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Vapor phase deposition of polymers in the presence of low vapor pressure liquids

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Vapor Phase Deposition of Polymers in the Presence of Low Vapor Pressure Liquids
PhD Thesis
Patrick Haller
August 2014

University of Southern California
Mork Family Department of Chemical Engineering and Material Science
Los Angeles, CA

2

Committee Members
Chair: Malancha Gupta
Pin Wang
Aiichiro Nakano

3

Executive Summary
This report outlines studies of the vapor-phase deposition of polymers in the presence of
low-vapor pressure liquids. Chapter 1 provides background information on the initiated
chemical vapor deposition (iCVD) process which is used to deposit the polymers. There is also
background information on other organic and inorganic vapor-phase deposition processes which
have been used with liquid substrates. Finally, there is information on the production and use of
polymer films and particles, both of which can be fabricated using iCVD in the presence of
liquids. Chapter 2 discusses results demonstrating that the polymer morphology at the liquid-
vapor interface depends on the surface tension interactions of the liquid and polymer, the
deposition rate, the viscosity of the liquid, and the deposition time. Chapter 3 focuses on the
fabrication of polymer films and includes chemical analysis and a novel method for the
fabrication of shaped, free-standing polymer films deposited onto liquid substrates from the
vapor-phase. Chapter 4 includes studies of polymer particle fabrication on liquid surfaces
including chemical analysis, particle engulfment, particle growth over time, and the effects of
liquid viscosity and polymer molecular weight on the particle size.

4

Table of Contents
1. Background
a. iCVD 5
b. Deposition onto Liquids 7
c. Polymer Film Fabrication and Applications 8
d. Polymer Particle Fabrication and Applications 10
2. Control of Polymer Morphology Deposited at Liquid-Vapor Interface
a. Introduction 12
b. Materials and Methods 13
c. Results and Discussion 16
i. Spreading Coefficient
ii. Polymer Deposition Rate
iii. Liquid Viscosity
iv. Deposition Time
d. Conclusions 25
3. Polymer Films
a. Introduction 26
b. Materials and Methods 28
c. Results and Discussion 31
i. Analysis of Films
ii. Free-standing Films
d. Conclusions 42
4. Polymer Particles
a. Introduction 43
b. Materials and Methods 45
c. Results and Discussion 47
i. Chemical Analysis
ii. Particle Engulfment
iii. Particle Growth over Time
iv. Liquid Viscosity
v. Polymer Molecular Weight
d. Conclusions 55
Acknowledgments 56
References 57

5

1. Introduction
1a. Initiated Chemical Vapor Deposition (iCVD)
The traditional iCVD process is a one-step, solvent-free process that can be used to
deposit a wide range of polymers including poly(2-hydroxyethyl methacrylate) (PHEMA),
1

poly(4-vinylpyridine) (P4VP),
2
poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA),
3
and
poly(o-nitrobenzyl methacrylate).
4
In the iCVD process, monomer and initiator molecules are
flowed into a chamber maintained at approximately 100 mTorr (Figure 1). The initiator
molecules are broken into free radicals by a heated filament array (typically set at 200-220°C).
The initiating radicals and monomer molecules diffuse to a stage cooled with a backside chiller
(typically set at 20-40°C) and polymerization occurs on the surface of the solid substrate. The
polymerization occurs via a free-radical mechanism and the deposited polymeric films are
chemically similar to solution-phase polymers.
5
Tert-butyl peroxide is generally used as the
initiator. Typically, iCVD is used to coat solid substrates,
6-8
but we recently demonstrated the
deposition of polymers onto liquids with low vapor pressures, including ionic liquids (ILs)
9, 10

and silicone oil (SiO).
11

6

Figure 1. Schematic showing the free radical mechanism associated with the traditional iCVD
process. I-I represents the initiator molecule and M represents the monomer.
7

1b. Other Vapor-Phase Deposition Processes onto Liquid Substrates
Ionic liquids have been used as liquid substrates in the vapor-phase deposition of
inorganic materials.
12,13
For example,
12
Borra et al. sputtered silver onto 1-ethyl-3-
methylimidazolium ethylsulphate in order to fabricate a high reflectivity surface. They found that
sputtering silver directly onto the IL surface led to a film consisting of colloidal particles with
diameters in the tens of nanometers. However, sputtering a thin layer of chromium followed by
silver led to a smooth film, which they attributed to the higher nucleation density of chromium.
They found that their coatings were stable after deposition. In another example, Torimoto and
co-workers found that sputtering gold onto IL surfaces resulted in the formation of gold
nanoparticles.
13
Nanoparticle size was found to be dependent on the type of IL used, while
nanoparticle concentration was dependent on the duration of the sputtering process. Silicone oil
has also been used as a substrate in inorganic vapor deposition processes.
14-16
Xie et al. described
a two-stage growth model for the deposition of gold and silver onto silicone oil surfaces with the
first stage involving nucleation and growth of molecules and the second involving diffusion
along the liquid surface.
15
Ye et al. proposed that nanoparticles are formed instead of films due to
an adsorbed layer of oil molecules preventing contact between particles.
16
Liquid substrates have
also been used in the vapor-phase deposition of organic parylene.
17
For example, Binh-Khiem et
al. deposited smooth, transparent parylene films onto glycerin, liquid paraffin, and 1,3,5-
trimethyl-1,1,3,5,5-pentaphenyl-trisiloxane silicone oil.
18

8

1c. Free-Standing Polymer Films
Free-standing polymer films are important for applications in optics,
19
sensing,
20

separations,
21,22
and tissue engineering. For example, the development of synthetic polymer
scaffolds to support transplanted cells in order to grow new tissue could save the lives of people
waiting for organ transplants.
23
Polymer thin films for use as in vivo cell supports must have
controllable degradation properties in order to retain their structure for a long enough time period
to foster the growth of new tissue without remaining in the body long-term. The polymer films
must also be fabricated as free-standing films because a solid substrate would compromise the
controlled degradation and could be toxic. One polymer that could potentially be used in tissue
engineering applications is poly(2-hydroxyethyl methacrylate) (PHEMA), which we can deposit
from the vapor-phase in our process.
24
Another potentially useful polymer which we can
fabricate is poly(N-isopropylacrylamide) (PNIPAAm), which undergoes a reversible phase
change at ~30°C, causing a sharp transition from hydrophilic to hydrophobic as the temperature
is increased above the transition.
25
As a result of this transition, PNIPAAm films adhere to
retinal tissue at physiological temperature but release at lower temperatures, so the films could
potentially be used as reversible adhesives for retinal implants.
26
A reversible adhesive would
enable the facile realignment of implants during surgery and the removal of malfunctioning
implants without damaging surrounding tissue. In order to adhere to body tissue with three-
dimensional geometry and avoid toxicity these polymer thin films must be free-standing.
Free-standing polymeric films are typically made by releasing the films from pH-
responsive or thermo-responsive sacrificial layers.
27,28
For example, Fujie et al. synthesized free-
standing thermoresponsive polymer films using a 3-step method that involved spin-coating
polysaccharide and poly(vinyl alcohol) layers onto a silicon substrate, peeling the film off of the
9

solid substrate, then releasing the polysaccharide layer by dissolving the poly(vinyl alcohol) in
water.
29
Free-standing films of different shapes were recently fabricated by Strook et al. using a
3-step method that involved microcontact printing patterned regions of self-assembled
monolayers, then adsorbing polyelectrolyte layers onto the hydrophobic regions, and finally
using a water-soluble sacrificial layer to release the films.
30
In certain cases, polymers can also
be removed from a solid substrate using non-aqueous methods such as peeling if the forces
between the polymer and the substrate are weak.
31
Our vapor-phase process offers a novel
method of fabricating free-standing polymer films, without the use of volatile solvents.

