires multiple steps such as spin coating polymers onto sacrificial layers and then removal of
the sacrificial layers using several steps of washing with various solvents.
154,155,156,157,158
The
fabrication process that we present in this paper is environmentally-friendly because no organic
solvents are used in any of the steps. The free-standing polymer films produced by our method
are highly pure and do not contain any residual monomer or solvent entrapment which will allow
48

biomedical researchers to use these films for in vivo applications such as tissue engineering,
surgical applications, and drug delivery.

3.3 Experimental

1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF
6
]) (97%, Aldrich), 1-
ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF
4
]) (97%, Aldrich), 1-butyl-3-
methylimidazolium tetrafluoroborate ([bmim][BF
4
]) (97%, Aldrich), poly(dimethyl siloxane)
(Xiameter PMX-200 350 cSt, Aldrich), 2-hydroxyethyl methacrylate (HEMA) (98%, Aldrich),
N-isopropylacrylamide (NIPAAm) (97%, Aldrich), 1H,1H,2H,2H-perfluorodecyl acrylate
(PFDA) (97%, Aldrich), and tert-butyl peroxide (TBPO) (98%, Aldrich) were used without
further purification. All depositions were carried out in a custom designed reaction chamber
(GVD Corp, 250 mm diameter, 48 mm height). For the deposition of PHEMA, the HEMA
monomer was heated to a temperature of 55°C, the stage temperature was maintained at 35°C
using a recirculating chiller, and the reactor pressure was kept constant at 110 mTorr. For the
deposition of PNIPAAm, the NIPAAm monomer was heated to a temperature of 60°C, the stage
temperature was maintained at 55°C using a recirculating chiller, and the reactor pressure was
kept constant at 100 mTorr. For the deposition of PPFDA, the PFDA monomer was heated to a
temperature of 50°C, the stage temperature was maintained at 35°C using a recirculating chiller,
and the reactor pressure was kept constant at 140 mTorr. For all depositions, a nichrome filament
array (80% Ni, 20% CR, Omega Engineering) was placed 32 mm above the substrate and was
resistively heated to 240°C. The TBPO initiator was maintained at room temperature and flowed
into the reactor at a rate of 0.92 sccm using a mass flow controller (Model 1479A, MKS).
49

The morphology of the polymer on the poly(dimethyl siloxane) silicone oil and the ILs
was tested by first dispensing 5 μL of liquid directly onto a silicon wafer. Contact angles were
then measured using a goniometer (ramé-hart Model 290-F1). After deposition, images of the
polymer on the droplets were taken using a microscope and a Nikon D3000 camera. The
continuity of the skins and particles was tested by subjecting the droplets to a continuous stream
of air. In order to make shaped films, outlines of shapes were first drawn onto the substrate with
a wax crayon using a ruler and then IL was dispensed into the interior of the wax outline using a
micropipette. For [bmim][ BF
4
], the shape was drawn onto an unmodified silicon wafer. For
[bmim][PF
6
] and [emim][BF
4
], the shape was drawn onto a silicon wafer which had first been
coated with a thin layer of PHEMA to increase the wetting of the IL into the corners of the
shapes. Silicone oil was then dispensed (5 μL) at each edge of the shape and allowed to slowly
spread over the wax and encircle the IL. After deposition of polymer, the polymer film was
removed from the IL either by inserting a razor underneath the film and lifting it off or by
submerging the entire substrate in silicone oil which allowed the film to float off the IL. The
thickness of the polymer films was determined from JEOL-6610 low-vacuum scanning electron
microscopy (SEM) images. For SEM sample preparation, the films were transferred onto a clean
silicon wafer and blown flush against the wafer. The wafer underneath the skin was then cracked
and mounted in a substrate holder such that the cross-section could be visualized. A thin gold
coating was sputtered onto the surface of the sample before imaging. Fourier transform infrared
spectroscopy (FTIR) (Thermo Nicolet iS10) was used to study the chemical composition of the
PHEMA and PNIPAAm films. Films were removed to a clean wafer by placing the wafer
surface on top of the polymer film and lifting the film off. The films were rinsed with methanol
and hexane before analysis. The FTIR of the PHEMA particles on silicone oil was measured by
50

first separating the PHEMA from the silicone oil through extraction with methanol and then drop
casting the resulting solution onto a clean wafer.

3.4 Results and Discussion

We studied the deposition of PHEMA onto 5 μL droplets of silicone oil, [bmim][PF
6
], 1-
butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF
4
]), and 1-ethyl-3-methylimidazolium
Figure 3-1. Images of 15 minutes of deposition of PHEMA onto A) [bmim][PF6], B)
[bmim][BF4], C) [emim][BF4], and D) silicone oil. E-H) The droplets were subjected to a
continuous stream of air to show that a continuous skin of PHEMA formed on the ILs but only
particles of PHEMA formed on the silicone oil.
51

