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Novel processing of liquid substrates via initiated chemical vapor depostion

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Novel processing of liquid substrates via initiated chemical vapor depostion

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Content Novel Processing of Liquid Substrates via Initiated
Chemical Vapor Deposition

by
Prathamesh Karandikar

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY IN
CHEMICAL ENGINEERING
December 2019

1

Committee Members
Dr. Malancha Gupta (Chair)
Dr. Aiichiro Nakano
Dr. Jayakanth Ravichandran

2

Executive Summary
This dissertation discusses novel processing techniques for low vapor pressure liquid
substrates such as ionic liquids (ILs) and silicone oils during the initiated chemical vapor
deposition (iCVD) process for the fabrication of polymer particles, polymer – IL gels, and
polymer films. Chapter 1 introduces the iCVD process and provides a background on vacuum
processes using sequential flow of precursors such as atomic layer deposition (ALD) and
molecular layer deposition (MLD). Chapter 2 demonstrates the fabrication of polymer – IL gel
beads by a sequential polymerization process using iCVD. First monomer was absorbed in IL
droplets which were kept spherical using a hydrophobic substrate, the reactor was then
evacuated, and the monomer absorbed in the IL was polymerized via free radicals to fabricate gel
beads. Chapter 3 describes a sequential process to fabricate polymer particles using iCVD. First
monomer droplets were condensed on a layer of silicone oil and the monomer was subsequently
polymerized to form polymer particles. The particle size was tuned by varying the viscosity of
the silicone oil resulting in solvent-less, surfactant-free fabrication of functional polymer
particles. Chapter 4 describes a reactor modification that allows the use of in situ sonic agitation
to set up standing waves on the liquid surface during iCVD. The effect of standing waves on the
growth and mechanical stability of fluoropolymer films was studied. The incorporation of a
crosslinker enabled the growth of continuous films on a moving vapor-liquid interface and the
mechanical stability of the polymer films was found to increase with film thickness and
crosslinking. Chapter 5 summarizes the conclusions of this dissertation and describes the future
work for the agitation of liquid substrates. Further reactor modifications to provide directionality
to the monomer flux to control the polymer morphology during iCVD are also described.
3

Acknowledgements
My sincere gratitude goes out first and foremost to my Ph.D. advisor Dr. Malancha Gupta
for her guidance in my research and for encouraging my professional growth. Her leadership,
involvement, and ability to recruit and manage talent is truly inspiring. I thank my dissertation
and qualifying committee members Dr. Jayakanth Ravichandran, Dr. Noah Malmstadt, and Dr.
Aiichiro Nakano for their feedback and help in formulating my thesis. The support and
encouragement of my lab mates made working towards a Ph.D. an inclusive experience.
My parents, Dr. Sadanand Karandikar and Trupti Karandikar, and family have been an
anchor point throughout my time away from home and they share this achievement with me for
their indefatigable encouragement and sacrifice during my journey. I thank my friends, Dr.
Nirakar Poudel, Baibhav Rawal, Ashrant Aryal, Achal Dhoj, Siddharth Bafna, Abhishek
Srinivasan, Dr. Alex Baldwin, Joycelyn Yip, Mahima Singh Deo, Allyson McGaughey, and Lu
Yu (Kiko) for our close ties during my time at the University of Southern California. The
campus community including the groups: Peaks and Professors and the Office of Wellness and
Health Promotion deserve special thanks for their service to the students.
I must also sincerely thank my professors at The Ohio State University and the friends I
made during my time in Ohio including Paul Goettemoeller, Cory Steinke, Dr. Andy Maxson,
Neal Gordon, Cambell Parrish, Zach Stephan, Jacob Michael, Dr. Matthew Borchers, Chris
Gehret, Steven Kerchmar, Blane Evans, Clark Siddle, Kathryn Blackburn, Emilee Landers, Dr.
Christina Crouch, Beverly Kremer and their families for hosting me on many holidays and
festive occasions. Finally, I must thank Sadhguru Jaggi Vasudev who has helped me transform
my perception of life and given me the tools for my physical, mental, and spiritual wellbeing.
4

Table of Contents
Chapter 1: Introduction ............................................................................................................... 9
1.1 initiated Chemical Vapor Deposition (iCVD) ........................................................................ 10
1.2 Vacuum Techniques Using Sequential Depositions ............................................................... 11
1.3 Liquid Substrates in Vacuum Deposition Processes ............................................................... 12
1.4 Polymer – Ionic Liquid Gels ................................................................................................... 14
1.5 Polymer – particles ................................................................................................................. 15
References ..................................................................................................................................... 16
Chapter 2: Fabrication of Ionic Liquid Gel Beads via Sequential Deposition ...................... 23
2.1 Abstract ................................................................................................................................... 24
2.2. Introduction ............................................................................................................................ 24
2.3 Experimental ........................................................................................................................... 26
2.4 Results and Discussion ........................................................................................................... 28
Conclusions ................................................................................................................................... 35
Acknowledgments......................................................................................................................... 35
References ..................................................................................................................................... 36
Chapter 3: Synthesis of Functional Particles by Condensation and Polymerization of
Monomer Droplets in Silicone Oils ........................................................................................... 38
3.1 Abstract ................................................................................................................................... 39
3.2 Introduction ............................................................................................................................. 39
3.3 Experimental ........................................................................................................................... 41
3.4 Results and Discussion ........................................................................................................... 46
Conclusion .................................................................................................................................... 58
Acknowledgments......................................................................................................................... 58
References ..................................................................................................................................... 59
Chapter 4: Effects of Standing Waves on the Growth and Stability of Vapor Deposited
Polymer Films.............................................................................................................................. 62
4.1 Abstract ................................................................................................................................... 63
4.2 Introduction ............................................................................................................................. 63
4.3 Experimental ........................................................................................................................... 65
4.4 Results and Discussion ........................................................................................................... 68
5

4.5 Conclusions ............................................................................................................................. 75
Acknowledgements ....................................................................................................................... 76
References ..................................................................................................................................... 76
Chapter 5: Conclusions and Future Work .............................................................................. 80
5.1 Conclusions ............................................................................................................................. 81
5.2 Future Work ............................................................................................................................ 82
References ..................................................................................................................................... 85

List of Tables and Figures
Figure 2-1: Contact angle images of a) a water droplet and b) a [emim][BF4] droplet on
chromatography paper coated with PPFDA fluoropolymer. ........................................................ 29

Figure 2-2: Top down images of a) a transparent droplet of [emim][BF4] on PPFDA coated
chromatography paper and b) an opaque gel bead composed of PHEMA and [emim][BF4] after
sequential deposition. c) Side view image of the gel bead. d) The gel bead held using a pair of
tweezers......................................................................................................................................... 30

Figure 2-3: FTIR spectra of the gel beads compared to a reference iCVD PHEMA film and
reference [emim][BF4]. Dashed lines indicate the location of C-H symmetric stretching (left) in
the imidazolium functionality of [emim][BF4] and C=O stretching (right) in the carbonyl
functionality of PHEMA. .............................................................................................................. 31

Figure 2-4: Top down images of a) a transparent droplet of [emim][BF4] on PPFDA coated
chromatography paper and b) the sample composed of PHEMA and [emim][BF4] after
simultaneous deposition showing a wrinkled PHEMA skin. c) Side view image of the sample. d)
The wrinkled PHEMA skin separated by tweezers. ..................................................................... 32

Figure 2-5: Contact angle images of a water droplet on the PPFDA coated chromatography
paper after a) simultaneous PHEMA deposition and b) sequential PHEMA deposition. ............ 33

Figure 2-6: a) The contact angle of the IL on the PPFDA coated substrate before sequential
deposition, b) the profile image of the gel after sequential deposition, c) the gel can be picked up
with tweezers, d) the contact angle of the IL on the PPFDA coated substrate before simultaneous
6

deposition, e) the profile image of the polymer film encapsulating the IL after simultaneous
deposition, and f) the polymer film partially peeled off the IL with a pair of tweezers ............... 34

Table 3-1: Size measurements of P4VP particles using dynamic light scattering ....................... 47

Figure 3-1: Representative size distributions for P4VP particles produced on 100 cst silicone oil
at Pm/Psat values of 1, 1.5, and 3.3. ................................................................................................ 47

Figure 3-2: Representative SEM images of P4VP particles produced on 100 cst silicone oil at
a) Pm/Psat = 1, b) Pm/Psat = 1.5, and c) Pm/Psat = 3.3. ...................................................................... 48

Table 3-2: UV-vis spectrometry data at 600 nm wavelength for P4VP particles in 100 cst
silicone oil. .................................................................................................................................... 49

Figure 3-3: Representative SEM images of P4VP particles produced on 100 cst silicone oil at
Pm/Psat = 1 with monomer condensation for a) 15 seconds, b) 45 seconds, and c) 90 seconds .... 50

Figure 3-4: a) Schematic of the reactor. b) Schematic of the monomer condensation and
polymerization process. The yellow spheres indicate monomer droplets and the white spheres
indicate polymer particles. ............................................................................................................ 51

Figure 3-5: a) Representative size distributions for P4VP particles produced at Pm/Psat = 1.5 on
100 cst and 1000 cst silicone oil and b) SEM image of a P4VP film produced at Pm/Psat = 1.5 on
100,000 cst silicone oil. ................................................................................................................ 52

Figure 3-6: Coalescence of 4-vinyl pyridine monomer droplets in 100 cst silicone oil at Pm/Psat =
1.5 in the absence of polymerization. ........................................................................................... 53

Figure 3-7: a) Representative size distributions for P4VP particles on 100 cst silicone oil
comparing particles produced via iCVD to particles produced at Pm/Psat = 1.5 and b) SEM
image of P4VP particles produced via iCVD on 100 cst silicone oil. .......................................... 54

Table 3-3: Size measurements of PHEMA particles using dynamic light scattering. ................. 55

7

Figure 3-8: Representative size distributions for PHEMA particles produced on 100 cst silicone
oil at Pm/Psat values of 1, 1.5, and 3.3. .......................................................................................... 55

Figure 3-9: Representative SEM images of PHEMA particles produced on the 100 cst silicone
oil at a)Pm/Psat = 1, b)Pm/Psat = 1.5, and c)Pm/Psat = 3.3................................................................. 56

Figure 3-10: a) Representative size distributions for PHEMA particles produced at Pm/Psat = 1.5
on 100 cst and 1000 cst silicone oil, and b) SEM image of a PHEMA film produced at Pm/Psat =
1.5 on 100,000 cst silicone oil. ..................................................................................................... 57

Figure 3-11: Coalescence of 2- hydroxyl ethyl methacrylate monomer droplets in 100cst silicone
oil at Pm/Psat = 1.5 in the absence of polymerization. ................................................................... 57

Figure 4-1. (a) Schematic of the modified iCVD reactor for in situ sonic modulation. (b) 5cSt
silicone oil lens suspended on a bath of 261 cSt Krytox mounted on the transducer inside the
modified reactor. ........................................................................................................................... 69

Figure 4-2. Phase diagram representing in situ modulation of a 5 cSt, 20 cSt, and 50 cSt silicone
oil lens on a bath of Krytox. The solid markers represent the threshold at which a circular (C)
lens transitions to a deformed lens (D) and the open markers represent the threshold at which a
deformed (D) lens transitions to a stable elongated shape (E). Representative images of the
different lens shapes are displayed within each region. ................................................................ 71

Figure 4-3. Images of real-time polymer deposition onto a 5 cSt silicone oil lens on a Krytox
bath during agitation regimes (C) – circular (260 Hz, 2.5 Vpp), (D) – deformed (260 Hz, 5 Vpp),
and (E) – elongated (260 Hz, 7Vpp). ............................................................................................ 73

Figure 4-4. (a) Cross-section and (b) top down image of a P(PFDA-co-EGDA) film grown on a
stationary 5 cSt silicone oil lens floating on Krytox and (c) cross-section and (d) top down image
of a P(PFDA-co-EGDA) film grown on an agitated 5 cSt silicone oil lens floating on Krytox.
The films were removed from the liquid surface before imaging. ............................................... 73

Figure 4-5. Agitation of P(PFDA-co-EGDA) films that encapsulate a 5 cSt silicone oil lens on a
Krytox bath as a function of film thickness and amplitude at 250 Hz. ......................................... 75

8

Figure 5-1: Average particle size as a function of deposition time for a) PnBA at a deposition
rate of 30 nm/min
2
and b) P4VP at a deposition rate of 40 nm/min ............................................. 83

Table 5-1: Average particle size on silicone oil viscosities of 100, 500, and 1000 cst for a)PnBA
at a deposition rate of 30 nm/min and a deposition time of 5 min and b) for P4VP at a deposition
rate of 40 nm/min and a deposition time of 5 min. ....................................................................... 84

