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Understanding structural changes of polymer films during vapor phase deposition

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Understanding structural changes of polymer films during vapor phase deposition

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Content Understanding Structural Changes
of Polymer Films During Vapor Phase Deposition
by
Stacey Bacheller
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMICAL ENGINEERING)
August 2024
Copyright 2024 Stacey Bacheller

ii
Acknowledgments
First, I would like to thank my advisor Dr. Malancha Gupta for believing in me and
arranging our first meeting in 2019. I am grateful for her guidance and support throughout the
last five years. I am appreciative for the USC Viterbi School of Engineering and Chevron
Corporation Partnership Program PhD Fellowship in energy resources which allowed me to
focus on my studies and research for the first two years. I feel lucky to have been able to explore
other research interests such as the sustainable textiles project with Paola Espinosa and
collaborate, although briefly, with the Childress group.
I would like to thank my fellow group members. I knew from the moment I toured the lab
that it would be a good fit. Specifically, I would like to thank Nareh Movsesian for allocating
time in her schedule to train me. Next, I would like to thank my co-authors on my first two
publications, Golnaz Dianat and Nicholas Welchert. I would also like to thank my committee
members Dr. Noah Malmstadt and Dr. Maral Mousavi for serving on my defense.
I also must acknowledge the unique PhD experience caused by the COVID-19 pandemic.
As a result, I have become a more thoughtful researcher and independent problem-solver. Lastly,
I am grateful to my parents for all their love and support. Thank you for believing in me and for
providing me the tools I needed to succeed in life and academics.

iii
Table of Contents
Acknowledgments......................................................................................................................... ii
List of Tables.................................................................................................................................. v
List of Figures............................................................................................................................... vi
Abstract......................................................................................................................................... ix
Chapter 1: Introduction............................................................................................................. 1
1.1 Initiated Chemical Vapor Deposition .................................................................................... 1
1.2 Modified Initiated Chemical Vapor Deposition .................................................................... 3
1.3 References............................................................................................................................. 6
Chapter 2: Synthesis of pH-Responsive Polymer Sponge Coatings and
Freestanding Films via Vapor Phase Deposition................................................ 10
2.1 Abstract................................................................................................................................ 10
2.2 Introduction ......................................................................................................................... 10
2.3 Experimental Section .......................................................................................................... 12
2.4 Results and Discussion........................................................................................................ 17
2.5 Conclusion........................................................................................................................... 31
2.6 Acknowledgment................................................................................................................. 32
2.7 References........................................................................................................................... 33
Chapter 3. Influence of Oblique Angle Deposition on Porous Polymer Film
Formation .............................................................................................................. 38
3.1 Abstract................................................................................................................................ 38
3.2 Introduction ......................................................................................................................... 38
3.3 Experimental Section .......................................................................................................... 40
3.4 Results and Discussion........................................................................................................ 42
3.5 Conclusion........................................................................................................................... 55
3.6 Acknowledgement............................................................................................................... 56
3.7 References........................................................................................................................... 56
Chapter 4. Vapor Phase Deposition of Porous Polymer Dendrites...................................... 59
4.1 Abstract................................................................................................................................ 59

iv
4.2 Introduction ......................................................................................................................... 59
4.3 Experimental Section .......................................................................................................... 62
4.4 Results and Discussion........................................................................................................ 64
4.5 Conclusion........................................................................................................................... 74
4.6 Acknowledgment................................................................................................................. 75
4.7 References........................................................................................................................... 76
Chapter 5. Concluding Remarks and Future Work ............................................................. 80
5.1 Conclusion........................................................................................................................... 80
5.2 Future work ......................................................................................................................... 81
5.2.1 Systematic study on sublimation step........................................................................... 81
5.2.2 Different Monomers..................................................................................................... 82
5.3 References........................................................................................................................... 84
Chapter 6. Full List of References …………………………………………………………85
6.1 References …………………………………………………………………………………85

v
List of Tables
Table 3-1. Lengths of region 1, 2, and 3 at different monomer capture temperatures.................. 51
Table 3-2. Lengths of region 1, 2, and 3 at different angles of deposition. .................................. 55
Table 4-1. Deposition conditions for samples, % area of coverage, and dendrite height............. 71

vi
List of Figures
Figure 1-1. Polymer thin film deposited using a) spin-coating and b) iCVD.7 .............................. 1
Figure 1-2. Schematic of iCVD process. ........................................................................................ 2
Figure 1-3. Schematic of sequential process for fabricating porous polymer membranes
using the modified iCVD process. 42................................................................................... 4
Figure 2-1. SEM images of a) pillared structure formed at a reactor pressure of 600 mTorr.
b) sponge structure formed at a reactor pressure of 350 mTorr. Cross-sectional SEM
images of c) pillared structure and d) sponge structure. ................................................... 19
Figure 2-2. a) SEM image of a freestanding P(MAA-co-EGDA) film (inset: dry 8 mm
diameter film). b) Image of the freestanding film after 5 seconds in pH 4 buffer. c)
Image of the freestanding film after 5 seconds in pH 10 buffer. ...................................... 21
Figure 2-3. SEM image of a) paper before coating and b) after coating with porous sponge
P(MAA-co-EGDA). Images of a mixture of Ponceau S (pink) and crystal violet
(purple) flowing through the c) uncoated paper and d) coated paper. .............................. 22
Figure 2-4. a) SEM image of the sponge coating before annealing (inset: image of coating
on a silicon wafer) and b) SEM image of the sponge coating after annealing (inset:
image of annealed coating on a silicon wafer).................................................................. 24
Figure 2-5. a) Top-down SEM image of the sponge coating fabricated on a dense 400 nm
layer of PGMA (inset: picture of the coating on the silicon wafer before the initial
tape test). b) Tape test results............................................................................................ 26
Figure 2-6. a) Schematic representation of the fabrication process to form a hierarchical
bilayer structure of a sponge bottom layer and a pillared top layer. b) Top-down
SEM image of the bilayer coating showing the top pillared layer. c) Cross-sectional
SEM image of the bilayer coating. Dashed line indicates the division between the
sponge and pillared layers. d) Top down and e) cross-sectional SEM image of the
collapsed bottom sponge layer after the dissolution of the top pillared layer in pH
10 solution......................................................................................................................... 29
Figure 2-7. SEM image of PMAA sponge coating a) before and b) after coating with
PPFDA (inset: Contact angle image). c) SEM image of freestanding P(MAA-coEGDA) sponge film coated with PPFDA after submersion in DI water. d)
Freestanding P(MAA-co-EGDA) sponge film................................................................. 31
Figure 3-1. a) Schematic of the extension used for oblique angle monomer delivery. b)
Schematic of the three regions of distinct morphologies. In-situ optical images
using a 30° monomer extension c) after 4 minutes of initial monomer capture at a
substrate temperature of -20°C and d) after 40 minutes of polymerization at a
substrate temperature of -10°C. e) The resulting porous PMAA film after
sublimation and removal from the reactor. (Blue-Region 1, Red- Region 2, YellowRegion 3)........................................................................................................................... 44
Figure 3-2. SEM images of a) region 1, b) region 2, and c) region 3 of the porous PMAA
film deposited using a 30° monomer extension and a monomer capture time of 4
minutes at a substrate temperature of -20°C followed by polymerization for 40
minutes at a substrate temperature of -10°C..................................................................... 46

vii
Figure 3-3. Optical images of a PMAA film grown without the monomer extension using a
monomer flow rate of a) 0.05 sccm and b) 0.3 sccm. SEM images of the
corresponding c) PMAA dendrite formed at 0.05 sccm and d) PMAA pillars formed
at 0.3 sccm. ....................................................................................................................... 47
Figure 3-4. Effect of the substrate temperature on monomer capture using a 30° monomer
extension. In situ optical images of the silicon wafer after 4 minutes of monomer
capture at substrate temperatures of a) 0°C, b) -10°C, and c) -20°C and
corresponding images after 40 minutes of polymerization at -10°C (d, e, f). .................. 49
Figure 3-5. Optical images of the porous PMAA films made using a 30° monomer
extension with a monomer capture temperature of a) 0°C, b) -10°C, and c) -20°C
and corresponding SEM images of (d,e,f) region 1, (g,h,i) region 2, and (j,k,l)
region 3 indicated in blue, red, and yellow, respectively. ................................................. 50
Figure 3-6. Effect of the monomer capture time using a 30° monomer extension and a
substrate temperature of -20°C. In situ optical images of the silicon wafer after a) 1
minute of monomer capture and, b) 2 minutes of monomer capture and
corresponding images (c,d) after 40 minutes of polymerization at -10°C........................ 51
Figure 3-7. Optical images of the porous PMAA films made using a 30° monomer
extension after a) 1 minute of monomer capture and b) 2 minutes of monomer
capture. SEM images of region 1 (c,d), region 2 (e,f), and region 3 (g,h) are
indicated in blue, red, and yellow, respectively. ............................................................... 52
Figure 3-8. SEM image of the intermediate structures found in region 2 using 1 minute of
monomer capture. ............................................................................................................. 53
Figure 3-9. Optical images of the porous PMAA films deposited at angles of a) 30° b) 50°
c) 70° and d) 90° using 4 minutes of monomer capture at a substrate temperature of
-20°C followed by 40 minutes of polymerization at a substrate temperature of -
10°C. Regions 1 (e,f,g,h), region 2 (i,j,k,l), and region 3 (m,n,o,p) are indicated in
blue, red, and yellow, respectively, where X is the geometric center. .............................. 54
Figure 4-1. a) Optical image of PMAA dendrites on a silicon wafer. SEM images of b)
characteristic branching of a representative dendrite, c) examples of different
dendritic shapes, and d) dendritic growth along the substrate edge. ................................ 66
Figure 4-2. SEM image of zoomed-in pores on the branch of a PMAA dendrite. ....................... 67
Figure 4-3. a) Optical image of PMAA dendrites on a scratched silicon wafer. SEM image
of b) dendrites along the scratch, c) dendrites along the wafer edge, and d) a
discrete dendrite not located on the scratch or edge. ........................................................ 68
Figure 4-4. Optical image of an uncoated silicon wafer and a wafer coated with 100 nm
PPFDA a) before deposition and b) after deposition. SEM image of PMAA
dendrites c,e) on the uncoated silicon wafer and d,f) on the coated wafer. ...................... 69
Figure 4-5. Optical image of PMAA dendrites on a silicon wafer with fingerprints at all
four corners. ...................................................................................................................... 70
Figure 4-6. Optical image of monomer capture on a silicon wafer after 1, 5, 15, and 30
minutes.............................................................................................................................. 71

viii
Figure 4-7. a) Optical image of PMAA dendrites, b) top-down SEM images of PMAA
dendrites, and c) cross-sectional SEM images of Sample A, Sample B, Sample C,
and Sample D.................................................................................................................... 72
Figure 4-8. FTIR spectra of the MAA monomer, a 400 nm dense PMAA control sample,
and Sample C. ................................................................................................................... 73
Figure 4-9. SEM image of Sample D coated with 400 nm of fluoropolymer and the
corresponding contact angle. ............................................................................................ 74

ix
Abstract
Polymer films are useful in providing surface functionality to improve and target specific
applications. Due to their various functionalities, polymer films are attractive for a range of
applications such as electronics, biomedical, energy, and separations. Traditional polymer
processing consists of solution-based methods such as dip-coating, spray-coating, and spincoating which can result in processing challenges such as surface tension effects and solvent
compatibility. In addition, solution-based processes are often multi-step and can result in residual
chemicals. Vapor-phase techniques such as initiated chemical vapor deposition (iCVD) offer a
more sustainable method of fabricating dense and porous polymer materials without the use of
harsh solvents and additives. Porous polymer materials can be synthesized through a modified
iCVD process. In this work, both the conventional and modified iCVD processes are discussed.
Chapter 1 introduces iCVD and the modified iCVD process and discusses the advantages
and applications. Chapter 2 describes the synthesis of porous polymer sponge coatings and
freestanding films. We showed how increasing the monomer flow rate caused the monomer to
condense before solidifying resulting in the sponge morphology, as compared to our previously
reported work on the porous polymer pillar structure. Chapter 3 applies the concept of oblique
angle deposition to the modified iCVD process to fabricate 3D porous polymer films with
different morphological regions. Adding an oblique angle extension to deliver the monomer, we
found three distinct regions. Region 1 at the center was characterized by porous polymer pillars
resembling those in the absence of the extension. Region 2 surrounded region 1 and consisted of
densified pillars as a result from monomer sublimating and recapturing downstream. Region 3,
furthest from the monomer extension, displayed dendritic polymer structures as a result of low

x
monomer concentration. Chapter 4 expands further on polymer dendritic growth. We studied the
diffusion-controlled nucleation and growth behavior of the monomer that resulted in the
branched structures. Chapter 5 offers concluding remarks and examples of future work for the
modified iCVD process.