10

1d. Polymer Particles

Our ability to control the polymer morphology could also enable us to deposit particles
for applications in photonics,
32
electronics,
33
and drug delivery
34-36
One method for the
fabrication of polymer nanoparticles is to dissolve pre-formed polymer in an organic solvent,
then mix the solution with a non-solvent and stabilizing agent, then remove the organic solvent
to form stabilized nanoparticles.
37
For example, Musyanovych et al. formed polymer
nanoparticles of poly(L-lactide) (PLA) and poly[(D,L-lactide)-co-glycolide] (PLGA) by
dissolving the polymers in chloroform and adding an aqueous surfactant solution. The
chloroform evaporated and left a suspension of particles with diameters ranging from 80-200 nm
and polydispersity indices of .1-.7.
38
Using a different method with pre-formed polymer, Meziani
formed poly(heptadecafluorodcylacrylate) (PHDFDA) nanoparticles by dissolving the PHDFDA
in supercritical CO
2
and passing the pressurized solution through a nozzle into ambient water,
leading to particles with an average diameter of 41±8 nm.
39
With the exception of super-critical
fluids, we have not found techniques for the fabrication of fluorinated polymer nanoparticles in
the literature, which is an additional advantage of our system. It is also possible to form polymer
nanoparticles during the polymerization process by polymerizing in emulsions. In conventional
emulsion polymerization, an emulsion of monomer is stabilized in water with a surfactant. A
water-soluble initiator is added to the emulsion and polymerization occurs only in the emulsions
and leads to the formation of particles.
40,41
For example, Yoon et al. studied the effect of the type
of initiator on the size and morphology of poly(N-vinylcarbazole) using toluene as the emulsion
solution getting a range of nanoparticle diameters from 50-200 nm.
42
In surfactant-free emulsion
polymerization, ionizable initiators or ionic co-monomers are used instead of a surfactant as a
stabilizer.
43
For example, Ozturk et al. synthesized PHEMA nanoparticles, using poly(vinyl
11

alcohol) as a stabilizer, with an average size of 150 nm and polydispersity index of 1.17.
44
By
comparison, our technique does not require the addition of any stabilizers.
Our method for the fabrication of polymer nanoparticles is environmentally benign
because it does not require the use of organic solvents. Instead we use non-volatile liquid
substrates, which can be reused. Furthermore, our technique does not suffer from solubility
problems. Polymer solubility is poor in supercritical fluids,
45
and emulsion polymerization
requires monomers which are insoluble in water and able to form emulsions. Conversely, our
technique does not require dissolution of polymer or emulsification of monomer. Finally, current
techniques require several steps, whereas our technique utilizes simultaneous polymerization and
particle formation.

12

2. Control of Polymer Morphology Deposited at Liquid-Vapor Interface

2a. Introduction
We have observed that the vapor-phase deposition of polymers onto liquid substrates can
result in the formation of polymer films or particles at the liquid-vapor interface. In this chapter,
we demonstrate the relationship between the polymer morphology at the liquid-vapor interface
and the surface tensions of the liquid and polymer, the liquid viscosity, the deposition rate, and
the deposition time. We show that the thermodynamically stable morphology is determined by
the surface tension interactions of the polymer and liquid. For short deposition times, stable
polymer films form when it is energetically favorable for the polymer to spread over the surface
of the liquid whereas polymer particles form when it is energetically favorable for the polymer
chains to aggregate. For some systems which do not strongly favor spreading or aggregation, we
observe that the initial morphology depends on the deposition rate. Particles form at low
deposition rates whereas unstable films form at high deposition rates. We also observe a
transition from particle formation to unstable film formation when we increase the viscosity of
the liquid or increase the deposition time. Our results provide fundamental understanding about
polymer growth at the liquid-vapor interface and can offer insight into the growth of other
materials on liquid surfaces. The insight gained from our study can allow for the production of
nanoparticles for applications in photonics,
46
electronics,
47
and drug delivery,
34-36
as well as films
for applications in sensing
20,21
and separations.
22

13

2b. Materials and Methods
Materials
Glycerol (EMD Chemicals), 1-ethyl-3-methylimidazolium tetrafluoroborate
([emim][BF
4
], 97%, Aldrich), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF
4
],
97%, Aldrich), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF
6
], 97%,
Aldrich), squalene (98%, Sigma), 1-decyl-3-methylimidazolium tetrafluoroborate ([dmim][BF
4
],
96.5%, Aldrich), low viscosity silicone oil (Xiameter PMX-200 350 cP, Aldrich), high viscosity
silicone oil (100,000 cP, Aldrich), 1H,1H,2H,2H-perfuorodecyl acrylate (PFDA, 97%,
Synquest), pentafluorophenyl methacrylate (PFM, 97%, Synquest), n-butyl acrylate (nBA, 99%,
Aldrich), n-butyl methacrylate (nBMA, 99%, Aldrich), 2-hydroxyethyl methacrylate (HEMA,
97%, Aldrich), 4-vinylpyridine (4VP, 95%, Aldrich) and tert-butyl peroxide (98%, Aldrich)
were used as received without further purification.

Polymer Depositions
Liquid droplets (5 µL) were dispensed from a micropipette onto silicon wafers which
were placed into a custom designed reaction chamber (250 mm diameter, 48 mm height, GVD
Corporation). For all depositions, the initiator was kept at room temperature and flowed into the
reactor at a rate of 0.95 sccm. Inside the reactor, a nichrome filament array (80% Ni, 20% Cr,
Omega Engineering) was held 32 mm above that stage and resistively heated to 200°C. The
temperature of the reactor stage was controlled using a re-circulating backside chiller. Specific
reaction conditions for each polymer can be found in Table 1. The deposition rate was
determined by measuring the film thickness on a reference silicon wafer using a profilometer
(Ambios Technology XP-2 stylus profilometer).
14

Table 1. Summary of deposition conditions used in this study.
Monomer
Monomer Jar
Temp. (°C)
Monomer Flow
Rate (sccm)
Stage Temp.
(°C)
Reactor
Pressure (mTorr)
D
r

(nm/min)
PFDA 50 1.16 35
55 3
80 10
100 27
120 37
PFM 25 1.54 20 100 10
nBA 25 4.15 10
300 10
450 23
600 29
750 43
900 51
nBMA 25 6.52 10
200 10
400 37
600 50
800 80
HEMA 50 0.77 35 100 10
4VP 25 4.10 25
210 6
520 10
830 33
1000 44

Imaging
The polymer deposited onto the liquid droplets was imaged using a stereo microscope at
10x total magnification. The P4VP particles shown in Figure 2 were separated from the silicone
oil surface using a 0.01% v/v solution of triton-X surfactant in water. After separation, 50 µL of
the solution was drop-cast onto a clean silicon wafer and allowed to dry under ambient
conditions. A thin gold coating was sputtered onto the sample and it was imaged with a JEOL-
6610 low-vacuum scanning electron microscope.

15

Dynamic Light Scattering Measurements
Dynamic light scattering was used to confirm the presence of P4VP particles deposited
on the surface of the silicone oil by analyzing the particles in the 0.01% v/v triton-X solution
with a Wyatt DynaPro Titan instrument. The particles had an average hydrodynamic radius of
128 nm and a polydispersity of 15.3%. DLS was also used to study PnBA particles which de-
wetted from the surface of [emim][BF
4
].

Contact Angle and Surface Tension Measurements
A goniometer (ramé-hart Model 290-F1) was used to measure the advancing and
equilibrium contact angles of the liquids on the polymers, the liquid-vapor surface tensions, and
the polymer-vapor surface tensions. Each measurement was taken 7 times and averaged. The
advancing contact angles were measured with an automated liquid dispensing system, and the
equilibrium contact angles were measured by dispensing 5 μL of liquid, allowing the droplets to
equilibrate for 5 min, and then measuring the contact angles. The pendant drop method was used
to measure liquid-vapor surface tensions.
50
The acid-base method was used to calculate polymer-
vapor surface tensions from the equilibrium contact angles of water, glycerol, and methylene
iodide on films of each polymer.
48
The errors in the measurements of the advancing contact
angles, the polymer-vapor surface tensions, and the liquid-vapor surface tensions were estimated
by averaging the standard deviations of each individual data point and were 2.0 degrees, 1.5
mN/m, and 1.0 mN/m, respectively. The error in the calculation of the spreading coefficient was
determined by propagating errors through Eq.1, which resulted in an estimated error of 3.5
mN/m.
49

16

2c. Results and Discussion
In order to examine the effect of surface tension on polymer deposition, we deposited a
range of polymers onto a range of liquids. Polymers were deposited at a rate of 10 nm/min for 5
minutes to minimize the effect of accumulated polymer on the liquid surface tension and
viscosity. The surface tension interactions between the polymer and liquid can be quantified by a
spreading coefficient (S), which is a measure of the free energy required for the polymer to
spread over the surface of the liquid
50
and is defined as the work of adhesion (W
A
) of the
polymer on the liquid minus the work of cohesion (W
C
) of the polymer: S = W
A
-W
C
.
51
For the
spreading of a polymer on a liquid, the work of adhesion (W
A
) is defined by the equation:
50

1
in which γ
PV
, γ
LV
, and γ
PL
are the polymer-vapor, liquid-vapor, and polymer-liquid surface
tensions, respectively. The work of cohesion is defined:
52

2
In order to solve for the spreading coefficient in terms of experimentally measurable quantities,
equations 1 and 2 are inserted into the equation for S and γ
PL
is solved in terms of the advancing
contact angle of the liquid on the polymer (Θ) using Young’s equation: γ
PL
= γ
PV
- γ
LV
*cos(Θ),
53

which gives:

3

17

Table 2. Summary of , S (bold) in mN/min, γ
LV
, and γ
PV
values.