tetrafluoroborate ([emim][BF
4
]) placed on a silicon wafer. In the iCVD process, monomer and
initiator molecules are flowed continuously into a vacuum chamber where the initiator molecules
are broken into free radicals by a heated filament array. Polymerization occurs on the surface of
the substrate via a free-radical mechanism.
159
In the case of the imidazolium-based ionic liquids,
HEMA monomer molecules can absorb into the ILs and polymerization can occur at both the
vapor-IL interface and within the bulk IL. In the case of silicone oil, HEMA molecules do not
appreciably absorb into the silicone oil and therefore polymerization should only occur at the
vapor-silicone oil interface. Figure 3-1 shows the images of the droplets taken after 15 minutes
of deposition. A continuous polymer skin that completely encapsulates the droplet formed on all
three ILs, while unconnected polymer particles formed on the silicone oil. An air stream was
applied to the droplets to demonstrate the continuous nature of the polymer skins on the ILs and
the granular nature of the polymer on the silicone oil. Fourier transform infrared spectroscopy
(FTIR) was used to study the chemical structure of the PHEMA deposited on silicone oil and the
ILs. Figure 3-2 shows that all of the spectra have the expected PHEMA peaks including the
broad O–H stretching peak from 3600-3100 cm
-1
, C–H stretching peaks from 3050 to 2800 cm
-1
,
the C=O peak from 1750 to 1685 cm
-1
, C–H bending from 1520 to 1350 cm
-1
, and C–O
stretching from 1310 to 1210 cm
-1
.
147
The spectra are nearly identical demonstrating that the
polymer is highly pure and that varying the liquid substrate does not affect the composition of
the polymer.
52

The different polymer morphology on the silicone oil versus the ILs can be exploited to
fabricate free-standing films. The skins that are formed on the IL droplet are connected to the
underlying silicon wafer and therefore cannot be removed without tearing. For example, Figure
3-3A shows that PHEMA completely encapsulates [bmim][PF
6
] droplets that are placed directly
onto silicon wafers. These encapsulated droplets do not move when the substrate is tilted at a 15
degree angle (Figure 3-3B). In order to make free-standing polymer films, we combined the ILs
and silicone oil onto a common substrate. Silicone oil was first dispensed onto silicon wafers and
allowed to spread over the wafer surface. The silicone oil completely wets the surface of the
Figure 3-2. FTIR spectra of A) a PHEMA film deposited onto a wafer, B) a PHEMA skin
formed on [bmim][PF6], C) a PHEMA skin formed on [bmim][BF4], D) a PHEMA skin formed
on [emim][BF4], and E) PHEMA particles deposited onto silicone oil.

53

silicon wafer forming a thin layer (~30 μm) onto which IL droplets can then be placed. Figure 3-
3C shows that a continuous PHEMA film forms on the IL droplet whereas only polymer
particles form on the surrounding silicone oil. Figure 3-3D shows that the droplet slides when
the substrate is tilted at a 15 degree angle. This verifies that the silicone oil acts as a lubricating
layer to prevent the polymer that forms on the IL from connecting to the underlying wafer. The
contact angles for [bmim][PF
6
], [emim][BF
4
], and [bmim][BF
4
] are 38°, 28°, and 19°
respectively on a silicon wafer (Figure 3-4A). When the IL is placed on top of the silicone oil, a
thin layer of oil remains between the IL and the underlying silicon wafer and the silicone oil
Figure 3-3. Images of 60 minutes of deposition of PHEMA onto a [bmim][PF6] droplet that was
placed on A) a silicon wafer and C) a silicon wafer covered with a layer of silicone oil. B,D) The
substrate was titled at a 15 degree angle after the deposition.

54

forms a concave meniscus on the side of the IL droplet. This meniscus makes it impossible to
measure a contact angle for the IL on the silicone oil, however Figure 3-4B shows that the trend
in wettability is the same on silicone oil as on a silicon wafer i.e. that [bmim][BF
4
] wets the most
and [bmim][PF
6
] wets the least. Without the use of silicone oil, the IL droplet spreads on the
silicon wafer during PHEMA deposition due to both monomer absorption into the IL and the
increased attraction between the IL and the PHEMA film formed on the silicon substrate
surrounding the droplet. In contrast, the use of silicone oil prevents the IL droplets from
spreading during PHEMA deposition. Comparison of Figure 3-4B and Figure 3-4C shows no
noticeable change in the curvature or diameter of the IL droplets after polymer deposition. We
believe that the meniscus acts as a barrier to prevent the spreading of the IL droplet during
deposition.
The shape of the free-standing polymer film can be controlled by patterning the IL and
silicone oil onto the substrate. Figure 3-5 shows a schematic of this fabrication method. First, an
outline of a shape is drawn onto the substrate using wax. The IL is then dispensed into the
Figure 3-4. Contact angle goniometer images of A) IL droplets on silicon wafers, B) IL droplets
on a layer of silicone oil, and C) the same droplets after 60 minutes of deposition of PHEMA.