1
Chapter 1: Introduction
1.1 Initiated Chemical Vapor Deposition
Polymer thin films are commonly fabricated using solution-based methods such as dipcoating1,2
, spin-coating3,4, and spray-coating.5,6 However, these processes face several processing
limitations such as solvent compatibility and surface tension effects. Vapor phase processing
methods such as initiated chemical vapor deposition (iCVD) are free from these disadvantages
and offer a more sustainable route to providing surface functionality. For example, spin-coating a
microtrench results in a polymer film at the bottom off the trench as a result of surface tension
effects from the solution phase (Figure 1-1).7 In comparison, the polymer film deposited using
iCVD results in a conformal coating along the top surface, trench side walls, and bottom. iCVD
is useful for depositing conformal polymer coatings onto complex substrates such as
microtrenches7,8, porous materials9,10, and flexible substrates.11,12
Figure 1-1. Polymer thin film deposited using a) spin-coating and b) iCVD.7
In the iCVD process (Figure 1-2), monomer and initiator vapors are introduced into a
cylindrical vacuum chamber.13,14 The initiator molecules, typically tert-butyl peroxide, are

2
thermally cleaved into free radicals using a Nichrome filament array heated to 200 to
300o
C.7,14,15 The substrate is generally cooled to approximately room temperature to promote
adsorption of monomer molecules onto the surface. The polymerization process occurs on the
substrate via a free radical polymerization mechanism. Propagation occurs through growing
polymer chains and termination occurs through recombination or with a primary radical
species.16 The monomer surface concentration is proportional to the ratio of the partial pressure
of the monomer to the saturation pressure of the monomer (PM/Psat). Typically, iCVD reactions
operate at PM/Psat values between 0.4 to 0.7.14,17,18 The kinetics of iCVD polymerization is
limited by monomer adsorption which can be increased by lowering the substrate
temperature.16,17
Figure 1-2. Schematic of iCVD process.
A major benefit of this vapor-phase technique is that it is free from harsh solvents and
additives that can impart impurities on the coated material, improving feasibility for sensitive
biological applications.15 Additionally, the moderate operating conditions such as the substrate
and filament temperature make it suitable for deposition onto delicate substrates and the low
energy conditions allow for full retention of monomer pendant groups compared to higher
energy vapor phase techniques such as plasma enhanced CVD.7,19 Full retention of functional

3
groups allow for precise control over surface properties to target a range of applications such as
electronic devices20,21, membranes for separations,22,23 and biomedical applications.24,25 The
iCVD process is an attractive method for surface modification because it provides desired
chemical functionality without changing the bulk properties of the material.26
1.2 Modified Initiated Chemical Vapor Deposition
Porous polymer materials have gained interest because of their potential for use in
environmental remediation, catalysis, energy storage, and biomedical application.27,28,29 They are
attractive because of their large surface area, pore sizes, and chemical functionality provided by
the polymer.27 Porous polymer materials are generally synthesized via solution-based methods
such as using solution-phase methods including electrospinning30, particulate leaching31,32,
freeze-casting33,34, phase separation35,36, cryopolymerization37,38, and copolymer selfassembly39,40 that require multiple steps and generate the porous structure through removal of the
solvent phase or porogen.
Our group has pioneered a modification to the iCVD process to fabricate porous polymer
materials using the vapor phase. In the traditional iCVD process, the partial pressure of the
monomer is kept below the saturation pressure of the monomer which results in conformal
polymer coatings as mentioned above.7,17 In the modified iCVD process, the partial pressure of
the monomer is increased above the saturation pressure of the monomer and the substrate
temperature is decreased below the freezing point of the monomer by adding a thermoelectric
cooler to the reactor. The monomer molecules solidify on the cooled substrate and are
polymerized either through a simultaneous method or a sequential method. In the simultaneous
method, the filament array is resistively heated before monomer introduction and the monomer

4
continues to flow during polymerization resulting in simultaneous monomer deposition and
polymerization.41 In the sequential method, the monomer is introduced first and then halted
before the filament is turned on to initiate polymerization (Figure 1-3).42,43 In both methods, after
polymerization the unreacted solid monomer is sublimated by allowing the substrate to return to
room temperature and complete sublimation is indicated by the reactor pressure returning to its
base pressure. For this work, we focus on the sequential method since it allows for more control
over the monomer deposition and polymerization step of the process.
Figure 1-3. Schematic of sequential process for fabricating porous polymer membranes using the
modified iCVD process. 42
This vapor-phase processing technique offers a sustainable, one-pot, bottom-up
fabrication method. We can adjust operating parameters in situ to fabricate hierarchical structures
and can provide additional surface functionality by combining the modified iCVD process with
traditional iCVD. For example, we demonstrate that we can fabricate superhydrophobic porous
films by depositing a thin film of poly(1H,1H,2H,2H-perfluorodecyl acrylate (PPFDA) onto the
porous poly(methacrylic acid) (PMAA) membranes. The thin film of PPFDA deposited using
traditional iCVD adds the desired chemical functionality to the synthesized porous polymer
membranes without changing the underlying porous structure.

5
Using the modified iCVD process, we can adjust operating conditions to form different
porous polymer structures. Previously, we have determined the effect of substrate temperature
during monomer capture and polymerization on the porous pillared membranes. Lowering the
substrate temperature of monomer capture results in higher nucleation on the surface and more
defined 3D pillars. Polymerization at higher temperatures result in greater inner porosity from
higher sublimation of unreacted monomer within the structures.42,43 In this work we demonstrate
that we can synthesize different porous morphologies by adjusting processing conditions further.
We can synthesize porous polymer sponge coatings and freestanding films by increasing the
monomer flow rate which result in the monomer condensing on the substrate before solidifying.
We show that we can fabricate porous polymer films with a thickness gradient and different
morphological regions by adding a monomer extension to deliver the monomer to the substrate.
We also establish a method for fabricating porous polymer dendrites by reducing the monomer
concentration in the reactor to result in a diffusion-limited nucleation and growth process.

6
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42 Dianat, G.; Movsesian, N.; Gupta, M. Vapor Deposition of Functional Porous Polymer
Membranes. ACS Applied Polymer Materials 2020, 2 (2), 98–104.
43 Dianat, G.; Movsesian, N.; Gupta, M. Process-Structure-Property Relationships for Porous
Membranes Formed by Polymerization of Solid Monomer by a Vapor-Phase Initiator.
Macromolecules 2018, 51(24), 10297-10303.

10
Chapter 2: Synthesis of pH-Responsive Polymer Sponge Coatings and
Freestanding Films via Vapor Phase Deposition
2.1 Abstract
Porous polymer materials are desired for a variety of applications such as catalysis,
filtration, sensing, optics, and biological engineering. In this work, we present a modified
initiated chemical vapor deposition (iCVD) process to synthesize sponge-like stimuli-responsive
poly(methacrylic acid) (PMAA) polymer coatings and freestanding films characterized by
micron-sized pores, pore interconnectivity, and varying heights. The carboxylic acid functional
group in the PMAA sponge allows for pH-responsive behavior ideal for biomedical applications
and separations. Thermal annealing was used to control the dissolution of the sponge coatings in
different pH environments. Ethylene glycol diacrylate was introduced during the fabrication
process to form freestanding sponges with pH-responsive swelling behavior and glycidyl
methacrylate was introduced during the fabrication process to increase durability of the sponge
coatings. Hierarchical structures with a bottom sponge layer and a top pillared layer were
fabricated in situ by varying the processing conditions. The sponge structures were coated with a
fluorinated polymer to fabricate superhydrophobic structures with self-cleaning properties. Our
fabrication process offers an all-dry one-pot method to synthesize polymer sponges with
different functionalities for a range of applications in drug delivery, sensor, and separation
applications.
2.2 Introduction
Three-dimensional porous polymer sponges have recently attracted growing interest for
applications in liquid separations, catalytic reactions, cell culturing, and drug delivery due to

11
their high surface area, high porosity, and pore interconnectivity.1,2,3,4,5,6 Porous polymer
materials are generally synthesized using solution-phase methods including electrospinning3,7,
particulate leaching8,9, freeze-casting10,11, phase separation12,13, cryopolymerization14,15, and
copolymer self-assembly16,17,18. In these processes, pores are typically created from the removal
of the solvent phase which leads to processing challenges such as solvent compatibility and
surface tension effects. Residual solvent, unreacted monomer, and additives present in these
solution-phase methods can result in impurities in the final material. For sensitive applications
such as use as commercial biomaterials, minor impurities can lead to failures in biocompatibility
testing.
Vapor phase techniques such as initiated chemical vapor deposition (iCVD) offer a more
sustainable route to synthesizing porous polymer materials since solvents and additives are not
required.19 In this paper, we show that we can adjust the iCVD process to create threedimensional polymer sponge coatings and self-standing films with interconnected pores. The
large surface area and channel spaces are advantageous for separation and biological
applications.1,5 The range in pore sizes are ideal for selective adsorption or encapsulation and
release of pharmaceuticals, genes, and proteins.1,20 Typically, functional porous polymer sponges
are produced in a multi-step process where first a solvent-based method is used to fabricate the
sponge structure and then the structure is coated with a chemical functionality in a second step.
For example, Chen et al. synthesized a superhydrophobic and superoleophilic sponge by
functionalizing a melamine sponge with polydimethylsiloxane through a solution-immersion
process for oil-water separation.21 Wu et al. fabricated magnetic superhydrophobic sponges for
oil and water separation by binding iron oxide nanoparticles to polyurethane sponges that were
modified by chemical vapor deposition followed by dip-coating in a fluoropolymer solution.22

12
The novelty of our process is that the sponge morphology and chemical functionality are
synthesized in a single, dry processing step. The high freezing point of methacrylic acid (MAA)
monomer (15 °C) allows for a bottom-up approach for fabricating a pH-responsive polymer
sponge. The carboxylic acid functional group present in the resulting poly(methacrylic acid)
(PMAA) sponge promotes a pH response that is useful for applications in adsorption and drug
delivery.23,24,25 PMAA is chosen for use in enteric drug release systems because at low pH, such
as the acidic environment of the stomach, PMAA encapsulated drugs remain charge neutral with
minimal swelling.20, 23,26 At high pH, such as the lower gastrointestinal tract, the carboxylic acid
deprotonates resulting in swelling and release of the drug. Controlled release of orally
administered drugs such as pain relievers and antibiotics offer numerous advantages. In
comparison to conventional dosage methods, controlled release systems require a lower drug
amount, help prevent degradation of the drug, minimize gastrointestinal side effects, and provide
a site-specific delivery.20,25,27 The carboxylic acid group in PMAA has also been shown to be
useful for environmental remediation. Humbeck et al. has shown that adding carboxylic acid
functional groups to diamondoid porous organic polymers promotes enhanced ammonia
adsorption.24 Similarly, hollow microporous nanospheres with poly(acrylic acid) lining were
shown to exhibit enhanced uptake of basic triphenylmethane dyes such as malachite green and
methyl violet by utilizing acid-base interactions. The removal of these toxic organic pollutants
from the environment are of growing concern since they are used in paper, printing, and textile
industries.25
2.3 Experimental Section
Methacrylic acid (MAA) (Aldrich, 99%), ethylene glycol diacrylate (EGDA)
(Polysciences, Inc.), glycidyl methacrylate (GMA) (Aldrich, ≥ 97.0), 1H,1H,2H,2H-

13
perfluorodecyl acrylate (PFDA) (C8PFA; SynQuest Laboratories, Inc., 97%), tert-butyl peroxide
(TBPO) (Aldrich, 98%), crystal violet (Aldrich, 90%), Ponceau S (Aldrich, 75%), and pH 4 and
10 buffer solutions (BDH, ACS Grade) were used as received without purification. The vapor
deposition process was performed in a cylindrical reactor chamber (GVD Corporation; 25 cm
diameter, 5 cm height). The reactor pressure was held under vacuum by a rotary vane vacuum
pump (Edwards E2M40) and a throttle valve controller (MKS 153D). The pressure was
measured by a capacitance manometer (MKS 622C01TDE Baratron). Porous polymer pillar and
sponge coatings were deposited onto silicon wafers (Wafer World) positioned on top of a
thermoelectric cooler (TEC) (Custom Thermoelectric). An adjustable DC power supply (Volteq
HY3010D) was used to control the TEC temperature. The reactor stage was set to the minimum
temperature of 10 °C using a recirculating chiller (Thermo Scientific NESLAB RTE 7). Initiator
and monomer flow rates were estimated by introducing the precursor into the system at room
temperature and 25 °C, respectively, setting the reactor pressure, and measuring the increase in
pressure of the system for 5 seconds after closing the valve to the vacuum pump.
To synthesize porous PMAA coatings, the TEC was kept at -20 °C during monomer
deposition. First, TBPO was allowed to flow into the reactor chamber to build reactor pressure
and then monomer was introduced into the system. For PMAA pillar coatings, TBPO flowed at a
rate of 0.6 sccm, the reactor pressure was set to 600 mTorr, and the MAA monomer flowed at a
rate of 0.3 sccm. For PMAA sponge coatings, TBPO flowed at a rate of 1.2 sccm, the reactor
pressure was set to 350 mTorr, and the MAA monomer flowed at a rate of 3.0 sccm. After 5
minutes of monomer deposition, the monomer flow was stopped, the TEC temperature was
raised to -10 °C, and the filament array (80% Ni, 20% Cr, Omega Engineering) was heated to
220 °C to initiate polymerization. Polymerization proceeded for 30 minutes, then TBPO flow