9

Chapter 1: Introduction

10

1.1 initiated Chemical Vapor Deposition (iCVD)
The initiated chemical vapor deposition (iCVD) process, is a solventless free radical
polymerization process that can be used to make a wide range of functional polymer films such
as poly(4-vinylpyridine) (P4VP),
1
poly(2-hydroxyethyl methacrylate) (PHEMA),
2
and
poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA).
3
Typically, gas phase monomer and
initiator precursors are simultaneously flown into a vacuum chamber at pressures between 50
and 1000 mTorr.
4
The initiator molecules are cleaved thermally into free radicals by a heated
filament array typically at temperatures between 200 – 250 °C. The substrate temperature is
maintained between 20 – 60 °C by a recirculating chiller and polymerization of adsorbed
monomer occurs via initiation, propagation and termination in a free-radical mechanism. The
surface concentration of monomer determines the polymer deposition rate and molecular weight.
Monomer concentration at the surface is dependent on the ratio of the monomer partial pressure
(PM) to the saturation pressure (PSat). PM is given by FM*Ptotal/Ftotal, where FM is the flow rate of
monomer, Ftotal is the total flow rate of monomer and initiator, and Ptotal is the reactor pressure,
typical flow rates are on the order of 1 – 10 standard cubic centimeters per minute (sccm). The
saturation pressure at a given stage temperature (Tstage) is approximated using the Clausius-
Clapeyron equation: PSat=Aexp(- HVap/RTstage). The PM/PSat can be increased by increasing the
monomer flowrate, reactor pressure, or lowering the stage temperature. Typically, the PM/Psat is
between 0.4 - 0.7 such that there would be 1-3 monolayers of monomer at the surface to provide
sufficient monomer concentration for a suitably fast deposition rate.
5
An in-situ laser
interferometer is used to monitor the thickness of the polymer deposited which eliminates line of
sight processing. Due to the all dry nature of the process iCVD is ideal for conformal coatings of
both hydrophobic and hydrophilic polymers on a variety of substrates such as particles,
6

membranes,
7
wires,
8
and high aspect ratio surfaces.
9

11

1.2 Vacuum Techniques Using Sequential Depositions
Atomic layer deposition (ALD) is a gas-phase thin-film deposition technique that relies
on sequential processing by alternate pulsing of precursor and reactant gases, separated by
evacuation steps or purge periods.
10
These precursors react with the surface of a substrate one at
a time in a sequential, self-limiting manner since there are only a finite number of reactive sites
allowing for precise thickness control at the Ångstrom or monolayer level.
11
For instance, in the
fabrication of Al2O3 trimethyl aluminum (TMA) is reacted with the hydroxyl groups on the
surface of the substrate and purged, this is then followed by the flow of H2O which reacts to
form a layer of Al2O3 with Ångstrom level growth per reaction cycle.
12
ALD is also used to
fabricate thin films
13
and nanoparticles
14
of precious metals such as Ru, Rh, Pd, Os, Ir, and Pt
and bimetallic nanoparticles
15
for applications in catalysis. Typically, molecular oxygen or H2O
is used as a reactant to remove the ligands associated with the metal precursor.
16
Similarly a
pulse of hydrogen is used to react with oxygen to convert metal oxide to pure metal.
13
The
typical processing temperatures for ALD range from 50 - 350 ℃ and pulse pressures are on the
order of 1-10 Torr while base pressure is in the millitorr range.
17

Molecular layer deposition (MLD) is considered as the organic counterpart to ALD for
growing pure polymers and metal based hybrid polymers.
18
A similar approach of using self-
terminating reactions is used to grow films using two or more precursors, for instance Yoshimura
et al. demonstrated the growth of 100 Å polyimide films by sequentially reacting pyromellitic
dianhydride and 2,4-diaminonitrobenzene.
19
The growth per reaction cycle was on the order of a
few Ångstroms showing molecular level control. Other polymer such as polyureas,
20

polyamides,
21
and polyethylene terephthalate
22
thin films have also been fabricated using an
12

MLD approach. The typical operating pressures can range from 10
-10
Torr to 10 Torr and the
operating temperatures range from 50 to 200 ℃.
23,20

1.3 Liquid Substrates in Vacuum Deposition Processes
Bottom up fabrication of inorganic materials with use of low vapor pressure liquid
substrates as novel substrates was reported by Ye et al. in 1996 when aluminum and silver films
were obtained via sputtering on silicone oil.
24
They found a percolation structure formed initially
that transitioned to a film after at longer deposition time due to the aggregation of clusters of
atoms. Similarly, Xie et al. described a two-stage growth model for the deposition of gold and
silver onto silicone oil.
25
The first stage involves nucleation and growth of clusters of atoms
which diffuse on the liquid surface to form ramified aggregates. As the nominal film thickness
increases the surface coverage increases forming a continuous film. Ionic liquids have also since
been used for sputtering processes to form both films
26
and nanoparticles.
27
Recently vegetable
oils such as castor and canola oils were also used as substrates for silver deposition.
28
It was
shown that the sputtering process parameters such as discharge voltage and the surface
coordination ability of the organic moieties of the oils determined whether metal nanoparticles or
metal films were formed. Typically sputter depositions are performed at low pressures in the
range 0.01 to 20 mTorr.
29

The most notable organic material to be deposited on liquid substrates using chemical
vapor deposition is poly(para-xylylene) (parylene). The deposition of parylene C on liquid
substrates was patented in 2006
30,31
and has been used to encapsulate liquids such as silicone oil,
poly(ethylene glycol), 1,2,6-trihydroxyhexane, and glycerol to fabricate intraocular lenses due to
its biocompatibility, optical, and barrier properties of parylene
32
. Parylene is deposited using the
Gorham process (1966),
33
typically the dimer dichloro[2,2]-paracyclophane is sublimated at 150
13

˚C and subsequently pyrolysed at 650 ˚C in a vacuum resulting in the reactive intermediate para-
xylylene. Polymerization occurs via chain propagation in a deposition chamber at room
temperature. This process is unique to organic parylene and its derivatives and cannot be used for
other functional monomers because of the high temperatures associated with sublimation and
pyrolysis.
4

In our group the iCVD process has been performed to deposit polymers on a wide range
of liquid substrates such as silicone oils, glycerol, squalene, Krytox, and ionic liquids (IL’s).
34

The deposition of a number of functional polymers such as P4VP, PHEMA, PnBA, PnBMA,
PPFDA, and PPFM was performed on the liquids which resulted in the formation of polymer
films or polymer particles depending on the spreading coefficient of the polymer on the liquids.
The spreading coefficient (S) can be written in terms of the liquid−vapor surface tension (γ LV),
the polymer−vapor surface tension (γPV), and the advancing contact angle of the liquid on the
polymer (θ) as S = γ LV*(1 + cos θ) − 2γPV.
35
Thermodynamically stable films were formed for
systems with a positive spreading coefficient and particles were formed for systems with a
negative spreading coefficient. The deposition rate and deposition time were found to contribute
to the polymer morphology at the vapor liquid interface.
34
The use of liquid substrates adds
complexity to the iCVD not only due to the surface tension and viscosity effects but also due to
monomer solubility effects since polymerization can occur at both the liquid−vapor interface as
well as within the liquid for monomers that are soluble in the liquid.
36
The solubility of the
monomer has also been exploited to fabricate heterogeneous films by using monomers that were
soluble and insoluble in the liquid.
37
Reactive IL’s have also been used as substrates to form
copolymers
38
using the iCVD process. Thus, the use of low vapor pressure liquid substrates has
created various avenues for polymer fabrication and processing with iCVD.
14

1.4 Polymer – Ionic Liquid Gels
Macromolecular networks of polymers in IL’s have attracted attention due to the
combination of unique material properties of ILs, such as high ionic conductivity,
39
wide
electrochemical window,
40
and thermal stability,
41
with the mechanical properties of polymers.
Compatible polymer – IL systems are of interest to the fabrication of gels since they consist of a
homogenous polymer network in the IL.
42,43,44
In such materials the ion transport is independent
of the segmental motion polymer chains leading to high ion conductivities at moderate
temperatures compared to conventional polymer electrolytes.
43, 43
Polymer-IL gels have been
utilized as fuel cell membranes,
45,46,47
lithium batteries
48,49
and dye sensitized solar cells.
50,51

Typically polymer-IL gels are prepared by solution phase free radical polymerization
52, 42, 44
and
self-assembly of triblock copolymers via co-solvent evaporation.
53,54,55
In-situ polymerization of
vinyl monomers in IL’s yields high molecular weight polymers and assists in the formation of
polymer gels at relatively low polymer concentrations for compatible systems, whereas phase
separation is observed when the polymer – IL systems are not compatible.
42,44
Reduced
termination rates, and increased propagation rates are observed during the polymerization of
monomers in IL’s resulting in the formation high molecular weight polymers compared to
traditional organic solvents.
56,57
In the co-solvent evaporation method the copolymer forms an
ion gel by self-assembly of an amphiphilic ABA-triblock copolymer, where the A segment is
solvatophobic and the B segment is solvatophilic and serves as a tie molecule between the
solvatophobic polymer domains that assemble in a micellar network.
58
For instance He et al.
reported that polystyrene-block-poly(ethylene oxide)-block-polystyrene, formed solid gels at
polymer concentrations as low as 5 wt % in 1-butyl-3-methylimidazolium hexafluorophosphate
compared to 10 – 30 % polymer wt.% in conventional polymer gels.
55
In Chapter 2 of this
15

dissertation a sequential vapor phase process to fabricate polymer – ionic liquid gel beads is
discussed.
1.5 Polymer – particles
Polymer nanoparticles with controlled sizes have applications in a wide array of
applications in a variety of fields that include inks, coatings, chromatography, protein synthesis,
catalysis,
59
photonics
60
and drug delivery.
61
Polymer particle synthesis is typically performed
using solution phase methods such as emulsion polymerization, dispersion polymerization and
suspension polymerization.
62
In emulsion polymerization the monomer is emulsified in the
reaction medium along with initiator using surfactants. The surfactant forms micelles above the
critical micelle concentration which serve as loci for the nucleation of polymer particles. The
total number of particles and hence final particle size is affected by the monomer
concentration,
63
surfactant concentration,
64,65
and type of surfactant.
66
The reaction is completed
when the monomer is consumed and sub-micrometer

particles are obtained
62,67,68
. However, the
removal of surfactant is critical to certain applications
69
and requires additional processing using
salts, acids, and alkalis.
70,71,72
In dispersion polymerization, the monomer is solubilized in an
organic solvent containing stabilizer such as poly(vinyl pyrrolidone) and an initiator, as a result a
homogeneous polymerization occurs and oligomer chains precipitate to form primary particle
nuclei.
73
Polymerization then proceeds as monomer absorbs into the primary particles which are
coated by the steric stabilizer. Typically, particles sizes of 1-30μm are obtained
62
and the method
also affords complex non-spherical particle shapes.
74,75
However the process is characterized by
long reaction times and relies on volatile organic solvents.
76, 74, 75
Suspension polymerization
uses vigorous stirring of monomer droplets in aqueous media in the presence of a steric
stabilizer. Typically polymer particles sizes in the range of 50 to 1500 μm
62
are obtained and
16

several process parameters, such as densities and viscosities of the continuous and dispersed
phases, interfacial tension, type and concentration of suspending agent, type of impeller,
and stirring speed, affect the particle size making the process relevant on the industrial scale.
77,78

Newer methods such as controlled/living radical polymerization techniques such as atom transfer
radical polymerization (ATRP) and reversible addition-fragmentation chain transfer
polymerization (RAFT) are subsets of emulsion polymerization and are able to control the molar
mass of the polymer and particle size to overcome the limitations of the short lifetimes and
termination associated with free radicals and to better control molecular weight distribution.
79,62

In Chapter 3 a surfactant free, solvent – less vacuum process for the synthesis of functional
polymer particles is described that uses free radicals to polymerize monomer droplets which are
condensed on a layer of silicone.
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23

Chapter 2: Fabrication of Ionic Liquid Gel Beads via Sequential
Deposition

P. Karandikar, M. Gupta. “Fabrication of ionic liquid gel beads via sequential deposition.” Thin
Solid Films, 2017, 635, 17−22.
24