In order to calculate S for each liquid—polymer system, we used a goniometer to
measure Θ, the pendant drop method
50
to measure γ
LV,
and the acid-base method
48
to calculate
γ
PV
. As shown in Table 2, we found that polymer films form when it is energetically favorable
for the polymer to spread over the surface of the liquid (S>0) whereas particles form when it is
energetically favorable for the polymer to aggregate and reduce the area of contact with the
liquid surface (S<0.) For example, the deposition of PPFDA onto [emim][BF
4
] (S=11.8) resulted
in polymer films (Figure 2A) whereas the deposition of P4VP onto SiO (S=-71.6) resulted in
polymer particles (Figure 2B). The P4VP polymer particles were recovered from the SiO surface
and imaged using scanning electron microscopy to confirm that discrete nanoparticles were
formed (Figure 2C). It is important to note that the low surface tension polymer PPFDA

Polymer
(γ
PV
in mN/m)

PPFDA
(13.6)
PPFM
(25.7)
PnBA
(35.1)
PnBMA
(35.4)
PHEMA
(48.5)
P4VP
(57.5)
Liquid
(γ
LV
in mN/m)
Glycerol
(63.4)
116.2°
8.2
99.6°
1.4
79.9°
4.3
81.7°
1.8
62.2°
-4.0
46.0°
-11.6
[emim][BF
4
]
(55.6)
107.4°
11.8
83.2°
10.8
71.4°
3.1
77.3°
-3.0
61.5°
-14.9
60.5°
-32.0
[bmim][BF
4
]
(45.6)
102.2°
8.8
80.3°
1.9
63.4°
-4.2
64.3°
-5.4
46.4°
-20.0
55.3°
-43.4
[bmim][PF
6
]
(45.1)
104.0°
7.0
85.7°
-2.9
55.0°
0.8
55.8°
-0.4
47.9°
-21.7
54.9°
-44.0
Squalene
(31.5)
86.1°
6.4
46.7°
1.7
53.5°
-15.5
30.1°
-12.0
27.7°
-37.6
22.1°
-54.3
[dmim][BF
4
]
(30.3)
88.4°
4.0
64.9°
-8.2
42.5°
-21.9
36.9°
-16.3
30.8°
-40.7
46.1°
-63.7
Silicone Oil
(22.8)
73.7°
2.0
31.1°
-9.1
50.2°
-32.8
39.5°
-30.4
23.6°
-53.3
25.2°
-71.7
18

(γ
PV
=13.6 mN/m) had positive spreading coefficients for all of the liquids and therefore films
were formed in all cases. These films remained clear and smooth for more than one month at
room temperature, indicating that the films are thermodynamically stable. Conversely, the high
surface tension polymers PHEMA (γ
PV
=48.5 mN/m) and P4VP (γ
PV
=57.5 mN/m) had negative
spreading coefficients for all of the liquids and formed particles in all cases. For polymers with
intermediate surface tensions, we found that the sign of the spreading coefficient strongly
depends on the surface tension of the liquid. For example, PnBA (γ
PV
=35.1 mN/m) has S=4.3
mN/m on the high surface tension liquid glycerol and formed a film, whereas PnBA has S=-32.8
mN/m on the low surface tension liquid silicone oil and formed particles. The error in the
spreading coefficient is ≈3.5 mN/m from the measurements of γ
LV
, γ
PV
, and θ, and likely
explains why the trend in the polymer morphology is not perfect for systems with S≈0.
Furthermore, we have previously shown that in some polymer-liquid systems the monomer can
absorb into the liquid
9
which would decrease both γ
LV
and the spreading coefficient. We have
ignored this effect in our calculations because for the systems with S>>0 that absorb monomer
(ie. nBA in [emim][BF
4
]) we always observe the formation of films indicating that the spreading
coefficient remains positive, whereas monomer absorption in systems with S<<0 (ie. HEMA in
[emim][BF
4
]) would only further favor the formation of particles.
19

The S value determines the thermodynamically preferred morphology of the polymer but
does not account for the effect of other reaction parameters on morphology, such as the polymer
deposition rate (D
r
). To study the effect of the deposition rate, we increased the deposition rate
by increasing the reaction pressure, and the deposition time was kept constant at 5 minutes. We
found that systems with S>>0 resulted in the formation of polymer films for all tested deposition
rates because it is energetically favorable for the polymer to spread and form a film. For
example, the deposition of PPFDA onto [emim][BF
4
] (S=11.8) resulted in the formation of
visible PPFDA films after 10, 3, 2, and 1 minutes for D
r
= 3, 10, 27, and 37 nm/min.,
respectively. Longer deposition times were needed at lower deposition rates because the films
were not visible until they reached a thickness of ~30 nm. These films remained visibly clear and
smooth for at least one month at room temperature, indicating thermodynamic stability. For
systems with S<<0, we found that particles formed for all deposition rates attainable by varying
the reaction pressure. For example, the deposition of P4VP onto SiO (S=-71.6) resulted in P4VP
particles for all D
r
= 6, 10, 33, and 44 nm/min. For some systems with S ≈0, we found that
particles formed at low deposition rates whereas films formed at high deposition rates.

Figure 2. Images showing a (A) PPFDA polymer film on [emim][BF
4
] (S=11.8) and (B) P4VP
polymer particles on SiO (S=-71.6). An SEM image (C) confirms that the P4VP polymer
formed discrete nanoparticles on SiO.

20

Table 3. Data showing that polymer morphology depends on deposition rate for systems with
S≈0.
Polymer Liquid
S
(mN/m)
Deposition Rate
(nm/min)
Morphology
PnBA [bmim][BF
4
] -4.2
10 Particles
23 Particles
29 Particles
43 Films
51 Films
PnBMA [bmim][PF
6
] -0.4
10 Particles
37 Particles
50 Films
80 Films
PnBA [bmim][PF
6
] 0.8
10 Particles
23 Films
29 Films
43 Films
51 Films

For example, Table 3 shows that the deposition of PnBA onto [bmim][BF
4
] (S=-4.2)
resulted in particles for D
r
= 10, 23, and 29 nm/min and clear films for D
r
= 43 and 51 nm/min.
We observed a similar morphology dependence on deposition rate for PnBA on [bmim][PF
6
]
(S=0.8 mN/m) and PnBMA on [bmim][PF
6
] (S=-0.4 mN/m). We also conducted a 1 minute
PnBA deposition onto [bmim][BF
4
] at D
r
=51 nm/min to match the amount of polymer deposited
in 5 minutes at D
r
=10 nm/min and we observed a film, which confirms that the film was formed
because of an increased rate of deposition rather than an increased amount of polymer. For the
systems with moderate spreading coefficients, we hypothesize that polymer particles form at low
deposition rates because the polymer chains are able to diffuse across the liquid surface and
aggregate before overlapping, whereas at high deposition rates the polymer chains have a higher
molecular weight
54
causing the chains to diffuse slower and overlap faster,
55,56
leading to the
formation of a film. In contrast, we did not observe a morphology dependence on the deposition
rate in the extreme cases because for systems with highly positive spreading coefficients it is
21