14
was halted, the TEC temperature was raised to 0 °C and then slowly to 18 °C to allow the
unreacted monomer to sublimate. Complete sublimation was confirmed by the reactor pressure
returning to a base pressure of 25 mTorr. To synthesize PMAA sponge coatings, the monomer
deposition, polymerization, and sublimation steps were carried out identical to PMAA pillar
coatings; however, the lower reactor pressure resulted in an increase in TBPO and MAA flow
rates.
For the fabrication of freestanding poly(methacrylic acid-co-ethylene glycol diacrylate)
P(MAA-co-EGDA) films, TBPO vapor flowed at a rate of 1.2 sccm and the reactor was set to
350 mTorr. The TEC was set to -20 °C, and MAA and EGDA vapors flowed at the same time at
rates of 3.0 sccm and 0.2 sccm, respectively. MAA flow was halted after 5 minutes, the TEC
temperature was increased to -10 °C, the filament was resistively heated to 220 °C to begin
polymerization, and the EGDA continued to flow for the first 15 minutes of polymerization to
ensure sufficient crosslinking. Polymerization continued for 15 minutes after halting EGDA flow
for a total polymerization time of 30 minutes. After 30 minutes, the filament was turned off,
TBPO flow was stopped, and the sample was sublimated at 0 °C. The films were delaminated
using an 8 mm biopsy punch (Healthlink). For the synthesis of the poly(methacrylic acid-comethacrylic anhydride) P(MAA-co-MAN) coatings, first PMAA sponge coatings were prepared
as described above and placed into an oven (VWR) at 175 °C for 20 minutes.
For the synthesis of the paper-based microfluidic devices, grade 1 chromatography paper
(Whatman) were cut into 1 cm by 4 cm strips. One strip was modified by deposition of porous
sponge P(MAA-co-EGDA). First, the TEC was set to -20 °C and MAA was introduced for 5
minutes. The TEC was increased to -10 °C, the filament was resistively heated to 220 °C to
begin polymerization, then after 15 minutes of polymerization, EGDA was introduced at a flow

15
rate of 0.2 sccm and allowed to flow for 15 minutes. TBPO and EGDA flows were stopped, the
filament was turned off, and the TEC was raised to 0 °C and then gradually raised to 18 °C to
permit sublimation. The uncoated and coated paper were analyzed by applying a 1 μL droplet of
a mixture of 2.5 mg/mL crystal violet and 0.25 mg/mL Ponceau S in buffered pH 10 solution
onto the paper and then placing one end of the paper in pH 10 buffer solution to promote
separation as the mobile phase wicked vertically up the paper.
To increase the adhesion of the sponge coatings, first a 400 nm dense PGMA layer was
deposited with the traditional iCVD process. Silicon wafers were placed on a TEC set to 10 °C,
TBPO and GMA were flowed into the system at rates of 0.6 sccm and 0.9 sccm, respectively,
and the reactor pressure was maintained at 60 mTorr. The filament was resistively heated to 220
°C to initiate polymerization. The thickness of the PGMA anchoring layer was monitored via an
in situ interferometer using a He–Ne laser (Industrial Fiber Optics, 633 nm). The filament was
turned off, the TEC was lowered to -20°C, TBPO flow was increased manually to 1.2 sccm, and
the reactor pressure increased to 350 mTorr. MAA was introduced into the system at a flow rate
of 3.0 sccm for 3 minutes. MAA flow was stopped, the TEC temperature was raised to -10 °C,
and the filament was set to 220 °C for 30 minutes to initiate polymerization. The filament was
turned off, the TBPO flow was stopped, and the sample sublimated for 30 minutes at 0 °C. For
the deposition of the top dense PGMA layer, TBPO flow was lowered manually to 0.6 sccm,
GMA was reintroduced at a flow rate of 0.9 sccm, the TEC was set to 10 °C, and the reactor
pressure set to 60 mTorr. The filament was resistively heated and polymerization proceeded for
40 minutes. The coating was annealed on a hot plate in air at 120 °C for 30 minutes. Tape tests
were performed using Scotch® (Magic) tape. An index finger was used to place tape on the

16
sample and then the tape was quickly removed. Each subsequent tape tests used a new piece of
tape.
For the fabrication of the hierarchical bilayer structure, the processing conditions for the
PMAA sponge and pillar were combined using a one-pot system. The bottom sponge layer was
fabricated through sequential crosslinking to form P(MAA-co-EGDA). First, the TEC was set to
-20 °C and MAA was introduced for 5 minutes. The TEC was increased to -10 °C and the
filament was resistively heated to 220 °C to begin polymerization. After 15 minutes of
polymerization, EGDA was flowed into the reactor at a rate of 0.2 sccm and allowed to flow for
15 minutes. TBPO and EGDA flows were stopped, the filament turned off, and the TEC was
raised to 0 °C and slowly to 18 °C to allow sublimation. Then, in situ the procedure for the
fabrication of PMAA pillars was followed.
Superhydrophobic PMAA sponges were fabricated by adding a 400 nm layer of
1H,1H,2H,2H-poly(perfluorodecyl acrylate) (PPFDA) using traditional iCVD. TBPO and PFDA
were flowed in at rates of 1.0 sccm and 0.3 sccm, respectively, and the reactor pressure set to 100
mTorr. The stage was set to 30 °C and the filament was set to to 240 °C to initiate
polymerization. The film thickness was monitored in situ on a reference silicon wafer. Scanning
electron microscopy (SEM Topcon Aquila, NOVA NanoSEM) with an accelerating voltage of 10
kV was used to visualize the samples. ImageJ, version 1.53k software was used to analyze the
SEM images to determine the porosity and pore sizes of the sponge structure. Average porosity
was determined through threshold analysis over 5 different images and pore sizes were measured
through manual selection of 1000 pores. Prior to SEM imaging, a layer of gold was sputter
coated on to all samples to prevent charging. Contact angle goniometry (ramé-hart 290) was used
to characterize the hydrophobicity of the fluorinated samples.

17
2.4 Results and Discussion
In the conventional iCVD process, monomer and initiator vapors are fed into a reactor
under vacuum and the monomer molecules adsorb to the surface of the substrate. A filament
array located inside the reactor is heated to thermally cleave the initiator molecules into free
radicals.28,29 These initiator radicals react with the adsorbed monomer molecules to begin
polymerization. Since the monomer partial pressure is kept below the saturation pressure,
polymerization results in dense polymer films. Our group has previously shown that we can
synthesize porous pillared membranes by modifying the initiated chemical vapor deposition
(iCVD) process parameters. The adjusted parameters include keeping the monomer partial
pressure below the triple point pressure and keeping the substrate temperature below the freezing
point of the monomer. 30,31 To fabricate the porous pillared PMAA coatings, first MAA monomer
was deposited onto a silicon wafer located on a thermoelectric cooler (TEC) set to -20 °C for 5
minutes, then the TEC temperature was raised to -10 °C, and the filament was turned on for 30
minutes to break the initiator molecules and initiate polymerization. Using a reactor pressure of
600 mTorr leads to the monomer depositing as frozen solid pillars. After polymerization, the
unreacted solid monomer was sublimated leaving a porous PMAA membrane with a pillar-like
structure as shown by the scanning electron microscope (SEM) image in Figure 2-1a. The
PMAA pillars possess dual-scale porosity with larger scale pores averaging tens of microns that
form from the spacing between the pillars and smaller micron size pores that form within the
pillars as a consequence of monomer sublimation, which is consistent with our previous
studies.32,33 In this paper, we demonstrate that decreasing the reactor pressure from 600 mTorr to
350 mTorr while maintaining all other conditions forms sponge-like structures as shown in
Figure 2-1b. The decrease in reactor pressure results in an increase of the monomer flow rate

18
from 0.3 sccm to 3.0 sccm which causes the monomer to condense before freezing since the local
monomer pressure increases above the triple point pressure. In comparison to the pillared
structure shown in Figure 2-1a, the sponge structure shown in Figure 2-1b possesses large void
spaces ranging from ten to one hundred microns and pore interconnectivity. The sponge structure
also has varying heights in comparison to the pillared structure as shown by the cross-sectional
SEM images in Figure 2-1c, d. Using ImageJ software to analyze the SEM images, the PMAA
sponge structure was determined to have an average porosity of 30%, an average pore size of 15
microns, and a nonuniform pore size distribution with diameters ranging from 1 to 100 microns.
The majority of the pores were smaller pores ranging from 1 to 10 microns found within the
larger pores.

19
Figure 2-1. SEM images of a) pillared structure formed at a reactor pressure of 600 mTorr. b)
sponge structure formed at a reactor pressure of 350 mTorr. Cross-sectional SEM images of c)
pillared structure and d) sponge structure.
Freestanding porous polymer films have applications as filters, membranes, tissue
scaffolds, and drug delivery systems.34,35 The sponge-like structure is advantageous for
biological applications because high porosity and pore interconnectivity allow for high drug
loading capacity.5,20 To fabricate freestanding polymer sponges that have pH-dependent swelling,

20
the crosslinker molecule ethylene glycol diacrylate (EGDA) was introduced simultaneously with
MAA during the fabrication process to form chemical crosslinks in order to prevent dissolution.
It has been shown that increasing the degree of crosslinking with EGDA improves mechanical
properties.36,37 We have previously shown that crosslinking with EGDA increases the mechanical
strength of the porous pillared PMAA coating to prevent cracking of the coating after thermal
annealing when deposited on flexible substrates such as polynorbornene rubber.36 For this study
and for practical bioapplications a limited degree of crosslinking is preferred to prevent
dissolution while maintaining the properties of the primary polymer.38 After 5 minutes of EGDA
and MAA monomer deposition, the MAA flow is stopped and the EGDA continues to flow for
the first 15 minutes of polymerization to ensure sufficient crosslinking. Crosslinking the sample
did not alter the sponge morphology as shown in Figure 2-2a. These crosslinked P(MAA-coEGDA) films easily delaminated from the silicon wafer substrate and were able to be cut into
freestanding films of 8 mm diameter using a biopsy punch. Simultaneous crosslinking allowed
delamination of the sponge likely because introducing EGDA and MAA simultaneously resulted
in a condensed EGDA monomer layer that did not freeze and thereby prevented the sponge layer
from adhering to the silicon wafer substrate. To confirm that crosslinking prevented the
dissolution of the freestanding sponge film, the P(MAA-co-EGDA) films were placed in buffer
solutions of pH 4 and pH 10. In both solutions, the films did not dissolve. In pH 4 buffer (Figure
2-2b), the film exhibited minimal swelling; however, in pH 10 buffer (Figure 2-2c), the film
swelled considerably. Swelling ratios of PMAA hydrogels are reported to abruptly increase in pH
solutions greater than 5.39 The reported pKa value of PMAA is 4.840,41; therefore, in pH solutions
greater than 5, the carboxylic acid group in PMAA ionizes causing electrostatic repulsion
between the negatively charged groups resulting in swelling of the polymer material. This pH-

21
dependent swelling is particularly useful for site-specific delivery of acid-degradable drugs and
enables PMAA to be an enteric polymer to control drug delivery. 23,26,38
Figure 2-2. a) SEM image of a freestanding P(MAA-co-EGDA) film (inset: dry 8 mm diameter
film). b) Image of the freestanding film after 5 seconds in pH 4 buffer. c) Image of the
freestanding film after 5 seconds in pH 10 buffer.
The carboxylic acid group in PMAA is also useful for separation applications since the
acid-base interactions are useful in the adsorption and removal of certain pollutants.24,25 To
demonstrate selective separation, we deposited the P(MAA-co-EGDA) sponge coating on
chromatography paper to fabricate a functionalized microfluidic device. Paper-based
microfluidic devices are low-cost diagnostic devices for point-of-care applications.42,43 The

22
devices can be improved through modification of the paper which allow for enhanced separation
of analytes. For example, the uncoated paper (Figure 2-3a) was unable to isolate the different
dyes in a mixture of crystal violet (purple) and Ponceau S (pink) as shown in Figure 2-3c, while
the coated paper (Figure 2-3b) was able to trap the cationic crystal violet due to electrostatic
interactions between the anionic PMAA and the cationic crystal violet (Figure 2-3d).
Figure 2-3. SEM image of a) paper before coating and b) after coating with porous sponge
P(MAA-co-EGDA). Images of a mixture of Ponceau S (pink) and crystal violet (purple) flowing
through the c) uncoated paper and d) coated paper.
Porous sponge coatings are useful in a variety of applications because they provide
chemical functionality without affecting the properties of the bulk material. For example, porous
polymer coatings have been applied to microneedles to improve transdermal drug delivery.44
The benefit of our bottom-up, vapor-phase fabrication process is the capability to modify the
surfaces of a wide variety of substrates without the use of harmful solvents. Since PMAA is a