55

outline. The wax barrier contains the IL within the shape because the IL does not wet the wax. In
the case of [bmim][BF
4
], the wax outline was drawn onto a bare silicon wafer. In the cases of
[bmim][PF
6
] and [emim][BF
4
], the wax outline was drawn onto a silicon wafer that was pre-
coated with PHEMA in order to increase the spreading of the IL into the corners of the shape.
Silicone oil was then added in multiple locations around the outside of the wax barrier and
allowed to spread over the barrier and encompass the IL. The silicone oil serves two purposes in
this fabrication process: it maintains the shape of the original IL droplet during deposition and it
prevents the polymer film from connecting to the underlying substrate. We would like to note
that there is no lubricating layer of silicone oil underneath the IL in this fabrication method since
the IL is dispensed before the silicone oil. Therefore the IL will not slide when the substrate is
Figure 3-5. The fabrication method for making shaped polymer films.

56

tilted. After deposition of polymer, the free-standing polymer film can be removed from the IL
either by inserting a razor underneath the film and lifting it off or by submerging the entire
substrate in silicone oil which allows the film to float off the IL.
Figure 3-6 shows the generality of our fabrication method for two different ILs and two
different polymers. A triangular PHEMA film was formed on [bmim][BF
4
] after 30 minutes of
deposition. The FTIR spectrum of the PHEMA film showed no difference from the spectrum of
PHEMA deposited on a silicon wafer, indicating the high purity of the film. The free-standing
film had an average thickness of 510 ± 64 nm at the edge of the triangle and 663 ± 35 nm at the
center. The increased thickness at the center of the film is caused by the integration of polymer
chains that form within the bulk IL since polymerization takes place simultaneously at both the
vapor–IL interface and within the IL at the conditions used for our study.
147
Similar to PHEMA,
the deposition of poly(N-isopropylacrylamide) (PNIPAAm) also results in polymer particles on
silicone oil and polymer skins on each of the three ILs. Therefore we can use our fabrication
Figure 3-6. Images and corresponding FTIR spectra of free-standing shaped films of A, B)
PHEMA formed on [bmim][BF4] and C, D) PNIPAAm formed on [bmim][PF6]. The films were
removed from the template and placed in a bath of silicone oil for imaging.
57

method to form shaped PNIPAAm films. A square PNIPAAm film was formed on [bmim][PF
6
]
after 135 minutes of deposition. The film had an average thickness of 445 ± 30 nm at the edge of
the square and 469 ± 41 nm at the center. The PNIPAAm film had the expected FTIR peaks:
asymmetric –CH
3
stretching at 2969 cm
-1
, asymmetric –CH
2
– stretching at 2931 cm
-1
, symmetric
–CH
3
stretching at 2880 cm
-1
, secondary amide C=O stretching at 1652 cm
-1
, –CH
3
and –CH
2
–
deformation at 1458 cm
-1
, and –CH
3
deformation at 1387 and 1366 cm
-1
.
160
Compared to the
PNIPAAm deposited onto a wafer, the shaped free-standing PNIPAAm film had a shift in the
location of the secondary amide N—H stretching from 1540 cm
-1
to 1575 cm
-1
. This is likely due
to the mobility of the PNIPAAm chains in the free-standing film that allows for hydrogen
bonding between the C=O and N–H groups.
161

In addition to PHEMA and PNIPAAm, we found that the deposition of several other
polymers including poly(o-nitrobenzyl methacrylate) (PoNBMA) and poly(pentafluorophenyl
methacrylate) (PPFM) also yields particles on silicone oil and skins on the ILs. The formation of
particles on silicone oil has also been examined in the deposition of silver,
162
copper,
163
gold,
164

and C
4
F
8
165
precursors. Ye et al. proposed that silver clusters that formed on silicone oil do not
merge into a film because an adsorbed layer of oil molecules surrounds the particles and thereby
prevents coalescence.
166
Similarly, we believe that a polymer skin does not form on the silicone
oil in cases where the silicone oil wets the polymer. For example, the silicone oil wets PHEMA,
PNIPAAm, PoNBMA, and PPFM (the contact angles of silicone oil on these polymers are 6°,
39°, 7°, and 24°, respectively) and the depositions of these polymers all result in the formation of
particles. After long deposition times (e.g., approximately 2 hours in the case of PHEMA), the
concentration of polymer particles becomes great enough to completely cover the oil surface
such that additional polymer can no longer interact with the underlying silicone oil and further
58

deposition results in a continuous film that grows on the layer of particles. In contrast to the
above polymers, we have found that the deposition of PPFDA results in continuous polymer
films on droplets of both silicone oil and all three imidazolium-based ILs (Figure 3-7A-D). We
believe that the formation of a continuous PPFDA skin on silicone oil is due to the poor wetting
between PPFDA and silicone oil, as silicone oil has a high contact angle on PPFDA (70°). When
PPFDA was deposited onto IL droplets placed on a layer of silicone oil, a continuous polymer
skin formed that encapsulated the two-liquid system and connected to the underlying silicon
Figure 3-7. Images of 30 minutes of deposition of PPFDA onto droplets of A) [bmim][PF6], B)
[bmim][BF4], C) [emim][BF4], and D) silicone oil placed on a silicon wafer. E) Images of
PPFDA deposited onto a droplet of [bmim][PF6] placed on a silicon wafer covered with a layer
of silicone oil and F) tilted at a 15 degree angle.