favorable for the polymer to spread over the liquid resulting in films, whereas for systems with
highly negative spreading coefficients it is likely that the chains diffuse and aggregate before
they can overlap resulting in particles. Unlike the stable films formed in systems with highly
positive spreading coefficients (i.e. PPFDA films), the films formed at high deposition rates for
systems with moderate spreading coefficients were not thermodynamically stable and eventually
de-wetted from the liquid surface. For example, the clear PnBA films deposited at D
r
=43 nm/min
onto [bmim][BF
4
] (S=-4.2 mN/m) de-wetted and formed particles after 24 hours.
Dynamic light scattering was also used to verify that the PnBA film that de-wetted from
the [bmim][BF
4
] surface formed particles. Immediately after deposition a [bmim][BF
4
] droplet
with a PnBA film was centrifuged, the IL was removed with a 25% v/v solution of methanol in
water, and the polymer was dispersed in a 0.01% v/v triton-X in water solution for analysis. The
results showed only the presence of triton-X micelles in water at 0.7 nm radius,
57
indicating that
there were no polymer particles. A separate PnBA film on [bmim][BF
4
] was left on the droplet
surface for 24 hours after deposition. The same separation and analysis procedure was
conducted, and the results showed the presence of particles with an average hydrodynamic radius
of 139 nm and a polydispersity of 77.5%, which verifies that particles formed due to the film de-
wetting from the liquid surface after the deposition.
The time it takes for polymer chains to diffuse on a liquid surface increases with liquid
viscosity.
58
We therefore hypothesized that we could favor the formation of unstable filmsinstead
of particles for systems with S<<0 by using higher viscosity liquids. We compared the deposition
of P4VP at 10 nm/min for 5 minutes onto 350 cP and 100,000 cP silicone oils to test the effect of
viscosity on the initial polymer morphology without significantly changing the surface tension of
the liquid.
59
We found that particles formed on the lower viscosity silicone oil (Figure 3A) and
22

clear films formed on the higher viscosity silicone oil (Figure 3B) demonstrating that very high
viscosity liquids can enable film formation for systems with S<<0. These films were unstable
due to the negative spreading coefficient and de-wetted within 5 days.

We studied the effect of the spreading coefficient, deposition rate, and liquid viscosity on
the initial polymer morphology using short 5 minute depositions in order to minimize the effects
of polymer accumulation at the liquid-vapor interface. We tested the effect of polymer
accumulation on polymer morphology by conducting longer depositions. For systems with S>>0,
films are initially formed and increasing the deposition time increases the film thickness. To
study the effect of deposition time in systems with S<<0, we deposited P4VP onto all of the
liquids at D
r
= 10 nm/min for deposition times ranging from 5 to 180 minutes. We found that
P4VP initially formed particles on all of the liquids, but at longer deposition times films formed
over the particles (Table 4), which is schematically shown in Figure 4A. For example, a 5
minute deposition of P4VP onto [emim][BF
4
] resulted in the formation of particles (Figure 4B)
while a 60 minute deposition resulted in a film over the particles (Figure 4C).

Figure 3. The deposition of P4VP at Dr = 10 nm/min. results in (A) particles on silicone oil
with 350 cP viscosity and (B) films on silicone oil with 100,000 cP viscosity.

23

Table 4. The deposition of P4VP onto a range of liquids results in the formation of particles
(shaded) after 5 minutes, but films form at longer deposition times (un-shaded).
Liquid θ
e

Deposition time (minutes)
5 15 30 45 60 90 120 180
[emim][BF
4
] 55.6 P F F F F F F F
[bmim][BF
4
] 47.8 P P P F F F F F
[bmim][PF
6
] 47.5 P P P F F F F F
Glycerol 23.4 P P P P F F F F
Squalene 17.0 P P P P P F F F
[dmim][BF
4
] 10.5 P P P P P P P F

Tambe et al. showed that the viscosity at a water-oil interface increased as a function of
particle surface coverage,
60
therefore we expect that the surface viscosity increases with the
accumulation of particles at the liquid-vapor interface. Similar to the formation of a P4VP film
on high viscosity silicone oil, the increase in surface viscosity likely enables the chains to
Figure 4. A schematic (A) showing that polymer films can form over polymer particles at
longer deposition times. For example, the deposition of P4VP onto [emim][BF
4
] results in (B)
particle formation after 5 minutes and (C) a film forms over the particles after 60 minutes.

24

overlap before they can aggregate leading to the formation of a film over the particles. The
accumulation of polymer at the liquid interface will also decrease the liquid surface tension
61

which favors the formation of particles, however the formation of a film over the particles
suggests that the effect of increased viscosity controls the polymer morphology at long
deposition times. We also observed that the time it takes for a film to form over particles
decreases with increasing equilibrium contact angle (θ
e
) of the liquid on the polymer. For
example, [emim][BF
4
] has a high contact angle on P4VP (θ
e
= 55.6°) and less than 15 min of
deposition was required to form a film, whereas [dmim][BF
4
] oil has a low contact angle on
P4VP (θ
e
= 10.5°) and more than 120 min of deposition was required to form a film. This is
likely because an increase in θ
e
between 0 and 90  increases the distance the particles protrude
from the interface
62
and increases the surface coverage, which results in a larger increase in
surface viscosity
63
and leads to the formation of films at shorter deposition times. These P4VP
films were unstable due to negative spreading coefficients and de-wetted within 24 hours.

25

2d. Conclusions
We have demonstrated that the thermodynamically stable morphology of the polymer
deposited onto liquid substrates depends on the spreading coefficient of the polymer on the
liquid. For systems with S>>0 it is energetically favorable for the polymer to spread across the
liquid surface to form a film, whereas for systems with S<<0 it is energetically favorable for the
polymer chains to aggregate into particles in order to reduce the area of contact with the liquid,
regardless of the deposition rate. For some systems with S≈0 the initial morphology depends on
the deposition rate, with particles forming at low deposition rates and unstable films forming at
high deposition rates. We also found that unstable films can be formed for systems with S<<0 by
increasing the viscosity either by using liquids with inherently high viscosities or by increasing
the deposition time to accumulate polymer particles at the liquid-vapor interface. This study has
enhanced our understanding of dynamic polymer-liquid interfacial phenomena and enables us to
control polymer morphology and deposit particles for applications in photonics,
32
electronics,
47

and drug delivery
34-36
as well as films for applications in sensing,
20,21
separations.
22
Furthermore,
the fundamental insight gained in this study may be applicable to other vapor-phase deposition
processes onto liquid surfaces such as parylene deposition and metal sputtering.

26

3. Polymer Films
3a. Introduction
Free-standing polymer films are important for applications in optics,
19
sensing,
20,21
separations,
22
and biomedical applications. For example, the development of synthetic polymer
scaffolds to support transplanted cells in order to grow new tissue could save the lives of people
waiting for organ transplants.
23
Polymer thin films for use as in vivo cell supports must have
controllable degradation properties in order to retain their structure for a long eno ugh time period
to foster the growth of new tissue without remaining in the body long-term. The polymer films
must also be fabricated as free-standing films because a solid substrate would compromise the
controlled degradation and could be toxic. One polymer that could potentially be used in tissue
engineering applications is poly(2-hydroxyethyl methacrylate) (pHEMA), which we can deposit
from the vapor-phase in our process.
24
Another potentially useful polymer which we can
fabricate is poly(N-isopropylacrylamide) (pNIPAAm), which undergoes a reversible phase
change at ~30°C, causing a sharp transition from hydrophilic to hydrophobic as the temperature
is increased above the transition.
25
As a result of this transition, pNIPAAm films adhere to retinal
tissue at physiological temperature but release at lower temperatures, so the films could
potentially be used as reversible adhesives for retinal implants.
26
A reversible adhesive would
enable the facile realignment of implants during surgery and the removal of malfunctioning
implants without damaging surrounding tissue. In order to adhere to body tissue with three-
dimensional geometry and avoid toxicity these polymer thin films must be free-standing.
Free-standing polymeric films are typically made by releasing the films from pH-
responsive or thermo-responsive sacrificial layers.
27,28
For example, Fujie et al. synthesized free-
standing thermoresponsive polymer films using a 3-step method that involved spin-coating
27

polysaccharide and poly(vinyl alcohol) layers onto a silicon substrate, peeling the film off of the
solid substrate, then releasing the polysaccharide layer by dissolving the poly(vinyl alcohol) in
water.
29
Free-standing films of different shapes were recently fabricated by Strook et al. using a
3-step method that involved microcontact printing patterned regions of self-assembled
monolayers, then adsorbing polyelectrolyte layers onto the hydrophobic regions, and finally
using a water-soluble sacrificial layer to release the films.
30
In certain cases, polymers can also
be removed from a solid substrate using non-aqueous methods such as peeling if the forces
between the polymer and the substrate are weak.
31
Our vapor-phase process offers a novel
method of fabricating free-standing polymer films, without the use of volatile solvents.