23
hydrophilic polymer that easily dissolves in aqueous environments, annealing is often used to
control the dissolution behavior of the polymer in different pH environments.33,45 In order to
tune the dissolution of the PMAA sponge coating, the coatings were first deposited onto silicon
wafers and then annealed at 175 °C for 20 minutes. During the annealing process, intramolecular
condensation reactions convert some of the water soluble methacrylic acid groups to methacrylic
anhydride (MAN) groups, which are less soluble. This results in the formation of the copolymer
poly(methacrylic acid-co-methacrylic anhydride) P(MAA-co-MAN).46 The annealing process
did not change the sponge structure of the coatings as shown in the SEM images before and after
annealing (Figure 2-4a and Figure 2-4b), but did slightly affect the color of the coatings. To
determine how the annealing process affects the solubility of the P(MAA-co-MAN) sponge
coatings, the annealed samples were placed in pH 4 and pH 10 buffer solutions. The coating
disappeared after only 20 minutes in pH 10 solution which is expected since PMAA is more
soluble in basic solutions whereas the coating required 7 days to dissolve in pH 4 solution. The
annealing process helps control dissolution of the sponge coating through partial conversion of
MAA to MAN, but does not prevent dissolution since the sample is not covalently crosslinked.
The P(MAA-co-MAN) coating can be permanently crosslinked by exposure to strongly
nucleophilic 1,3-diaminopropane vapors.19,33,47

24
Figure 2-4. a) SEM image of the sponge coating before annealing (inset: image of coating on a
silicon wafer) and b) SEM image of the sponge coating after annealing (inset: image of annealed
coating on a silicon wafer).
For applications such as oil/water separation48 and wound dressings33,49, it is important
for the porous coating to have good adhesion to the underlying substrate. We have previously
shown that we can increase the adhesion of the pillared coatings shown in Figure 1a to
underlying substrates through the incorporation of the epoxide-containing glycidyl methacrylate
(GMA) monomer into the fabrication process.36 We examined whether this approach could also
work to enhance the adhesion of the sponge coatings to the underlying substrate. First, 400 nm of
a dense poly(glycidyl methacrylate) (PGMA) base layer was deposited onto a silicon wafer using
the traditional iCVD process. Then, the PMAA sponge was deposited onto the PGMA layer.
After sublimation of the unreacted MAA monomer, an additional dense layer of PGMA was
deposited on top of the PMAA sponge coating using the traditional iCVD process. The addition
of a top layer of dense PGMA onto the PMAA sponge did not affect the morphology of the
coating as shown in Figure 2-5a. Annealing was performed on a hot plate in air at 120 °C for 30
minutes. The dense PGMA base layer helps provide anchoring to the substrate through a reaction

25
between the epoxide group present in PGMA and the hydroxyl groups on the silicon wafer.36,50,51
The PGMA adheres to the sponge layer by annealing at a temperature higher than the glass
transition temperature of PGMA (61.3 °C) which promotes diffusion and entanglement of the
PGMA chains with PMAA.36,50 Adhesion of the sponge to the underlying silicon wafer was
confirmed by a tape test. Scotch® tape was applied with an index finger to contact the sample
and then quickly removed. A fresh piece of tape was used after every test. The annealed sponge
coating demonstrated good adhesion to the substrate by passing 5 tape tests as shown in Figure
2-5b. During the sixth tape test, a corner of the coating was removed by the tape. Without the
annealed PGMA base and top layers, the PMAA sponge coating fails the tape test immediately.

26
Figure 2-5. a) Top-down SEM image of the sponge coating fabricated on a dense 400 nm layer
of PGMA (inset: picture of the coating on the silicon wafer before the initial tape test). b) Tape
test results.
The iCVD parameters such as stage temperature and reactor pressure can be tuned in situ
to allow for the synthesis of unique structures. For example, we were able to synthesize a bilayer
hierarchical structure by combining our pillar and sponge fabrication conditions in situ. These
types of hierarchical structures are commonly found in nature such as on the Lotus leaf and

27
gecko feet and serve as inspiration for synthetic research because of their advantageous surface
properties such as enhanced hydrophobicity and adhesion.52,53,54 Hierarchical structures are
characterized by a combination of nano- and macroscopic length scales which provide high
surface area and high flux which are promising properties for applications in catalysis,
separations, and bioengineering.2,16,54 To make our bilayer hierarchical structure, we used the
process shown in Figure 2-6a. First, a crosslinked sponge layer was formed. Crosslinking is
important in order to prevent dissolution of the sponge layer during the fabrication of the pillared
layer. In contrast to the fabrication process to make crosslinked freestanding films, the EGDA
was introduced sequentially after the MAA instead of simultaneously to ensure that the coating
did not delaminate. To fabricate the crosslinked sponge, first MAA was deposited for 5 minutes
at a reactor pressure of 350 mTorr with a corresponding monomer flow rate of 3 sccm and then
the filament was turned on to begin polymerization. After 15 minutes, EGDA was brought in for
the final 15 minutes of polymerization. The unreacted MAA monomer was then allowed to
sublimate. After sublimation, PMAA pillars were deposited onto the sponge layer in situ by
depositing MAA for 5 minutes at a reactor pressure of 600 mTorr with a corresponding monomer
flow rate of 0.3 sccm. The filament was turned on for 30 minutes to allow for polymerization and
then the unreacted MAA monomer was sublimated. The SEM images in Figure 2-6b and Figure
2-6c confirm the fabrication of a bilayer coating consisting of a bottom sponge layer and a top
pillared layer. The bilayer coating has a total thickness of approximately 200 microns which is
expected since the sponge layer is approximately 90 microns thick under similar processing
conditions and the pillar layer is approximately 100 microns thick under similar processing
conditions. To confirm the synthesis of two layers, the bilayer sample was placed in pH 10 buffer
solution. The top pillar layer dissolved as expected since it was not crosslinked, and the

28
crosslinked sponge layer remained. The SEM image of the dried bottom sponge layer shown in
Figure 2-6d,e is only 50 microns thick because the structure collapsed after submersion in the
buffer solution and subsequent drying.

29
Figure 2-6. a) Schematic representation of the fabrication process to form a hierarchical bilayer
structure of a sponge bottom layer and a pillared top layer. b) Top-down SEM image of the
bilayer coating showing the top pillared layer. c) Cross-sectional SEM image of the bilayer

30
coating. Dashed line indicates the division between the sponge and pillared layers. d) Top down
and e) cross-sectional SEM image of the collapsed bottom sponge layer after the dissolution of
the top pillared layer in pH 10 solution.
By adding a low surface energy coating onto the sponge structures, we can synthesize
superhydrophobic sponge coatings and freestanding sponge films that retain their physical
structure after exposure to aqueous solutions. Superhydrophobic coatings are useful in providing
self-cleaning properties against contaminants.55,56 Superhydrophobic materials are typically
synthesized by combining a rough surface with a low surface energy material.53,57 The large void
spaces and varying heights of our sponge coatings and free-standing sponge films provide ideal
surface roughness for creating superhydrophobic structures. Compared to other fabrication
processes, our synthesis method is one-pot because we can combine our modified iCVD process
with our traditional iCVD process to coat the sponge structures with the fluoropolymer
poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA). As shown in Figure 2-7a,b, modifying the
PMAA sponge coating with 400 nm of dense PPFDA using the traditional iCVD process did not
change the underlying structure of the sponge coating. The static contact angle and hysteresis of
the coated sponge was found to be 144 ± 4° and 6°, respectively. (Figure 2-7b inset). The
addition of the PPFDA coating allows for preservation of the sponge-like structure in aqueous
environments. For example, as shown in Figure 5e, after being submerged in buffer solution, the
crosslinked sponge base layer did not dissolve, however it did collapse, losing its original
structure. Figure 2-7c shows an SEM image of a freestanding P(MAA-co-EGDA) sponge film
coated with 400 nm of PPFDA using the traditional iCVD process that retained its sponge-like
structure after being submerged in pH 4, pH 10, and deionized water. The superhydrophobic

31
freestanding film repelled water droplets and remained floating when placed in water as shown
in Figure 2-7d.
Figure 2-7. SEM image of PMAA sponge coating a) before and b) after coating with PPFDA
(inset: Contact angle image). c) SEM image of freestanding P(MAA-co-EGDA) sponge film
coated with PPFDA after submersion in DI water. d) Freestanding P(MAA-co-EGDA) sponge
film
2.5 Conclusion
In this work, we fabricated PMAA sponge coatings and freestanding films by modifying
the initiated chemical vapor deposition process. The porous morphology of the sponge is
attractive for applications such as separations and drug delivery because of the range in pore

32
sizes and pore interconnectivity. The carboxylic acid present in PMAA provides the sponge with
a pH-responsive behavior that is beneficial for enteric drug release systems and environmental
remediation of pollutants. We demonstrated that we can fabricate sponges with pH-responsive
dissolution and swelling through thermal annealing and chemical crosslinking, respectively. We
were able to improve the adhesion of the coatings to the underlying substrate via addition of the
epoxide containing monomer glycidyl methacrylate into the fabrication process. By adjusting in
situ the process parameters of the modified iCVD process, we were able to synthesize a bilayer
coating consisting of a sponge bottom layer and a pillared top layer in situ. In addition, we were
able to provide additional chemical functionality to the PMAA sponge coatings and freestanding
films through a top coating of poly(1H,1H,2H,2H-perfluorodecyl acrylate) using a one-pot
system.
2.6 Acknowledgment
This work was supported by a fellowship from the Chevron Corporation (USC-CVX UPP).

33
2.7 References
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Multifunctional Material Platforms toward Task-Specific Applications. Advanced
Materials 2018, 31 (4), 1802922.
2 Parlett, C. M. A.; Wilson, K.; Lee, A. F. Hierarchical Porous Materials: Catalytic
Applications. Chemical Society Reviews 2013, 42 (9), 3876–3893.
3 Sun, B.; Long, Y.-Z.; Yu, F.; Li, M.-M.; Zhang, H.-D.; Li, W.-J.; Xu, T.-X. Self-Assembly of a
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Chapter 3. Influence of Oblique Angle Deposition on Porous Polymer Film
Formation
3.1 Abstract
In this study, we applied oblique angle deposition to a modified initiated chemical vapor
deposition (iCVD) process to synthesize porous poly(methacrylic acid) (PMAA) films. During
the modified iCVD process, frozen monomer is first captured on a cooled substrate, then
polymerization occurs via a free radical polymerization mechanism, and finally the excess
monomer is sublimated resulting in a porous polymer film. We found that delivering the
monomer through an extension at an oblique angle resulted in porous films with three
morphological regions. Region 1 is located nearest to the monomer extension outlet and consists
of porous polymer pillars, region 2 consists of densified pillars which occur due to recapturing
and polymerization of the sublimated monomer, and region 3 is located furthest from the
monomer extension outlet and consists of dendritic structures which occur due to low monomer
concentration. We investigated the role of substrate temperature and monomer deposition time on
the growth process. We found that changing the extension angle influenced the location of the
regions and the film coverage across the substrate. Our results provide useful guidelines for
tuning the structures within porous polymer films by varying the angle of monomer delivery.
3.2 Introduction
Oblique angle deposition (OAD) has been used extensively to fabricate porous inorganic
films via physical vapor deposition (PVD).1,2,3 During OAD, the material flux is delivered at an
oblique angle relative to the surface of the substrate which typically results in oriented columns.
Similar to OAD, glancing angle deposition (GLAD) is often used to fabricate nanostructured

39
semiconductor films with helical, zig-zag, and S-shaped patterns by rotating the substrate while
the material flux is being delivered at an oblique angle.4,5,6 OAD and GLAD are often combined
with PVD of inorganic materials such as metals, oxides, and semiconductors because the
shadowing effect results in isolated structures. Recent studies have incorporated OAD and
GLAD with the chemical vapor deposition (CVD) of organic materials to fabricate structured
parylene-c (PPX-C) nanorods.7,8,9,10 CVD is generally used to generate conformal coatings
because of the small sticking coefficient of the incident flux which causes redistribution to the
shadowed regions, however He et al. showed that the oblique angle polymerization of PPX-C
through CVD results in isolated structures because of the high growth rate at the tips of the
surface structures combined with the significantly lower growth rate in the shadowed region.11,12
Continuing to understand the growth mechanism of organic materials using OAD is important
for the development of advanced polymer materials. We recently extended the concept of OAD
to the initiated chemical vapor deposition (iCVD) process.13 During the iCVD process,
monomer and initiator vapors are introduced into a reactor chamber and a heated filament array
thermally cleaves the initiator molecules into free radicals which react with adsorbed monomer
molecules to initiate polymerization.
14,15 Our study showed that combining OAD with the iCVD
process led to the formation of patterned dense films with thickness gradients and the monomer
flow rate and vapor flow angle impacted the location of the thickest point of the films.
In this study, we applied OAD to a modified iCVD process to study the growth of porous
poly(methacrylic acid) (PMAA) films. To synthesize porous polymer materials using the
modified iCVD process, a thermoelectric cooler is added to the reactor to decrease the
temperature of the substrate below the freezing point of the monomer to promote solidification of
the monomer vapor.
16,17,18,19 After initial monomer capture, the filament is heated to cleave the