2.1 Abstract
We demonstrate the fabrication of gel beads composed of ionic liquid (IL) and polymer.
The IL droplets are kept spherical during the deposition process by placement onto
chromatography paper coated with fluoropolymer. The deposition process then occurs in two
steps. In the first step, the monomer is absorbed into the IL droplet. In the second step, the
initiator radicals are introduced. This sequential deposition process allows polymerization to
primarily occur within the liquid droplet and therefore the beads are not attached to the
underlying substrate and can be easily removed.
2.2. Introduction
The initiated chemical vapor deposition (iCVD) process is a solventless polymerization
process which is typically used to coat solid surfaces.
1,2
We recently demonstrated that we can
use the iCVD process to deposit polymer onto liquid substrates with low vapor pressures such as
silicone oils and ionic liquids (ILs).
3,4
The deposition of polymer onto these liquids has
complexity due to surface tension effects and solubility effects. The surface tension effects
determine whether polymer deposition leads to the formation of films or particles at the surface
of the liquid.
5
Monomer solubility in the liquid allows for interesting dynamics due to different
polymerization kinetics in the bulk liquid.
6
For example, higher molecular weight polymers and
increased propagation rate coefficients have been reported for free radical polymerizations in IL
media compared to organic solvents.
7,8,9,10

ILs have attracted considerable attention due to such properties as high ionic
conductivity
11
, wide electrochemical window
12
, thermal stability
13
, and low vapor pressure
14

making them useful for diverse applications such as energy storage and utilization
15,16
,
radioactive waste handling
17,18
, and carbon capture.
19
Integration of polymers within ILs is
25

important for combining the chemical functionality of ILs with the mechanical properties of
polymers.
20
We have previously demonstrated two methods to incorporate polymers with ILs. In
the first method, we encapsulated IL droplets within polymer shells composed of
poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA), poly(ethylene glycol diacrylate)
(PEGDA), and poly(1H,1H,2H,2H-perfluorodecyl acrylate-co-ethylene glycol diacrylate)
(P(PFDA-co-EGDA))
21
. The IL droplets were first stabilized into spherical shapes by rolling
them in poly(tetrafluoroethylene) (PTFE) particles and then they were placed on a bed of PTFE
particles during deposition in order to prevent bridging between the deposited polymer and the
underlying substrate. The PTFE particles that were used to stabilize the droplet shape were
incorporated into the polymer shell after deposition. The shells were robust enough to remove
the IL inside shell and replace it with another liquid. In the second method, we fabricated thin gel
films of thicknesses from 3 to 20 μm via iCVD polymerization of monomer 2- hydroxyethyl
methacrylate (HEMA) within 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF4].
6

Since HEMA is soluble in [emim][BF4], the monomer can both adsorb and absorb into the IL
and therefore polymerization occurs on both the liquid surface and within the interior liquid. A
transition from a viscous liquid to a gel was observed with increasing polymer concentration.
Molecular weight analysis of the films by gel permeation chromatography showed the presence
of low molecular weight polymer chains which formed on the surface and high molecular weight
polymer chains which formed within the interior liquid.
In the above gel fabrication study, the monomer and initiator vapors were introduced
simultaneously and therefore polymerization occurred at both the surface and within the interior.
In this work, we fabricate gel beads using a sequential deposition process. Sequential depositions
are used in processes such as atomic layer deposition (ALD) and molecular layer depositions
26

(MLD) to fabricate thin films.
22,23
The film thickness can be precisely controlled due to the
sequential exposure of the substrate to precursors. The sequential processing also provides
conformal films on substrates with complex geometries. In ALD and MLD, the precursors are
reacted to the underlying substrate. In a typical iCVD process, there is no reaction with the
underlying substrate and therefore sequential processing does not lead to film formation because
the monomer or initiator will get pumped out between steps. However, we can use sequential
processing on a liquid substrate because the liquid can act as a monomer sink. To fabricate the
gel beads, the monomer was first absorbed into the IL and then the initiator was introduced to
polymerize the monomer. Since the majority of the polymerization occurs within the liquid, this
sequential deposition prevents the formation of a film around the IL and therefore allows the
beads to be easily removed from the underlying substrate. Instead of using PTFE particles to
stabilize the droplet shape during polymerization, we introduce the concept of using fluorinated
chromatography paper as our substrate in order to use surface tension interactions to keep the IL
droplets spherical.
2.3 Experimental
2.3.1 Polymer Deposition
2- Hydroxyethyl methacrylate (HEMA) monomer (98% Aldrich), 1H,1H,2H,2H-perfluorodecyl
acrylate (PFDA) monomer (97% Aldrich), 1-Ethyl-3-methylimidazolium tetrafluoroborate
([emim][BF4]), and Di-tert-butyl peroxide (TBPO) (98% Aldrich) were used as received without
further purification. All polymer depositions were performed in a custom built reactor (25 cm
diameter, 48 mm height) (GVD Corporation) equipped with a recirculating chiller to adjust the
substrate temperature. A nichrome filament array (80% Ni, 20% Cr, Omega Engineering) was
placed 3.5 cm above the substrate. Deposition rates were monitored by in situ laser (633 nm
27

helium-neon) interferometry (Industrial Fiber Optics) on a reference silicon wafer. Polymer
depositions of poly (1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) were performed on a 10cm
x 6cm piece of chromatography paper (GE Healthcare Life Sciences Whatman™) at a rate of
10nm/min to alter the wetting properties of the paper. A deposition time of 40 minutes was
chosen to deposit polymer for a total thickness of 400 nm in order to ensure a uniform coating of
fluoropolymer over the individual fibers of the porous chromatography paper. The base pressure
before starting the deposition was 5.73 Pa. The deposition was performed at a pressure of 10.67
Pa, the PFDA monomer jar was maintained at 50 °C, and the TBPO flowrate was kept at 0.5
sccm using a mass flow controller (Type 1152C MKS). The stage temperature was maintained at
30 °C and the filament array was heated to a temperature of 250 °C.
A two-step process was performed to create the gel beads. 1.5 μl of [emim][BF4] was
pipetted onto a 2cm x 2cm square of chromatography paper coated with PPFDA and placed
inside the reactor for monomer absorption. In the first step, HEMA was absorbed into the droplet
of IL for 1 hr. The HEMA monomer jar was heated to 40 °C, the stage temperature was
maintained at 25 °C, and the reactor pressure was held at 20 Pa. Following this absorption step,
the monomer flow was turned off and the reactor was evacuated for 10 minutes. In the second
step, the initiator was introduced into the reactor for 1 hour at a pressure of 133.32 Pa with a
flow rate of 2.5 sccm and the filament array was heated to a temperature of 250°C.
For the control experiments of simultaneous deposition, poly (2- hydroxyethyl
methacrylate) (PHEMA) was deposited for 1 hour at a rate of 20nm/min as measured on a
reference silicon wafer by the simultaneous flow of monomer and initiator on a 1.5 μl droplet of
[emim][BF4] placed on PPFDA coated chromatography paper. The reactor pressure was held at
16 Pa, the HEMA monomer jar was heated to 40°C, the TBPO flow rate was set to 0.5 sccm, and
28

the stage temperature was maintained at 25°C. The filament array was resistively heated
continuously throughout the process at 250°C.
The sequential and simultaneous depositions of PHEMA on the IL droplet were also
performed using conventional substrates. A silicon wafer coated with 200 nm of PPFDA was
used to replace the chromatography paper coated with PPFDA. A 1.5 μl droplet of [emim][BF4]
was dispensed on the PPFDA coated silicon wafer and processed using the sequential and
simultaneous depositions as described above.
2.3.2 Analysis
A contact angle goniometer (Ramé -Hart Model 290-F1) was used to measure the contact
angles of water and IL on chromatography paper coated with PPFDA before and after the
deposition of PHEMA. 1.5 μl drops of the liquids were dispensed on the paper. Profile images of
the samples were taken using the goniometer camera.
Fourier transform infrared spectroscopy (FTIR) was conducted on a reference iCVD film
of PHEMA, [emim][BF4], and the gel beads using a Thermo-Scientific Nicolet instrument. The
spectrum for the IL was collected by placing 1 μl of the liquid between two pieces of silicon
wafers and subtracting the background spectra of the two wafers without IL. The spectrum for
the gel beads were collected by placing eight gel beads between two pieces of silicon wafers and
subtracting the background spectra of the two wafers without the beads.
2.4 Results and Discussion
2.4.1 Fabrication of Beads via Sequential Deposition
29

In order to fabricate beads with a spherical shape, we placed the IL droplets on a
substrate with a low surface energy in order to achieve high contact angles for the IL. The
substrate that we used was chromatography paper coated with a thin layer of PPFDA. Due to the
conformal nature of the iCVD process, the individual fibers of the chromatography paper are
coated and therefore the porous morphology of the chromatography paper is kept intact [24]. The
surface roughness of the chromatography paper combined with the low surface energy of the
PPFDA coating allows us to achieve high contact angles of IL. The contact angles of water
(Figure 2-1a) and [emim][BF4] (Figure 2-1b) on the PPFDA coated chromatography paper were
130 ± 6° and 131 ± 5 °, respectively, whereas both water and [emim][BF4] completely soak the
uncoated chromatography paper.

Figure 2-1: Contact angle images of a) a water droplet and b) a [emim][BF4] droplet on chromatography
paper coated with PPFDA fluoropolymer.

After we pipetted the IL droplets onto the PPFDA coated chromatography paper, we then
implemented a sequential free radical polymerization process in our iCVD reactor to form the gel
beads. In the first step, we absorbed HEMA monomer into the IL droplets in the absence of free
radicals for one hour. In the second step, we introduced the initiator and turned on the filament
array to create free radicals for one hour. There was no monomer flow in the second step which
allows for the sequential process to utilize the monomer absorbed in the IL as the main source of
reactants for polymerization. In the sequential deposition process, the monomer at the surface of
30

the IL is negligible since it is not replenished by the continuous flow of monomer into the
reaction chamber. Thus polymerization occurs mostly within the IL phase and the gel beads can
therefore be easily removed from the PPFDA coated chromatography paper after deposition
since there is no significant polymer coating tethering the beads onto the surface of the substrate.
Figure 2-2a shows that the IL droplet is transparent before monomer absorption. After
monomer absorption and subsequent polymerization, the transparent IL droplet is transformed
into an opaque gel bead as shown in Figure 2-2b. We confirmed that the droplet shape is
preserved during the polymerization process as seen in the profile image of the gel bead on the
coated paper in Figure 2-2c. The contact angle of the gel beads on the PPFDA coated paper
before removing from the paper was measured to be 130 ± 3° which is similar to the contact
angle of the original IL droplet on the PPFDA coated paper. We demonstrate that the gel beads
can be lifted and handled with ease using tweezers in Figure 2-2d.

Figure 2-2: Top down images of a) a transparent droplet of [emim][BF4] on PPFDA coated
chromatography paper and b) an opaque gel bead composed of PHEMA and [emim][BF4] after
sequential deposition. c) Side view image of the gel bead. d) The gel bead held using a pair of tweezers.

2.4.2 Chemical characterization of the gel beads
a b c
d
31

We used FTIR analysis to confirm the incorporation of PHEMA within the [emim][BF4]
droplet. Figure 2-3 shows the absorbance spectra for [emim][BF4], a reference iCVD PHEMA
film deposited onto a silicon wafer, and the gel beads. The presence of polymer within the gels
was confirmed by the presence of the characteristic carbonyl stretching around 1720 cm
-1
in both
the PHEMA iCVD film and the gels. The presence of ionic liquid within the gel beads was
confirmed by the presence of the symmetric stretching of the C-H bonds in the imidazolium
functionality between 3000 – 3250 cm
-1
.
25,26
This data demonstrates that the gel beads are
composed of both the polymer and the IL.

Figure 2-3: FTIR spectra of the gel beads compared to a reference iCVD PHEMA film and reference
[emim][BF4]. Dashed lines indicate the location of C-H symmetric stretching (left) in the imidazolium
functionality of [emim][BF4] and C=O stretching (right) in the carbonyl functionality of PHEMA.

2.4.3 Simultaneous Deposition
In order to demonstrate the significance of the sequential process for the formation of the beads,
we performed simultaneous iCVD of PHEMA onto IL droplets on PPFDA coated paper.
Monomer was continuously flowed in the presence of free radicals for one hour to deposit 1.2
32

microns of PHEMA onto the IL droplets. Since both precursors are introduced at the same time,
polymerization can occur at both the surface of the liquid and within the interior of the liquid.
The polymerization at the liquid surface leads to a wrinkled polymer skin around the IL that
connects the droplet to the underlying substrate. In addition, significant deformation of the
droplet morphology was observed as seen in the top down (Figure 2-4b) and profile images
(Figure 2-4c). The change in the droplet morphology during the processing occurs due to the
deposition of PHEMA onto the PPFDA coated paper surrounding the IL droplet. The change in
the surface tension of the fibers causes the IL droplets to start wetting the paper during the
simultaneous deposition. As a result the liquid wicks into the paper and the polymer film
collapses which gives it a wrinkled appearance. The polymer skins that are formed in the
simultaneous process encapsulate the liquid and can be separated from the IL mechanically using
tweezers as seen in Figure 2-4d.