59

wafer (Figure 7E). Therefore the IL droplet did not move when the substrate was tilted at a 15
degree angle (Figure 7F) which is in contrast to the deposition of PHEMA which resulted in a
continuous film on only the IL surface (Figure 3-3C,D). Therefore while free-standing films
could be formed using PHEMA, PNIPAAm, PoNBMA, and PPFM, they could not be formed
using PPFDA.

3.5 Conclusions

We have demonstrated that the deposition of both PHEMA and PNIPAAm on silicone oil
results in the formation of polymer particles, whereas deposition onto imidazolium-based ILs
results in polymer skins which completely encapsulate the ILs. We exploited this difference in
polymer morphology to fabricate ultrathin free-standing polymer films of different shapes by
combining the ILs and silicone oil onto a common substrate. Our study reveals some very
interesting surface tension effects: when the IL is placed on top of the silicone oil, a thin layer of
oil remains between the IL and the underlying silicon wafer and the silicone oil forms a concave
meniscus on the side of the IL droplet. This meniscus helps maintain the shape of the IL and
thereby the shape of the resulting polymer. FTIR analysis shows that the free-standing polymer
films are highly pure (free of residual monomer, IL, and silicone oil) which will enable their use
for biomedical applications. The free-standing PNIPAAm films have many potential uses due to
their temperature-responsive hydrophilicity.
167,168

Our fabrication process is environmentally-friendly because no organic solvents are used
in any of the steps and ionic liquids are non-volatile, non-flammable, and can be easily recycled.
We demonstrated the generality of our fabrication method across a range of imidazolium-based
60

ILs ([bmim][PF
6
], [bmim][BF
4
], and [emim][BF
4
]). Our ability to produce free-standing polymer
films of controlled shape, size, and thickness is useful for a wide variety of applications in
biosensing, biomimicry, and separations. In addition to PHEMA and PNIPAAm, we have found
that the depositions of several other polymers including PoNBMA and PPFM also yield particles
on silicone oil and skins on ILs. This allows us to extend our fabrication method to make light-
responsive
169
and click-active polymer films.
170
Furthermore, films with multiple functionalities
(e.g., mechanical strength, temperature-responsive swelling, photoresponsive solubility) can be
made by sequentially stacking polymers.

3.6 Acknowledgments

This work was supported by the National Science Foundation under Grant No. EEC-
0310723, the Mork Family Graduate Fellowship (R.F.F.), and the James H. Zumberge Faculty
Research and Innovation Fund.

3.7 References

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63

4. Two Stage Growth of Polymer Nanoparticles at the Liquid-Vapor
Interface by Vapor Phase Polymerization

Citation: Submitted to Langmuir

64

4.1 Abstract

In this paper, we study the growth of polymer nanoparticles that are formed on the
surface of silicone oils via initiated chemical vapor deposition. The average radius of the
particles can be increased by decreasing the silicone oil viscosity, increasing the deposition time,
or increasing the deposition rate. The time series data indicates that there are two stages for the
particle growth. Particle nucleation occurs in the first stage and the particle size is dependent on
the liquid viscosity and deposition rate. Particle growth occurs in the second stage, during which
the particle size is dependent only on the amount of deposited polymer. This two-step process
allows us to make core-shell particles by sequentially depositing different polymers. The benefits
of our nanoparticle synthesis process are that solvents and surfactants are not required and the
size of the nanoparticles can be controlled over a wide range of radii with a relatively narrow
distribution.

65

4.2 Introduction

Polymer nanoparticles have a wide range of applications in fields such as
pigmentation,
171
photonics,
172
electronics,
173
separations,
174
and drug delivery.
175
The majority of
these applications require particles of controlled sizes and functionality which makes it important
to develop new fabrication methods that allow these parameters to be tuned. The common
methods for making polymer nanoparticles can be placed into two main categories: the
polymerization of monomers
176,177,178
and the dispersion of pre-formed polymer.
179,180,181
The
approaches that utilize the polymerization of monomers include emulsion,
176,182
surfactant-free
emulsion,
177,183
and controlled/living radical polymerization.
178,184,185
These techniques generally
contain an aqueous phase, a water soluble initiator, and monomer. Conventional emulsion
polymerization uses a surfactant as a stabilizer and the initiator and surfactant concentrations are
varied to control particle size.
186,187
For example, Muñoz-Bonilla et al. used emulsion
polymerization to make polymethyl methacrylate nanoparticles with an average particle diameter
of 130 - 460 nm with coefficients of variation (C.V.) from 0.05 – 0.14 by altering the surfactant
concentration.
188
Surfactant free emulsion uses ionizable initiators or ionic co-monomers as
stabilizers instead of surfactants which has the advantage of avoiding the need for the difficult
removal of stabilizing surfactants, however it is difficult to prepare monodisperse and precisely
controlled particle sizes.
189
Controlled/living radical polymerization techniques such as atom
transfer radical polymerization (ATRP)
178,184
and reversible addition and fragmentation transfer
chain polymerization (RAFT)
185
use an emulsion system, however an additional control agent is
added to control the molar mass of the polymer and particle size.
190
For example, Oh et al.
polymerized oligo(ethylene glycol) monomethyl ether methacrylate via ATRP using a
miniemulsion system to synthesize particles with an average diameter of 188 - 207 nm with a
66