40
initiator molecules in order to start the polymerization process. After polymerization, the
filament is turned off and the unreacted monomer is sublimated. In our current work, the
monomer flow was delivered through an extension at different angles with respect to the
substrate. We found that delivering the monomer using the extension resulted in porous films
with three regions of distinct morphologies. Changing the extension angle influenced the
location of the regions and the film coverage across the substrate.
Understanding the formation of porous polymer materials is advantageous for the
advancements of new materials. For example, porous polymer materials have been utilized for a
range of applications including separations, catalysis, and biomedical applications.20,21,22
Significant advancements have been made to synthesize materials with carefully tuned pore sizes
and structures, however these synthesis methods face challenges since they generally require
multiple time-consuming steps and often involve the removal of an organic solvent phase or
porogen to generate the size-specific voids.23,24 Our modified iCVD process allows us to
synthesize porous structures via a one-pot vapor phase technique, offering a simple and
sustainable route free from solvents and solution-phase processing challenges such as solvent
compatibility and surface tension effects. In a single processing step, we are able to synthesize
porous films with a combination of pore sizes and structures while simultaneously providing
chemical functionality useful for applications such as adsorption of contaminants and drug
delivery due to the carboxylic acid group present in PMAA films.25,26,27,28
3.3 Experimental Section
Methacrylic acid (MAA, 99%) and tert-butyl peroxide (TBPO, 98%) were purchased and
used as received from Sigma-Aldrich without further purification. The porous polymer films
were deposited on silicon wafers (Wafer World) using a custom-designed cylindrical reactor

41
chamber (GVD Corporation; 25 cm diameter, 5 cm height). The substrate was cooled by setting
the reactor stage to 10°C using a recirculating chiller (Thermo Scientific NESLAB RTE 7) and
additional cooling was achieved using a thermoelectric cooler (TEC) (Custom Thermoelectric)
controlled by an adjustable DC power supply (Volteq HY3010D). The reactor pressure was
maintained under vacuum conditions through use of a rotary vane vacuum pump (Edwards
E2M40) and a throttle valve controller (MKS 153D) and was measured by a capacitance
manometer (MKS 622C01TDE Baratron).
The TBPO flow rate was metered using a mass flow controller (MKS 1479A) and
maintained at 0.6 sccm for all experiments. The MAA flow was controlled using a needle valve
and was directed through a monomer extension consisting of ¼ in. inner diameter stainless steel
tubing. The monomer extension was sealed to the monomer inlet of the reactor chamber using
polytetrafluoroethylene tape (1/4 in., TaegSeal). The monomer extension had bend angles of
150°, 130°, 110°, and 90° that correspond to angles of 30°, 50°, 70°, and 90° with respect to
delivery to the substrate. The height of the monomer extension was held constant for all four
monomer extension angles and the line-of-sight distance was 12 mm, 8 mm, 7 mm, and 6 mm
from the monomer extension outlet to the substrate for the angles of 30°, 50°, 70°, and 90°,
respectively.
To fabricate porous PMAA coatings using the modified iCVD process, first TBPO was
introduced at 0.6 sccm. Then, the reactor pressure was set to 600 mTorr for monomer capture and
polymerization. For the typical OAD experiments, the MAA flow rate was introduced through
the monomer extension into the reactor at a flow rate of 0.05 sccm. During monomer capture, the
TEC temperature was set to -20 °C. After 4 minutes of monomer capture, the MAA flow was
halted, the temperature of the TEC is increased to -10 °C, and the filament array (80% Ni, 20%

42
Cr, Omega Engineering) was heated to 240 °C to break the initiator molecules in order to begin
polymerization. After 40 minutes of polymerization, the TBPO flow was stopped and the TEC
temperature was increased to 0 °C to allow the unreacted monomer to sublimate. During
sublimation, the TEC temperature was slowly increased to room temperature and complete
sublimation of the monomer was verified by the reactor pressure returning to a base pressure of
22 mTorr. To test the effect of the monomer capture temperature and the monomer capture time,
the TEC temperature was set to -10 °C and 0 °C and shorter capture times of 1 and 2 minutes
were used. To verify polymer dendrite growth, all experimental conditions remained the same
except the monomer extension was removed. To verify polymer pillar growth without the use of
the monomer extension, the MAA flow rate was increased to 0.3 sccm while maintaining all
other conditions.
The porous PMAA films were imaged using scanning electron microscopy (NOVA
NanoSEM) with an accelerating voltage of 10 kV. A layer of platinum was sputtered onto the
films prior to SEM imaging to prevent charging. ImageJ version 1.53k software was used to
compare the thickness of the pillared region grown with a 30° monomer extension and without
the monomer extension. Optical images of the porous PMAA films were taken in situ and after
removal from the reactor using an iPhone 12 Pro camera.
3.4 Results and Discussion
The monomer flow was delivered through an angled extension with respect to the
substrate as shown in Figure 3-1a where X shows the geometric center which is the line-of-sight
impingement point of the monomer flow and A is the shortest path from the outlet of the
extension to the substrate. To study the effect of OAD on the growth of the porous PMAA
polymer film, we first used a 30° monomer extension and a monomer flow rate of 0.05 sccm. A

43
low flow rate was used with the extension since our preliminary experiments showed that high
flow rates generate a high localized pressure that forces the monomer to condense rapidly on the
surface. This phenomenon was confirmed by our previous computational fluid dynamics model
that showed a pressure spike near the extension outlet.13 The monomer was initially captured and
frozen on a substrate cooled to -20°C. After 4 minutes of monomer capture, the monomer flow
was halted, the substrate temperature was increased to -10°C, and the filament array was heated
to 240°C to cleave the initiator molecules to begin polymerization of the solid monomer. After
40 minutes of polymerization, the initiator flow was stopped, the filament array was turned off,
and the unreacted monomer was allowed to sublimate. As shown in Figure 3-1b, delivering the
monomer using the extension resulted in porous films with three regions of distinct
morphologies due to spreading of the monomer during polymerization as shown by the
comparisons of the sample before and after polymerization (Figure 3-1c and 3-1d). Figure 3-1c
shows the region of initial monomer capture before polymerization. Figure 3-1d shows that
some of the initially captured monomer is sublimated and recaptured to areas of less monomer
concentration during the polymerization process. The resulting polymer film is therefore spread
outward from the initial region of monomer capture as shown in Figure 3-1e.

44
Figure 3-1. a) Schematic of the extension used for oblique angle monomer delivery. b)
Schematic of the three regions of distinct morphologies. In-situ optical images using a 30°
monomer extension c) after 4 minutes of initial monomer capture at a substrate temperature of -
20°C and d) after 40 minutes of polymerization at a substrate temperature of -10°C. e) The
resulting porous PMAA film after sublimation and removal from the reactor. (Blue-Region 1,
Red- Region 2, Yellow-Region 3)
Scanning electron microscopy (SEM) was used to analyze the films and it was found that
the film consisted of three regions of different morphologies. Region 1 is a circular region found
below the monomer extension outlet encompassing the points A and X as shown in Figure 3-1b.
The thickest point of the film occurs between points A and X as a result of the low monomer
flow rate which is consistent with our previous study of the growth of dense poly(2-hydroxyethyl
methacrylate) films using the monomer extension at low monomer flow rates.13 Region 1 is
characterized by pillars as shown in Figure 3-2a which is similar to those formed in our earlier

45
studies in the absence of the monomer extension at higher flower rates.17,18,29,30,31 The pillars do
not appear to have any preferential orientation with respect to the angle of delivery unlike OAD
of oxides, metals, and parylene-c which result in highly structured films.7,8,9,10,11 Instead of
orienting relative to the angle of monomer delivery, the polymer pillars in our films orient
parallel to the direction of freezing which is perpendicular to the surface. This is consistent with
freeze-casting studies that use unidirectional freezing to synthesize aligned porous polymer
structures.32,33
Region 2 is located in a concentric ellipse outside of region 1 and consists of denser
pillars (Figure 3-2b) in comparison to the pillars found in region 1. The denser pillars are caused
by monomer sublimating from region 1 and recapturing to areas of lower monomer
concentration. The most nucleation occurs directly below the extension outlet in region 1
resulting in more defined three-dimensional pillar structures whereas in region 2 the monomer
concentration is lower resulting in less nucleation on the surface which results in denser, twodimensional structures. Region 3 (Figure 3-2c) occurs furthest away from the extension and
consists of dendrites, or tree-like growths. The monomer concentration is the least in region 3
and the dendrites form as a result of small, scattered nucleation sites that continue to grow during
polymerization. The polymer dendrites form similar to ice crystal dendrites as a result of
branching instability, or when an initial crystal sprouts branches as a result of the ice growth
being diffusion limited.34,35

46
Figure 3-2. SEM images of a) region 1, b) region 2, and c) region 3 of the porous PMAA film
deposited using a 30° monomer extension and a monomer capture time of 4 minutes at a
substrate temperature of -20°C followed by polymerization for 40 minutes at a substrate
temperature of -10°C.
In order to study polymer growth with and without the extension, we deposited without
the monomer extension using flow rates of 0.05 sccm and 0.3 sccm while holding all other
parameters constant. In the iCVD process, the monomer surface concentration is proportional to
the partial pressure of the monomer which is a function of the monomer flow rate and substrate
temperature.36,37 At 0.05 sccm, there is a low monomer concentration across the entire reactor
which leads to the formation of dendrites across the whole substrate as shown in Figure 3-3a.
Since these dendrites are similar to those formed in region 3 using the extension at the same flow
rate, it can be confirmed that region 3 has a low monomer concentration which leads to dendrite
formation. At 0.30 sccm, there is a higher monomer concentration across the entire reactor which
leads to the formation of uniform pillars across the whole substrate as shown in Figure 3-3b.
Since these pillars are similar to those formed in region 1 using the extension at lower flow rate
of 0.05 sccm, it can be confirmed that region 1 has a high monomer concentration leading to
pillar growth. Despite the higher monomer flow rate, the porous PMAA pillars grow at a rate of

47
approximately 25 microns per minute without the monomer extension, in comparison to
approximately 90 microns per minute at the thickest point in region 1 with the use of the 30°
monomer extension at a lower monomer flow rate because the monomer extension generates a
high local monomer concentration in region 1 which leads to taller pillars.
Figure 3-3. Optical images of a PMAA film grown without the monomer extension using a
monomer flow rate of a) 0.05 sccm and b) 0.3 sccm. SEM images of the corresponding c) PMAA
dendrite formed at 0.05 sccm and d) PMAA pillars formed at 0.3 sccm.
Next, we studied the effect of different substrate temperatures during initial monomer
capture by depositing the MAA monomer at -20°C, -10°C, and 0°C using the 30° monomer
extension while holding all other conditions constant. The monomer was captured for 4 minutes
(Figure 3-4a,b,c), the substrate temperature was increased to -10 °C, and then polymerization
proceeded for 40 minutes. In Figure 3-4a, there is minimal monomer capture on the wafer after 4

48
minutes of monomer deposition at the highest substrate temperature of 0°C, while the most
monomer is captured at -20°C (Figure 3-4c) since the lower substrate temperature promotes
more monomer capture which is consistent with our earlier studies without the monomer
extension.17,30,31 Before polymerization, the substrate temperature is set to -10°C for all of the
samples; therefore, it is important to note that the substrate temperature is decreased for the
sample captured at 0°C, maintained for the sample captured at -10°C, and increased for the
sample captured at -20°C. Despite these adjustments to the substrate temperature, all three
samples exhibit spreading during polymerization as a result of the monomer sublimating and
recapturing (Figure 3-4d,e,f).