28

3b. Materials and Methods
Materials
The 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF
6
]) ionic liquid (97%,
Aldrich), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF
4
]) ionic liquid (97%,
Aldrich), 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF
4
]) ionic liquid (97%,
Aldrich), 2-hydroxyethyl methacrylate (HEMA) monomer (98%, Aldrich), 1H,1H,2H,2H-
perfluorodecyl acrylate (PFDA) monomer (97%, Aldrich), N-isopropylacrylamide (NIPAAm)
monomer (97%, Aldrich), and tert-butyl peroxide (TBPO) initiator (98%, Aldrich) were used
without further purification.
Polymer Depositions
Ionic liquid droplets (5 µL) were dispensed from a micropipette onto 4 inch silicon
wafers which were placed into a custom designed reaction chamber (250 mm diameter, 48 mm
height). A nichrome filament array (80% Ni, 20% Cr, Omega Engineering) was resistively
heated to 200°C and the distance between the filament array and the substrate was kept constant
at 32 mm. For depositions of films used for FTIR analysis, the HEMA monomer was heated to
55°C in a jar and flowed into the reactor at a rate of 0.52 sccm, TBPO was kept at roo m
temperature and flowed into the reactor at a rate of 2.52 sccm, and the reactor pressure was
maintained at 0.50 Torr. For poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) depositions,
the PFDA monomer was heated to 50°C and flowed into the reactor at a rate of 0.28 sccm,
TBPO was kept at room temperature and flowed into the reactor at a rate of 3.30 sccm, and the
reactor pressure was maintained at 0.35 Torr. For the free-standing film depositions, the HEMA
monomer was heated to a temperature of 55°C, the stage temperature was maintained at 35°C
using a recirculating chiller, and the reactor pressure was kept constant at 110 mTorr. For the
29

deposition of PNIPAAm, the NIPAAm monomer was heated to a temperature of 60°C, the stage
temperature was maintained at 55°C using a recirculating chiller, and the reactor pressure was
kept constant at 100 mTorr. The TBPO initiator was maintained at room temperature and flowed
into the reactor at a rate of 0.92 sccm using a mass flow controller (Model 1479A, MKS).
Imaging
The droplets were imaged using a stereo microscope at 10x total magnification. The
thickness of the polymer skins was determined using a JEOL-6610 low-vacuum scanning
electron microscope. Ionic liquid was removed from the droplets by puncturing the skin near the
edge and using compressed air to force the IL though the hole. The silicon substrate underneath
the skin was cracked and mounted in a substrate holder such that the cross-section could be
visualized. A thin gold coating was sputtered onto the surface of the sample before imaging.
Film thicknesses on silicon were determined using a profilometer (Ambios Technology XP-2
stylus profilometer).

Fourier Transform Infrared Spectroscopy
Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet iS10) was used to study
the chemical composition of the polymer films. The polymer was removed from the surface of
the [bmim][PF
6
] drops using forceps and rinsed with solvent before analysis.

Fabrication of Shaped Polymer Films
In order to make shaped films, outlines of shapes were first drawn onto the substrate with
a wax crayon using a ruler and then IL was dispensed into the interior of the wax outline us ing a
micropipette. For [bmim][BF
4
], the shape was drawn onto an unmodified silicon wafer. For
30

[bmim][PF
6
] and [emim][BF
4
], the shape was drawn onto a silicon wafer which had first been
coated with a thin layer of PHEMA to increase the wetting of the IL into the corners of the
shapes. Silicone oil was then dispensed (5 μL) at each edge of the shape and allowed to slowly
spread over the wax and encircle the IL. After deposition of polymer, the polymer film was
removed from the IL either by inserting a razor underneath the film and lifting it off or by
submerging the entire substrate in silicone oil which allowed the film to float off the IL.

31

3c. Results and Discussion
The deposition of PHEMA onto droplets of [bmim][PF
6
] for 15 minutes at a stage
temperature of 35°C results in the formation of continuous polymer films, which is shown in
Figure 5A. The films completely encapsulate the liquid droplets and can be torn (Figure 5B)

We used Fourier transform infrared spectroscopy (FTIR) to characterize the PHEMA
films. Figure 6 compares the FTIR spectrum of a PHEMA film deposited on a silicon wafer
using traditional iCVD and the spectrum of a PHEMA skin deposited on the [bmim][PF
6
]
droplet at a stage temperature of 35°C using iCVD. The spectra are nearly identical,
demonstrating that the films have the same composition. Both spectra have the expected major
peaks: O—H stretching from 3600-3100 cm
-1
, C—H stretching from 3050-2800 cm
-1
, C=O
stretching from 1770-1700 cm
-1
, C—H bending from 1520-1350 cm
-1
, and C—O stretching from
1310-1210 cm
-1
.
64
The absence of additional peaks in the spectrum of the PHEMA film
deposited by iCVD indicates the high purity of the polymer film. We note that these polymer
films were rinsed multiple times with methanol prior to FTIR analysis, and that subsequent tests
have shown that there is IL present in the polymer film prior to rinsing.
Figure 5. Images of a PHEMA film on a [bmim][PF
6
] droplet at a stage temperature of
35°C after (A) 15 minutes of deposition, in which (B) a hole could be torn. Scale bars
represent 1 mm.

32

Figure 7. Expected reaction events occurring A) at the vapor–IL interface and B) within the IL.

Figure 6. FTIR spectra of PHEMA films deposited onto (A) a silicon wafer and (B)
[bmim][PF
6
].

33

PHEMA polymerization takes place simultaneously at both the vapor–[bmim][PF
6
]
interface and within the [bmim][PF
6
], as shown schematically in Figure 7. The rate of
polymerization at the interface is controlled by the diffusion of monomer molecules to the cooled
substrate surface.
5
The temperatures at the vapor-[bmim][PF
6
] interface and the vapor-silicon
interface are similar and therefore we expect similar deposition rates at both surfaces. Table 5
shows the thicknesses of the PHEMA skins. The thickness at the edge of the skin is similar to the
thickness of the PHEMA film on the silicon surface, however the thickness at the center of the
skin is significantly greater due to contributions from polymer chains that form within the bulk
IL. Within 90 seconds of PHEMA deposition, there are polymer pieces floating at the center of
the surface of the [bmim][PF
6
] droplet. These polymer pieces form as HEMA monomer absorbs
into the [bmim][PF
6
] layer and propagation occurs within the IL. At a critical molecular weight,
the growing PHEMA chains become insoluble in the [bmim][PF
6
] and float to the top because
the density of PHEMA is lower (1.21 g/ml)
65
than that of [bmim][PF
6
] (1.37 g/ml).
66
The
increased thickness at the center of the PHEMA skins accounts for their rough appearance.
Table 5. Thickness of PHEMA skins deposited onto [bmim][PF
6
] at a stage temperature of 35°C
after 15 minutes.
Thickness of PHEMA Skins (nm)
Deposition Center Edge
1 247 108
2 143 75
3 193 136
4 221 106
5 165 94

34

In the iCVD process, the monomer and initiator vapors flow simultaneously and
continuously throughout the deposition. The sequential flow of monomer vapor followed by
initiator vapor does not lead to polymerization because the monomer molecules will not stay
adsorbed to the surface of the substrate and instead will be pumped out of the system before the
introduction of the initiator vapor. When liquid substrates are added into the process, however,
the IL can act as a monomer trap. The IL can hold the monomer molecules after the flow of
monomer vapor has been stopped and excess monomer vapor has been pumped out of the
reactor. When initiator vapor is flowed into the system, polymerization can only occur within the
IL in which monomer molecules are trapped. To test our hypothesis, HEMA monomer and
initiator vapors were flowed into the reactor for 15 minutes at a stage temperature of 35°C,
without turning on the filament. This allowed the HEMA molecules to absorb into the
[bmim][PF
6
] without reacting. The reactor was then pumped down for 15 minutes to remove
excess monomer vapor. Initiator vapor was then introduced into the reactor for 15 minutes with
the filament array turned on but without the introduction of additional monomer. The sequential
flow of HEMA and initiator at a stage temperature of 35°C led to the formation of floating
PHEMA polymer pieces (Figure 8). There was no polymerization on the surrounding silicon.
35

The formation of polymer pieces instead of a continuous film is to be expected since
polymerization can only occur within the [bmim][PF
6
] in which monomer molecules are trapped.
We note that the ability to use the sequential flow of monomer and initiator to polymerize within
the IL layer will only work with monomers that are soluble within the IL.

We also deposited PPFDA films onto [bmim][PF
6
] at a stage temperature of 35°C, as
shown in Figure 9. We used FTIR spectroscopy to characterize the PPFDA films. Figure 10
Figure 9. Image of PPFDA deposited onto [bmim][PF
6
].