C.V. of 0.03 – 0.09.
191
A common problem with controlled/living radical polymerization is the
presence of a residual control agent in the product.
192

In this paper, we study the nucleation and growth of nanoparticles that are made at the
vapor—liquid interface via initiated chemical vapor deposition (iCVD). In the iCVD process,
vapor phase monomer and initiator molecules are flowed into a vacuum chamber where the
initiator is thermally decomposed into radicals by a resistively heated filament array. The
monomer and initiator radicals adsorb to the surface of the substrate where polymerization
occurs via a free radical mechanism.
193,194
Our group recently showed that iCVD onto low vapor
pressure liquids including ionic liquids and silicone oils can lead to the formation of
nanoparticles.
195,196,197
In comparison to solution phase methods for fabricating polymer
nanoparticles, the iCVD technique does not use volatile organic solvents or
surfactants/stabilizers which are difficult to remove post-processing. A wide variety of
functionalities can also be achieved by varying the monomer.
In this study, we elucidate the growth mechanism associated with nanoparticle formation
by systematically studying the effects of the silicone oil viscosity, deposition time, and
deposition rate. We show that the particles grow in two stages: 1) during the nucleation stage, the
polymer chains initially diffuse and aggregate together and 2) in the growth stage, additionally
deposited polymer chains join the nucleated particles. The mechanistic insights from our studies
enable us to form core-shell particles by sequential deposition. Polymer core-shell particles have
been studied for a wide range of applications such as drug delivery,
198
emulsifiers,
199
and
sensors.
200
Our process allows us to make core-shell particles using a variety of polymers and
affords the ability to easily control the thicknesses of the core and shell layer independently.

67

4.3 Experimental

The precursors n-butyl acrylate (99%, Aldrich), di-tert-butyl peroxide (98%, Aldrich),
and o-nitrobenzyl methacrylate (95%, Polysciences) were used as received without further
purification. All polymer depositions were performed within a custom built iCVD reactor
chamber (GVD Corp, 250 mm diameter, 48 mm height). For poly(n-butyl acrylate) (PnBA)
particle synthesis, 0.5 mL of silicone oil (100, 500, and 1000 cst at 25°C) (polydimethylsiloxane,
Aldrich) was pipetted onto a glass microscope slide. The slides were then placed on the reactor
stage which was maintained at a temperature of 10 °C by a recirculating chiller. The samples
were given 30 minutes to equilibrate to the stage temperature before the monomer and initiator
were introduced into the reactor. Di-tert-butyl peroxide (TBPO) vapor was introduced at room
temperature through a mass flow controller at a rate of 1.7 sccm and the nBA monomer was
introduced at room temperature using a needle valve to control for a rate of 9.0 sccm. The reactor
pressure was maintained at 150, 300, and 400 mTorr to achieve deposition rates of 10, 30, and 60
nm/min, respectively. A nichrome filament array (80% Ni, 20% Cr, Omega Engineering) was
resistively heated to 240 °C to initiate the polymerization. For the deposition time series
experiments, the time was measured starting from when the filament temperature reached 180 °C
which is the temperature at which the initiator begins to thermally decompose. Deposition rates
were obtained by measuring the thickness of the polymer deposited on a reference silicon wafer
using an in situ 633 nm helium-neon laser interferometer (Industrial Fiber Optics). The
molecular weight distributions were measured using gel permeation chromatography (GPC)
using polymer deposited on a reference wafer which has previously been shown to match the
molecular weight of polymer deposited on liquid substrates.
201
Tetrahydrofuran (99%,
Mallinckrodt) was used as the eluent with a flow rate of 0.5 mL/min. The GPC setup used an
68

HPLC pump (Agilent 1200 series) and a column with a separation range of 2 kDa – 2 MDa
(Shodex, LF-804). The inline refractive index detector (Wyatt Optilab rEX) and multi-angle light
scattering detector (Wyatt Dawn Helios) were used to measure the absolute molecular weight.
The equilibrium contact angles of silicone oil on the polymer films were measured using a
goniometer (ramé-hart Model 290-F1).
Dynamic light scattering (DLS) (Wyatt DynaPro Titan) was used to measure the particle
size distribution with a temperature controlled microsampler set to 25°C. The silicone oil was
collected after the deposition of the particles by dripping it off the slide into microcentrifuge
tubes. These oil samples collected from the slides were used without further modification. For
the DLS measurement, 20 μL of these oil samples were pipetted into a quartz cuvette and run for
20 acquisitions with an acquisition time of 10 seconds per acquisition. The reported particle size
was averaged from 6 samples, 2 each in 3 separate depositions, with the error bars representing
the standard deviation of the average. Scanning electron microscopy (SEM) was used to
visualize the particles using a JEOL JSM-7001F SEM with an acceleration voltage of 15 kV.
First the particles were separated from the oil solution by mixing with a 0.01% v/v solution of
Triton X (laboratory grade, Aldrich) in filtered DI water. The water phase was then drop cast
onto a silicon wafer at ambient conditions. These samples were sputter coated with platinum for
30 seconds before imaging to prevent charging.
For the core-shell particle experiments, the PnBA core was first deposited as previously
described at a reactor pressure of 300 mTorr for 2.5 minutes. After the PnBA deposition, the
reactor was opened to the atmosphere and one sample was removed to measure the PnBA core
size before the deposition of poly(o-nitrobenzyl methacrylate) (PoNBMA). For PoNBMA
depositions, the reactor stage was set to 30 °C and the samples were given 30 minutes to
69