49
Figure 3-4. Effect of the substrate temperature on monomer capture using a 30° monomer
extension. In situ optical images of the silicon wafer after 4 minutes of monomer capture at
substrate temperatures of a) 0°C, b) -10°C, and c) -20°C and corresponding images after 40
minutes of polymerization at -10°C (d, e, f).
The SEM images in Figure 3-5 show that the different monomer capture temperatures did
not affect formation of the three morphological regions. Three-dimensional pillars (Figure 3-
5d,e,f), densified pillars (Figure 3-5g,h,i), and dendrites (Figure 3-5j,k,l) were found on all three
samples. The major difference between the three samples is the location of each region as shown

50
in Table 3-1. The distance of each region was measured from the left-edge of the silicon wafer.
Region 1 for all three capture temperatures is located in approximately the same area since the
monomer extension was maintained in the same position. All three regions follow the same trend
of increasing along the x-direction with decreasing substrate temperature since lowering the
temperature led to better surface coverage. Smaller dendrites could be formed at a monomer
capture temperature of -20°C as shown in Figure 3-5l since region 3 extends far from the
extension outlet.
Figure 3-5. Optical images of the porous PMAA films made using a 30° monomer extension
with a monomer capture temperature of a) 0°C, b) -10°C, and c) -20°C and corresponding SEM
images of (d,e,f) region 1, (g,h,i) region 2, and (j,k,l) region 3 indicated in blue, red, and yellow,
respectively.

51
Table 3-1. Lengths of region 1, 2, and 3 at different monomer capture temperatures.
Monomer Capture
Temperature (℃)
Region 1 (mm) Region 2 (mm) Region 3 (mm)
0 6-10 4-13 0-18
-10 6-11 4-14 0-20
-20 6-13 4-19 0-25
Figure 3-6. Effect of the monomer capture time using a 30° monomer extension and a substrate
temperature of -20°C. In situ optical images of the silicon wafer after a) 1 minute of monomer
capture and, b) 2 minutes of monomer capture and corresponding images (c,d) after 40 minutes
of polymerization at -10°C.

52
Figure 3-7. Optical images of the porous PMAA films made using a 30° monomer extension
after a) 1 minute of monomer capture and b) 2 minutes of monomer capture. SEM images of
region 1 (c,d), region 2 (e,f), and region 3 (g,h) are indicated in blue, red, and yellow,
respectively.
To study the effect of the monomer capture time, monomer was deposited for 1 minute
and 2 minutes using a monomer capture temperature of -20°C and the 30° monomer extension
while holding all other conditions constant. The lower capture temperature of -20°C was used
since it promotes greater nucleation on the substrate (Figure 3-6a,b). The SEM images in Figure
3-7 verify the presence of the three regions, however, the structures in all regions display open
porosity compared to the samples formed by capture of monomer for 4 minutes. In addition, the
sample made using 1 minute of monomer capture contained intermediate structures composed of

53
dendritic edges with pillars in the middle found in region 2 at less magnification as shown in
Figure 3-8 which were not seen for the samples with longer capture times of 2 and 4 minutes.
Figure 3-8. SEM image of the intermediate structures found in region 2 using 1 minute of
monomer capture.
We also studied the effect of the angle of the monomer extension. We deposited the
polymer using monomer extension angles of 30°, 50°, 70°, and 90° while holding all other
conditions constant. Monomer was captured for 4 minutes at -20°C and polymerization
proceeded for 40 minutes at -10°C. For all four angles of deposition, the SEM images (Figure 3-
9) of the three regions follow the same trends as with the 30° monomer extension. Region 1 is
characterized by pillar growth and the different monomer extension angles did not affect the
orientation of the pillars since the direction of freezing remained constant. Region 2 for all four
angles display pillar densification and region 3 for all four angles contain dendritic structures.
Although the SEM images of each region for the different angles are comparable, the locations
of the outer regions vary depending on the angle as shown in Table 3-2. The location and length
of region 1 is constant for all four angles of deposition since the extension is placed such that the

54
geometric center is the same for all four monomer extension angles. However, as the angle
decreases, regions 2 and 3 increase in size along the x-axis which is consistent with our previous
experimental results and computational fluid dynamics modeling of nonporous film growth using
oblique angle iCVD which showed broadening of the concentration profile as the angle of the
monomer vapor flow decreases.
Figure 3-9. Optical images of the porous PMAA films deposited at angles of a) 30° b) 50° c) 70°
and d) 90° using 4 minutes of monomer capture at a substrate temperature of -20°C followed by

55
40 minutes of polymerization at a substrate temperature of -10°C. Regions 1 (e,f,g,h), region 2
(i,j,k,l), and region 3 (m,n,o,p) are indicated in blue, red, and yellow, respectively, where X is the
geometric center.
Table 3-2. Lengths of region 1, 2, and 3 at different angles of deposition.
� (°) Region 1 (mm) Region 2 (mm) Region 3 (mm)
30 6-13 4-19 0-25
50 6-12 3-18 0-21
70 6-11 3-16 0-18
90 6-12 4-14 1-16
3.5 Conclusion
The effect of adding an oblique angle monomer inlet extension to the modified iCVD
process to synthesize porous PMAA films was studied. We found that the oblique angle delivery
system resulted in films with three different morphological regions. Region 1 resembled the
porous polymer pillars grown in the absence of the monomer extension. Region 2 displayed
pillar densification as a result of monomer sublimating and recapturing during polymerization
and region 3 contained dendritic structures as a result of low monomer concentration far from the
extension. We studied the effect of the substrate temperature and time during monomer capture
and compared four different monomer extension angles. Our results showed that decreasing the
substrate temperature led to better surface coverage. Decreasing the time of monomer capture
resulted in structures with more open porosity. The three morphological regions were found for
all four monomer extension angles, however, the film broadened along the x-direction as the
monomer extension angle decreased. The results of this study demonstrate that oblique angle
deposition can be used to fabricate three-dimensional porous polymer materials with different
morphologies.

56
3.6 Acknowledgement
We thank the Gabilan Distinguished Professorship in Science and Engineering for funding.
3.7 References
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Chapter 4. Vapor Phase Deposition of Porous Polymer Dendrites
4.1 Abstract
The synthesis of hierarchical structures with different length scales is useful for designing
self-cleaning surfaces, adhesives, templates for catalysis, and sensors. In this work, we study the
formation of three-dimensional porous poly(methacrylic acid) dendritic structures using vapor
phase deposition. These polymer dendrites have a hierarchical structure composed of branched
finger-like features with small scale pores inside these features. These structures are prepared by
limiting the concentration of methacrylic acid monomer introduced into the reactor which results
in diffusion-controlled nucleation and growth on the substrate. The nucleation of the dendritic
structures can be controlled through changes in surface energy of the substrate. For example,
physical defects can be used to enhance nucleation and manipulate the location of dendrite
growth, and the use of a low surface energy coating can be used to limit nucleation. The
coverage on the substrate and the thickness of the dendrites can be systematically tuned through
varying processing parameters. The results of this study provide a one-step, solventless synthesis
process for fabricating polymer materials that can be used to build next-generation surfaces and
templates.
4.2 Introduction
Hierarchical structures such as those found on the surface of a lotus leaf and on gecko
feet have features across multiple length scales that provide a high surface area ideal for sensors,
separations, catalysis, energy storage, and superhydrophobicity.1,2,3,4,5,6 Inspired by the lotus leaf
structure, Lu et al. created a highly porous low-density polyethylene surface with a high contact

60
angle of 173.0 ± 2.5° as a result of hierarchical micro- and nanostructures.7 Similarly, the selfcleaning nature of the lotus leaf has also been mimicked in the fabrication of surfaces with
antifouling and antimicrobial properties.8 Hierarchical surfaces are extensively reported for
superhydrophobic applications, but are promising for many other applications that benefit from a
high surface area and features of varying size scales. For example, Kim et al. synthesized a
hydrogen bonding catalyst using a melamine-based porous organic polymer with a hierarchical
structure that allowed for a large number of available bonding sites.5 Dendritic structures are a
type of hierarchical structure characterized by finger-like features, high levels of branching, high
surface area, and abundant edges and corners that are useful for a broad range of applications
including sensors, electronics, and catalysis.9,10,11 Dendritic structures form through diffusionlimited aggregation of particles during nonequilibrium processes which result in the complex,
branched structures.12,13,14,15 Polymer dendrites are generally synthesized via material specific,
solution-based methods such as self-assembly. For example, Sun et al. fabricated fractal patterns
by using a solvent evaporation process to self-assemble specific structural architectures of
nonlinear multihydrophilic block copolymers.16 These types of solution-based methods offer
control through aggregation of predefined geometries, but are limited by factors such as multiple
steps, solubility, and surface tension effects.16,17,18, In this study, we are able to generate
diffusion-limited conditions using a vapor-phase process to form dendritic polymer structures
with inner pores. The benefit of using a vapor-phase polymer processing method is that it is free
from templates, porogens, additives, and solvents. In addition, the low processing temperatures
and bottom-up synthesis allow for deposition onto temperature-sensitive substrates and threedimensional surfaces.19,20

61
Our method for fabricating porous polymer dendrites involves generating diffusionlimited conditions using a modified initiated chemical vapor deposition (iCVD) process. The
traditional iCVD process is typically used to fabricate thin, conformal polymer films by imitating
solution-phase free-radical polymerization in the vapor phase. During the iCVD process,
monomer and initiator gases are introduced into a vacuum reactor, monomer molecules adsorb
onto a cooled substrate, and a heated filament array thermally cleaves the initiator molecules into
free radicals which then react with the monomer molecules to begin polymerization. Our group
has shown that we can modify the iCVD process to fabricate porous poly(methacrylic acid)
(PMAA) coatings by lowering the substrate temperature below the freezing point of the
methacrylic acid (MAA) monomer. 21,22,23 The methacrylic acid vapor goes through a phase
change, then the solid monomer is polymerized, and the unreacted monomer is sublimated. We
have shown that we can adjust reactor conditions to fabricate porous polymer films with pillarlike24,25 and sponge-like morphologies.26 This modified iCVD process can be extended to other
iCVD monomers with relatively high freezing points, such as N-isopropylacrylamide, as
previously reported.21
We recently applied the concept of oblique angle deposition to our modified iCVD
process by adding an extension to deliver the monomer at an angle to the substrate which
resulted in porous polymer coatings with distinct morphological regions.27 The film closest to the
monomer extension outlet consisted of porous pillar-like structures resembling those grown in
the absence of the extension and dendritic structures were formed furthest away from the
extension as a result of low monomer concentration. In this work, we further investigate the
formation of these dendritic structure by limiting the monomer concentration across the entire
reactor and studying the shape of the structures formed at physical defects such as scratches. We

62
also observe how chemical patterning and the monomer capture time affect the growth of the
dendritic structures. Lastly, we show that the dendritic structures can be functionalized to
fabricate hydrophobic surfaces. Our hierarchical PMAA dendritic structures have potential to be
used as templates for both organic and inorganic materials. For example, dendritic gold
structures have been synthesized using a templated growth process on a thin film of polystyreneb-poly(4-vinylpyridine) for detection of organic molecules at very low concentrations.10 Zhang
et al. fabricated gold nanoparticles using a polymer ionic liquid as both the template and
stabilizer.28 In addition to being used as a physical template, due to the presence of the carboxylic
acid functional groups, the pH-responsive nature of the PMAA dendrites can be utilized for the
precise design of advanced materials. For example, Wang et al. demonstrated the advantage of
pH-responsive PMAA to design lithium-ion battery anode materials through self-assembly by
exploiting the swelling and coiling behavior of PMAA in different pH solutions.29
4.3 Experimental Section
Methacrylic acid (MAA) (Aldrich, 99%), 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA)
(C8PFA; SynQuest Laboratories, Inc., 97%), and tert-butyl peroxide (TBPO) (Aldrich, 98%),
were purchased and used as received without further purification. The porous polymer dendrites
were deposited onto silicon wafers (Wafer World) using a custom-designed cylindrical reactor
chamber (GVD Corporation; 25 cm diameter, 5 cm height). The substrate was cooled using a
thermoelectric cooler (TEC) (Custom Thermoelectric) controlled by an adjustable DC power
supply (Volteq HY3010D) placed on the reactor stage set to 10°C using a recirculating chiller
(Thermo Scientific NESLAB RTE 7). The reactor vacuum pressure was maintained using a

63
rotary vane vacuum pump (Edwards E2M40) and a throttle valve controller (MKS 153D) and
measured by a capacitance manometer (MKS 622C01TDE Baratron).
To fabricate porous PMAA dendrites on silicon wafers using the modified iCVD process,
first the initiator TBPO was introduced at 0.6 sccm using a mass flow controller (MKS 1479A).
Then, the reactor pressure was set and maintained at 600 mTorr during monomer deposition and
polymerization. The TEC was set to -20 °C and MAA was introduced at a low flow rate of 0.05
sccm for 4 minutes for most samples. To investigate the effect of the monomer capture time,
MAA was introduced for 1, 5, 15, and 30 minutes. After monomer deposition, the MAA flow
was terminated, the TEC temperature was increased to -10 °C, and the filament array (80% Ni,
20% Cr, Omega Engineering) was resistively heated to 260 °C to cleave the initiator molecules
and begin polymerization. After 30 minutes of polymerization, the TBPO flow was stopped, and
the TEC temperatures was increased to 0 °C and then gradually to room temperature to allow the
unreacted monomer to sublimate. For the samples with monomer capture times of 15 and 30
minutes, polymerization proceeded for 40 and 50 minutes, respectively, to ensure sufficient
polymerization of the monomer. Complete sublimation of unreacted monomer was confirmed by
the reactor pressure returning to a base pressure of 22 mTorr.
The scratch pattern on the silicon wafer substrate was applied using a diamond cutter pen.
Using the traditional iCVD process, PPFDA was deposited onto a silicon wafer and on the
sample consisting of PMAA dendrites formed using a monomer capture time of 30 minutes.
TBPO and PFDA were introduced into the reactor at flow rates of 1.0 sccm and 0.3 sccm,
respectively, and the reactor pressure was set to 90 mTorr. The reactor stage was maintained at 30
°C and the filament was resistively heated to 260 °C to initiate polymerization. The film