Figure 2-4: Top down images of a) a transparent droplet of [emim][BF4] on PPFDA coated
chromatography paper and b) the sample composed of PHEMA and [emim][BF4] after simultaneous
deposition showing a wrinkled PHEMA skin. c) Side view image of the sample. d) The wrinkled PHEMA
skin separated by tweezers.
33

This is in contrast to the gel beads produced by the sequential process in which the polymer
forms a homogenous phase with the IL. In the simultaneous iCVD process, there is continuous
flow of monomer into the reaction chamber and therefore the concentration of monomer
adsorbed at the surface of the IL is expected to be in equilibrium with the gas phase
concentration. Thus unlike sequential deposition, the concentration of monomer adsorbed on the
surface of the IL is expected to be significantly higher leading to the formation of a polymer skin
encapsulating the IL. We used contact angle goniometry to confirm that the wetting properties of
the PPFDA coated paper were indeed altered after simultaneous deposition of PHEMA. Water
droplets completely wet the paper after simultaneous deposition (Figure 2-5a). This verifies that
the deformation of the shape of the droplet during simultaneous deposition is due to PHEMA
deposition onto the PPFDA coated paper. We also measured the contact angle of water on the
PPFDA coated paper after sequential deposition of PHEMA to compare the two processes. We
found that the contact angle was reduced to 75 ± 19 ° (Figure 2-5b) which indicates that there is
still sufficient fluorinated polymer functionality to render the surface hydrophobic relative to
uncoated paper. We hypothesize that there is some adsorption of monomer on the paper surface
during the absorption step of the sequential process which is polymerized during the initiation
step. This results in partial coating of the paper with PHEMA which alters the contact angle of
water.

Figure 2-5: Contact angle images of a water droplet on the PPFDA coated chromatography paper after a)
simultaneous PHEMA deposition and b) sequential PHEMA deposition.
34

2.4.4 Sequential and simultaneous deposition on conventional PPFDA coated substrates
In order to demonstrate the effect of the surface roughness and porosity of the chromatography
paper on the sequential and simultaneous deposition, we replaced the PPFDA coated
chromatography paper with a PPFDA coated silicon wafer. The IL has a lower contact angle of
102 ± 3° on the PPFDA coated silicon wafer as shown in Figure 2-6a. Gel formation was
observed after the sequential deposition as shown in Figure 2-6b however the contact angle of
the gel changed slightly during processing due to a partial PHEMA deposition on the PPFDA
surface. The gel can be picked up using a pair of tweezers as shown in Figure 2-6c since there is
no polymer coating tethering it to the wafer. The simultaneous deposition results in a smooth
polymer skin encapsulating the IL as shown in Figure 2-6e. The uniform deposition of PHEMA
on the PPFDA coated silicon wafer causes the IL to wet the wafer significantly during deposition
as shown in Figure 2-6e. The polymer skin tethers the IL to the surface of the wafer and can be
mechanically separated from the IL with a pair of tweezers as shown in Figure 2-6f.

Figure 2-6: a) The contact angle of the IL on the PPFDA coated substrate before sequential deposition, b)
the profile image of the gel after sequential deposition, c) the gel can be picked up with tweezers, d) the
contact angle of the IL on the PPFDA coated substrate before simultaneous deposition, e) the profile
image of the polymer film encapsulating the IL after simultaneous deposition, and f) the polymer film
partially peeled off the IL with a pair of tweezers
35

These results are analogous to those seen on the porous substrate, however in the case of
simultaneous iCVD on the silicon wafer we do not observe wrinkling of the polymer film
because the silicon wafer is impermeable and prevents the IL from soaking into the substrate.
Conclusions
We have demonstrated that gel beads can be made by placing IL droplets on
chromatography paper coated with fluoropolymer to maintain the liquid at a high contact angle
during deposition. Polymerization must be performed sequentially. Monomer is absorbed into
the ionic liquid droplets in the first step and the initiator is introduced in the second step. We
have shown that this sequence is necessary in order to: 1) prevent polymerization on the
chromatography paper during deposition which can change the morphology of the liquid droplet
and 2) avoid the formation of a polymer skin around the liquid droplet which can connect the
beads to the underlying substrate. In the sequential process, polymerization occurs mostly within
the liquid droplet and therefore the resulting gel beads can be easily lifted from the underlying
substrate.
Acknowledgments
This work was supported by the U.S. Department of Energy, Office of Science, Basic
Energy Sciences under Award #DE-SC0012407.

36

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6. Frank-Finney, R. J.; Bradley, L. C.; Gupta, M. Formation of Polymer-Ionic Liquid Gels Using
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7. Schmidt-Naake, G.; Woecht, I.; Schmalfuß, A.; Glück, T. V. Free Radical Polymerization in
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9. Harrisson, S.; Mackenzie, S. R.; Haddleton, D. M. Unprecedented solvent-induced
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10. Kubisa, P. Ionic liquids as solvents for polymerization processes—Progress and challenges.
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14. Ahrenberg, M.; Beck, M.; Neise, C.; Keßler, O.; Kragl, U.; Verevkin, S.P.; Schick, C. Vapor
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ionic liquids. Electrochim. Acta 2007, 53, 1911-1919.
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1-8.

38

Chapter 3: Synthesis of Functional Particles by Condensation and
Polymerization of Monomer Droplets in Silicone Oils

P. Karandikar, M. Gupta. “Synthesis of Functional Particles by Condensation and
Polymerization of Monomer Droplets in Silicone Oils.” Langmuir, 2017, 33, 7701−7707.
39

3.1 Abstract
The synthesis of poly (4-vinyl pyridine) and poly (2-hydroxyethyl methacrylate) polymer
particles in silicone oil using a sequential vapor phase polymerization method in which monomer
droplets were first condensed onto a layer of silicone oil and subsequently polymerized via a free
radical mechanism is reported. The viscosity of the silicone oil was systematically varied. At
lower viscosities, a heterogeneous particle size distribution was produced where small particles
were formed by engulfment of the monomer droplets at the liquid surface and large particles
were formed by coalescence of the monomer droplets inside the liquid layer. Coalescence could
be inhibited by increasing the viscosity of the silicone oil leading to a decreased average radius
and a narrower size distribution of the polymer particles. The advantages of our method for the
fabrication of polymer particles are that it does not require surfactants or organic solvents.
3.2 Introduction
Polymer particles have uses in a wide range of applications such as catalysis
1
, photonics
2

and drug delivery
3
. Polymer particles are typically synthesized by emulsion
4
or dispersion
polymerization
5
. In emulsion polymerization, a hydrophobic monomer is dispersed in an aqueous
phase and polymerized using an initiator. Emulsions are susceptible to degradation via Ostwald
ripening, a process by which small monomer droplets disappear due to monomer diffusion into
the bulk phase, therefore the use of surfactants or stabilizers is necessary to control particle
size
6,7
. In dispersion polymerization, a homogenous mixture of monomer, initiator, and stabilizer
is reacted in a solvent and the resulting polymer precipitates to form particle nuclei since it is
insoluble in the solvent. Subsequent particle growth is controlled by addition of stabilizers or
cross-linkers after the nucleation stage
8,9
. Block-copolymer self-assembly is also another route to
40

tune polymer morphology
10,11
which involves tailoring the hydrophilic to hydrophobic block
ratios and solvent interactions
12
.
The initiated chemical vapor deposition (iCVD) process is a solventless free radical
polymerization method which can be used to deposit functional polymers. Monomer and initiator
molecules are flown simultaneously into a vacuum reactor where thermal decomposition of the
initiator produces free radicals. The ratio of the monomer partial pressure (Pm) to the monomer
saturation pressure (Psat) can be used to control the deposition rate of polymer onto the substrate.
A Pm/Psat value of less than 1 is used for the deposition of thin dense coatings and films
13
. We
recently used the iCVD process to deposit polymers onto low vapor pressure liquids such as
silicone oils and ionic liquids
14, 15
. We observed that the polymer morphology at the vapor-liquid
interface was governed by surface tension and viscosity leading to the formation of polymer
films and particles
16
.
Here we report a sequential deposition process exploring the use of Pm exceeding Psat to
synthesize polymer particles by first condensing monomer droplets on silicone oil and then
polymerizing the monomer. This allows us to independently control the monomer condensation
step and the polymerization step, which is in contrast to the conventional iCVD process which
utilizes simultaneous flow of monomer and free radicals to deposit polymer. We studied the
polymerization of two different monomers, 4 - vinyl pyridine (4VP) and 2 - hydroxyethyl
methacrylate (HEMA), since these polymers have a variety of useful properties. For example,
poly (4 – vinyl pyridine) (P4VP) is pH responsive
17
and has been used in CO2 capture
18
while
poly (2-hydroxyethyl methacrylate) (PHEMA) is a hydrogel known for its biocompatibility
19,20
.
In this study we show that the particles size is governed by engulfment of monomer droplets at
the liquid surface and coalescence within the liquid layer. Suspension polymerization methods
41

typically yield particles with average diameters ranging from 5 μm to several millimeters in
which the suspension of monomer droplets is mechanically agitated and stabilizers such as poly
(vinyl alcohol) are used to control the particle size
21, 22
. In contrast, our process produces
particles with average diameters ranging from 0.1 to 0.5 μm and does not require stirring or
stabilizers. Our technique also offers many additional advantages such as the ability to tune the
particle size by changing the substrate viscosity and the ability to tune the polymer functionality
by varying the monomer precursor.
3.3 Experimental
Silicone oil (100 cst, 1000 cst, and 100,000 cst Sigma-Aldrich), 4-vinyl pyridine (4VP)
(95% Sigma-Aldrich), 2-hydroxyethyl methacrylate (HEMA) (98% Sigma-Aldrich), di-tert-butyl
peroxide (TBPO) (98% Sigma-Aldrich), and hexanes (98% Sigma-Aldrich) were used without
further purification. Volumetric mixtures (1:1 v/v) of monomer and silicone oil were prepared to
assess the solubility of HEMA and 4VP in 100 cst silicone oil. The mixtures were allowed to
stand for 24 hours for complete phase separation before FTIR (Thermo-Scientific Nicolet iS10)
was performed on the oil phase. Absence of characteristic vibrations of pyridine for 4VP and
carbonyl stretching of HEMA in the silicone oil phase indicated that both monomers are
insoluble in the silicone oil.
Polymerization was performed in a custom designed vacuum reactor (250 mm diameter,
48 mm height, GVD Corporation). A nichrome filament array (80% Ni, 20% Cr, Omega
Engineering) was used to generate free radicals by thermal cleavage of TBPO. The reactor
pressure was measured using a pressure transducer (MKS Baratron
®
capacitance manometer
622A01TDE) and set point was achieved and maintained by an automated butterfly valve. A
42

recirculating chiller was used to maintain the stage temperature of the reactor. The base pressure
of the reactor was 36 mTorr.
In order to fabricate the liquid layers, 10 μL of silicone oil (100 cst, 1000 cst, 100,000
cst) was pipetted onto a 1 cm x 1 cm piece of silicon wafer for a nominal thickness of 100 μm
and this sample was placed inside the reactor. In order to fabricate the poly (4-vinyl pyridine)
(P4VP) particles, first 4VP monomer was condensed on the silicone oil. The silicone oil
substrate was cooled to -10 ˚C using a thermoelectric cooler (TEC) (Custom Thermoelectric)
while the stage was maintained at 20 ˚C using the recirculating chiller. The temperature of the
silicone oil was allowed to equilibrate for 30 minutes before exposure to monomer vapors. Since
the silicone oil was cooler than the reactor wall and the stage, the monomer only condensed on
the silicone oil. The monomer jar was heated to 25 ˚C to produce a flowrate of 13 sccm and the
initiator flow rate was set to 0.7 sccm using a mass flow controller. The monomer was condensed
for 15 seconds by setting the reactor pressure to 150 mTorr, 225 mTorr, and 500 mTorr to
achieve Pm/Psat values of 1, 1.5, and 3.3 respectively. In the second step of the process, the
monomer flow was turned off which was followed by increasing the initiator flow rate to 5 sccm
and setting the reactor pressure to 1 Torr as the filament array was resistively heated to 250 °C
for 15 minutes to generate free radicals. The reactor pressure was set to 1 Torr to prevent the
evaporation of the condensed monomer during the second step of the process. In order to
fabricate poly (2-hydroxy ethyl methacrylate) (PHEMA) particles, first HEMA monomer was
condensed on the silicone oil. The silicone oil substrate was cooled to 0 ˚C using a
thermoelectric cooler (TEC) (Custom Thermoelectric) while the stage was maintained at 20 ˚C
using the recirculating chiller. The temperature of the silicone oil was allowed to equilibrate for
30 minutes before exposure to monomer vapors. The monomer jar was heated to 40 ˚C to
43