Figure 8. Image of PHEMA deposited after saturating the [bmim][PF
6
] with HEMA
monomer and subsequently flowing initiator at a stage temperature of 35°C.

36

compares the FTIR spectra of a PPFDA film deposited onto a silicon wafer using traditional
iCVD and a PPFDA skin deposited onto a [bmim][PF
6
] droplet using ILiCVD. The spectra are
nearly identical, demonstrating that the films have the same composition. Both spectra have the
expected major peaks: C=O stretching at 1741 cm
-1
, asymmetric CF
2
stretching at 1242 cm
-1
,
symmetric CF
2
stretching at 1207 cm
-1
, and a CF
2
CF
3
peak at 1153 cm
-1
.
3
The absence of
additional peaks in the spectrum of the PPFDA film deposited using ILiCVD indicates the high
purity of the polymer film.

In contrast to the PHEMA skins, the PPFDA skins have a more uniform thickness as
shown in Table 6. This suggests that PPFDA polymerization begins evenly across the vapor–IL
interface. The thickness of the skins is the same as the thickness of the PPFDA film on the
silicon surface confirming that the deposition rate is the same at the vapor–IL interface and at the
silicon–vapor interface.

Figure 10. FTIR spectra of PPFDA films deposited onto (A) a silicon wafer and (B)
[bmim][PF
6
].

37

Table 6. Thickness of PPFDA skins deposited onto [bmim][PF
6
] at a stage temperature of 35°C
after 15 minutes.
Thickness of PPFDA Skins (nm)
Deposition Center Edge
1 108 120
2 153 147
3 120 107
4 138 125
5 107 108

The PHEMA films shown in Figure 5 and PPFDA films shown in Figure 9 are connected
to the underlying solid substrate. For example, Figure 11A shows that PHEMA completely
encapsulates [bmim][PF
6
] droplets that are placed directly onto silicon wafers. These
encapsulated droplets do not move when the substrate is tilted at a 15 degree angle (Figure
11C). In order to fabricate films for applications, we added a lubricating layer of silicone oil
underneath the IL droplet. The PHEMA will not form films on the lubricating layer, enabling the
films to be released from the substrate. In order to make free-standing polymer films, we
combined the ILs and silicone oil onto a common substrate. Silicone oil was first dispensed onto
silicon wafers and allowed to spread over the wafer surface. The silicone oil completely wets the
surface of the silicon wafer forming a thin layer (33 μm) onto which IL droplets can then be
placed. Figure 11B shows that a continuous polymer film forms on the IL droplet whereas only
polymer particles form on the surrounding silicone oil. Figure 11D shows that the droplet slides
when the substrate is tilted at a 15 degree angle. This verifies that the silicone oil acts as a
lubricating layer to prevent the polymer that forms on the IL from connecting to the underlying
wafer.

38

The shape of the free-standing polymer film can be controlled by patterning the IL and
silicone oil onto the substrate. Figure 12 shows a schematic of this fabrication method. First, an
outline of a shape is drawn onto the substrate using wax. The IL is then dispensed into the
outline. The wax barrier contains the IL within the shape because the IL does not wet the wax. In
the case of [bmim][BF
4
], the wax outline was drawn onto a bare silicon wafer. In the cases of
[bmim][PF
6
] and [emim][BF
4
], the wax outline was drawn onto a silicon wafer that was pre-
coated with PHEMA in order to increase the spreading of the IL into the corners of the shape.
Figure 11. Images of a 60 minute deposition of PHEMA onto a [bmim][PF
6
] droplet that was
placed on A) a silicon wafer and B) a silicon wafer covered with a layer of silicone oil. C,D)
The substrate was titled at a 15 degree angle after the deposition.

39

Silicone oil was then added in multiple locations around the outside of the wax barrier and
allowed to spread over the barrier and encompass the IL. The silicone oil serves two purposes in
this fabrication process: it maintains the shape of the original IL droplet during deposition and it
prevents the polymer film from connecting to the underlying substrate. We would like to note
that there is no lubricating layer of silicone oil underneath the IL in this fabrication method since
the IL is dispensed before the silicone oil. Therefore the IL will not slide when the substrate is
tilted. After deposition of polymer, the free-standing polymer film can be removed from the IL
either by inserting a razor underneath the film and lifting it off or by submerging the entire
substrate in silicone oil which allows the film to float off the IL.

Figure 12. The fabrication method for making shaped polymer films.

40

Figure 13 shows the generality of our fabrication method for two different ILs and two
different polymers. A triangular PHEMA film was formed on [bmim][BF
4
] after a 30 minute
deposition. The FTIR spectrum of the PHEMA film showed no difference from the spectrum of
PHEMA deposited on a silicon wafer, indicating the high purity of the film. The free-standing
film had an average thickness of 510 ± 64 nm at the edge of the triangle and 663 ± 35 nm at the
center. The increased thickness at the center of the film is caused by the integration of polymer
chains that form within the bulk IL since PHEMA polymerization takes place simultaneously at
both the vapor–IL interface and within the IL at the conditions used for our study.
9
Similar to
PHEMA, the deposition of poly(N-isopropylacrylamide) (PNIPAAm) also results in polymer
particles on silicone oil and polymer skins on each of the three ILs. Therefore we can use our
fabrication method to form shaped PNIPAAm films. A square PNIPAAm film was formed on
[bmim][PF
6
] after a 135 minute deposition. The film had an average thickness of 445 ± 30 nm at
the edge of the square and 469 ± 41 nm at the center. The PNIPAAm film had the expected
FTIR peaks: asymmetric –CH
3
stretching at 2969 cm
-1
, asymmetric –CH
2
– stretching at 2931
cm
-1
, symmetric –CH
3
stretching at 2880 cm
-1
, secondary amide C=O stretching at 1652 cm
-1
, –
CH
3
and –CH
2
– deformation at 1458 cm
-1
, and –CH
3
deformation at 1387 and 1366 cm
-1
.
67

Compared to the PNIPAAm deposited onto a wafer, the shaped free-standing PNIPAAm film
had a shift in the location of the secondary amide N—H stretching from 1540 cm
-1
to 1575 cm
-1
.
This is likely due to the mobility of the PNIPAAm chains in the free-standing film that allows
for hydrogen bonding between the C=O and N–H groups.
68

41

Figure 13. Images and corresponding FTIR spectra of free-standing shaped films of A, B)
PHEMA formed on [bmim][BF
4
] and C, D) PNIPAAm formed on [bmim][PF
6
]. The films
were removed from the ionic liquid template and placed in a bath of silicone oil for imaging.

42

3d. Conclusion
We can deposit continuous polymer films on liquid substrates with controlled thicknesses
which are free of liquid impurities. As discussed in Chapter 2, we can also deposit polymer
particles. We exploited this difference in polymer morphology to fabricate ultrathin free-standing
polymer films of different shapes by combining the ILs and silicone oil onto a common
substrate. FTIR analysis shows that the free-standing polymer films are highly pure (free of
residual monomer, IL, and silicone oil) after rinsing which will enable their use for biomedical
applications. The free-standing PNIPAAm films have many potential uses due to their
temperature-responsive hydrophilicity.
69,70

Our fabrication process is environmentally-friendly because no organic solvents are used
in any of the steps and ionic liquids are non-volatile, non-flammable, and can be easily recycled.
We demonstrated the generality of our fabrication method across a range of imidazolium-based
ILs ([bmim][PF
6
], [bmim][BF
4
], and [emim][BF
4
]). Our ability to produce free-standing polymer
films of controlled shape, size, and thickness is useful for a wide variety of applications in
biosensing, biomimicry, and separations. In addition to PHEMA and PNIPAAm, we have found
that the deposition of several other polymers including poly(o-nitrobenzyl methacrylate) and
poly(pentafluorophenyl methacrylate) also yield particles on silicone oil and skins on ILs. This
allows us to extend our fabrication method to make light-responsive
4
and click-active polymer
films.
71
Furthermore, films with multiple functionalities (e.g., mechanical strength, temperature-
responsive swelling, photoresponsive solubility) can be made by sequentially stacking polymers.