equilibrate to this temperature. The TBPO was introduced into the reactor chamber at room
temperature through a mass flow controller at a rate of 1.7 sccm and the oNBMA monomer was
introduced into the reactor chamber from a source jar set at 75 °C to achieve a flow rate of 1.2
sccm. The reactor pressure was maintained at 120 mTorr and the filament array was heated to
240 °C to initiate the polymerization. After a 25 min PoNBMA deposition, the core-shell particle
size was measured by DLS and then the same solution was exposed to a handheld UV lamp (250
Watt, UV-Technik) for 1 hour to convert the PoNBMA to poly(methacrylic acid) (PMAA). The
oil was then thoroughly mixed with pH 8 buffer solution (BDH) to dissolve the PMAA shell.
The oil and buffer solution phase separated and the oil phase was collected using a separatory
funnel and the particle size was measured again.

4.4 Results and Discussion

We chose to study the deposition of PnBA onto silicone oil as our model system. Since
the n-butyl acrylate monomer does not absorb into the silicone oil, polymerization will occur
only at the vapor-liquid interface. We have previously found that the spreading coefficient
determines whether a polymer will form particles or a continuous film when it is deposited via
iCVD onto a liquid surface.
196
The spreading coefficient can be written as S = γ
LV
(1+cosθ) - 2γ
PV

where γ
LV
is the liquid−vapor surface tension, θ is the advancing contact angle of the liquid on
the polymer, and γ
PV
is the polymer−vapor surface tension. The spreading coefficient of PnBA
on silicone oil is negative, therefore the polymer wants to reduce its contact area with the liquid
and particles will form.

The polymer particles can either remain at the liquid surface or
completely submerge within the liquid.
197
The energy required for a particle to submerge into the
liquid is proportional to (1-cosθ
e
)
2
, where θ
e
is the equilibrium contact angle of the liquid on the
70

polymer.
202
The silicone oil has an equilibrium contact angle of 21° on PnBA, therefore it is
energetically favorable for the particles to remain at the liquid surface rather than submerge.
For our study, we first pipetted the silicone oil onto glass slides and then deposited PnBA
via iCVD. After the deposition, the liquid was poured off the slide and the size distribution of the
particles was measured using DLS with the silicone oil as the solvent. Figure 4-1A shows an
SEM image of PnBA particles from a 5 minute deposition onto silicone oil. The particles were
separated from the oil using an aqueous surfactant solution and then drop-casted onto a silicon
Figure 4-1. A) SEM image and B) DLS histogram of PnBA particles from a 5 minute
deposition at a deposition rate of 30 nm/min onto 100 cst silicone oil.
71

wafer. The image shows the spherical shape of the nanoparticles and confirms the size measured
by DLS (figure 4-1B).
In order to study the mechanism of particle formation, we first deposited PnBA onto a
range of silicone oil surfaces with different viscosities. Since the PnBA polymer particles form
by the diffusion and aggregation of polymer chains at the liquid-vapor interface, our hypothesis
is that the particle size can be varied by altering the viscosity of the silicone oil. An advantage of
using silicone oil as our liquid substrate is that it allows for the variation of the liquid viscosity
without changing the chemistry or significantly changing the surface tension of the liquid.
Figure 4-2 shows the results of 5 minute depositions of PnBA onto a range of silicone oils with
viscosities of 100, 500, and 1000 cst at a constant deposition rate of 30 nm/min as measured on a
reference silicon wafer. The radius increases as the viscosity decreases likely because at lower
Figure 4-2. A) Overlaid histograms of the particle size distributions on silicone oil viscosities of
100, 500, and 1000 cst at a deposition rate of 30 nm/min and a deposition time of 5 minutes. B)
Table of the average radius and size distribution for each viscosity.

72

viscosities, polymer chains are able to diffuse faster leading to a greater amount of chains
aggregating. Yu and coworkers have shown experimentally that increasing the viscosity of the
liquid decreases the lateral diffusion coefficient of proteins at the liquid surface.
203
Our results
are in agreement with our previous work that studied the deposition of cross-linked
poly(ethylene glycol diacrylate) polymer films onto silicone oil and saw an increase in the lateral
feature size of the microstructures in the film with decreasing viscosity due to the increased rate
of diffusion of the polymer on the surface.
204

We studied the particle size as a function of time in order to determine if the particles
continue to grow or whether new particles are nucleated. We deposited the polymer from 0.5 to
Figure 4-3. Overlaid histograms of the particle size distribution for a range of deposition
times from 0.5 to 30 minutes on a 1000, 500, and 100 cst silicone oil substrate at a
deposition rate of 30 nm/min.
73