produce a flowrate of 0.25 sccm and the initiator flow rate was set to 0.7 sccm using a mass flow
controller. The monomer was condensed for 15 seconds by setting the reactor pressure to 70
mTorr, 100 mTorr, and 225 mTorr to achieve Pm/Psat values of 1, 1.5, and 3.3 respectively. In
the second step monomer flow was turned off and polymerization was performed at 1 Torr by
resistively heating the filament array to 250 °C for 15 minutes with an initiator flow rate of 5
sccm.
Coalescence was demonstrated in the absence of polymerization by condensing 4VP
monomer for 15 seconds at a Pm/Psat of 1.5 on the 100 cst silicone oil using the same conditions
as described above. Subsequently the monomer flow was turned off and the reactor pressure was
increased to 1 Torr with an initiator flow rate of 5 sccm for 15 minutes, however the filament
array was not heated. Similarly, the coalescence of HEMA droplets was demonstrated by
condensing the monomer for 15 seconds at a Pm/Psat of 1.5 on the 100 cst silicone oil and
subsequently increasing the reactor pressure to 1 Torr without heating the filament array for 15
minutes.
For the iCVD control experiments, the monomer and initiator were simultaneously flown
into the reactor with continuous resistive heating of the filament array at 250 °C for 15 minutes.
A reference silicon wafer was placed inside the reactor to monitor the polymer deposition rate
via in situ interferometry using a 633 nm helium-neon laser (Industrial Fiber Optics). For the
P4VP deposition on the 100 cst silicone oil, the monomer jar was heated to 25 ˚C, the initiator
flow rate was set to 0.7 sccm, the reactor pressure was set to 500 mTorr, and the stage
temperature was maintained at 25˚C resulting in a Pm/Psat of 0.3 and a deposition rate of 20
nm/min. For the deposition of PHEMA on the 100 cst silicone oil, the monomer jar was heated
to 40 ˚C, the initiator flow rate was set to 1sccm, the reactor pressure was set to 150 mTorr, and
44

the stage was maintained at 25 ˚C resulting in a Pm/Psat of 0.3 and a deposition rate of 20 nm/min.
The equilibrium contact angle of silicone oil on iCVD films of PHEMA and P4VP deposited on
the silicon wafer was determined using a goniometer (Ramé -Hart Model 290-F1). 5 μl of 100 cst
silicone oil was dispensed on the polymer films and allowed to equilibrate for 5 minutes. The
contact angle for both polymers was determined to be zero based on the complete spreading of
silicone oil.
A quartz crystal microbalance (QCM) (Sycon Instruments) with 6MHz gold plated
crystal was used to measure the gas phase monomer adsorption and absorption of 4VP and
HEMA in the 100 cst silicone oil during iCVD. First the adsorption for 4VP and HEMA was
measured on the bare crystal at 25 ˚C using the same reactor pressure, stage temperature, and
Pm/Psat as the iCVD control experiments listed above, however the filament array was not turned
on to prevent polymerization on the crystal. A thin layer of silicone oil (550 Å) was then spread
onto the gold crystal and the experiment was repeated to measure the total amount of monomer
adsorbed and absorbed in the silicone oil. The mass uptake was allowed to equilibrate, which
occurred within 10 minutes. The concentrations of 4VP and HEMA measured on the bare crystal
(0.14 μg/cm
2
and 0.21 μg/cm
2
respectively) were similar to those measured in the presence of
silicone oil (0.15 μg/cm
2
and 0.20 μg/cm
2
respectively) indicating that there is no significant
absorption of monomers in the silicone oil during the iCVD process.
Dynamic light scattering (DLS) (Wyatt DynaPro Titan) was used to measure the particle
size distribution. Three depositions were performed at each Pm/Psat value (1, 1.5, and 3.3) and for
the control iCVD experiments for P4VP and PHEMA. The silicone oil collected after each
polymerization was transferred from the silicon wafer to a quartz cuvette sample holder using a
pipette and a razor blade. In order to collect a sufficient volume of silicone oil for the DLS
45

measurements, three 1 cm x 1 cm wafers were placed inside the reactor for each deposition. The
measurement displayed a regularization graph with a hydrodynamic particle radius and
polydispersity which represents the width of the distribution determined using cumulants
analysis
23
. Each measurement consisted of 20 acquisitions that were collected and analyzed in
DYNAMICS
TM
– version 7.1.7.16 (Wyatt Technology Corp.) software. Three measurements
were performed on the samples collected from each deposition to calculate the mean particle
radius, standard deviation of the mean particle radius (S.D.), and a coefficient of variation
(C.V.). The C.V. represents the standard deviation of the particle size distribution of all
measurements divided by the mean particle radius and is used to quantify the width of the
distribution. An Agilent model 8453 UV – visible spectrophotometer was used to measure the
absorbance of the P4VP particle suspension in the 100 cst silicone oil. Disposable plastic
cuvettes with a path length of 10 mm were used to measure the absorbance at 600 nm
wavelength. A sample volume of 0.5 ml was prepared by collecting silicone oils from multiple
depositions. A blank measurement consisting of pure silicone oil was performed before
analyzing each sample. Three measurements were performed to calculate the average absorbance
and standard deviation for each sample.
Scanning electron microscopy (SEM) (JEOL-7001) was performed to image the polymer
particles. Samples for SEM were prepared by rinsing the liquid layers collected after the
polymerization process with hexanes in order to remove the silicone oil from the silicon wafer.
In order to prevent dissolution of the P4VP polymer particles, ultraviolet (UV) crosslinking was
performed with a 250 MW Hg lamp (UV-technik) for 20 minutes prior to rinsing the silicone oil
with hexanes. The PHEMA and P4VP films were imaged as fabricated on the 100,000 cst
silicone oil liquid layers without any further treatment.
46

X-ray photoelectron spectroscopy (XPS) was performed to analyze the atomic
composition of the P4VP films produced on the 100,000 cst silicone oil using a Kratos Axis
Ultra DLD instrument with a monochromatic Al Kα x-ray source. The P4VP films were lifted
off the surface of the 100,000 cst silicone oil using double sided carbon tape and soaked in
hexanes overnight to remove residual silicone oil before performing XPS. Survey spectra were
acquired from 0 to 900 eV with a step size of 1 eV and by performing five scans.
3.4 Results and Discussion
The polymer particles were produced by first condensing monomer on a layer of silicone
oil and then subsequently polymerizing the monomer by turning on the filament array to cleave
the initiator molecules into free radicals. Monomer condensation occurs as the monomer partial
pressure (Pm) approaches the saturation pressure (Psat) and therefore the process must be carried
out at Pm/Psat values equal to or greater than unity. In order to systematically study the effect of
Pm/Psat on the average size of P4VP particles, we condensed 4VP monomer at Pm/Psat values of 1,
1.5, and 3.3 on 100 cst silicone oil. The value of Pm/Psat was varied by keeping the substrate
temperature constant while increasing the total reactor pressure. The monomer was condensed
for 15 seconds and then the filament array was turned on for 15 minutes to polymerize the
monomer. The dynamic light scattering (DLS) data shown in Table 3-1 and Figure 3-1 indicate
that Pm/Psat does not have a significant effect on the average particle radius and the particle size
distribution in this range of process conditions. This is likely because as the reactor pressure
reaches the set value, the monomer starts condensing at the vapor-liquid interface as Pm/Psat
approaches unity and these droplets are then cloaked and engulfed by the silicone oil. This
mechanism of cloaking and engulfment was studied by Anand et al. for the condensation of
water droplets on silicone oils
24
. They found that droplets formed at the vapor-liquid interface
47

were cloaked by a thin layer of oil within 10
-16
to 10
-4
seconds leading to engulfment into the
bulk oil. The minimum free energy required for this engulfment is given by ∆G = πr
2
γ (1 -
|cosθe|)
2
where r is the particle radius, γ is the liquid – vapor surface tension, and θe is the
equilibrium three phase contact angle
25
. Since 4VP droplets are readily wet by silicone oil, the
contact angle for our system is zero and the engulfment of monomer droplets is
thermodynamically favorable
26,27
. The fast wetting dynamics and engulfment prevents
accumulation and growth of the droplets at the vapor-liquid interface thereby leading to similar
particle size distributions for all Pm/Psat values studied.

Table 3-1: Size measurements of P4VP particles using dynamic light scattering

Figure 3-1: Representative size distributions for P4VP particles produced on 100 cst silicone oil at P m/P sat
values of 1, 1.5, and 3.3.

The SEM images of the particles (Figure 3-2) show a spherical morphology and a
heterogeneous size distribution in agreement with the size distribution provided by DLS. We
Sample
Mean
Radius
(nm)
Standard
Deviation
(nm)
Coefficient of
Variation
100 cst, P m/P sat = 1 233 70 0.14
100 cst, P m/P sat = 1.5 193 30 0.18
100 cst, P m/P sat = 3.3 257 54 0.15
1000 cst, P m/P sat = 1.5 68 9 0.07
100 cst, iCVD, P m/P sat = 0.3 137 14 0.06
48

observed that the SEM samples produced at higher Pm/Psat values had a greater coverage of
polymer particles on the silicon wafer likely because a longer period of time was required to
achieve the set point leading to a longer duration of monomer condensation.

Figure 3-2: Representative SEM images of P4VP particles produced on 100 cst silicone oil at a)
P m/P sat = 1, b) P m/P sat = 1.5, and c) P m/P sat = 3.3.

We investigated the increase in particle concentration by measuring the absorbance of the
particle suspension in silicone oil using UV-vis spectroscopy at 600 nm wavelength (Table 3-2).
The absorbance data suggests that the concentration of polymer particles in the suspension is
49

increasing as a function of Pm/Psat since a higher particle concentration causes reduced
transmittance due to light scattering in the visible region
28
. In order to further investigate the
effect of condensation time on the particle coverage and size distribution, we performed our
process at Pm/Psat = 1 for monomer condensation times of 15 seconds, 45 seconds and 90
seconds. We did not observe a significant effect on the particle size distribution as expected
because of the fast wetting dynamics and engulfment at the silicone oil surface. The SEM images
(Figure 3-3) show that there is a significant increase in particle coverage on the silicon wafer
with increasing condensation time which is consistent with the Pm/Psat data. We also found an
increase in UV-vis absorbance with increasing condensation time (Table 3-2) confirming that the
higher particle coverage on the wafer (Figure 3-2) is a result of longer duration of monomer
condensation.
Sample
Condensation
time
Absorbance
Units (AU)
±
Standard
Deviation
P
m
/P
sat
= 1
15 seconds 0.14 ± 0.02
P
m
/P
sat
= 1.5
15 seconds 0.18 ± 0.01
P
m
/P
sat
= 3.3
15 seconds 0.25 ± 0.01
P
m
/P
sat
= 1
45 seconds 0.33 ± 0.07
P
m
/P
sat
= 1
90 seconds 0.91 ± 0.06

Table 3-2: UV-vis spectrometry data at 600 nm wavelength for P4VP particles in 100 cst silicone oil.
50

Figure 3-3: Representative SEM images of P4VP particles produced on 100 cst silicone oil at P m/P sat = 1
with monomer condensation for a) 15 seconds, b) 45 seconds, and c) 90 seconds

Whereas the smaller particles at the lower end of the distribution are formed by the
cloaking and engulfment of monomer droplets nucleated at the vapor-liquid interface, we
hypothesize that the larger particles at the higher end of the distribution are formed within the
bulk liquid by the process of coalescence, in which monomer droplets merge to minimize
interfacial surface area with the bulk phase (Figure 3-4).
51

Figure 3-4: a) Schematic of the reactor. b) Schematic of the monomer condensation and polymerization
process. The yellow spheres indicate monomer droplets and the white spheres indicate polymer
particles.

Since Brownian motion of the monomer droplets contributes significantly towards
coalescence
29
, increasing the viscosity of the silicone oil should inhibit coalescence and thereby
alter the particle size distribution. We studied the P4VP particle size distribution for 1000 cst,
10,000 cst, and 100,000 cst silicone oils at a Pm/Psat value of 1.5. Compared to the 100 cst
silicone oil, the average particle size was significantly smaller for the 1000 cst silicone oil (Table
1) and the particle size distribution was also narrower as indicated by the coefficient of variation
(C.V.) which represents the width of the particle size distribution (Figure 3-5). The higher
viscosity inhibits coalescence of monomer droplets in the bulk silicone oil and therefore prevents
the formation of large particles at the higher end of the distribution, however the formation of
small particles at the lower end of the distribution is not affected since their formation is
governed by monomer nucleation at the vapor-liquid interface which is independent of the
viscosity of the liquid layer which is in agreement with the work of Anand et al.
24
Particles also
formed on the 10,000 cst silicone oil.
52

For the 100,000 cst silicone oil, we observed a transition from particle formation to film
formation (Figure 3-5). We hypothesize that since the viscosity is increased by three orders of
magnitude, the monomer droplets accumulate at the surface of the liquid and do not completely
engulf into the silicone oil leading to a rough polymer film at the surface of the liquid. The
atomic composition of this film was analyzed using XPS and was found to be similar (84.31% C,
7.04% N, 8.65% O, 0.00 %Si) to a reference P4VP film fabricated on a silicon wafer using iCVD
(84.97% C, 8.63% N, 5.59% O, 0.82% Si) which suggests that the silicone oil was not integrated
into the film.

Figure 3-5: a) Representative size distributions for P4VP particles produced at P m/P sat = 1.5 on 100 cst
and 1000 cst silicone oil and b) SEM image of a P4VP film produced at P m/P sat = 1.5 on 100,000 cst
silicone oil.

In order to study the effect of polymerization on coalescence, 4VP monomer was
condensed on 100 cst silicone oil for 15 seconds at a Pm/Psat of 1.5 and coalescence was allowed
to occur in the absence of free radicals for 15 minutes by keeping the filament off. We observed
the formation of large monomer droplets that sank to the bottom of the silicone oil and settled on
the silicon wafer (Figure 3-6). The diameters of these droplets were on the order of millimeters,
which is several orders of magnitude higher than the diameters of the polymer particles produced
53

when the filament was turned on. Since the density of 4VP (0.975 g/mL) is higher than the
density of 100 cst silicone oil (0.96 g/mL), the monomer droplets sink when droplet diameters
grow above 1 μm as gravitational forces become dominant compared to Brownian motion
29
. This
phenomenon was not observed when we polymerized the monomer after condensation by free
radicals, because polymerization inhibits coalescence of monomer droplets beyond the micron
scale. Tao et al also reported that polymerization inhibits coalescence while using supersaturated
monomer vapors to produce polymer films
30
.

Figure 3-6: Coalescence of 4-vinyl pyridine monomer droplets in 100 cst silicone oil at P m/P sat = 1.5 in the
absence of polymerization.

In contrast to our process, the conventional iCVD process is a simultaneous
polymerization process where monomer adsorbed at the silicone oil surface is polymerized to
form polymer particles by diffusion and aggregation of polymer chains at the vapor-liquid
interface.
16,31,32
We performed conventional iCVD of P4VP on 100 cst silicone oil which resulted
in a smaller average particle size and a lower C.V. (Table 3-1) compared to our process on the
100 cst silicone oil. Since the equilibrium contact angle for silicone oil on P4VP is zero, it is
energetically favorable for the P4VP particles to engulf into the liquid layer similar to the
monomer droplets however they cannot coalesce to minimize interfacial area and are likely to
54

undergo elastic collision. Engulfment into the bulk oil stabilizes the particle size, since it
prevents further growth of polymer particles by aggregation of polymer chains being deposited at
the surface of the liquid and a narrower particle size distribution is attained for the conventional
iCVD process (Figure 3-7). The particles formed by the conventional iCVD process represent the
characteristic size at which engulfment occurs and are comparable in size with the small particles
formed by our process likely due to similar wetting dynamics and surface tension interactions of
the silicone oil with the monomer and the polymer.

Figure 3-7: a) Representative size distributions for P4VP particles on 100 cst silicone oil comparing
particles produced via iCVD to particles produced at Pm/Psat = 1.5 and b) SEM image of P4VP particles
produced via iCVD on 100 cst silicone oil.

In order to investigate the generality of our process for other monomers, we condensed HEMA
on 100 cst silicone oil at Pm/Psat values of 1, 1.5 and 3.3 for 15 seconds and polymerized the
monomer for 15 minutes. Based on the average particle size data (Table 3-3) and representative
particle size distributions (Figure 3-8), there was no significant effect of Pm/Psat on the formation
of PHEMA particles due to the zero contact angle of this system similar to the P4VP particles.

55

Sample
Mean
Radius (nm)
Standard
Deviation (nm)
Coefficient of
Variation
100 cst, Pm/Psat = 1 213 26 0.14
100 cst, Pm/Psat = 1.5 230 28 0.17
100 cst, Pm/Psat = 3.3 201 43 0.16
1000 cst, Pm/Psat = 1.5 90 4 0.07
100 cst, iCVD, Pm/Psat = 0.3 153 10 0.07

Table 3-3: Size measurements of PHEMA particles using dynamic light scattering.

Figure 3-8: Representative size distributions for PHEMA particles produced on 100 cst silicone oil at
P m/P sat values of 1, 1.5, and 3.3.

SEM images (Figure 3-9) indicate a spherical morphology and a heterogeneous particle
size distribution in agreement with the DLS data. Unlike P4VP, the PHEMA particles appear
partially melted in the SEM images most likely because their morphology was damaged upon
exposure to the electron beam
33
as a result of the lower glass transition temperature of PHEMA.
We then performed the process at a Pm/Psat value of 1.5 on 1000 cst silicone oil and this resulted
in a smaller average particle size (Table 3-3) and a narrower particle size distribution compared
to the 100 cst silicone oil due to the inhibition of coalescence (Figure 3-10). A transition to film
formation was also observed for PHEMA on the 100,000 cst silicone oil similar to P4VP (Figure
3-10).
56

Figure 3-9: Representative SEM images of PHEMA particles produced on the 100 cst silicone oil at
a)P m/P sat = 1, b)P m/P sat = 1.5, and c)P m/P sat = 3.3.

The conventional iCVD process was also performed for PHEMA on 100 cst silicone oil
which resulted in a smaller average particle size (Table 3-3) and a narrower particle size
distribution as compared to our process. Finally, we also demonstrated the coalescence of
HEMA on 100 cst silicone oil in the absence of polymerization which resulted in millimeter-
sized monomer droplets similar to 4VP (Figure 3-11). The density of HEMA (1.073 g/mL) is
also higher than that of the silicone oil and resulted in sinking of monomer droplets due to
gravitational forces. These droplet diameters are at least a few orders of magnitude greater than
57

the diameters of the PHEMA particles which further verifies that polymerization inhibits
coalescence.

Figure 3-10: a) Representative size distributions for PHEMA particles produced at P m/P sat = 1.5 on 100
cst and 1000 cst silicone oil, and b) SEM image of a PHEMA film produced at P m/P sat = 1.5 on 100,000 cst
silicone oil.

Figure 3-11: Coalescence of 2- hydroxyl ethyl methacrylate monomer droplets in 100cst silicone oil at
P m/P sat = 1.5 in the absence of polymerization.

58

Conclusions
Polymer particles were synthesized by condensing monomer onto silicone oils and
subsequently polymerizing the monomer in a vacuum reactor. The oversaturation did not
significantly affect the particle size distribution in the range of Pm/Psat values used in this study.
Nucleation and engulfment at the vapor-liquid interface determines the small particle sizes at the
lower end of the distribution and coalescence of monomer droplets determines the large particle
sizes at the higher end of the distribution. Increasing the viscosity of the silicone oil inhibits
coalescence and prevents the formation of large particles resulting in a narrower particle size
distribution. Rough polymer films can be formed using extremely high viscosities which prevent
monomer droplets from completely submerging into the liquid. The generality of the process was
established using two different monomers, HEMA and 4VP. The trends in average particle size
and particle size distribution were similar for both PHEMA and P4VP.
Acknowledgments
This work was supported by the U.S. Department of Energy, Office of Science, Basic
Energy Sciences under Award #DE-SC0012407.

59

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62

Chapter 4: Effects of Standing Waves on the Growth and Stability of
Vapor Deposited Polymer Films

P. Karandikar, M. M. De Luna, M. Gupta. “Effects of Standing Waves on the Growth and
Stability of Vapor Deposited Polymer Films.” ACS Applied Polymer Materials, 2019.
63

4.1 Abstract
We demonstrate that liquid substrates can be agitated in situ via sound during vapor phase
polymerization. The effects of standing waves on the growth of polymer films at the vapor-liquid
interface were studied by depositing poly(1H,1H,2H,2H-perfluorodecyl acrylate) and
poly(1H,1H,2H,2H-perfluorodecyl acrylate-co-ethylene glycol diacrylate) onto an agitated
silicone oil lens. It was found that continuous polymer films could be formed by the addition of a
crosslinker. We also tested the mechanical stability of the polymer films on agitated liquids and
found that the robustness of the films increased with the addition of crosslinker and increasing
film thickness.
4.2 Introduction
Vacuum deposition processes such as atomic layer deposition (ALD),
1
molecular layer
deposition (MLD),
2
chemical vapor deposition (CVD),
3
physical vapor deposition (PVD),
4
and
their variants are important for the bottom-up fabrication of organic and inorganic materials.
Several modifications have recently emerged to move substrates during these processes to allow
for improved conformality, scalability, and tuning of the morphology. For example, large
quantities of nanoparticles have been conformally coated using ALD with rotary reactors,
5
and
fluidized bed reactors equipped with mechanical stirring.
6,7
Fluidized bed reactors,
8
rotary
reactors,
9,10
and loud speakers
10,11
have also been used to agitate particles during plasma
polymerization. Pursel et al. used motors to rotate and tilt substrates relative to the direction of
monomer flux during Parylene C deposition to create new columnar morphologies.
12
Similarly,
substrates have been tilted and rotated to fabricate inorganic materials such as C-shaped
magnesium fluoride
13
and slanted columnar chromium
14
thin films.
64

The initiated chemical vapor deposition (iCVD) process can be used to deposit polymers
onto surfaces via a free radical mechanism.
15,16,17
The monomer and initiator are flowed into a
vacuum chamber and a heated filament array cleaves the initiator molecules into free radicals to
begin the polymerization process.
18
We recently demonstrated that the iCVD process can be used
to deposit polymers onto low vapor pressure liquids such as silicone oils, ionic liquids, and
glycerol.
19,20,21
The deposited polymer chains are mobile on liquid surfaces which allows for
different morphologies compared to deposition onto solid substrates. We showed that the
thermodynamically preferred morphology is governed by the spreading coefficient (𝑆 ) of the
polymer at the vapor-liquid interface which is given by 𝑆 = 𝛾 𝐿𝑉
∗ (1 + cos 𝜃 ) − 2𝛾 𝑃𝑉
, where

γLV is the liquid-vapor surface tension, γPV is the polymer-vapor surface tension, and θ is the
advancing contact angle of the liquid on the polymer.
20,21
The formation of thermodynamically
stable polymer films is observed when the spreading coefficient is positive since it is
energetically favorable for polymer chains to spread over the liquid surface.
Our previous work has been performed on stationary liquids. Although we encounter
passive motion due to polymer diffusion and wetting during deposition onto liquids, our current
study is the first example of active motion in these systems. Although Lau et al. developed a
rotary reactor to coat particle beds of glass microspheres with poly(glycidyl methacrylate),
22

there are no reports of active motion of liquid substrates during iCVD. Active motion can be
used to control diffusion and aggregation during deposition on liquid surfaces. For instance,
Rogov et al. circulated liquids via gravity driven laminar flow and a rotating cylinder during
magnetron sputtering of Au onto vacuum oil to control the size and concentration of nanoparticle
aggregates by tuning the liquid exposure time in the deposition zone and mixing the liquid.
23,24

65

Here for the first time we modify an existing iCVD chamber to actively modulate liquids
in situ via sound. The modulation of liquids via sound was first reported by Faraday in 1831
when a hydrodynamic instability was generated at the liquid surface in the form of waves by the
vertical oscillation produced by the application of a violin bow at the edge of a fluid filled
container.
25
These instabilities referred to as standing waves can generate a variety of patterns in
a wide range of fluids including water,
26
silicone oils,
27
glycerol,
28
and mercury.
25
We
demonstrate that acoustic resonance can be utilized to modulate the vapor-liquid and liquid-
liquid interface to study the growth and stability of thin films during vapor phase deposition.
4.3 Experimental
Experimental setup
A 3 cm x 3 cm x 0.6 cm aluminum container was mounted on top of the surface
transducer (SparkFun Electronics) to hold the liquids. A piece of silicon was taped to the bottom
of the container to provide better optical properties for visualizing the liquids. The function
generator (Tektronix AFG 3021 C) and a DC power supply were connected to the surface
transducer and TEC (Custom Thermoelectric), respectively, through a KF 16 feed port on the
reactor wall via a six-prong adapter. To prepare the bath, 3 ml of 261 cSt Krytox (Sigma
Aldrich) was pipetted into the aluminum container and the reactor was evacuated to a base
pressure of 20 mTorr to remove air bubbles for one hour before dispensing a 50 μl lens of
silicone oil (5, 20, 50, or 100 cSt, Sigma Aldrich) (Figure 4-1b) on the Krytox.
Polymer depositions
The liquids were pre-cooled to 10 °C inside the reaction chamber at base pressure for 30
minutes by setting the TEC to 10 °C and keeping the stage temperature at 30 °C. The di-tert-
66

butyl peroxide (TBPO) (Sigma Aldrich) initiator was introduced at 0.3 sccm using a mass flow
controller (MKS Type 1479A). The 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (SynQuest)
monomer jar was heated to 45 °C to produce a flow rate of 0.1 sccm. The ethylene glycol
diacrylate (EGDA) (Polysciences, Inc.) monomer jar was heated at 40 °C to produce a flowrate
of 0.1 sccm. The filament array (Nichrome, 80% Ni, 20% Cr, Omega Engineering) was heated to
250 °C to start polymerization while the reactor pressure was maintained at 70 mTorr for PPFDA
deposition and 60 mTorr for P(PFDA-co-EGDA) deposition. The deposition rate was calibrated
via in situ laser interferometry (633 nm He-Ne laser, Industrial Fiber Optics) by performing the
deposition in the absence of agitation on a reference silicon wafer mounted on the transducer
since the vibrations introduced noise into the laser signal. The pictures of polymer deposition on
moving liquids and thin film agitation inside the reactor were taken by an iPhone X camera
(Apple Inc.) mounted on a tripod stand outside the reactor on top of the quartz viewport.
Magnified view images were taken after the deposition using a stereoscopic microscope
(NATIONAL) by carefully removing the liquid container from the deposition chamber.
X-ray photoelectron spectroscopy
The chemical composition of a representative P(PFDA-co-EGDA) film on a reference
silicon wafer was analyzed using X-ray photoelectron spectroscopy (Kratos Axis Ultra DLD)
with a monochromatic Al K  X-ray source. Survey spectra were taken from 0 to 900 eV with a
step size of 1 eV a total of five times. The chemical composition of the film was 20.6 %O, 49.3
%C, and 20.1 %F. The total fluorine content was attributed to PPFDA and the excess oxygen and
carbon content was attributed to PEGDA to determine the relative ratio of PPFDA to PEGDA in
the copolymer.
Contact angle goniometry
67

A contact angle goniometer (ramé-hart Model 290-F1) was used to measure the
advancing and equilibrium contact angles of the liquids (Krytox or 5 cSt silicone oil) on the
polymer films (PPFDA or P(PFDA-co-EGDA)). The contact angles were measured by
dispensing 5 µL of the liquid onto the polymer films and the average of 5 measurements was
used. For the advancing contact angle, the tilting base method and was used and the contact
angle before the droplet rolled off was measured. The measured advancing contact angles of the
PPFDA-Krytox, P(PFDA-co-EGDA)-Krytox, and P(PFDA-co-EGDA)-silicone oil systems were
48º, 46º, and 62º, respectively. The liquid-vapor surface tension of the liquids was measured
using the pendant drop method. The surface tension of the Krytox and silicone oil was measured
to be 19.3 mN/m and 22.1 mN/m, respectively. The polymer-vapor surface tension was
calculated using the acid-base method by measuring the equilibrium contact angle of water,
glycerol, and methylene iodide on each polymer. The surface tension of PPFDA and P(PFDA-
co-EGDA) were calculated to be 13.6 and 15.5 mN/m, respectively. The spreading coefficient of
the PPFDA-Krytox, P(PFDA-co-EGDA)-Krytox, and P(PFDA-co-EGDA)-silicone oil systems
were calculated to be 4.5 mN/m, 1.1 mN/m, and 1.4 mN/m, respectively. The spreading
parameter for silicone oil on Krytox is given by S = γ2 – (γ1 + γ12) where γ1 is the surface tension
of silicone oil (21 mN/m), γ2 is the surface tension of Krytox (16 mN/m), and γ12 is the
interfacial tension between the two liquids (6 mN/m). These values were measured by the
pendant drop method. The equilibrium thickness of the lens, ec, can then be calculated using the
equation
1
2
𝜌 ̃gec
2
= |S| where 𝜌 ̃ = (ρ1/ρ2) * (ρ2 - ρ1), g is gravitational acceleration and ρ1 and ρ2
are the density of silicone oil and Krytox respectively.
Thickness measurement
68

The poly(1H,1H,2H,2H-perfluorodecyl acrylate-co-ethylene glycol diacrylate) (P(PFDA-
co-EGDA)) films were grown on the surface of stationary and agitated silicone oil (regime D) by
depositing polymer for 4 minutes. The films were removed from the liquid surface with tweezers
and rinsed with hexane to remove the silicone oil. The top surface and cross-section of the films
were imaged via SEM (JEOL-7001). We measured the cross-sectional thickness at five points on
each image. The average thickness of the film was found to be 120 nm ± 22 nm on the stationary
liquid and 115 nm ± 38 nm on the agitated liquid.
4.4 Results and Discussion
The unmodified iCVD chamber has a cylindrical geometry (250 mm diameter, 48 mm
height). We increased the reactor height to 124 mm using a 76 mm spacer flange to allow for
additional space for a surface transducer (30 mm height) to be mounted on the reactor stage
(Figure 4-1a). For our experiments, the filament array was heated to 250 ℃ to generate free
radicals. Since radiative heat intensity is inversely proportional to the square of the distance from
the heat source
29
and the amount of monomer adsorbed on the surface of the substrate decreases
with increasing temperature,
17
the filament array was elevated to 106 mm above the stage to
allow the heat to be dissipated. A thermoelectric cooler (TEC) was placed between the
transducer and the stage to control the temperature of the liquid substrates to 10 °C and the
frequency and amplitude input to the transducer was supplied by a function generator.
69

Figure 4-1. (a) Schematic of the modified iCVD reactor for in situ sonic modulation. (b) 5cSt silicone oil
lens suspended on a bath of 261 cSt Krytox mounted on the transducer inside the modified reactor.

Pucci et al. recently produced Faraday waves by agitating a floating lense of a low
viscosity liquid via sound in a bath of a more viscous insoluble fluid that provided a flexible
boundary to modulate the lens.
30
We used a similar two-liquid system consisting of a floating
lens of low viscosity (5 cSt) silicone oil in a bath of higher viscosity (261 cSt) Krytox for our
study (Figure 4-1b). The silicone oil has a density of 0.913 g/ml and is immiscible in the bath
and therefore floats on the Krytox which has a higher density of 1.90 g/ml. Using an equation to
balance buoyancy and capillary forces,
31
the thickness at the center of the lens is approximately
2.2 mm.
We demonstrate that the liquid lens can be modulated in real time inside the reactor under
a vacuum of 30 mTorr as shown in Figure 4-2. The lens is initially in a circular morphology (C).
The lens was deformed by driving the transducer sinusoidally with the function generator. We
applied a range of frequencies from 50 – 300 Hz with increments of 10 Hz and amplitudes in the
range of 0-10 Volts peak to peak (Vpp) with increments of 0.5 Vpp. Although we observed
perturbations on the surface of the lens, no distinct wave pattern was observed until a threshold
of 230 Hz and 10 Vpp. At this threshold, known as the Faraday threshold,
32
a visible concentric
70

standing wave pattern was observed which marks a shift to a deformed lens morphology (D). At
240 Hz, the Faraday threshold appeared at 5 Vpp. At this frequency, increasing the amplitude
resulted in several transient shapes until a stable elongated shape with standing waves (E) was
observed at 7 Vpp. Increasing the amplitude further resulted in increased elongation of the lens
into a worm-like shape with standing waves as shown in Figure 4-2. Similar transitions from (C)
through (E) were observed at higher frequencies, however no transitions were observed at 280
Hz. A local minimum in the amplitude (1.5 Vpp) required for a transition from (C) to (D) was
observed at 250 Hz. Our hypothesis is that the resonant frequencies are between 230 Hz to 270
Hz for the 5 cSt silicone oil for the range of amplitudes in our study. The competition between
the radiation pressure exerted by the standing waves and the capillary response due to surface
tension on the lens is defined by a dimensionless control parameter given by ρω
2
ζ
2
/(4*σ/R)
where ρ is the lens density, ω is the wave angular frequency, ζ is the wave amplitude, σ is the
surface tension, and R is the radius of the lens.
32
The value for the control parameter is unity at
the phase boundary between (C) and (D). The control parameter increases above unity when the
amplitude is increased resulting in elongation of the lens.
32
We observed similar transitions in a
lens of 20 cSt silicone oil, however the transitions from (C) through (E) occurred only at 250 Hz
and at higher amplitudes of 2.5 Vpp for (C) to (D) and 3 Vpp for (D) to (E) compared to 1.5 and
2.5 Vpp for the 5 cSt silicone oil. The 50 cSt silicone oil also undergoes transitions from (C)
through (E) only at 250 Hz between 6.5 and 7 Vpp. The 100 cSt silicone oil lens could not be
deformed using the surface transducer due to high viscosity which prohibits modulation in the
range of frequencies and amplitudes applied. The amplitude required to modulate the lens at 250
Hz increases as the viscosity of the lens increases which is consistent with viscous damping
observed in standing waves.
33,34

71

Figure 4-2. Phase diagram representing in situ modulation of a 5 cSt, 20 cSt, and 50 cSt silicone oil lens
on a bath of Krytox. The solid markers represent the threshold at which a circular (C) lens transitions to a
deformed lens (D) and the open markers represent the threshold at which a deformed (D) lens
transitions to a stable elongated shape (E). Representative images of the different lens shapes are
displayed within each region.

The reactor modifications allow us to study the effect of standing waves on the growth of
polymer films at the vapor-liquid interface. We chose to deposit linear poly(1H,1H,2H,2H-
perfluorodecyl acrylate) (PPFDA) and crosslinked poly(1H,1H,2H,2H-perfluorodecyl acrylate-
co-ethylene glycol diacrylate) (P(PFDA-co-EGDA)) since both these polymers form continuous
films on the surfaces of stationary silicone oil and Krytox due to positive spreading coefficients.
We chose to suspend 5 cSt silicone oil on the Krytox bath since the low viscosity provides the
least resistance to modulation over a wide range of frequencies. The frequency and amplitude of
agitation was held at a fixed value throughout the polymer deposition to generate different lens
morphologies ((C), (D), or (E)) (Figure 4-3). In each case, he polymer was deposited for 4
minutes for a total film thickness of approximately 100 nm (Figure 4-3). Figure 4-3 shows the
film growth as a function of time as captured on camera. The linear PPFDA did not form a
continuous film over the surface of the moving liquid in all the regimes of agitation ((C), (D),
72

and (E)) as demonstrated by the PPFDA fragments that were observed on the surface of the lens
after deposition (Figure 4-3). The P(PFDA-co-EGDA) formed a continuous film over both the
liquids thereby encapsulating the silicone oil lens in all the regimes of agitation ((C), (D), and
(E)) as shown in the magnified images (Figure 4-3). The deposition of P(PFDA-co-EGDA) was
found to distort the initial lens morphology by damping the standing waves. The damping effect
became more prominent with increasing film thickness, leading to less visible waves (Figure 4-
3). The chemical composition of a representative P(PFDA-co-EGDA) film was found to be 53%
PPFDA and 47% PEGDA using X-ray photoelectron spectroscopy. Several studies have shown
that the mechanical stability of soft materials can be improved by in situ crosslinking.
35,36,37
Our
hypothesis is that the addition of the crosslinker EGDA allows for bridging of polymer chains
and provides mechanical robustness, which makes growth of the film on the moving liquids
possible. The thickness of the crosslinked films grown on the stationary and agitated liquids were
found to be 120 nm ± 22 nm and 115 nm ± 38 nm, respectively, indicating that the film thickness
was not affected by the agitation of the liquid surface (Figure 4-4). Mechanically induced
wrinkles in polymer films have applications in tuning wetting and adhesive properties, and for
controlling cellular morphology.
38
The surface topography of the agitated film retained its
wrinkled morphology after removal from the liquid surface compared to smooth films grown on
a stationary liquid (Figure 4-4). The buckled domains in the dry state are on the order of microns
which does not match the millimeter-scale periodicity of the standing waves on the oil lens
during processing likely due to relaxation of the polymer film during transfer from the liquid
surface. Our in situ modulation set up can also be applied to other vapor deposition systems such
as atomic layer deposition, plasma deposition, and sputtering. In the case of metal and inorganic
depositions, it is possible that the millimeter scale periodicity will remain.
73

Figure 4-3. Images of real-time polymer deposition onto a 5 cSt silicone oil lens on a Krytox bath during
agitation regimes (C) – circular (260 Hz, 2.5 Vpp), (D) – deformed (260 Hz, 5 Vpp), and (E) – elongated
(260 Hz, 7Vpp).

Figure 4-4. (a) Cross-section and (b) top down image of a P(PFDA-co-EGDA) film grown on a stationary 5
cSt silicone oil lens floating on Krytox and (c) cross-section and (d) top down image of a P(PFDA-co-
EGDA) film grown on an agitated 5 cSt silicone oil lens floating on Krytox. The films were removed from
the liquid surface before imaging.

Polymer thin films suspended on deformable substrates can be used to study the
mechanical properties of polymers.
39,40
Our modified reactor allows us to study the effect of
74

standing waves on films that are deposited on liquid surfaces, which is useful for understanding
the mechanical strength of these films. First, we deposited P(PFDA-co-EGDA) at various
thicknesses (25 nm, 100 nm, 300 nm, and 1 μm) on the stationary 5 cSt silicone oil lens
suspended on the Krytox bath prior to turning on the transducer. The crosslinked polymer
encapsulated the silicone oil lens by forming a continuous film over both the liquids. A new film
was fabricated prior to agitation at 240, 250, or 260 Hz and the amplitude was ramped from 0-10
Vpp in increments of 0.5 Vpp. The crosslinked film pinned the lens into a circular shape, thereby
inhibiting the elongation observed in the absence of polymerization as shown in Figure 4-2. The
films did not rupture at 240 Hz or 260 Hz between amplitudes of 0-10 Vpp thereby keeping the
lens encapsulated. The films were found to rupture at amplitudes of 6, 7.5, and 8.5 Vpp at 250
Hz for thicknesses of 25, 100, and 300 nm, respectively, leading to the ejection of small liquid
droplets (Figure 4-5). The films rupture only at 250 Hz likely because there is a local minimum
in the amplitude required for standing waves at this frequency (Figure 4-2). The force exerted by
the standing waves on the films increases with amplitude and can be estimated by the control
parameter given above. The thicker films can withstand higher amplitudes before rupture,
indicating that the thicker films are more robust. This is consistent with the work of Menon and
coworkers who studied the buckling of polymer thin films suspended on liquid interfaces and
found that the stiffness of the films defined via a bending modulus increases with thickness.
39,40

The thickest film (1 μm) did not break at 250 Hz and the standing waves were less visible due to
damping (Figure 4-5). The linear PPFDA films did not offer significant resistance to modulation
and ruptured upon agitation from the (C) to (D) regime at all the frequencies and thicknesses
tested. These results indicate that the crosslinker is essential for maintaining the structural
integrity of the films during agitation via standing waves.
75

Figure 4-5. Agitation of P(PFDA-co-EGDA) films that encapsulate a 5 cSt silicone oil lens on a Krytox bath
as a function of film thickness and amplitude at 250 Hz.
4.5 Conclusions
In summary, we demonstrate sonic modulation of liquid substrates inside a modified
iCVD reactor. Our set up allows us to independently control the thickness of the deposited
polymer and the frequency and amplitude of agitation which enables the study of structure-
property relationships. We studied the effects of standing waves on the growth of polymer films
and tested the mechanical stability of the films during sonic modulation. We found that the
formation of continuous fluoropolymer films on a moving liquid surface was enabled by
copolymerization with a crosslinking monomer. We also agitated polymer films on the liquid
surface after deposition and found that the mechanical robustness was enhanced by incorporation
of crosslinker and increasing the film thickness. Future work can explore different polymer
functionalities and liquids to tune the wettability at the vapor-liquid interface to further study the
76

growth of polymers on moving liquids. Our agitation module is also adaptable to other vacuum
deposition processes to enable in situ modulation of liquid substrates.
Acknowledgements
M. D. L. is supported by a National Science Foundation Graduate Research Fellowship under
grant DGE-1418060.
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80

Chapter 5: Conclusions and Future Work

81

5.1 Conclusions
This thesis has expanded on novel processing methods for liquid substrates during iCVD.
Previous work has utilized deposition conditions that typically introduce monomer and free
radicals simultaneously at Pm/Psat values less than unity on quiescent liquids. This work has
aimed to study how sequential deposition approaches where monomer and free radicals are
introduced sequentially, monomer oversaturation (at Pm/Psat > 1), and active motion in liquid
substrates can be used to control the deposition of polymer and its morphology.
Chapter 2 demonstrates that monomer solubility in the IL can be used to absorb monomer
into the liquid and polymerize the absorbed monomer to form a polymer-IL gel bead. A
superhydrophobic surface was used to maintain the spherical shape of the bead during
polymerization, simultaneous introduction of monomer and free radicals does not result in a gel
bead since the polymer is deposited on the superhydrophobic surface thereby changing the
contact angle of the IL on the substrate during polymerization.
Chapter 3 utilizes the oversaturation of monomer to condense monomer droplets on the
liquid surface which are subsequently polymerized to form polymer particles. The monomer
droplets can undergo coalescence which results in a larger average particle size and a more
heterogenous size distribution compared to polymer depositions at Pm/Psat < 1. Increasing the
viscosity of the silicone oil was found to inhibit coalescence and the formation of large particles
thereby controlling the particle size distribution.
Chapter 4 implements a reactor modification to enable the modulation of the liquid
surface via standing waves with the use of sound. This allowed the deposition of polymer films
on a moving liquid surface and the standing waves were found to inhibit the formation of
continuous polymer films in the case of linear PPFDA. Copolymerization with a crosslinker
82

resulted in continuous polymer films over the moving liquid surface emphasizing the importance
of in situ crosslinking during the growth of polymer films. The modification also allowed the
testing of the mechanical stability of deposited crosslinked films of various thickness and the
amplitude of failure was found to increase with increasing film thickness.
5.2 Future Work
During iCVD on liquid substrates the surface tension, viscosity and process conditions
such as polymer deposition rate and deposition time govern the morphology of the polymer
formed
1
. It was shown that when the spreading coefficient of the polymer on the liquid surface is
negative and the polymer chains aggregate to form particles whereas polymer films are observed
the spreading coefficient is positive. However, the liquid was kept stationary during the polymer
deposition and therefore perturbing the liquid surface could significantly alter the morphology of
the resulting polymer. In Chapter 4 the effects of liquid agitation on film formation were
demonstrated using PPFDA as a model system. Applying the reactor modification to study
particle formation during the agitation of the liquid surfaces should be the next step to progress
this research and further control particle growth. In addition, reactor modifications to the vapor
delivery system such as the use of a nozzle to add directionality to the monomer flux could allow
for the fabrication of polymer films with novel morphologies.
2

83

Preliminary data (Figure 5-1) shows that the polymer particle size exhibits different
growth trends with deposition time depending on the equilibrium contact angle of the polymer on
the liquid. When the contact angle is non zero as is the case for poly(n-butyl acrylate) (PnBA)
(Figure 5-1a) the average particle radius increases with increasing deposition time
3,4
, however
when the contact angle is zero as is the case for poly(4-vinyl pyridine) (P4VP) (Figure 5-1b)
there is no significant increase in the average particle radius with increasing deposition time.
Figure 5-1: Average particle size as a function of deposition time for a) PnBA at a deposition rate of 30
nm/min
3
and b) P4VP at a deposition rate of 40 nm/min

We also observed that the average particle size decreases with an increase in the silicone
oil viscosity (Table 5-1). Based on these results we hypothesize that polymer particle formation
is governed by the energetics of engulfment and the diffusion and aggregation of polymer chains
at the vapor - liquid interface. The free energy required for a particle to submerge into a liquid is
given by ∆G = πr
2
γ (1 - |cosθe|)
2
where r is the particle radius, γ is the liquid – vapor surface
tension, and θe is the equilibrium three phase contact angle, thus it is more favorable for a
particle with a zero contact angle to submerge compared to particles with non-zero contact
angles
4
. The diffusion and aggregation of polymer chains is inversely proportional to the
viscosity of the liquid as described by Rouse or Reptation models which define the diffusivity of
polymer chains as a ratio of thermal energy (kBT) and the chain friction factor ( f ) which is
a) b)
84

related to the monomeric friction coefficient ( ζ )
5
. The characteristic relaxation time of polymer
chains through a distance comparable to the radius of gyration is estimated between 1 – 10
-10
seconds for melts and dilute solutions of polymers
4,6
. We expect to alter the particle growth by
agitating the liquid via acoustic resonance since this will affect the diffusion and aggregation of
polymer chains as well as the energetics of the vapor-liquid interface.

a) PnBA b) P4VP
Viscosity
(cst)
Average
Radius (nm)
±
Standard
Deviation
Viscosity
(cst)
Average
Radius (nm)
±
Standard
Deviation
100 160±22 100 107±14
500 86±6 500 43±4
1000 66±11 1000 35±5

Table 5-1: Average particle size on silicone oil viscosities of 100, 500, and 1000 cst for a)PnBA at a
deposition rate of 30 nm/min and a deposition time of 5 min and b) for P4VP at a deposition rate of 40
nm/min and a deposition time of 5 min.
85

References

1. Haller, P. D.; Bradley, L. C.; Gupta, M. Effect of Surface Tension, Viscosity, and Process
Conditions on Polymer Morphology Deposited at the Liquid-Vapor Interface. Langmuir 2013,
29, 11640-11645.
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engineered anisotropic nanofilm with unidirectional wetting properties. Nat. Mater., 2010, 9,
1023–1028.
3. Frank-Finney R. J.; Gupta, M. Two-Stage Growth of Polymer Nanoparticles at the Liquid–
Vapor Interface by Vapor-Phase Polymerization. Langmuir, 2016, 32, 11014−11020.
4. Haller, P. D.; Gupta, M. Synthesis of Polymer Nanoparticles via Vapor Phase Deposition onto
Liquid Substrates. Macromol. Rapid Commun. 2014, 35, 2000−2004.
5. Monroy, F.; Hilles, H. M.; Ortega, F.; Rubio R. G. Relaxation Dynamics of Langmuir
Polymer Films: A Power-Law Analysis. Phys. Rev. Lett. 2003, 91, 268302(4)
6. Hiemenz, P.C.; Lodge, T.P. Polymer Chemistry, 2nd edition; Boca Raton: CRC Press, 2007.

Abstract (if available)

Abstract This dissertation discusses novel processing techniques for low vapor pressure liquid substrates such as ionic liquids (ILs) and silicone oils during the initiated chemical vapor deposition (iCVD) process for the fabrication of polymer particles, polymer – IL gels, and polymer films. Chapter 1 introduces the iCVD process and provides a background on vacuum processes using sequential flow of precursors such as atomic layer deposition (ALD) and molecular layer deposition (MLD). Chapter 2 demonstrates the fabrication of polymer – IL gel beads by a sequential polymerization process using iCVD. First monomer was absorbed in IL droplets which were kept spherical using a hydrophobic substrate, the reactor was then evacuated, and the monomer absorbed in the IL was polymerized via free radicals to fabricate gel beads. Chapter 3 describes a sequential process to fabricate polymer particles using iCVD. First monomer droplets were condensed on a layer of silicone oil and the monomer was subsequently polymerized to form polymer particles. The particle size was tuned by varying the viscosity of the silicone oil resulting in solvent-less, surfactant-free fabrication of functional polymer particles. Chapter 4 describes a reactor modification that allows the use of in situ sonic agitation to set up standing waves on the liquid surface during iCVD. The effect of standing waves on the growth and mechanical stability of fluoropolymer films was studied. The incorporation of a crosslinker enabled the growth of continuous films on a moving vapor-liquid interface and the mechanical stability of the polymer films was found to increase with film thickness and crosslinking. Chapter 5 summarizes the conclusions of this dissertation and describes the future work for the agitation of liquid substrates. Further reactor modifications to provide directionality to the monomer flux to control the polymer morphology during iCVD are also described.

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Creator Karandikar, Prathamesh (author)

Core Title Novel processing of liquid substrates via initiated chemical vapor depostion

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 09/23/2019

Defense Date 07/30/2019

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

Tag chemical vapor deposition,ionic liquids,OAI-PMH Harvest,polymer films,polymer gels,polymer nano particles

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Advisor Gupta, Malancha (committee chair), Nakano, Aiichiro (committee member), Ravichandran, Jayakanth (committee member)

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