64
thickness was monitored on a reference silicon wafer using a He–Ne laser (Industrial Fiber
Optics, 633 nm).
The porous polymer dendrites were imaged using scanning electron microscopy (NOVA
NanoSEM) with an accelerating voltage of 10 kV. A layer of platinum was sputtered onto the
films prior to SEM imaging to prevent charging. The chemical composition was confirmed using
a Fourier transform infrared (FTIR) spectrometer (Nicolet iS10, Thermo Scientific). The PMAA
dendrites were compared against MAA liquid monomer and a reference wafer with 400 nm of
PMAA deposited using the traditional iCVD process. Optical images of the porous PMAA
dendrites were taken using an iPhone 12 Pro camera. The spacing between the dendrites, the
branch length, the height of the structures, and the pore sizes were measured using ImageJ
software version 1.54g. The percent area of surface coverage and number of particles were
calculated by adjusting the threshold and using particle analysis on ImageJ. Contact angle
goniometry (rame-́hart 290) with 10 µL deionized water droplets was used to measure
hydrophobicity by averaging 10 measurements across the sample.
4.4 Results and Discussion
To create conditions for diffusion-controlled nucleation and growth on the substrate, we
limited the amount of monomer introduced into the reactor. As shown in Figure 4-1a, using a low
monomer flow rate of 0.05 sccm led to the formation of discrete three-dimensional PMAA
dendrites across the entire surface of the silicon wafer as opposed to a uniform layer of pillared
structures that result when higher monomer concentrations are used.22,23,25 ImageJ software was
used to determine that the spacing between the PMAA dendrites ranged from 10 microns to 1
mm. The dendritic structures form in a similar manner to ice dendrites as a result of diffusion-

65
limited growth leading to branched, tree-like structures.12,13,14,30 As shown in Figure 4-1b, the
characteristic dendritic branches extend off the nucleation center and have varying lengths with
an average of approximately 400 microns. Since dendrites grow during nonequilibrium
conditions, the porous polymer dendrites form in many different shapes as shown in Figure 1c.
The dendrites can grow and attach to neighboring structures, can grow in spiral shapes with the
fingers extending in different directions, and some dendrites are nearly symmetrical, with the
fingers extending equally in all directions. Since the silicon wafer substrates are cut into
rectangles using a diamond cutter, the sides of the substrate have surface roughness that
encourages nucleation along the edges, as shown in Figure 4-1d, which is consistent with crystal
growth at defects.31

66
Figure 4-1. a) Optical image of PMAA dendrites on a silicon wafer. SEM images of b)
characteristic branching of a representative dendrite, c) examples of different dendritic shapes,
and d) dendritic growth along the substrate edge.
The polymer dendrites possess a hierarchical structure composed of branches extending
out of the nucleation center and small pores inside the branches and the nucleation center as
shown in Figure 4-2. The smaller scale pores form during the sublimation of the unreacted
monomer which occurs both during polymerization and after polymerization. These pores are
similar to the ones formed in porous pillared PMAA structures using the modified iCVD
process.21,22,23 Our systematic studies for the porous pillared structures showed that the pores
within the structures can be tuned by adjusting the polymerization temperature which is
proportional to the sublimation rate.23 Larger pores form at higher sublimation rates which is
consistent with studies of the deposition of poly(p-xylylene) on sublimating ice particles.32

67
ImageJ software was used to measure the pore sizes across 100 pores and the pores were found
to range from 70 nm to 30 microns in diameter with the majority of pores being between 1 and
10 microns.
Figure 4-2. SEM image of zoomed-in pores on the branch of a PMAA dendrite.
To further study dendritic nucleation and growth, we investigated the effect of changing
the surface energy on the substrate through both physical and chemical patterning. Based on
nucleation theory, nucleation on the surface can be enhanced through using high surface energy
materials such as metals or through the addition of surface roughness.33 For example,
topographical surface defects provide a cavity for molecules to nucleate and gather.34,35 To
determine how the addition of surface roughness impacts dendritic growth, a diamond cutter pen
was used to produce a scratch pattern across the silicon wafer substrate. During monomer
capture, we observed that the monomer first begins to grow along the scratch as predicted since
the surface defect enhances nucleation by lowering the free energy barrier.34 Figure 4-3a shows
the SEM image of the sample after polymerization and sublimation of the unreacted monomer.
The polymer preferentially grew along the scratch lines and along the cut edge of the wafer
which is consistent with crystal nucleation studies.34,35 The dendrites display the characteristic

68
branching along the scratch (Figure 4-3b) and along the cut wafer edge (Figure 4-3c). This
technique can potentially be used to fabricate different patterns by etching dots, circles, parallel
lines, and intersecting lines. The majority of the dendrite growth occurs along the scratches,
however, a few discrete dendrites (Figure 4-3d) are formed elsewhere on the wafer because
although the scratches encourage nucleation at that location, the growth is not inhibited
elsewhere.
Figure 4-3. a) Optical image of PMAA dendrites on a scratched silicon wafer. SEM image of b)
dendrites along the scratch, c) dendrites along the wafer edge, and d) a discrete dendrite not
located on the scratch or edge.
To study the effect of changing the surface energy of the substrate, we studied dendrite
formation on a wafer that was coated with the fluorinated polymer poly(1H,1H,2H,2Hperfluorodecyl acrylate) (PPFDA) using traditional iCVD (Figure 4-4a). PPFDA has a very low
surface energy (9.3 mN/m) 36 compared to the uncoated silicon wafer (100 mN/m).37 As shown in
Figure 4-4b, the coated substrate experiences less nucleation and growth compared to the

69
uncoated substrate. ImageJ software was used to determine that the percent area of coverage on
the coated substrate is 11%, whereas the coverage on the uncoated substrate is 25%. The
dendrites on the uncoated (Figure 4-4c,e) and coated substrate (Figure 4-4d,f) have finger-like
features; however, the dendrites on the coated substrate are more compact, with shorter branches
less than 100 microns long in contrast to 200-300 microns on the uncoated substrate. The PPFDA
coating delays nucleation and diffusion on the surface as shown in ant-icing applications.38
Effective anti-icing coatings significantly delay nucleation on the surface, but they are unable to
fully prevent ice formation over extended periods of time.33,39
Figure 4-4. Optical image of an uncoated silicon wafer and a wafer coated with 100 nm PPFDA
a) before deposition and b) after deposition. SEM image of PMAA dendrites c,e) on the uncoated
silicon wafer and d,f) on the coated wafer.
The dendrite growth is extremely sensitive to imperfections on the surface as
demonstrated with the scratch pattern. In Figure 4-4b, there is a section of increased dendritic

70
growth on the coated wafer caused from a surface defect during handling of the substrate. Minor
surface imperfections such as fingerprints become obvious during monomer deposition since the
monomer nucleates preferentially on the defect in a similar manner to topographical defects like
the mechanical scratch pattern. To demonstrate this, we intentionally pressed a gloved finger
onto all four corners of a silicon wafer substrate and deposited the polymer dendrites. As shown
in Figure 4-5, increased dendritic growth occurs at the location of the fingerprints due to the
increased surface energy relative to the smooth silicon wafer leading to a higher number of
polymer dendrites. In the middle of the substrate without any surface contamination, the dendrite
spacing is similar to that shown Figure 4-1a.
Figure 4-5. Optical image of PMAA dendrites on a silicon wafer with fingerprints at all four
corners.
To further understand the mechanism associated with dendritic growth, we systematically
varied the monomer capture time. We first observed the substrate over a 30-minute period
without polymerization to study the effect of monomer capture time on the monomer nucleation
behavior (Figure 4-6). ImageJ threshold particle analysis on the substrates determined that the

71
number of monomer nucleation sites as indicated by the number of particles was consistent
across all four monomer captures times with a minor decrease at 30 minutes as a result of some
of the sites combining over time. The monomer coverage on the substrate increased from 2 % to
40 % from 1 minute to 30 minutes as the monomer nucleation sites grew over time. Next, we
captured monomer for 1 min (Sample A), 5 min (Sample B), 15 min (Sample C), and 30 min
(Sample D) and then followed the polymerization and sublimation procedure (see Table 4-1). For
Samples C and D, polymerization proceeded for 40 and 50 minutes, respectively, to ensure
sufficient polymerization of the monomer. For Sample A, most of the captured monomer is
sublimated during polymerization, leaving a few small microstructures. For Sample B, the
sample resembles those in Figure 4-1 in which monomer is captured for 4 minutes. Increasing
the time of monomer capture results in greater coverage on the substrate since the monomer has
extended time to grow through diffusion resulting in the formation of larger and taller PMAA
dendrites as shown in Figure 4-7c. Image analysis of the samples after polymerization confirmed
that the percent area of coverage and height of the dendrites increased with monomer capture
time as shown in Table 4-1.
Figure 4-6. Optical image of monomer capture on a silicon wafer after 1, 5, 15, and 30 minutes.
Table 4-1. Deposition conditions for samples, % area of coverage, and dendrite height.

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Sample Monomer
Capture Time
(min)
Polymerization Time
(min)
% Area of
Coverage After
Polymerization
Dendrite
Height
(µm)
A 1 30 0.1 14 ± 9
B 5 30 20 56 ± 6
C 15 40 35 96 ± 8
D 30 50 60 192 ± 22
Figure 4-7. a) Optical image of PMAA dendrites, b) top-down SEM images of PMAA dendrites,
and c) cross-sectional SEM images of Sample A, Sample B, Sample C, and Sample D.

73
FTIR analysis was used to verify the chemical composition of the porous PMAA
dendrites by comparing the spectra of the unreacted MAA monomer, a 400 nm dense PMAA
film, and Sample C (Figure 4-8). The spectra of the dense PMAA film and Sample C match
confirming polymerization due to the removal of the peak at 1636 cm–1 attributed to C═C
stretching found only in the MAA monomer spectra.36 In both the reference dense PMAA film
and Sample C, the expected absorbance peaks were found such as the broad OH absorption
between 2500 and 3500 cm–1 along with the characteristic carboxylic acid C═O stretching peak
around 1700 cm–1
.
40 Additionally, the spectra also confirm the presence of asymmetric
−CH3 stretching at 2993 cm–1
, asymmetric −CH2– stretching at 2944 cm–1
, −CH3 and −CH2–
deformation at 1455 cm–1
, and −CH3 deformation at 1389 cm–1
.
41
Figure 4-8. FTIR spectra of the MAA monomer, a 400 nm dense PMAA control sample, and
Sample C.
The hierarchical structure of the dendrites can be used to engineer surfaces with different
properties such as self-cleaning. To demonstrate this, we deposited 400 nm of PPFDA
fluoropolymer using traditional iCVD onto Sample D. The coating did not alter the structure of
the PMAA dendrites as confirmed through SEM imaging (Figure 4-9). The contact angle on a

74
silicon wafer that has been coated with PPFDA coating is 120 ± 2°, which agrees with previously
reported values.26,36 The contact angle on the sample consisting of fluorinated PMAA dendrites is
137± 3° confirming that the enhanced surface roughness improved hydrophobicity.
Figure 4-9. SEM image of Sample D coated with 400 nm of fluoropolymer and the
corresponding contact angle.
4.5 Conclusion
Vapor phase deposition was used to fabricate porous polymer dendrites by limiting the
amount of monomer available for deposition. The dendrites are characterized by branches and
small-scale pores within the structures and are a result of the diffusion-limited nucleation and
growth of the monomer. Nucleation can be enhanced through surface defects such as scratches,
fingerprints, and rough edges and can be limited by coating the substrate with a low surface
energy material. Increasing the monomer capture time resulted in more surface coverage on the
substrate and taller dendrites. The benefit of this vapor-phase method is its lack of solvents and
ability to be used as a template as demonstrated by the increase in hydrophobicity after the
addition of a fluorinated coating. These polymer dendrites have potential for use in advanced

75
applications such as sensing and catalysis because of their high surface area, porosity, and high
levels of branching. In comparison to lotus leaf inspired hierarchical structures, the PMAA
dendrites lack nanoscale features. For future work, we plan to systematically study sublimation
rate and polymerization temperature to understand how these parameters affect pore size in order
to fabricate nanoscale features. Future studies will also focus on increasing the resolution of the
patterning method by combining surface defects with low surface energy coatings and by
employing higher monomer capture temperatures to decrease nucleation on the surface.
4.6 Acknowledgment
We thank the Gabilan Distinguished Professorship in Science and Engineering for
funding.

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3 Reinhardt, H.; Kroll, M.; Karstens, S. L.; Schlabach, S.; Hampp, N. A.; Tallarek, U. Nanoscaled
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from Block Copolymers. Science (New York, N.Y.) 2013, 341 (6145), 530–534.
5 Kim, J.-J.; Lim, C.-R.; Reddy, B. M.; Park, S.-E. Hierarchical Porous Organic Polymer as an
Efficient Metal-Free Catalyst for Acetalization of Carbonyl Compounds with
Alcohols. Molecular Catalysis 2018, 451, 43–50.
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Polymers: Highly Enhanced Gas Uptake and Transport through Templated Synthesis. Chemical
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7 Lu, X.; Zhang, C.; Han, Y. Low-Density Polyethylene Superhydrophobic Surface by Control of
Its Crystallization Behavior. Macromolecular Rapid Communications 2004, 25 (18), 1606–1610.
8 Jiang, R.; Hao, L.; Song, L.; Tian, L.; Fan, Y.; Zhao, J.; Liu, C.; Ming, W.; Ren, L. Lotus-LeafInspired Hierarchical Structured Surface with Non-Fouling and Mechanical Bactericidal
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9 Shin, S.; Gu, M.-L.; Yu, C.-Y.; Jeon, J.; Lee, E.; Choi, T.-L. Polymer Self-Assembly into
Unique Fractal Nanostructures in Solution by a One-Shot Synthetic Procedure. Journal of the
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12 Witten, T. A.; Sander, L. M. Diffusion-Limited Aggregation, a Kinetic Critical
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13 Oaki, Y.; Imai, H. Experimental Demonstration for the Morphological Evolution of Crystals
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Synthesized through Rapid Oxidative Polymerization. Polymer Journal 2019, 51 (1), 11–18.
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Aggregates and Flowerlike Arrays of Polystyrene-B-Poly(Acrylic Acid)
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20 Cheng, C.; Gupta, M. Surface Functionalization of 3D-Printed Plastics via Initiated Chemical
Vapor Deposition. Beilstein Journal of Nanotechnology 2017, 8, 1629–1636.
21 Seidel, S. W.; Kwong, P.; Gupta, M. Simultaneous Polymerization and Solid Monomer
Deposition for the Fabrication of Polymer Membranes with Dual-Scale
Porosity. Macromolecules 2013, 46 (8), 2976–2983.
22 Seidel, S.; Gupta, M. Systematic Study of the Growth and Morphology of Vapor Deposited
Porous Polymer Membranes. Journal of Vacuum Science & Technology A: Vacuum, Surfaces,
and Films 2014, 32 (4), 041514.
23 Dianat, G.; Movsesian, N.; Gupta, M. Process-Structure-Property Relationships for Porous
Membranes Formed by Polymerization of Solid Monomer by a Vapor-Phase
Initiator. Macromolecules 2018, 51, 10297-10303.
24 Dianat, G.; Gao, K.; Prevoir, S.; Gupta, M. Scratch-Resistant Porous Polymer Coatings with
Enhanced Adhesion to Planar and Curved Substrates. ACS Applied Polymer
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25 Dianat, G.; Movsesian, N.; Gupta, M. Vapor Deposition of Functional Porous Polymer
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26 Bacheller, S.; Dianat, G.; Gupta, M. Synthesis of PH-Responsive Polymer Sponge Coatings
and Freestanding Films via Vapor-Phase Deposition. ACS Applied Polymer
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27 Bacheller, S.; Welchert, N. A.; Gupta, M. Influence of Oblique Angle Deposition on Porous
Polymer Film Formation. Langmuir 2023, 39 (4), 1507–1514.
28 Zhang, J.; Meng, L.; Zhao, D.; Fei, Z.; Lu, Q.; Dyson, P. J. Fabrication of Dendritic Gold
Nanoparticles by Use of an Ionic Polymer Template. Langmuir 2008, 24 (6), 2699–2704.
29 Wang, H.; Madumali Kalubowilage; Bossmann, S. H.; Amama, P. B. Design of Highly Porous
Fe3O4@Reduced Graphene Oxide via a Facile PMAA-Induced Assembly. RSC
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32 Tung, H.-Y.; Guan, Z.-Y.; Liu, T.-Y.; Chen, H.-Y. Vapor Sublimation and Deposition to Build
Porous Particles and Composites. Nature Communications 2018, 9 (1), 2564.
33 Na, B.; Webb, R. L. A Fundamental Understanding of Factors Affecting Frost
Nucleation. International Journal of Heat and Mass Transfer 2003, 46 (20), 3797–3808.
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Chapter 5. Concluding Remarks and Future Work
5.1 Conclusion
In this dissertation, we discussed several approaches to fabricating porous polymer
materials with different structural properties using vapor phase deposition. Traditional iCVD and
modified iCVD provide a solvent-free, one-pot method for synthesizing polymer materials and
coatings. In Chapter 2, we showed how to adjust processing parameters of the modified iCVD
process to synthesize porous polymer sponge coatings and freestanding films. By increasing the
monomer flow rate, the monomer condensed on the substrate before solidifying resulting in the
porous sponge structure. In comparison to the previously reported porous pillar structure, the
polymer sponge consisted of large void spaces, pore interconnectivity, and varying heights. We
took advantage of the condensation behavior of the monomer to fabricate freestanding crosslinked films by simultaneously introducing the cross-linker EGDA with MAA. We also provided
a method of improving adhesion to the substrate by incorporating the epoxide containing
monomer GMA to fabricate a robust sponge coating. We offered possible applications for the
synthesized polymer sponge by demonstrating enhanced separation of charged dyes in a
microfluidic device and uses as a superhydrophobic material after coating with PPFDA.
In Chapter 3, we applied the concept of oblique angle deposition to the modified iCVD
process to fabricate porous polymer materials with different morphological regions. We
systematically studied the effect of the monomer extension angle, the monomer capture
temperature, and time of monomer capture. We found that the extension angle did not result in
any apparent orientation relative to the material flux, unlike that found in structured inorganic
materials fabricated using OAD with physical vapor deposition. Regardless of the monomer

81
extension angle and capture temperature and time, we showed the presence of three regions of
different morphologies. Region 1 resembled the pillars grown in the absence of the extension,
region 2 showed densified pillars, and region 3 displayed branch-like, dendritic structures.
In Chapter 4, we continued to investigate dendritic polymer structures by limiting the
monomer concentration across the entire reactor. Dendritic structures are often found in far from
equilibrium processes and modeled by diffusion-limited aggregation of particles. In our system,
the polymer dendrites form as a result of the diffusion-controlled nucleation and growth behavior
of the monomer. Since dendritic growth is largely influenced by the nucleation behavior of the
monomer, we showed that we can enhance nucleation by adding surface defects and decrease
nucleation by lowering the surface energy of the substrate. To further understand the mechanism
of dendritic growth, we studied the effect of monomer capture time on monomer and polymer
coverage. We found that increasing the capture time resulted in more surface coverage and taller
polymer dendrites.
5.2 Future work
5.2.1 Systematic study on sublimation step
The benefit of the modified iCVD process is the ability to fabricate porous polymer
materials without the use of multiple steps and solvents. However, one of the major limitations of
the modified iCVD process is the inability to control pore sizes and limit the broad pore size
distribution. Our current approach fabricates porous pillared structure with a bimodal pore size
distribution with larger pores between the structures and smaller pores within the structure as
result of monomer sublimation.1,2 The sponge structure has a large pore size distribution, with
the smallest pore size measured to be about 1 micron. Incorporation of nano-scale pores would

82
be advantageous in fabricating hierarchical structures that encompass length scales from nano to
macro. We believe systematically studying polymerization time and temperature to control the
sublimation rate will provide key information on how these process parameters affect pore sizes.
Studies on the deposition of poly-para-xylylene on sublimating ice particles show how
controlling the sublimation of the ice template resulted in control over the final particle size and
inner pore sizes.3 In our system, sublimation occurs both during polymerization and after during
the final sublimation step. The rate of sublimation is increased using higher substrate
temperatures and extending the amount of time. Previously, we have studied the effect of these
process parameters during polymerization on the porous pillared structure but have not
investigated adjusting the rate of sublimation after polymerization. In our current experimental
procedure, sublimation occurs after initiator flow is halted, the filament is turned off to end
polymerization, and the butterfly valve to maintain the reactor pressure is opened. To prevent
loss of structural integrity, the substrate is slowly increased to room temperature and the reactor
pressure is monitored until it returns to its base vacuum pressure. The typical sublimation step
for fabrication of the porous pillar structures takes approximately 1 to 2 hours. To provide more
control of the final sublimation step we can systematically study the effect of the final
sublimation step on the final structure by lowering the pressure and substrate temperature
incrementally until return to base pressure and room temperature. This approach will limit the
sublimation rate by controlling the reactor pressure, temperature, and time more precisely and
should result in smaller inner pore sizes.
5.2.2 Different Monomers
In this work, we discuss the fabrication of porous PMAA materials with different
structures. The methacrylic acid monomer is attractive for use in the modified iCVD process

83
because of its relatively high freezing point of 15 °C. MAA allows for formation of pHresponsive polymer membranes. Our research group has previously shown that the technique can
also be extended to other monomers by depositing N-isopropylacrylamide (NIPAAm) which has
a freezing point of 63 °C.
4 PNIPAAm offers temperature-responsive hydrophilicity that has
potential for different applications.5 To target other applications and deposit different structures,
we can continue to explore other monomers for use in the modified iCVD process. We have
started to explore the deposition of the monomer N-vinyl-2-pyrrolidone (VP) which has a
freezing point of around 14°C. Thin PVP films demonstrate biocompatibility and have shown
uses in the pharmaceutical industry.6 Other monomers follow the same mechanism of solidifying
followed by polymerization of the solid monomer due to their relatively high freezing points;
however, the final structures may vary. Continuing to expand and study other monomers in this
system, will provide additional insight on the fabrication of porous polymers to fabricate
different structures and broaden the scope of applications.

84
5.3 References
1 Seidel, S.; Gupta, M. Systematic Study of the Growth and Morphology of Vapor Deposited
Porous Polymer Membranes. Journal of Vacuum Science & Technology A: Vacuum, Surfaces,
and Films 2014, 32 (4), 041514.
2 Dianat, G.; Movsesian, N.; Gupta, M. Process-Structure-Property Relationships for Porous
Membranes Formed by Polymerization of Solid Monomer by a Vapor-Phase
Initiator. Macromolecules 2018, 51, 10297-10303.
3 Tung, H.-Y.; Guan, Z.-Y.; Liu, T.-Y.; Chen, H.-Y. Vapor Sublimation and Deposition to Build
Porous Particles and Composites. Nature Communications 2018, 9 (1), 2564.
4 Seidel, S. W.; Kwong, P.; Gupta, M. Simultaneous Polymerization and Solid Monomer
Deposition for the Fabrication of Polymer Membranes with Dual-Scale
Porosity. Macromolecules 2013, 46 (8), 2976–2983.
5 Alf, M. E.; Godfrin, P. D.; Hatton, T. A.; Gleason, K. K. Sharp Hydrophilicity Switching and
Conformality on Nanostructured Surfaces Prepared via Initiated Chemical Vapor Deposition
(ICVD) of a Novel Thermally Responsive Copolymer. Macromolecular Rapid
Communications 2010, 31 (24), 2166–2172.
6 Chan, K.; Kostun, L. E.; Tenhaeff, W. E.; Gleason, K. K. Initiated Chemical Vapor Deposition
of Polyvinylpyrrolidone-Based Thin Films. Polymer 2006, 47 (20), 6941–6947.

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Abstract (if available)

Abstract Polymer films are useful in providing surface functionality to improve and target specific applications. Due to their various functionalities, polymer films are attractive for a range of applications such as electronics, biomedical, energy, and separations. Traditional polymer processing consists of solution-based methods such as dip-coating, spray-coating, and spin-coating which can result in processing challenges such as surface tension effects and solvent compatibility. In addition, solution-based processes are often multi-step and can result in residual chemicals. Vapor-phase techniques such as initiated chemical vapor deposition (iCVD) offer a more sustainable method of fabricating dense and porous polymer materials without the use of harsh solvents and additives. Porous polymer materials can be synthesized through a modified iCVD process. In this work, both the conventional and modified iCVD processes are discussed.

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Creator Bacheller, Stacey (author)

Core Title Understanding structural changes of polymer films during vapor phase deposition

School Viterbi School of Engineering

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Degree Conferral Date 2024-08

Publication Date 07/16/2024

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