30 minutes onto each viscosity at a constant deposition rate of 30 nm/min. Figure 4-3 shows the
overlaid histograms taken from DLS measurements representing the size distribution of the
particles formed at each deposition time. The histograms shift to a larger particle size rather than
broadening. The absence of a tail at the lower particle sizes suggests that a significant amount of
new particles are not being formed continuously throughout the deposition. Figure 4-4 shows
plots of the average particle radius as a function of deposition time. For all viscosities, the
average radius increased as a function of deposition time. Since the particles remain at the
silicone oil surface and the rate of polymerization is constant, if the additionally deposited
Figure 4-4. Particle size as a function of deposition time at a deposition rate of 30 nm/min
using a silicone oil viscosity of a) 1000, b) 500, and c) 100 cst.
74

polymer chains diffuse to existing particles instead of nucleating new particles, then the growth
of the particle radius should scale with deposition time to the 1/3 power, representing a constant
mass growth rate. The data for each time series was fit to an equation of the form r(t) = A·t
(1/3)
,
where r is the particle radius, t is the deposition time, and A is a fitted scaling constant. The fit
was performed using a nonlinear regression and the coefficient values were determined by
iterative least squares estimation. The fitted values of A are 39, 56, and 100 for the 1000, 500,
and 100 cst silicone oil, respectively. The corresponding R-squared values are 0.97, 0.96, and
0.95 which shows that the equation is a good fit for the particle size data. The fitted data and the
lack of increase in the coefficient of variation, as shown in the tables in figure 4-4, suggest that
the particles form in a two-stage mechanism, the initial nucleation and then growth by
accumulation of all additionally deposited polymer.
In order to determine how the molecular weight of the polymer chains affects the
nucleation and growth of the particles, we systematically varied the deposition rate. In the iCVD
process, deposition rate and molecular weight are coupled by their dependence on the monomer
concentration adsorbed to the substrate surface. This concentration has been shown to be
proportional to the ratio of the partial pressure of the monomer to the saturation pressure of the
Table 4-1. Particle size at each viscosity for a range of deposition rates with deposition time
controlled to a constant deposition thickness of 300 nm.

75

monomer.
193
We therefore varied the deposition rate by varying the monomer partial pressure by
changing the total reactor pressure while keeping the saturation pressure constant. The deposition
rates were 10, 30, and 60 nm/min. Since our results in figure 4-4 show that the particle radius
grows as more polymer is deposited, we varied the deposition time to keep the total amount of
polymer deposited constant at a thickness of 300 nm as measured on a reference wafer. Table 4-
1 displays the results showing that at each viscosity, the particle radius increased with increasing
deposition rate. Since there was a change in the radius with a constant amount of polymer
deposited, we can conclude that the deposition rate affects the number of particles being
nucleated. The deposition rates of 10, 30 and 60 nm/min had corresponding estimated weight
average molecular weights of 9.5, 51.7, and 131.9 kDa, respectively. Increasing the molecular
weight decreases chain diffusion which should result in smaller particles, however it leads to
larger particles which is the opposite trend to the viscosity data indicating that chain diffusion is
not the only factor in nucleation. The relative change in molecular weight is larger compared to
the corresponding relative change in the deposition rate meaning there are a smaller number of
polymer chains being deposited at any given time at the larger deposition rate. This smaller
concentration of polymer chains during the nucleation stage leads to a smaller number of
particles being nucleated and therefore, the additionally deposited polymer aggregates to fewer
particles leading to a larger size. Figure 4-5 shows the average particle size as a function of
deposition time for the deposition rates of 10, 30, and 60 nm/min and a silicone oil viscosity of
1000 cst. The data was again fitted to the equation of the form r(t) = A·t
(1/3)
resulting in fitted
values for A of 26, 39, and 65 for the 10, 30, and 60 nm/min deposition rates, respectively. The
corresponding R-squared values are 0.98, 0.97, and 0.99 showing a good fit to data which
supports the previous time series data suggesting a two-stage mechanism.
76

To further validate the two-stage mechanism, we varied the deposition rate during the
growth stage. The particle growth in the second stage should only be dependent on the total
amount of polymer deposited after nucleation and not on the deposition rate. We nucleated the
particles by depositing PnBA for 5 minutes onto 1000 cst silicone oil at a deposition rate of 10
nm/min. We chose 5 minutes since our data in Figure 4-4 showed that new particles are not
being nucleated at that time. The deposition rate was then set to 10, 30, and 60 nm/min for 25,
8.5, and 4 min, respectively, to keep the total amount of polymer deposited constant at 300 nm as
Figure 4-5. Particle size as a function of deposition time for a range of deposition rates with a
silicone oil viscosity of 1000 cst.

Table 4-2. Particle size for depositions where the deposition rate was initially the same but
then varied during the deposition.