43

4. Polymer Particles
4a. Introduction
In Chapter 2, we discussed the use of vapor-phase polymerization onto liquid substrates
to form polymer films or polymer particles and in this chapter the studies are extended to the
growth of polymer particles. Polymer particles have not been studied in vapor-phase systems but
they are widely synthesized in the liquid phase. One method for the fabrication of polymer
nanoparticles is to dissolve polymer in an organic solvent, mix the solution with a non-solvent
and stabilizing agent, then remove the organic solvent to form nanoparticles.
37-39
Using this
method Musyanovych et al. formed polymer nanoparticles of poly(L-lactide) (PLLA) particles
with diameters ranging from 80-460 nm and polydispersity indices (PDI) ranging from .06-.43.
38

Another method to make polymer nanoparticles is to form emulsions of the monomer followed
by polymerization in the emulsions, resulting in particle formation.
40-44,72
For example, He et al.
synthesized poly(methyl methacrylate) particles with diameters ranging from 14-45 nm and PDI
ranging from .092-.197.
72

Although we are the first group to study the mechanism of vapor-deposited polymer
particle growth on liquid surfaces the deposition of inorganic particles including silver,
12,15,16

gold,
13
and copper
14
has been studied. For example, Xie et al. described a two-stage growth
model for the deposition of gold and silver onto silicone oil surfaces with the first stage
involving nucleation and growth of molecules and the second involving diffusion along the
liquid surface.
15
Additionally, Torimoto and co-workers found that sputtering gold onto IL
surfaces resulted in the formation of gold nanoparticles, and that the particle concentration
increased with the duration of the sputtering process.
13
Although our work focuses on polymer
44

deposition, the mechanistic insight gained in our study may improve the understanding of other
deposition processes onto liquid substrates.
Compared to solution phase nanoparticle fabrication techniques, our deposition approach
offers one-step fabrication, fast reaction times (~15 minutes), room temperature reactions (25
°
C), and does not require a stabilizer or volatile solvent, making our process environmentally
benign. In this study we have elucidated the effects of particle submersion, deposition time,
polymer MW, and liquid viscosity on particle growth, which enables us to synthesize particles of
a desired size for applications in fields as diverse as catalysis,
73,74
photonics,
32
and drug
delivery.
34-36

45

4b. Materials and Methods
Materials
The silicone oils (Xiameter PMX-200 200 cSt, Aldrich) and (5, 100, 350, and 500 cSt,
Aldrich) liquids as well as the 2-hydroxyethyl methacrylate (HEMA, 97%, Aldrich), 4-
vinylpyridine (4VP, 95%, Aldrich), and n-butyl acrylate (nBA, 99%, Aldrich) monomers and
tert-butyl peroxide (98%, Aldrich) initiator were used as received without further purification.
The poly(dimethyl siloxane) with cross-linker (Silgard 184, Dow Corning) was also used as
received.
Polymer Depositions
Ionic liquid droplets (5 µL) were dispensed from a micropipette onto 4 inch silicon
wafers which were placed into a custom designed reaction chamber (250 mm diameter, 48 mm
height). A nichrome filament array (80% Ni, 20% Cr, Omega Engineering) was resistively
heated to 200°C and the distance between the filament array and the substrate was kept constant
at 32 mm. For PHEMA depositions, the HEMA monomer was heated to 50°C in a jar and flowed
into the reactor at a rate of 0.8 sccm, TBPO was kept at room temperature and flowed into the
reactor at a rate of 0.95 sccm, the reactor pressure was maintained at 0.10 Torr and the stage
temperature was maintained at 35°C. For poly(4-vinylpyridine) (P4VP) depositions, the 4VP
monomer was heated to 25°C and flowed into the reactor at a rate of 4.1 sccm, TBPO was kept at
room temperature and flowed into the reactor at a rate of 0.95 sccm, the reactor pressure was
maintained at 0.52 Torr, and the stage temperature was maintained at 25°C. For the MW studies
the pressure was varied from 0.10 Torr to 0.98 Torr. For poly(n-butyl acrylate) (PnBA)
depositions, the nBA monomer was maintained at room temperature and flowed into the reactor
at a rate of 12.1 sccm, TBPO was kept at room temperature and flowed into the reactor at a rate
46

of 0.95 sccm, the reactor pressure was maintained at 0.50 Torr, and the stage temperature was
maintained at 10°C. For the MW studies the pressure was varied from 0.09 Torr to 0.90 Torr.
Fourier Transform Infrared Spectroscopy
Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet iS10) was used to study
the on silicone oil by first separating the PHEMA from the silicone oil through extraction with
methanol and then drop casting the resulting solution onto a clean wafer.
Scanning Electron Microscopy
The solid PDMS samples from the engulfment studies were sputtered with a thin gold
coating and the samples were imaged with a JEOL-6610 low-vacuum scanning electron
microscope.
Particle Size and Polymer MW Measurements
The P4VP and PnBA particles deposited onto silicone oil were separated by mixing the
liquid substrate with a 0.01% (v/v) triton-X in water. The mixture formed clear layers of silicone
oil and water, from which the water layer was removed and centrifuged for 10 minutes at 13,000
RPM in order to remove bubbles. The samples were then analyzed using a Wyatt DynaPro Titan
dynamic light scattering instrument. The polymer MW was measured for the same samples by
allowing the water to evaporate, dissolving the polymer in MeOH, and measuring the MW using
the DLS instrument.

47

4c. Results and Discussion
The Fourier transform infrared spectra (FTIR) in Figure 14 show that a PHEMA film
deposited onto a silicon wafer and PHEMA particles deposited onto a silicone oil surface have
the same composition. Both spectra have the expected major peaks: O—H stretching from 3600-
3100 cm
-1
, C—H stretching from 3050-2800 cm
-1
, C=O stretching from 1770-1700 cm
-1
, C—H
bending from 1520-1350 cm
-1
, and C—O stretching from 1310-1210 cm
-1
. The absence of
additional peaks in the spectrum from the PHEMA particles suggests high purity.

In order to understand the mechanism by which polymer particles grow at the liquid-
vapor interface, we studied the location of the particles relative to the interface during the
deposition process. Colloidal particles at liquid-vapor interfaces may remain at the interface or
submerge into the bulk liquid,
75
which we hypothesize has a significant impact on the growth
mechanism of the particles because the polymer chains can aggregate at the liquid-vapor
interface to increase the particle size, but aggregation may be prevented in the bulk liquid due to
stabilization by the liquid molecules. The amount of energy required for a particle to submerge
Figure 14. FTIR spectra of a PHEMA film deposited onto a silicon wafer and PHEMA
particles recovered from a silicone oil surface

48

into a bulk liquid from the liquid-vapor interface is:

, where r is
radius of particle, γ
LV
is liquid-vapor surface tension, and θ
e
is equilibrium contact angle.
76

According to the equation for ΔG it is energetically favorable for particles with θ
e
> 0° to remain
at the liquid-vapor interface, whereas there is no energetic barrier keeping particles with θ
e
= 0°
at the liquid-vapor interface and the particles will submerge into the liquid. Thus, we studied two
polymers on which PDMS has different contact angles in order to determine whether or not the
polymer particles submerge during the deposition process. The uncured PDMS liquid has θ
e
=
21° on PnBA and we expect that polymer particles remain at the liquid-vapor interface, whereas
the PDMS liquid has θ
e
= 0° on the P4VP so we expect that the particles submerge into the
liquid. We deposited the polymers on onto the uncured PDMS for 5 minutes, allowed the PDMS
to cure, and observed the PDMS using SEM. The images show that there are PnBA particles at
the liquid-vapor interface but not in the bulk liquid (Figure 15A, B) indicating that when θ
e
> 0°
the particles will not submerge. Conversely, there are P4VP particles in the bulk liquid but not at
the liquid-vapor interface (Figures 15C, D) indicating that when θ
e
= 0° the particles will
submerge.

49

Figure 15. Top-down (A) and cross-sectional (B) images of PnBA deposited onto PDMS
indicate that the PnBA particles remain at the liquid-vapor interface, but top-down (C) and cross-
sectional (D) images of P4VP deposited on PDMS indicate that the P4VP particles are
submerged into the bulk liquid.

In order to understand how both unsubmerged and submerged particles grow as a
function of deposition time we deposited PnBA and P4VP onto silicone oil for times ranging
from 1-90 minutes at a deposition rate of 10 nm/min and measured the size of the particles using
dynamic light scattering (DLS). The silicone oil was selected because it has the same chemical
structure as the PDMS but does not have added cross-linker, and neither monomer absorbs into
the silicone oil so polymerization will not occur in the bulk liquid.
9
The silicone oil has a θ
e
=
13° on PnBA so the particles will remain at the interface, whereas the silicone oil has θ
e
= 0° on
P4VP so the particles will submerge into the liquid. We found that the particle size increased as a

50

function of time for PnBA on silicone oil, but the particle size did not change as a function for
P4VP on silicone oil (Table 7).
Table 7. The radii and PDIs of PnBA and P4VP particles deposited onto silicone oil as a
function of deposition time.
PnBA P4VP
Time (min.) Radius (nm) PDI Radius (nm) PDI
1 121 .08 157 .30
3 153 .14 150 .45
5 169 .24 151 .45
15 189 .36 164 .27
45 215 .49 162 .36
90 251 .39 162 .19

We hypothesize that the PnBA particles grow as a function of time because the particles
remain at the liquid-vapor interface and polymer chains continue to aggregate over time and
increase the particle size. Conversely, the P4VP particles stop growing once the particles are
submerged into the silicone oil. In order for the particles to stop growing within 1 min of
deposition the particles must become submerged within this time, which is consistent with
previous studies of the rate of polymer particle submersion into liquids.
77
The PDI of the PnBA
particles ranges from .08-.49 and the PDI of the P4VP particles ranges from .19-.45 (Table 7),
which are high compared to the solution phase methods discussed in the introduction.
72
In order
for polymer particles to form in our system the polymer chains must diffuse across the liquid
surface and aggregate. We hypothesize that the high PDI in our particles results from the random
nature of the aggregation process, which leads to polymer particles with very different numbers
of polymer chains per particle. In order to test this hypothesis, we measured the MW of the
PnBA and P4VP deposited onto silicone oil for all deposition times and found that the MW of
the PnBA was constant at ~40 kDa with PDI of ~.03 and the MW of the P4VP was constant at
~40 kDa with PDI of ~.02. Thus, the PDI of the particles is much higher than that of the
51

individual chains, suggesting that the main contribution to the high PDI in the particles is
variation in the number of polymer chains per particle resulting from the aggregation process.
In order for polymer chains to aggregate and form polymer particles the polymer chains
must diffuse across the liquid surface, and the rate of diffusion depends on the viscosity of the
liquid.
58
Thus, we directly tested the effect of polymer diffusion on polymer particle size by
depositing PnBA and P4VP onto silicone oils with viscosities ranging from 5-500 cP for 1
minute at a rate of 10 nm/min. We found that the size of both the unsubmerged PnBA particles
and the submerged P4VP particles decreased with increasing viscosity (Figure 16A, B), showing
that the polymer particle size increases with an increasing diffusion rate at the liquid-vapor
interface. The trend is similar for both unsubmerged and submerged systems, which suggests that
the effect of viscosity on the growth process is more significant at the liquid-vapor interface than
in the bulk liquid. In order to confirm that the difference in particle size is due to differences in
the number of chains per particle, we measured the MW and found that the PnBA had a MW of
~40 kDa on each silicone oil and the P4VP had a MW of ~40 kDa on each silicone oil. Since the
viscosity of the liquid does not affect the polymer MW, we hypothesized that the silicone oil
surface does the significantly affect the polymerization kinetics. In order to test this we measured
the MW of the PnBA and P4VP on solid substrates from the same depositions and found that the
MW was the same as on the silicone oils, confirming that the silicone oil surface does not affect
polymerization kinetics.

52

Figure 16. The size of both (A) PnBA and (B) P4VP particles deposited onto silicone oils
decrease with increasing liquid viscosity.

In the previous experiments the polymer MW was constant, but we expect that the
polymer MW will affect the size of the polymer particles. In order to test the effect of MW on
the size of polymer particles deposited at the liquid-vapor interface, we conducted 1 minute
depositions of PnBA and P4VP onto silicone oil with 200 cP viscosity using a range of reaction
pressures. In the iCVD process the ratio of monomer partial pressure to monomer saturation
pressure (P
M
/P
Sat
) controls the monomer concentration at the substrate surface and thus the MW.
5

We varied the reaction pressure for both polymers and found that the MW of both polymers
increased with increasing P
M
/P
Sat
(Figure 17A,B). An increase in polymer MW also resulted in
an increase in particle radius for both polymers (Figure 17C,D).

53

Figure 17. The MW of (A) PnBA and (B) P4VP deposited onto silicone oil increases as a
function of P
M
/P
Sat
. The size of the (C) PnBA and (D) P4VP particles deposited onto the silicone
oil also increases as function of P
M
/P
Sat
.

54

4d. Conclusions
We have studied the growth mechanism of polymer particles deposited at the liquid-
vapor interface via vapor phase polymerization. We have found that polymer particles can either
remain at the liquid-vapor interface during the deposition process or submerge into the bulk
liquid, depending on the contact angle of the liquid and the polymer. Polymer particles which
remain at the liquid-vapor interface during the deposition process grow larger as a function of
deposition time, whereas polymer particles which submerge into the liquid do not grow as a
function of deposition time. For both submerged and unsubmerged polymer liquid systems we
have found that the size of the polymer particles decreases as a function of liquid viscosity and
increases as a function of polymer molecular weight. Our results provide fundamental
understanding of the location of polymer particles and the aggregation behavior of polymer
chains at the liquid-vapor interface, enabling us to fabricate polymer particles with controlled
size for potential applications in catalysis, photonics, and drug delivery.

55

Acknowledgments
I gratefully acknowledge the support of my advisor, Malancha Gupta, as well as the rest
of the Gupta lab group. I also acknowledge the Mork Family Department of Chemical
Engineering and Materials Science, the Viterbi School of Engineering, the National Science
Foundation, and the American Chemical Society for financial support.

56

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Abstract (if available)

Abstract This report outlines studies of the vapor‐phase deposition of polymers in the presence of low‐vapor pressure liquids. It includes background information on the initiated chemical vapor deposition (iCVD) process which is used to deposit the polymers. There is also background information on other organic and inorganic vapor‐phase deposition processes which have been used with liquid substrates. Finally, there is information on the production and use of polymer films and particles, both of which can be fabricated using iCVD in the presence of liquids. The report discusses results demonstrating that the polymer morphology at the liquid‐vapor interface depends on the surface tension interactions of the liquid and polymer, the deposition rate, the viscosity of the liquid, and the deposition time. It also demonstrates the fabrication of polymer films and includes chemical analysis and a novel method for the fabrication of shaped, free‐standing polymer films deposited onto liquid substrates from the vapor‐phase. Studies of polymer particle fabrication on liquid surfaces were conducted including chemical analysis, particle engulfment, particle growth over time, and the effects of liquid viscosity and polymer molecular weight on the particle size.

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Fabrication of silicon-based membranes via vapor-phase deposition and pyrolysis of organosilicon polymers

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Simultaneous monomer deposition and polymerization at low substrate temperatures for the formation of porous polymer membranes

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The patterning of polymer thin films on porous substrates via initiated chemical vapor deposition

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Selective deposition of polymer coatings onto structured surfaces

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Fabrication of functional porous membranes via polymerization of solid monomer by a vapor-phase initiator

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Chemical vapor deposition of graphene: synthesis, characterization, and applications

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Real time surface analysis of complex oxide thin films during pulsed laser deposition

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The selective area growth and coalescence of indium phosphide nanostripe arrays on silicon through MOCVD for NIR monolithic integration

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Phase change heterostructures for electronic and photonic applications

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Fabrication and study of organic and inorganic optoelectronics using a vapor phase deposition (VPD)

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Plasma emission across metal nanoparticle surfaces and semiconductor -liquid interface characterizations

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Fabrication, deposition, and characterization of size-selected metal nanoclusters with a magnetron sputtering gas aggregation source

Asset Metadata

Creator Haller, Patrick David (author)

Core Title Vapor phase deposition of polymers in the presence of low vapor pressure liquids

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 08/04/2014

Defense Date 06/12/2014

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

Tag liquid substrates,OAI-PMH Harvest,particles,polymer,surface energy,thin films,vapor phase deposition

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Gupta, Malancha (committee chair), Nakano, Aiichiro (committee member), Wang, Pin (committee member)

Creator Email patrick.d.haller@gmail.com,phaller@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-451517

Unique identifier UC11287002

Identifier etd-HallerPatr-2757.pdf (filename),usctheses-c3-451517 (legacy record id)

Legacy Identifier etd-HallerPatr-2757-0.pdf

Dmrecord 451517

Document Type Dissertation

Format application/pdf (imt)

Rights Haller, Patrick David

Type texts

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

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

Repository Name University of Southern California Digital Library

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