77

measured on a reference silicon wafer. Table 4-2 shows that the average particle size from the
three sets of experiments are all within error of each other validating that particle size during the
growth stage is only dependent on the amount of polymer deposited. As a control, we reversed
the conditions so that the nucleation deposition rate was 30 nm/min and after 8.5 minutes the
deposition rate was altered to 10 nm/min for an additional 5 minutes (Table 4-3). The resulting
particle size is larger since the deposition rate during the nucleation stage is larger, which was
shown in Figure 4-5 to increase the initial nucleated particle size.
The advantage of our growth technique is that it can be extended to produce polymer
core-shell particles by depositing a second polymer during the growth stage. For example, we
selected PoNBMA as our second polymer to make the shell layer since it has a negative
spreading coefficient and the silicone oil has a non-zero contact angle (29°) and therefore it is
also a system that will form particles that remain at the liquid surface. Additionally, PoNBMA is
a UV-responsive polymer where exposure to UV radiation causes the o-nitrobenzyl groups to be
cleaved to form a carboxylic acid and a 2-nitrosobenzaldehyde byproduct, thereby converting
PoNBMA to PMAA which can be dissolved in ph-8 buffer.
205
To make the core-shell particles,
we first deposited a PnBA core onto 500 cst silicone oil for 2.5 min and then deposited a
PoNBMA shell for 25 min. The average radius of these core-shell particles was measured by
Table 4-3. Particle size for depositions where the order of the deposition rates was reversed.

78

DLS to be 101 ± 5 nm. After these particle sizes were characterized, the oil dispersions were
exposed to a UV lamp for 1 hour to convert the PoNBMA shell to PMAA and then mixed with a
buffer solution to remove the PMAA. The PnBA core remained dispersed in the oil phase due to
the hydrophobicity of the polymer and the samples were again characterized by DLS and found
to have an average radius of 68 ± 3 nm which is similar to the average radius of the PnBA core
(64 ± 3 nm) before PoNBMA deposition. This demonstrates that our method can be used to
make core-shell particles where the size of the core and shell layers can be independently
controlled.

4.5 Conclusions

The formation of PnBA nanoparticles on the surface of silicone oil was studied. The
distribution and rate of particle growth suggest that the growth of the PnBA particles consists of
a two-stage mechanism: initial nucleation and subsequent growth as additional polymer chains
diffuse to the particle. The initial particle size is determined by the ability of the polymer chains
to aggregate. In the second stage, the particle size increases with the total amount of additionally
deposited polymer. Our process allows for the synthesis of particles of a controlled size with an
average radius of 30 – 300 nm and a relatively narrow distribution with a C.V. of 0.20.
Figure 4-6. Schematic of our core-shell deposition process with PnBA as the core and
PoNBMA as the shell. The shell layer can be exposed to UV light and then dissolved.

79

Understanding of this growth mechanism allows for using this method to form core-shell
particles by sequentially depositing two different polymers.

4.6 Acknowledgments

This work was supported by the U.S. Department of Energy, Office of Science, Basic
Energy Sciences under Award #DE-SC0012407. We thank Dr. Shuxing Li and the USC
NanoBiophysics Core Facility for assistance with GPC and DLS.

80

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Abstract (if available)

Abstract The initiated chemical vapor deposition (iCVD) process is a vapor phase is a technique used to deposit functional polymer coatings typically onto solid substrates. Our group is the first to introduce low vapor pressure liquids, such as ionic liquids and silicone oils, as substrates into the iCVD process. The use of liquid substrates adds additional complexity in the form of solubility and surface tension effects. Precursors that are soluble in the liquid have the ability to absorb into the liquid as well as adsorb to the surface and create two distinct locations of polymerization. The polymerization that occurs at the surface can result in polymer films or particles depending on the surface tension interactions between the polymer and liquid. Section 1 introduces the iCVD process and provides background on past works that studied vacuum depositions with liquid substrates as well as the applications and current methods of the polymer materials made using liquid substrates: polymer-ionic liquid gels, free standing films, and nanoparticles. Section 2 investigates the polymerization that occurs within an ionic liquid layer, focusing on the difference between polymerization within the layer and at the surface, the effect of polymer accumulation within the liquid layer, and the formation of gels. Section 3 demonstrates the different polymer morphologies formed at a variety of liquid surfaces and uses these properties to develop a method of forming shaped free standing films using a two-liquid system. Section 4 studies the growth mechanism of polymer nanoparticles formed at the liquid surface, the understanding of which can be used to synthesize of nanoparticles of a controlled size and to form polymer core-shell particles.

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Asset Metadata

Creator Frank-Finney, Robert J. (author)

Core Title Formation of polymer gels, films, and particles via initiated chemical vapor deposition onto liquid substrates

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 06/30/2016

Defense Date 05/04/2016

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

Tag iCVD,liquid substrates,OAI-PMH Harvest,polymer morphology

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 frankfin@usc.edu,rob.frankfinney@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-259191

Unique identifier UC11279608

Identifier etd-FrankFinne-4487.pdf (filename),usctheses-c40-259191 (legacy record id)

Legacy Identifier etd-FrankFinne-4487.pdf

Dmrecord 259191

Document Type Dissertation

Format application/pdf (imt)

Rights Frank-Finney, Robert J.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA