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Scale-up of vapor-phase deposition of polymers: towards large-scale processing

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Scale-up of vapor-phase deposition of polymers: towards large-scale processing

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SCALE-UP OF VAPOR-PHASE DEPOSITION OF POLYMERS:
TOWARDS LARGE-SCALE PROCESSING

by

Christine Cheng

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

August 2020

ii
Acknowledgements
This work was made possible by the many people who contributed to funding, ideas, and
support. Firstly, I would like to thank my advisor Malancha Gupta for her continued guidance and
mentorship throughout these years. I would also like to thank my fellow members of the Gupta
Polymers Lab who have worked with me, including Mark De Luna, Nareh Movsesian, Golnaz
Dianat, and Prathamesh Karandikar. I would like to acknowledge the Alfred E. Mann Institute for
Biomedical Engineering at the University of Southern California and the ARCS Foundation for their
financial support.
I am grateful for the continued encouragement from my family, especially my parents and
my brother, Zhongli Cheng. Many friends provided invaluable support to me, especially Natalie
Shih, Seorim Song, Lori Tamura, Karel Lee, Valentina Fung, Vito Prasad, and Ryan Ko. I am
grateful for all of the fellow women in engineering who have inspired and encouraged me
throughout the years, including Ashley Guo, Chelsea Appleget, Zumra (Peksaglam) Seidel, Alexa
Hudnut, and Emily Reed. I am thankful for the rest of my chemical engineering doctoral cohort,
Alireza Delfarah, Andre Kovach, and Sarah (Katz) Schechter, for going through this journey
together. To all my friends in the running, climbing, hiking, and yoga communities, thank you for all
the time you have spent being active with me. I would be remiss to not thank the many educators
throughout my life who nurtured my intellectual curiosity since childhood, especially Leslie Streight
and Don Juang. Thank you all.

iii
Table of Contents
Acknowledgements ii
List of Figures iv
List of Tables v
Abstract vi
Chapter 1. Introduction 1
1.1 Process Scale-up 1
1.2 Initiated Chemical Vapor Deposition 2
1.3 Soft Material Implants 4
1.4 Scale-up of Initiated Chemical Vapor Deposition 5
1.5 Initiated Plasma-Enhanced Chemical Vapor Deposition 6
1.6 Selectively Wetting Multilayer and Janus Membranes 6
1.7 References 8
Chapter 2. Surface Functionalization of 3D-Printed Plastics via Initiated Chemical Vapor Deposition 16
2.1 Abstract 16
2.2 Introduction 17
2.3 Results and Discussion 19
2.4 Conclusions 28
2.5 Experimental Details 28
2.6 Acknowledgments 31
2.7 References 31
Chapter 3. Roll-to-Roll Surface Modification of Cellulose Paper via Initiated Chemical Vapor Deposition 36
3.1 Abstract 36
3.2 Introduction 37
3.3 Experimental Details 39
3.4 Results and Discussion 42
3.5 Conclusions 50
3.6 Acknowledgments 51
3.7 References 51
Chapter 4. Solvent-Free Synthesis of Selectively Wetting Multilayer and Janus Membranes 57
4.1 Abstract 57
4.2 Introduction 58
4.3 Experimental Details 60
4.4 Results and Discussion 63
4.5 Conclusions 72
4.6 Acknowledgements 73
4.7 References 73
Chapter 5. Future Directions 78
5.1 Grafted Functional Coatings for Soft Material Implants 78
5.2 Roll-to-Roll Deposition of Patterned Coatings 79
5.3 Mechanistic Studies of Initiated Plasma-Enhanced Chemical Vapor Deposition 80
5.4 Hybrid Metal and Polymer Films 80
5.5 References 81

iv
List of Figures
Figure 1-1. (a) A cylindrical iCVD reactor. (b) Schematic of iCVD process. Initiator (I-I) and monomer (M) vapors
enter the vacuum chamber and react on the substrate surface to form a polymer film. 2
Figure 1-2. Microtrenches with polymer coatings applied using (a) spin-coating and (b) iCVD. 3
Figure 2-1. Schematic of the iCVD process. The 3D printed substrate (white lattice) is placed on a silicon wafer
piece on a temperature-controlled stage. Initiator (I-I) and monomer (M) in vapor phase are introduced into
the reactor and the filament array is heated to thermally cleave the initiator. 19
Figure 2-2. (a) Static contact angles for a 7.5 mm tall PLA lattice that was uncoated and coated with PPFDA. (b)
FTIR spectra of the PPFDA film (top) and liquid monomer (bottom). Dashed lines correspond to the CF2
and CF3 signals, and the asterisks in the monomer spectrum correspond to the vinyl signals. (c)
Representative XPS survey spectrum of the top of the lattice coated with PPFDA. 20
Figure 2-3. SEM images of the lattice before (left) and after (right) PPFDA coating. 22
Figure 2-4. PLA lattices coated with PPFDA (left), uncoated (center), and coated with P(HEMA-co-EGDA) (right)
in water. 25
Figure 2-5. Water droplets (colored with blue food coloring) on an uncoated lattice and on a 25 mm tall PLA
lattice coated with P(HEMA-co-EGDA). 26
Figure 2-6. Sequential deposition of P(HEMA-co-EGDA) and PPFDA. (a) Nut, bolt, and comb in the iCVD
reactor. Water droplets (colored with blue food coloring) on ABS substrates (b) without coating, (c) coated
with P(HEMA-co-EGDA), and (d) coated with PPFDA. 27
Figure 3-1. (a) Photograph of the roll-to-roll module in an iCVD reactor chamber. A 3 m-long roll of
chromatography paper is mounted on the module. (b) Schematic showing that the substrate rolls from the
feed roll, through the primary deposition zone (shown in blue), and onto the receiver roll. 42
Figure 3-2. Atomic compositions from XPS survey spectra for uncoated paper, PPFDA (theoretical), and on the
top and bottom of roll-to-roll paper coated with PPFDA at multiple points along the length of the coated
paper. 44
Figure 3-3. SEM images of paper (a and b) before and (c and d) after roll-to-roll coating with PPFDA. 44
Figure 3-4. (a) Images of water droplets on uncoated chromatography paper, stationary paper that was coated with
PPFDA, and paper that was coated with PPFDA in a roll-to-roll deposition. (b) Contact angle measurements
of the top (triangles) and bottom (squares) of the roll-to-roll paper coated with PPFDA as a function of
distance along the length of the paper. 45
Figure 3-5. (a) Atomic compositions from XPS survey spectra across the width of the coated paper at the center of
the roll (1.5 m). (b) Contact angle measurements across the width of the top (triangles) and bottom (squares)
of the coated paper at various lengths (0.1, 1.5, and 2.9 m). 47
Figure 3-6. (a) Image of an origami bowl made with paper that was coated with PPFDA in a roll-to-roll deposition.
The bowl is filled with water that is dyed with blue food coloring. (b–d) Time series images of dyed solvents
rolling off of a SLIPS material at a 15° tilt. The solvents used were (b) water dyed with red food coloring, (c)
isopropanol dyed with green food coloring, and (d) chloroform dyed with ethyl violet. 48
Figure 3-7. Images of a mixture of Ponceau S (pink) and crystal violet (violet) on paper. Separations are shown on
uncoated paper, stationary paper coated with xPMAA via iCVD, and roll-to-roll paper coated with xPMAA at
different distances along the length of the paper. The arrow indicates the direction of eluent flow. 49
Figure 4-1. (a) Image of the plasma in the iPECVD reactor chamber. (b) Schematic of the free-radical
polymerization mechanism. (c) FTIR spectra of the TVTSO monomer (bottom) and PTVTSO polymer
deposited using iPECVD (top). The dashed lines correspond to the peaks associated with the vinyl bonds.
SEM images of Whatman 1 paper (d) before and (e) after coating with PTVTSO. 64
Figure 4-2. (a) Chromatography paper taped down on the reactor stage during PTVTSO deposition. (b) To test the
wetting behavior of paper coated with PTVTSO, strips are cut from the coated paper samples and dipped in
dyed water. (c) Wicking behavior of Whatman 1 CHR (thin) and 3MM CHR (thick) chromatography paper
taped to the stage and coated with PTVTSO at varying deposition times. 65
Figure 4-3. Atomic compositions from XPS survey spectra of PTVTSO coatings on Whatman 3MM CHR paper. 66
Figure 4-4. Wicking behavior of chromatography paper elevated to 2.5 cm above the stage and coated with
PTVTSO at varying deposition times. 67
Figure 4-5. (a) Schematic of the compact roll-to-roll module in the iPECVD system. Contact angle goniometry
measurements (b) along the length (inset: representative contact angle image) and (c) across the width of
Whatman 1 CHR paper coated with PTVTSO via roll-to-roll iPECVD. (d) Origami bowl folded from roll-to-
roll coated hydrophobic paper and filled with dyed water. 69
v
Figure 4-6. (a) Whatman 3MM CHR paper coated with PTVTSO via roll-to-roll iPECVD at varying roll speeds.
Contact angle measurements (b) along the length (inset: representative contact angle image) and (c) across the
width of thick paper coated at a roll speed of 8.0 cm/min. 71
Figure 5-1. Patterned PONBMA channels filled with dyed liquid. The polymer was deposited in our lab using
iCVD. 79

List of Tables
Table 2-1. XPS survey spectra for PLA lattices. 22

vi
Abstract
Two vapor-phase deposition techniques for fabricating polymer coatings are studied in this
work. Chapter 1 introduces the initiated chemical vapor deposition (iCVD) and initiated plasma-
enhanced chemical vapor deposition (iPECVD) processes and the motivation behind coating large-
scale soft material substrates.
Chapter 2 demonstrates deposition of polymers onto 3D printed plastic substrates. 3D
printing is a useful fabrication technique because it offers design flexibility and rapid prototyping.
The ability to functionalize the surfaces of 3D printed objects allows the bulk properties, such as
material strength or printability, to be chosen separately from surface properties, which is critical to
expanding the breadth of 3D printing applications. In this work, we studied the ability of iCVD to
coat 3D printed shapes composed of poly(lactic acid) and acrylonitrile butadiene styrene. The
thermally insulating properties of 3D printed plastics pose a challenge to the iCVD process due to
large thermal gradients along the structures during processing. In this study, processing parameters
such as the substrate temperature and the filament temperature were systematically varied to
understand how these parameters affect the uniformity of the coatings along the 3D printed objects.
The 3D printed objects were coated with both hydrophobic and hydrophilic polymers. Contact
angle goniometry and X-ray photoelectron spectroscopy were used to characterize the
functionalized surfaces. These results can enable the use of iCVD to functionalize 3D printed
materials for a range of applications such as tissue scaffolds and microfluidics.
Chapter 3 investigates the use of iCVD for coating large areas of flexible materials. Tuning
surface properties of flexible materials enhances the versatility of existing materials, giving them new
functions for applications in textiles, filtration, flexible electronics, and sensors. However, traditional
surface modification methods are typically solvent-based, which limits the range of substrates that
can be coated. In this work, we demonstrate the ability to use roll-to-roll processing to continuously
vii
modify the surface properties of large areas of flexible substrates using iCVD, which is an all-dry
process. We designed and built a roll-to-roll module that can be used to uniformly coat 1500 cm2 of
chromatography paper in a single deposition. Rolls of paper were coated with a fluoropolymer and
an ionizable polymer, and the coated paper was used for origami, nonstick surfaces, and paper-based
microfluidic devices.
Chapter 4 studies the deposition of hydrophobic coatings onto porous materials for the
fabrication of membranes with asymmetric and symmetric wetting properties. iPECVD was used to
deposit an organosilicon polymer coating onto chromatography paper of different thicknesses. The
hydrophobic organosilicon polymer serves as an environmentally-friendly alternative to fluorinated
polymers, and the all-dry coating process does not use solvents. We demonstrate that the deposition
time, thickness of the paper, and orientation of the paper relative to the stage affect the symmetry of
the wettability. The chemical functionality of the deposited polymer was characterized via Fourier-
transform infrared spectroscopy and X-ray photoelectron spectroscopy. The hydrophobicity and
wetting behavior of the coated paper were characterized with contact angle goniometry and wicking
studies, respectively. We also demonstrate that we can scale up the process by using a roll-to-roll
module. Our ability to systematically tune the wettability of materials allows for the fabrication of
multilayer and Janus membranes that are useful for applications including filtration, smart textiles,
flexible sensors, and wound dressings.
Chapter 5 discusses future directions for this work, including grafting polymer coatings onto
soft material implants, investigating roll-to-roll patterned coatings, studying the reaction mechanisms
in iPECVD to enhance functionality retention and tune coating conformality, and depositing hybrid
inorganic/organic films.
1
Chapter 1. Introduction
1.1 Process Scale-up
Chemical vapor deposition (CVD) of polymers is important for a variety of applications,
including flexible electronics,1,2,3 wound dressings,4,5 paper-based microfluidic analytical devices,6,7
and desalination membranes.8,9 Scaling up CVD processes can enable the high-throughput
manufacturing of coated substrates in large quantities for realistic commercial applications. The
scale-up of vapor phase coating techniques including atomic layer deposition (ALD),10 plasma-
enhanced chemical vapor deposition (PECVD),11 oxidative chemical vapor deposition (oCVD),12
and initiated chemical vapor deposition (iCVD)13 have been previously demonstrated. For example,
Parsons and colleagues demonstrated high-throughput ALD of alumina coatings on textiles using
spatial ALD, in which substrates were moved through alternating zones of precursor exposure.10
PECVD has been used on the commercial scale since the 1980s, expanding from its origins in the
semiconductor industry to other applications in soft materials. Wertheimer and coworkers designed
a roll-to-roll process for coating polymer substrates.14 PECVD of polymers compared to iCVD has
disadvantages that include damage to the substrate via etching, poor retention of deposited
functional groups, and nonconformal coatings.40 Gleason and coworkers reported custom
modifications to a large roll-to-roll PECVD reactor to scale up oCVD of a conductive polymer12 and
iCVD of poly(glycidyl methacrylate).13 In this work, we studied the deposition of polymers onto
large-scale substrates using two different deposition techniques, iCVD and initiated plasma-
enhanced chemical vapor deposition (iPECVD). Technology transfer from the research bench to
the industrial scale is very limited, and this work aims to bridge that gap.
2
1.2 Initiated Chemical Vapor Deposition

Figure 1-1. (a) A cylindrical iCVD reactor. (b) Schematic of iCVD process. Initiator (I-I) and
monomer (M) vapors enter the vacuum chamber and react on the substrate surface to form
a polymer film.
Initiated chemical vapor deposition (iCVD) is a one-step, solvent-free polymerization
technique used to fabricate polymer thin films and coatings (Figure 1-1a).15,16 iCVD is a powerful
method for producing functional polymer coatings, because it avoids many surface tension problems
that traditional liquid-based coating techniques face, such as pore clogging, film dewetting, solvent
leaching, and nonconformal coating.15,17
In the iCVD process (Figure 1-1b), monomer and initiator, di-t-butyl peroxide (TBPO),
vapors are introduced into the reactor chamber under vacuum. The monomer adsorbs to the surface
of the temperature-controlled substrate. The heated filament array thermally cleaves the peroxide
bond in TBPO, generating t-butoxy radicals that adsorb to the substrate surface and initiate free
radical polymerization with the desired monomer. The rate of reaction in iCVD is limited by
adsorption of monomer to the substrate, and a lower substrate temperature results in a faster
polymerization rate.16
Temperature-controlled stage
Heated filament array
Flow in
initiator Flow out vapors
to vacuum
pump
Flow in
monomer
I-I
I •
I •
I •
Substrate
M
M M IMM •
(a)
(b)
3
A variety of functional polymers can be deposited. Stimuli-responsive polymers, such as UV-
cleavable poly(o-nitrobenzyl methacrylate)18,19 and thermally-switchable poly(N-
isopropylacrylamide),20 have been deposited using iCVD. Additionally, the surface energy of
substrates can tuned using hydrophobic polymers, such as poly(1H,1H,2H,2H-perfluorodecyl
acrylate),21,22 and hydrophilic polymers, including poly(2-hydroxyethyl methacrylate)23,24 and
poly(vinyl pyrrolidone).25 Cross-linking monomers such as ethylene glycol diacrylate23,25 and divinyl
benzene26,27 can also be introduced during the polymerization to enhance the deposition kinetics or
the mechanical strength of the resulting polymer film. Additionally, polymers with moieties that can
be used for further reactivity, such as poly(propargyl methacrylate),28,29 poly(glycidyl methacrylate),30
poly(furfuryl methacrylate),31 and poly(pentafluorophenyl methacrylate),32,33 can be deposited.

Figure 1-2. Microtrenches with polymer coatings applied using (a) spin-coating and (b)
iCVD.
Because iCVD is a vapor-phase process, surface tension effects associated with traditional
liquid-phase processing techniques are not present, resulting in conformal coatings on a variety of
structured surfaces, including microtrenches (Figure 1-2),34,35 microporous membranes,36,37 and
chromatography paper.19,22 Though other chemical vapor deposition processes are also commonly
used to fabricate coatings, they have drawbacks compared to iCVD. ALD has been used to deposit
thin, conformal coatings, but it is primarily used to deposit inorganic materials, most commonly
alumina.38,39 PECVD has been used to deposit polymers but can result in damage to the substrate via
etching, poor retention of deposited functional groups, and nonconformal coatings.40 In contrast,
4
iCVD can be used to fabricate polymer coatings with conformal coverage, good retention of
functional groups, and no damage to the substrate.

1.3 Soft Material Implants
The implantable medical devices market is a hundred-billion dollar industry in the United
States alone, and millions of Americans currently live with medical implants, including cardiac
pacemakers and joint replacements. One area of focus for this work is 3D printed implants.41 There
is significant interest in utilizing 3D printing of soft materials to create personalized, inexpensive
medical implants.42 Fabricating affordable, custom implants is a possibility because 3D printing is an
inherently flexible process. One challenge in developing 3D printed medical implants is that the
plastics used for 3D printing are hydrophobic, which is problematic because hydrophobic surfaces
are susceptible to biofouling.43 Biofouling in implants can lead to implant rejection or malfunction of
biosensors, and consequently implant failure.44,45 Implant failure results in removal of the device and
can lead to infections, which reduce patient quality of life. Thus, it is desirable to tune the surface
properties of 3D printed implants to improve integration of the implants into patients.
In addition to 3D printed implants, another closely related area of focus for this work is
hydrocephalus shunts. Hydrocephalus is a chronic medical condition in which cerebrospinal fluid
accumulates in the brain. This condition is incurable, and patients are generally treated using an
implanted shunt system that drains the fluid into a different part of the body for the entirety of their
lives.46 Due to the extended period of time these devices are implanted within the patients, these
shunt systems are prone to failure, with complications including mechanical failure and obstructions.
An estimated 50% of hydrocephalus shunts in pediatric patients fail within the first two years of
placement, subsequently leading to repeated surgical procedures.47 Shunt failure manifests as non-
specific symptoms, like headaches and blurred vison, and cannot be predicted. As such, if
5
hydrocephalus shunts were coated with functional polymers that prevent biofouling, improved
cellular differentiation, and could be personalized to the patient, it would revolutionize how
hydrocephalus patients are treated. In collaboration with physicians at Children’s Hospital Los
Angeles (CHLA) and other researchers at USC, I am working to develop the next generation
hydrocephalus shuts to achieve this end. The current implant I am working on is made of Parylene-
C, which is bioinert and biocompatible, but it is also hydrophobic and susceptible to biofouling.
Both the hydrocephalus shunt and 3D printed implants have similar surface properties and
thus face similar challenges that include biofouling and eventually implant failure. By modifying the
properties of the surface of implanted polymer structures with functionalized coatings will prevent
biofouling and extend the lifetime of the implant. This improvement will reduce fatalities and
improve patient quality of life for the millions of patients that live with implants.

1.4 Scale-up of Initiated Chemical Vapor Deposition
Previous work to scale up the iCVD process involved redesigning several PECVD reactor
components and resulted in significant changes to the iCVD process, including moving the substrate
vertically, which gives rise to a convective flow pattern different than that in a conventional
horizontal iCVD reactor. In this work, we demonstrate the ability to coat large areas of flexible
materials in a nonmodified iCVD chamber by designing a compact roll-to-roll module from low-
cost, readily available materials. With our compact roll-to-roll module, we can coat several meters of
substrate in a single deposition without modifying the iCVD process. The process is easily
modifiable, and different flexible substrates of varying lengths can be coated using iCVD, because it
is not a line-of-sight-process. We demonstrate the versatility of our system by conformally coating 3
m-long rolls of chromatography paper with a hydrophobic polymer and a cross-linked ionizable
6
polymer. We demonstrate the functionality of the coated paper by fabricating slippery liquid-infused
surfaces (SLIPS), microfluidic channels for chromatographic separations, and origami structures.

1.5 Initiated Plasma-Enhanced Chemical Vapor Deposition
iPECVD is an all-dry polymerization technique that is analogous to iCVD. In iPECVD, a
low-power plasma is utilized to preferentially cleave initiator molecules into radicals. These radicals
react with monomer molecules with unsaturated bonds, resulting in the deposition of a thin polymer
coating on the substrate surface. iPECVD has been shown to achieve high deposition rates of 30
nm/min, compared to 2.5 nm/min without the initiator.48 Polymers deposited via iPECVD can
achieve high functional group retention if a low plasma power (20–50 W) is used with the initiator
tert-butyl peroxide.48

1.6 Selectively Wetting Multilayer and Janus Membranes
The ability to tune the symmetry of wettability for porous materials, such as textiles and
membranes, is important for enhancing their performance and functionality. For example, porous
Janus materials have asymmetric properties on two sides typically caused by different surface
chemistries.49,50,51 Janus membranes with asymmetric wetting can be used for directional transport of
liquids,52 and they have been used as separation membranes,53,54,55 moisture-wicking fabrics,56,57,58 and
medical wound dressings.59,60,61 Janus membranes can be made by either fabricating an asymmetric
material during membrane formation or by post-modification of a membrane to impart asymmetric
functionality.49,50 Vapor-phase techniques are ideal for asymmetric modification of porous materials
because they do not use solvents. For example, Waldman et al. fabricated Janus membranes using
atomic layer deposition of hydrophilic alumina onto hydrophobic polypropylene,62 Tian et al. reacted
hydrophobic perfluorooctyl-trichlorosilane vapor with hydroxy groups on hydrophilic cellulose,63
7
and Cheng et al. modified a hydrophilic copper mesh with copper(II) hydroxide nanowires with a
hydrophobic fluorosilane vapor.64 In this work, we demonstrate a scalable, versatile, and solvent-free
vapor-phase modification technique to apply hydrophobic polymer coatings onto hydrophilic
porous materials. Our fabrication process can be easily extended to other precursors to deposit
hydrophilic polymer coatings65,66 onto hydrophobic porous materials.
Although perfluoroalkyl coatings are very effective for making hydrophobic surfaces, they
have also been shown to degrade into byproducts that resist biodegradation and accumulate in
human and animal tissue.67,68 As a result, many companies have begun to phase out the use of
fluorinated coatings, and the United States Environmental Protection Agency has instituted an
action plan to address the public health impact of perfluoroalkyl substances.69 A potential
replacement for fluorinated polymers are organosilicon polymers, which are hydrophobic and have
low toxicity and environmental effects.70,71 Coclite and Gleason recently demonstrated that initiated
plasma-enhanced chemical vapor deposition (iPECVD) can be used to deposit organosilicon
polymers.48,72 Because iPECVD does not use solvents and can be used to deposit organosilicon
polymers with low environmental impacts, it is an attractive process for sustainably fabricating
hydrophobic coatings for Janus membranes.
In this work, we used iPECVD to deposit an organosilicon polymer onto porous cellulose
paper to fabricate asymmetric and symmetric porous materials, including Janus membranes and
multilayer membranes with selective wettability. We also demonstrated the scalability of the
deposition process using a compact roll-to-roll module.73 These membranes fabricated via iPECVD
have applications in a variety of fields, including filtration and separation membranes, microfluidic
diagnostic devices, and smart wound dressings. By using other monomers that can be vaporized, we
can use iPECVD to deposit polymers with different functionalities, such as charged polymers for
separations and pH-responsive polymers.
8
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9

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10

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Chapter 2. Surface Functionalization of 3D-Printed Plastics via Initiated
Chemical Vapor Deposition
Published as: Cheng, C.; Gupta, M. Beilstein J. Nanotechnol. 2017, 8, 1629–1636.
2.1 Abstract
3D printing is a useful fabrication technique because it offers design flexibility and rapid
prototyping. The ability to functionalize the surfaces of 3D printed objects allows the bulk
properties, such as material strength or printability, to be chosen separately from surface properties,
which is critical to expanding the breadth of 3D printing applications. In this work, we studied the
ability of the initiated chemical vapor deposition (iCVD) process to coat 3D printed shapes
composed of poly(lactic acid) and acrylonitrile butadiene styrene. The thermally insulating properties
of 3D printed plastics pose a challenge to the iCVD process due to large thermal gradients along the
structures during processing. In this study, processing parameters such as the substrate temperature
and the filament temperature were systematically varied to understand how these parameters affect
the uniformity of the coatings along the 3D printed objects. The 3D printed objects were coated
with both hydrophobic and hydrophilic polymers. Contact angle goniometry and X-ray
photoelectron spectroscopy were used to characterize the functionalized surfaces. Our results can
enable the use of iCVD to functionalize 3D printed materials for a range of applications such as
tissue scaffolds and microfluidics.

17
2.2 Introduction
Three-dimensional printing (3DP) is a useful fabrication technique that offers rapid and low-
cost prototyping, high levels of design complexity, and resolution on the micron scale.1,2 These
attractive features have led to applications of 3DP in diverse fields including tissue engineering,2,3
microfluidics,4 robotics,5 and batteries.6,7 3DP involves a computer-aided design of the target
structure sliced into 2D layers and printed layer-by-layer.2,3 Four methods of 3DP are most common.
Fused deposition modeling (FDM) involves heating a feed filament past the melting point of the
material and extruding it onto a platform, which moves progressively downwards as layers are
printed.8,9 Inkjet printing deposits droplets of ink onto a platform, with ink flow regulated by a
piezoelectric actuator.10,11 Selective laser sintering uses a laser beam to heat a layer of powder above
its melting point, fusing it to the previous layers, and then new powder is subsequently rolled over
the printed object.12,13 In stereolithography (SLA), a laser or UV beam selectively hardens layers of
photocurable resin and then the object is covered with another layer of fresh resin.14,15
Though the number of printable functional materials is burgeoning,1,16,17 tuning the material
properties within the constraints of printability is still a challenge. This limitation presents a problem
for application-driven print objects, because consideration of material printability must supersede
other functionalities, such as biocompatiblity or stimuli-responsiveness. Thus, controlling post-
printing surface properties is critical to expanding the breadth of 3DP applications, because it allows
for tuning of bulk properties, such as cost-effectiveness or structural rigidity, independently of
sophisticated surface functionalization. For example, in scaffolds for bone tissue engineering,
angiogenesis is a major challenge, because printed scaffolds have hydrophobic surface properties and
do not promote cellular differentiation.3,18 Surface modification of printed scaffolds can allow for the
tuning of surface functionalization to promote vascularization and tissue regeneration while
maintaining control over the mechanical robustness of the bulk structure. Hong et al. demonstrated
18
that simply dipping polycaprolactone/poly(lactic-co-glycolic acid) 3D scaffolds in mussel adhesive
proteins promoted cellular adhesion, proliferation, and differentiation, showing that a facile surface
modification improved the viability of using 3D printed scaffolds for tissue engineering
applications.18 In another example of surface functionalization, Wang et al. reported a method for
modifying the surfaces of 3DP structures fabricated via SLA by using a UV-curable resin with an
embedded alkyl-bromide initiator from which atom transfer radical polymerization was initiated.19,20
They demonstrated that complex 3D printed structures could be coated with hydrophobic polymers
and various metals. However, this coating technique is limited to photocurable resins into which the
polymerization initiator has already been incorporated, which restricts surface modification to only
SLA printed objects and wastes unused initiator embedded within the bulk structure. The breadth of
materials and feature sizes of 3D printed objects presents a challenge to finding a universal method
for surface functionalization.
Initiated chemical vapor deposition (iCVD) is a technique that can be used to deposit
functional polymer coatings.21,22 In the iCVD process, monomer and tert-butyl peroxide (TBPO)
initiator are introduced in the vapor phase to a reactor chamber under vacuum, whereupon the
initiator is thermally cleaved by a heated filament array. Monomer and initiator radicals adsorb to
substrates on a cooled stage where polymerization occurs. The molecular weight increases with
decreasing substrate temperature and typical molecular weights are in the range of 50,000 to
200,000.23,24 The iCVD process is solvent-free and therefore surface tension effects are avoided,
allowing for conformal coating on complex surfaces such as mictrotrenches25 and nanopore
membranes.26 Since the rate of reaction in iCVD is limited by adsorption of monomer to the
substrate, a lower substrate temperature results in a faster polymerization rate.24 Thus, the thermally
insulating properties of macro-scale 3D printed plastics pose a challenge to the iCVD process.
Although there have been previous reports of iCVD deposition onto thermally insulating substrates
19
such as tissue wipes,27 glass,28 and poly(ethylene naphthalate),29 these substrates were typically less
than 1 mm in thickness and therefore the thermal gradients were modest. In contrast to these
previous studies, our 3D printed objects are over 5 mm in thickness and therefore the significant
thermal gradients may impact the deposition process.
In this study, we printed the 3D objects using both poly(lactic acid) (PLA) and acrylonitrile
butadiene styrene (ABS) in order to study the generality of the coating process for modifying the
surfaces of different plastics. We investigated the deposition of poly(1H,1H,2H,2H-perfluorodecyl
acrylate) (PPFDA)23 and poly((2-hydroxyethyl methacrylate)-co-(ethylene glycol diacrylate))
(P(HEMA-co-EGDA))30 onto 3D objects of a variety of shapes and sizes to study the capabilities
and limitations of the iCVD process. X-ray photoelectron spectroscopy (XPS) and contact angle
goniometry were used to study the surface properties before and after coating.

2.3 Results and Discussion

Figure 2-1. Schematic of the iCVD process. The 3D printed substrate (white lattice) is
placed on a silicon wafer piece on a temperature-controlled stage. Initiator (I-I) and
monomer (M) in vapor phase are introduced into the reactor and the filament array is heated
to thermally cleave the initiator.
A schematic of the iCVD deposition process onto 3D printed substrates is shown in Figure
2-1. To systematically study the uniformity of the iCVD coatings, PPFDA was deposited onto 3D
printed PLA lattices of 7.5 mm in height. PPFDA was chosen as a model polymer because it is easily
discernable from the underlying substrate via XPS.23 Additionally, the relatively high water contact
angle on flat PPFDA (121°)31 compared to that on flat PLA (72.5°)32 allows for the use of contact
20
angle goniometry to verify polymer deposition. Substrates were printed with PLA because of its ease
of printing, low cost, and prior use in biomedical applications.33 A silicon wafer piece was placed
under the substrate to visually observe the penetration of polymer through the lattice.

Figure 2-2. (a) Static contact angles for a 7.5 mm tall PLA lattice that was uncoated and
coated with PPFDA. (b) FTIR spectra of the PPFDA film (top) and liquid monomer
(bottom). Dashed lines correspond to the CF2 and CF3 signals, and the asterisks in the
monomer spectrum correspond to the vinyl signals. (c) Representative XPS survey spectrum
of the top of the lattice coated with PPFDA.
To measure the change in hydrophobicity of the 7.5 mm PLA lattice after the deposition of
PPFDA, contact angle changes were monitored (Figure 2-2a). Variations in contact angle
21
measurements at the top and bottom of the pieces and among different pieces can be attributed to
slight variations in geometry during the printing process. During deposition, the top side was closer
to the heated filament array and the bottom side was placed on a silicon wafer piece on the stage.
After coating, the contact angle changed from 110°  2° to 122°  2° at the top of the lattice and
from 103°  2° to 131°  8° at the bottom, indicating that both the top and bottom of the lattice
were coated with PPFDA. The contact angles are higher than that of flat PLA and flat PPFDA due
to surface roughness.34 Penetration of polymer through the lattice was also confirmed by deposition
on the silicon wafer piece underneath the lattice. We used Fourier transform infrared spectroscopy
(FTIR) (Figure 2-2b) to compare the spectra of the PPFDA film deposited on the silicon wafer (top)
and the liquid monomer (bottom). The peaks at 1250, 1200, and 1150 cm-1 in the polymer confirm
the presence of the CF2 and CF3 groups and the absence of signal from the vinyl bond in the
polymer spectrum at 1640, 1620, 1410, 1400, 1300, 1080, 986, and 971 cm-1 indicates that all the
vinyl bonds were completely reacted. Additionally, the presence of PPFDA at the top of the lattice
was verified using X-ray photoelectron spectroscopy (XPS) to analyze the chemical composition of
the surface (Figure 2-2c). The survey spectrum of the top of the PPFDA coated lattice had atomic
percentages of 51.2% F, 6.2% O, and 42.6% C on a hydrogen-free basis, which agreed well with the
theoretical composition of PPFDA (53.1% F, 6.3% O, 40.6% C) rather than that of PLA (40% O,
60% C) indicating that there is at least 5 nm of PPFDA coating at the top of the lattice since XPS
probes the top 5 nm of the surface. Scanning electron microscopy (SEM) images of the lattice
(Figure 2-3) reveal that the appearance before modification (left) and after PPFDA coating (right)
are similar since the thickness of the polymer coating is much smaller than the feature size.
22

Figure 2-3. SEM images of the lattice before (left) and after (right) PPFDA coating.
Table 2-1. XPS survey spectra for PLA lattices.
Sample
Stage
Temperature (°C)
Filament
Temperature (°C)
Position % F % O % C
Reference
PPFDA
53.1 6.3 40.6
S1 15 250
Top 43.8 8.1 48.1
Bottom 49.6 6.6 43.8
S2 35 250
Top 46.4 6.6 47.0
Bottom 51.1 6.2 42.7
S3 45 250
Top 53.0 7.0 40.0
Bottom 51.6 6.2 42.2
F1 35 220
Top 50.7 6.9 42.4
Bottom 50.1 6.5 43.4
F2 35 190
Top 49.7 6.8 43.5
Bottom 53.9 6.3 39.8
H1 15 250
Top 27.9 15.7 56.4
Bottom 52.9 6.4 40.7
H2 45 250
Top 43.8 7.9 48.3
Bottom 53.3 6.5 40.2
Absorption 45 250
Top 19.0 15.3 65.7
Bottom 26.2 13.8 60.0

The stage temperature can impact the thermal gradient during polymerization. The
concentration of monomer at the surface of the substrate increases with decreasing temperature as
previously shown by quartz crystal microbalance experiments by Lau and Gleason.24 At a given stage
temperature, we expect different temperatures at the top and bottom of the 3D printed objects due
to the heat from the filament array. To systematically study this effect, we studied depositions at
stage temperatures of 15, 35, and 45 °C (Table 2-1). For these stage temperatures, the temperature at
the bottom of a 7.5 mm lattice was measured to be 31, 43, 48 °C, respectively and the temperature at
23
the top of the lattice was measured to be 62, 77, and 80 °C, respectively. These 30–35 °C
temperature differences are due to the large height and thermally insulating properties of the PLA
lattice. After PPFDA deposition at a stage temperature of 15 °C (S1), the contact angle of the lattice
increased from 105°  2° to 126°  5° at the top and from 99°  8° to 131°  4° at the bottom.
After PPFDA deposition at a stage temperature of 35 °C (S2), the contact angle at the top of the
lattice increased from 109°  5° to 127°  3° at the top and from 104°  2° to 139°  3° at the
bottom. After PPFDA deposition at a stage temperature of 45 °C (S3), the contact angle of the
lattice increased from 111°  6° to 125°  4° at the top and from 105°  2° to 133°  4° at the
bottom. These contact angle increases indicate that the lattices were coated at both the top and
bottom at the three stage temperatures, despite the large temperature gradients. To further verify the
presence of the PPFDA coatings, XPS was used to measure the atomic composition at the top and
bottom of the lattices (Table 2-1). For the three stage temperatures, the atomic compositions of the
bottom match well with the theoretical composition of PPFDA, again indicating that there is at least
5 nm of coating at the bottom of the lattice, however the top sides of S1 and S2 have slightly less
fluorine indicating less coating.
Another challenge for iCVD onto plastic materials is the potential for precursor molecules
to absorb into the substrate. To verify that our XPS signals are due to polymer and not due to
monomer, a PLA lattice was placed in the reactor and exposed to the same deposition conditions
except without the presence of initiator. Polymerization does not occur because of the absence of
the initiator, but the heated filament causes the same thermal gradients in the PLA lattice that were
present during depositions. After monomer exposure, the contact angle changed from 106°  3° to
119°  5° at the top of the lattice and from 107°  4° to 119°  4° at the bottom of the lattice. This
contact angle increase indicates that some monomer was absorbed into the lattice. XPS of the
24
sample (Table 2-1) shows the presence of a fluorine signal, which is absent in PLA, confirming the
presence of monomer in the lattice. However, this fluorine signal is much less than that for the
PPFDA polymer. Therefore, we can conclude that although there may be some monomer
absorption during deposition, the large fluorine signals from the samples S1-S3 match the theoretical
PPFDA values and therefore confirm the presence of a polymer coating of more than 5 nm.
To decrease thermal gradients during the deposition of PPFDA onto the PLA lattices, the
filament temperature can be reduced. We therefore investigated whether a uniform coating could
still be achieved with lower filament temperatures. The filament temperature was lowered to 220 °C
(F1) and the contact angles changed from 106°  5° to 127°  3° at the top of the lattice and from
102°  6° to 140°  3° at the bottom of the lattice. The filament temperature was further lowered to
190 °C (F2) and the contact angles changed from 106°  3° to 128°  2° at the top of the lattice and
from 99°  6° to 133°  5° at the bottom of the lattice. At both filament temperatures, the top and
bottom of the lattice exhibited contact angle increases, indicating that the lattices could be coated at
lower filament temperatures. Additionally, XPS of the lattices (Table 2-1) showed that the atomic
composition agreed well with that of PPFDA, indicating that there is at least 5 nm of PPFDA at the
top and bottom of both samples F1 and F2.
To further investigate the effects of thermal gradients, the lattice size was increased to 25
mm, which reaches to 6 mm below the filament array. For stage temperatures of 15 and 45 °C, the
temperature at the top of the lattice was measured to be 97 and 103 °C, respectively. At a stage
temperature of 15 °C (H1), the contact angle changed from 107°  5° to 118°  4° at the top of the
lattice and from 103°  8° to 125°  5° at the bottom of the lattice. XPS of the sample (Table 2-1)
showed that the atomic composition at the bottom agreed well with PPFDA, however the
composition at the top was similar to the signal found for monomer absorption, indicating that
25
deposition did not occur. For a stage temperature of 45 °C (H2), the contact angle changed from
101°  3° to 123°  2° at the top of the lattice and from 93°  7° to 134°  6° at the bottom of the
lattice. XPS of the sample (Table 2-1) showed that the bottom was coated, however the decreased
fluorine coating at the top demonstrated less coating. These samples indicate that the thermal
gradients in very tall 3D objects can inhibit polymerization close to the filament. These thermal
effects could be combatted by increasing the height of the filament array, optimizing substrate
orientation, or lowering the substrate temperature.

Figure 2-4. PLA lattices coated with PPFDA (left), uncoated (center), and coated with
P(HEMA-co-EGDA) (right) in water.
To demonstrate the generality of the iCVD process for depositing different functional
polymers onto 3D printed substrates, 7.5 mm tall lattices were also coated with a hydrophilic, cross-
linked polymer. PHEMA was chosen as the model hydrophilic polymer because it is biocompatible
and has been used in biomedical applications.35,36 However, because PHEMA is water soluble, a
small amount of cross-linker EGDA was incorporated during the deposition to ensure that the
hydrophilic polymer coating would not dissolve in water. As shown in Figure 2-4, an uncoated PLA
lattice did not sink in water, despite PLA having a density of 1.25 g/cc. Since the uncoated PLA is
hydrophobic, the pores of the lattice remained filled with air instead of wetting readily, which
sufficiently reduced the overall density of the lattice causing it to float. Similarly, a lattice coated with
PPFDA did not sink, because its enhanced hydrophobicity caused its pores to also remain filled with
26
air. Unlike the hydrophobic lattices, the lattice coated with P(HEMA-co-EGDA) wicked water into
its pores because of the hydrophilicity and the lattice sank in the water. To demonstrate the efficacy
of the incorporated cross-linker for preventing dissolution of the polymer coating, the lattice coated
with P(HEMA-co-EGDA) was soaked in water for three days and then was dried and placed back
into the water, whereupon it sank, demonstrating the retention of its hydrophilicity.

Figure 2-5. Water droplets (colored with blue food coloring) on an uncoated lattice and on
a 25 mm tall PLA lattice coated with P(HEMA-co-EGDA).
To study the limitations of the iCVD process for coating macro-scale plastics, P(HEMA-co-
EGDA) was deposited onto a 25 mm tall lattice. In Figure 2-5, the bottom of the lattice was placed
on the silicon wafer piece on the stage and the top of the lattice was nearest to the filament. From
Figure 2-5, the bottom of the lattice exhibited hydrophilic properties verifying that it was coated
with P(HEMA-co-EGDA). The middle and top of the lattice wicked water, but less readily than the
bottom of the lattice, indicating partial polymer coverage. Comparison of the droplets on the coated
lattice show that the coated lattice is more hydrophilic than the uncoated lattice.
27

Figure 2-6. Sequential deposition of P(HEMA-co-EGDA) and PPFDA. (a) Nut, bolt, and
comb in the iCVD reactor. Water droplets (colored with blue food coloring) on ABS
substrates (b) without coating, (c) coated with P(HEMA-co-EGDA), and (d) coated with
PPFDA.
A unique feature of the iCVD process is facile layering of polymer coatings with different
chemistries, which allows for tuning of surface properties. To demonstrate this feature, substrates
were coated with a hydrophilic copolymer followed by a hydrophobic polymer. Substrates were
printed with ABS to demonstrate the generality of the substrate material. A comb, nut, and bolt
28
were all coated in the same deposition to show that objects with complex features can be easily
coated using the iCVD process (Figure 2-6a). The uncoated ABS substrate surfaces were
hydrophobic (Figure 2-6b). The substrates were first coated with P(HEMA-co-EGDA), after which
the comb, nut, and bolt all were readily wetted (Figure 2-6c). Following the coating with hydrophilic
polymer, the substrates were then coated with PPFDA, and the substrate surfaces regain
hydrophobicity (Figure 2-6d). These sequential depositions of polymers with different chemistries
show that the substrate surface properties can be readily tuned using the iCVD process.

2.4 Conclusions
The iCVD process was used to modify the surfaces of 3D printed polymer substrates with
complex geometries. The lattices studied were 7.5 and 25 mm tall, which were significantly larger
than insulating substrates that were coated in previous iCVD reports. Both hydrophobic (PPFDA)
and hydrophilic (P(HEMA-co-EGDA)) polymer coatings were deposited onto substrates made of
PLA and ABS. Thermal gradients over PLA lattices were studied and shown to decrease polymer
coverage on 25 mm substrates, but these effects can be overcome by optimizing substrate
orientation and lowering the substrate temperature. Additionally, the surface properties of the
substrates can be tuned using sequential polymer depositions. This coating process can be
generalized to modify the surface properties of a variety of 3D printed materials for potential
applications in tissue grafting, microfluidics, and electronics.

2.5 Experimental Details
1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (SynQuest Laboratories, 97%), 2-
hydroxyethyl methacrylate (HEMA) (Aldrich, 97%), ethylene glycol diacrylate (EGDA)
(Polysciences, Inc.), and tert-butyl peroxide (TBPO) (Aldrich, 98%), were used as received without
29
further purification. 3D printed PLA lattices (Invention’s Hub, Mission Hills, CA) were also used as
received. The comb, nut, and bolt were printed on a MakerBot Replicator 2X using ABS filament in
True Red.
Polymerization was carried out in a custom-built iCVD reactor (GVD Corporation, 250 mm
diameter, 48 mm height). Substrates were placed on silicon wafer pieces (Wafer World, 100 mm) on
a stage that was temperature-controlled by a backside recirculating chiller (Thermo Scientific
NESLAB RTE 7). The orientation of the lattice during polymer deposition was controlled for
consistency. In FDM, the first layer is printed onto a heated, flat build plate which causes the
bottom of the layer to be discernably flatter than subsequent extruded layers. This first printed layer
was placed facing downwards during all iCVD depositions. Prior to polymer deposition, the stage
was cooled to 14 C for an hour to reduce the temperature of the substrates. Reactor pressure was
achieved by a rotary vane vacuum pump (Edwards E2M40) controlled by a throttle valve (MKS
153D) and measured with an ambient temperature capacitance manometer (MKS 622C01TDE
Baratron). Monomers were loaded into stainless steel jars and subsequently attached to the reactor
chamber. To achieve appropriate monomer vapor pressure, PFDA was heated to 50 °C (for S1-S3,
F1-F2, H1-H2) or 60 °C for all other depositions; HEMA was heated to 50 °C, and EGDA was
heated to 35 °C. TBPO was kept at room temperature of 25 °C and introduced into the reactor
using a mass flow controller (MKS Type 1152C).
Immediately before polymer deposition, the stage temperature was raised to the appropriate
temperature for deposition, which was 35 °C unless otherwise stated. During deposition, a nichrome
filament (Omega Engineering, 80%/20% Ni/Cr) array held at 31 mm above the substrates was
resistively heated to 250 °C, unless otherwise stated, to thermally cleave the peroxide bond of the
initiator. The deposition rate was monitored in situ via interferometry on a reference silicon wafer
using a He-Ne laser (Industrial Fiber Optics, 633 nm). For S1-S3, F1-F2, and H1-H2, PFDA and
30
TBPO were introduced into the reactor at flow rates of 0.26 and 1.8 sccm, respectively. Reactor
pressure was maintained at 50 mTorr, and deposition was carried out for 1 hour. For the other
depositions of the hydrophobic coating, PFDA and TBPO were introduced into the reactor at flow
rates of 0.6 and 1.0 sccm, respectively. Reactor pressure was maintained at 100 mTorr and
deposition proceeded for 1 hour. For the deposition of the cross-linked hydrophilic coating, HEMA
was introduced at a flow rate of 1.0 sccm, EGDA was introduced at 0.14 sccm, and TBPO was
introduced at 1.3 sccm. Reactor pressure was maintained at 130 mTorr and deposition was carried
out for 1.5 hours. To coat samples with multiple polymer layers, the substrates were removed from
the reactor, rinsed with deionized water, and characterized between polymer layers.
Contact angles were measured on a goniometer (ramé-hart 290) with 10 L droplets of
deionized water. Five measurements were taken per sample and averaged and profile images were
taken using the goniometer camera. Additionally, because the lattice structure consists of alternating,
crosshatched layers, the structure has visible grooves depending on the viewing orientation.
Therefore, to measure contact angles, the lattice was oriented such that the grooves were orthogonal
to the goniometer camera. The chemical functionality of samples was studied using a Fourier
transform infrared spectrometer (Thermo Scientific i510), with a resolution of 4 scans over a total of
32 scans. The surface composition of samples was studied using X-ray photoelectron spectrometer
(Kratos Axis Ultra DLD) with a monochromatic Al K  source. Survey spectra were taken from 0 to
800 eV in steps of 1 eV, averaged over five scans. Sample morphology was studied using a scanning
electron microscope (Topcon Aquila), and samples were sputtered with a thin layer of silver
(Cressington Sputter Coater 108) prior to imaging.

31
2.6 Acknowledgments
This work was supported by the National Science Foundation under Award No. 1332394.
The authors thank June Park for her assistance.

2.7 References

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Chapter 3. Roll-to-Roll Surface Modification of Cellulose Paper via
Initiated Chemical Vapor Deposition
Published as: Cheng, C.; Gupta, M. Ind. Eng. Chem. Res. 2018, 57, 11675–11680.
3.1 Abstract
Tuning surface properties of flexible materials enhances the versatility of existing materials,
giving them new functions for applications in textiles, filtration, flexible electronics, and sensors.
However, traditional surface modification methods are typically solvent-based, which limits the
range of substrates that can be coated. In this work, we demonstrate the ability to use roll-to-roll
processing to continuously modify the surface properties of large areas of flexible substrates using
initiated chemical vapor deposition, which is an all-dry process. We designed and built a roll-to-roll
module that can be used to uniformly coat 1500 cm2 of chromatography paper in a single
deposition. Rolls of paper were coated with a fluoropolymer and an ionizable polymer, and the
coated paper was used for origami, nonstick surfaces, and paper-based microfluidic devices.

Initiator
Monomer
Vapors
out
Feed roll
Receiver roll
Filament array
Origami SLIPS
Microfluidic
channel
37
3.2 Introduction
The surface modification of flexible materials is important for a variety of applications,
including flexible electronics,1,2,3 wound dressings,4,5 paper-based microfluidics,6,7 and desalination
membranes.8,9 Typical methods for modifying flexible materials are solution-based;2—9 however,
solvent effects can limit the efficacy of these processes. Finding a common solvent that is
compatible with both the substrate and the coating is not always possible, and surface-tension
effects prevent conformal coating of micro- or nanoscale features. As a result, vapor-phase
processes such as initiated chemical vapor deposition (iCVD)10,11 and plasma enhanced chemical
vapor deposition (PECVD)12,13 are preferred for coating structured surfaces.
In the iCVD process,10,11 monomer vapor and initiator di-tert-butyl peroxide (TBPO) vapor
are introduced into a reactor chamber under vacuum. The peroxide bond in TBPO is thermally
cleaved by a heated filament array, initiating free radical polymerization with the desired monomer.
The polymer is deposited onto substrates at ambient temperatures which prevents degradation of
the substrates during the coating process. Because iCVD is a vapor-phase process, surface tension
effects are avoided, resulting in conformal coatings on a variety of structured surfaces including
microtrenches14,15 and microporous membranes.16,17 A variety of functional polymers can be
deposited to fabricate nonstick surfaces,18,19 antibiofouling materials,20,21 and paper-based
microfluidics.22,23
Scaling up the iCVD process can enable the high-throughput manufacturing of coated
substrates in large quantities for realistic commercial applications such as scalable production of
paper-based microfluidic analytical devices, bandages, and flexible electronics. The scale-up of vapor
phase coating techniques including atomic layer deposition (ALD),24 PECVD,25 and oxidative
chemical vapor deposition (oCVD)26 has been previously demonstrated. For example, Parsons and
38
colleagues demonstrated high-throughput ALD of alumina coatings on textiles using spatial ALD, in
which substrates were moved through zones of precursor exposure.24 PECVD has been used on the
commercial scale since the 1980s, expanding from its origins in the semiconductor industry to other
applications in soft materials. Wertheimer and coworkers designed a roll-to-roll process for coating
polymer substrates.27 The PECVD of polymers compared to iCVD has disadvantages that include
damage to the substrate via etching, poor retention of deposited functional groups, and
nonconformal coatings.28 Gleason and coworkers reported custom modifications to a large roll-to-
roll PECVD reactor to scale up oCVD of a conductive polymer26 and iCVD of poly(glycidyl
methacrylate).29 These modifications involved redesigning several reactor components and resulted
in significant changes to the iCVD process, including moving the substrate vertically, which gives
rise to a convective flow pattern different than that in a conventional horizontal iCVD reactor.
In this paper, we demonstrate the ability to coat large areas of flexible materials in a
nonmodified iCVD chamber by designing a compact roll-to-roll module from low-cost, readily
available materials. With our compact roll-to-roll module, we can coat several meters of substrate in
a single deposition without modifying the iCVD process. We demonstrate the versatility of our
system by conformally coating 3 m-long rolls of 5 cm-wide chromatography paper with a
hydrophobic polymer, poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA), and a cross-linked
ionizable polymer, poly(methacrylic acid-co-ethylene glycol diacrylate) (xPMAA). The uniformity of
the coating along the length and the width of the substrate was confirmed using X-ray photoelectron
spectroscopy (XPS) and contact angle goniometry, and the conformality of the polymer on the
cellulose fibers was verified with scanning electron microscopy (SEM). We demonstrate the
functionality of the coated substrates by fabricating slippery liquid-infused surfaces (SLIPS),
microfluidic channels, and origami structures.

39
3.3 Experimental Details
1H,1H,2H,2H-Perfluorodecyl acrylate (PFDA) (SynQuest Laboratories, 97%), methacrylic
acid (MAA) (Aldrich, 99%), ethylene glycol diacrylate (EGDA) (PolySciences, Inc.), TBPO (Aldrich,
98%), silicon wafers (Wafer World 119), Krytox 1525 (Aldrich), food color (Kroger), ethyl violet
(Sigma), crystal violet (Aldrich, 90%), Ponceau S (Aldrich, 75%), pH 10 buffer solution (BDH, ACS
grade), and grade 1 chromatography paper (Whatman) were used as received.
To build the roll-to-roll module, two motors (Uxcell DC gear motor 12 V 2 rpm) were
mounted onto L-shaped brackets (Uxcell 37 mm motor mounting bracket holder). A strip of
plexiglass (Source One Premium, 1/16 in.) was laser cut, and the two brackets were then mounted
onto the ends, approximately 12.5 cm apart. Metal rods (3 mm diameter, 6 cm length) were
connected to the motors with an aluminum alloy shaft coupler (Uxcell), and the rods served as
spools for the flexible substrate. All parts for the roll-to-roll module were purchased from Amazon
for $70. The motors were connected with copper wire to external power supplies with variable
voltages (VOLTEQ HY3010D).
The iCVD process was carried out in a cylindrical reactor (25 cm diameter, 5 cm height)
(GVD Corporation) under vacuum maintained by a rotary vane vacuum pump (Edwards E2M18).
Reactor pressure during depositions was controlled using a throttle valve (MKS 153D) controlled by
a capacitance manometer (MKS 622C01TDE Baratron). Monomers were loaded into stainless steel
jars that were then mounted onto the reactor, and the jars were heated to achieve appropriate vapor
pressure for deposition. PFDA was heated to 50 °C; MAA was kept at 25 °C, and EGDA was
heated to 45 °C. The initiator TBPO was kept at 25 °C, and its flow rate was controlled using a mass
flow controller (MKS Type 1152C). The reactor stage temperature was controlled using a backside
recirculating chiller (Thermo Scientific NESLAB RTE 7) and was kept at 30 °C.
40
During depositions, the nichrome filament array (Omega Engineering, 80%/20% Ni/Cr)
held at 4.6 cm above the stage was resistively heated to 230 °C to thermally cleave TBPO, initiating
free radical polymerization. For depositions of PPFDA, monomer and initiator were introduced into
the reactor at 0.3 and 2 sccm, respectively, and the reactor pressure was maintained at 70 mTorr. For
depositions of xPMAA, MAA, EGDA and initiator were introduced into the reactor at 5, 0.1, and 2
sccm respectively, and the reactor pressure was kept at 250 mTorr. The deposition rate was
monitored on a reference silicon wafer piece in situ using interferometry with a He—Ne laser
(Industrial Fiber Optics, 633 nm) and was 4 nm/min for PPFDA and 8 nm/min for xPMAA. For
roll-to-roll depositions, polymerization was carried out for 3 min on the substrate before starting the
rolling, for 2 h onto the moving substrate, and for an additional 3 min on the substrate after rolling.
For the deposition of PPFDA onto stationary paper, polymer was deposited for 1 h. For the
deposition of xPMAA onto stationary paper, polymer was deposited for 10 min. The temperature of
the paper during a deposition was measured by mounting a thermocouple to the paper in the middle
of the primary deposition zone and depositing PPFDA for 30 min. Distances along the length of the
paper were defined such that the part of the roll that was first coated was 0 m, and distances across
the width of the paper were defined such that the edge of the paper closest to the gas feed was 0 cm.
Atomic compositions of deposited PPFDA coatings and uncoated paper were characterized
using an X-ray photoelectron spectrometer (Kratox Axis Ultra DLD) with a monochromatic Al Kα
source. Survey spectra were taken from 800 to 0 eV in 1 eV steps and averaged over 5 scans. Spectra
were referenced to 284.8 eV for the C–C peak. Morphology of paper surfaces were imaged using a
scanning electron microscope (JSM 7001F) at an operation voltage of 20 kV. Deposited PPFDA
coatings were also characterized using a contact angle goniometer (ramé-hart 290). Reported contact
angles were averaged over 20 measurements with a drop volume of 5 L of deionized water, and
41
error bars represent one standard deviation below and above the average. Droplet profile images
were captured using the goniometer camera. Camera images were taken using a Nikon D3000.
Origami was performed on three 5  5 cm squares of paper taken from near the center of
the roll-to-roll paper coated with PPFDA. The three squares were folded and joined together to
form a single bowl. The SLIPS material was made from paper cut from near the center of the roll-
to-roll paper coated with PPFDA. Krytox was added to the fluorinated paper via pipet to form a
SLIPS material, which was placed on a glass slide and tilted at an angle of 15° using a tilting-base
goniometer. Solvents were dispensed in 30 L droplets. Paper-based microfluidic devices were made
from roll-to-roll paper coated with xPMAA by cutting 1 cm-wide strips along the length of the
paper (the length of the strip was 5 cm, spanning the width of the roll). The ability of uncoated and
coated paper to separate dyes was analyzed by applying 1 L droplets of a buffered pH 10 solution
of 2.5 mg/mL crystal violet and 0.25 mg/mL Ponceau S onto the paper. One end of the channel
was placed in buffered pH 10 solution such that the eluent wicked vertically up the paper.

42
3.4 Results and Discussion

Figure 3-1. (a) Photograph of the roll-to-roll module in an iCVD reactor chamber. A 3 m-
long roll of chromatography paper is mounted on the module. (b) Schematic showing that
the substrate rolls from the feed roll, through the primary deposition zone (shown in blue),
and onto the receiver roll.
We designed a compact roll-to-roll module that could be inserted into an iCVD reactor
without any modification to the reactor chamber (Figure 3-1a). Two motors are used to move the
substrate during the deposition. The substrate unwinds from the feed roll, passes through the
primary deposition zone (12.5 cm long) with the top of the substrate facing the filament array and
the bottom of the substrate facing the stage, and winds onto the receiver roll (Figure 3-1b). The
velocity at which the substrate travels through the primary deposition zone can be varied between
2.5–6 cm/min for our roll-to-roll module. In the primary deposition zone, the substrate is
a)
b)
Heated
filaments
Motor
Motor
Vapor
inlet
Outlet
to
pump
Rolling
direction
Length
Width
Receiver
roll
Feed
roll
Primary
deposition
zone
43
suspended 1 cm above the stage to maintain sufficient tension in the paper for rolling. The heated
filament array is placed above the roll-to-roll module at 4.6 cm above the stage. The filaments
directly above the motors were removed to prevent touching the motors which have a height of 4.8
cm above the stage. The addition of the roll-to-roll module did not require changes to the iCVD
process itself, beyond the modification of the filament array, which makes the module an attractive
way to scale up existing iCVD processes.
We chose to coat Whatman #1 chromatography paper as a model substrate because it has
several properties that make it attractive for roll-to-roll processing: (i) it is flexible and can be shaped
into rolls, (ii) it is porous and therefore we can study whether the polymer coating penetrates
through the entire thickness of the paper (180 m), (iii) it is foldable and can be shaped after
coating, and (iv) it can be used for applications such as paper-based analytical devices and filtration.
We placed a 3 m-long roll of 5 cm-wide paper into the reactor and continuously coated it with
PPFDA in a single deposition. In the iCVD process, the deposition rate of the polymer can be tuned
by changing the ratio of the monomer partial pressure to the monomer saturation pressure (PM/Psat),
which is proportional to the amount of monomer adsorbed to the substrate surface.30 In our
process, the temperature of the suspended paper substrate (68 °C) was higher than that of the
reference silicon wafer (30 °C) placed on the stage. The value of PM/Psat was 0.1 on the silicon wafer,
therefore we rolled the substrate at the slowest velocity (2.5 cm/min) to ensure that the substrate
was coated. The entire roll of paper was coated in 2 h. The processing time could be decreased by
increasing substrate velocity while increasing the deposition rate via increasing the flow rate of the
monomer or increasing the reactor pressure.
44

Figure 3-2. Atomic compositions from XPS survey spectra for uncoated paper, PPFDA
(theoretical), and on the top and bottom of roll-to-roll paper coated with PPFDA at multiple
points along the length of the coated paper.

Figure 3-3. SEM images of paper (a and b) before and (c and d) after roll-to-roll coating
with PPFDA.
We chose PPFDA as a model polymer because the presence of polymer coating along the
length and on both sides of the paper could be verified by XPS analysis. The fluorine content along
the length of the roll-to-roll coated paper (Figure 3-2) was attributed to the presence of PPFDA
(40.6% C, 6.3% O, and 53.1% F) since the uncoated paper is composed of cellulose which had a
measured composition of 60.5% C and 39.5% O and did not contain fluorine. The fluorine content
Uncoated Paper
Roll-to-Roll Coated
Paper (1.5 m)
Top
Bottom
100 µm
a)
b)
c)
d)
100 µm
100 µm 100 µm
45
along the length of the coated paper ranged from 45–52% F, which is slightly lower than that
expected for PPFDA likely because the thickness of the PPFDA coating on the paper is less than
the probe depth of XPS (about 5–10 nm), and therefore, the spectrometer is also sampling the
underlying paper. Comparisons of the atomic compositions along the length of the coated paper on
both sides show no significant variation, which indicates the uniformity of the PPFDA coating on
the paper during the roll-to-roll process. The topography of the paper after roll-to-roll coating was
characterized via SEM (Figure 3-3). Coated paper cut from the middle of the roll was imaged, and
the morphology of the paper was found to be unchanged compared to uncoated paper, confirming
that the individual fibers in the paper are conformally coated.

Figure 3-4. (a) Images of water droplets on uncoated chromatography paper, stationary
paper that was coated with PPFDA, and paper that was coated with PPFDA in a roll-to-roll
deposition. (b) Contact angle measurements of the top (triangles) and bottom (squares) of
the roll-to-roll paper coated with PPFDA as a function of distance along the length of the
paper.
0 0.5 1 1.5 2 2.5 3
Distance (m)
135
140
145
150
Contact Angle ( ° )
Top
Bottom
b)
a)
Uncoated
Paper
Stationary
Coated Paper
Roll-to-Roll
Coated Paper
(1.5 m)
Top
Completely
wets
142 º ± 2 º 141 º ± 3 º
Bottom
144 º ± 3 º 143 º ± 3 º
46
The uniformity of the PPFDA coating along the length of the roll-to roll coated paper was
further verified using contact angle measurements. Whereas uncoated chromatography paper wets
immediately (Figure 3-4a), the PPFDA coating on the roll-to-roll coated paper modifies the paper to
become hydrophobic (Figure 3-4b). There are no significant trends or variations among the contact
angles along the length of coated paper, confirming that the entire roll is coated uniformly, and the
contact angles on the top and bottom sides of the paper are comparable, demonstrating that the
coating has penetrated through the thickness of the paper. The contact angles are comparable to
those on stationary coated paper (Figure 3-4a), indicating that rolling the substrate does not affect
the polymerization process.
47

Figure 3-5. (a) Atomic compositions from XPS survey spectra across the width of the
coated paper at the center of the roll (1.5 m). (b) Contact angle measurements across the
width of the top (triangles) and bottom (squares) of the coated paper at various lengths (0.1,
1.5, and 2.9 m).
The uniformity of the polymer coating across the width of the paper was also studied. Figure
3-5a shows that the atomic compositions across the width of the coated paper at the center of the
roll ranged from 48–56% F and showed no trend, which demonstrates that the paper is coated
uniformly across the width. Additionally, contact angles across the width of the paper were taken at
multiple points along the length of the roll (Figure 3-5b) and there is no significant trend, further
demonstrating the uniformity of the coating across the width.
48

Figure 3-6. (a) Image of an origami bowl made with paper that was coated with PPFDA in a
roll-to-roll deposition. The bowl is filled with water that is dyed with blue food coloring. (b–
0 s 1 s 8 s
Water
Isopropanol
0 s 4 s 8 s
Chloroform
0 s 1 s 2 s
c)
b)
d)
5 mm
5 mm
5 mm
1 cm
a)
49
d) Time series images of dyed solvents rolling off of a SLIPS material at a 15° tilt. The
solvents used were (b) water dyed with red food coloring, (c) isopropanol dyed with green
food coloring, and (d) chloroform dyed with ethyl violet.
To demonstrate the hydrophobic properties of the coated paper, a section from the center
of the roll was folded into a bowl shape using origami (Figure 3-6a). The bowl was able to hold
water because of the surface interaction between the hydrophobic paper and the water, whereas
water seeps out of a bowl composed of uncoated paper. Additionally, the coated paper was used to
make SLIPS materials. The Aizenberg group has shown that porous substrates with low surface
energy can be infused with perfluorinated liquids to form SLIPS.31,32 Whereas the PPFDA-coated
paper is hydrophobic (repels water), the SLIPS material is omniphobic (repels polar and nonpolar
liquids). We made SLIPS materials by taking a section of coated paper from the center of the roll
and infusing it with perfluorinated liquid (Krytox). For our SLIPS material at a 15° tilt, water (Figure
3-6b), isopropanol (Figure 3-6c), and chloroform (Figure 3-6d) all rolled off within seconds, whereas
the latter two solvents do not roll off paper coated with PPFDA.

Figure 3-7. Images of a mixture of Ponceau S (pink) and crystal violet (violet) on paper.
Separations are shown on uncoated paper, stationary paper coated with xPMAA via iCVD,
and roll-to-roll paper coated with xPMAA at different distances along the length of the
paper. The arrow indicates the direction of eluent flow.
To demonstrate that a variety of polymers can be deposited using our roll-to-roll module, we
studied the deposition of xPMAA which can be used for the fabrication of functional paper-based
microfluidic devices. Paper-based microfluidic devices are useful for point-of-care diagnostics,33,34,35
Uncoated
Paper
Stationary
Paper
Roll-to-Roll Coated Paper
0.1 m 1.5 m 2.9 m
2.5 mm
50
and the carboxylic acid group of MAA allows for separation of analytes because it is deprotonated in
solution.36,37,38 During the iCVD process, the reactor pressure and the monomer flow rates were
selected such that the PM/Psat matched that of PFDA. After coating the roll, microfluidic channels
were cut at three distances along the length of the roll. The xPMAA coating separated a mixture of
crystal violet and Ponceau S in pH 10 buffer solution because the anionic xPMAA coating trapped
the cationic crystal violet as the anionic Ponceau S eluted upward, whereas the uncoated paper was
unable to separate the dyes (Figure 3-7). The separation on the three roll-to-roll coated channels was
comparable to that on stationary paper coated via iCVD, indicating that the paper was coated
uniformly along the entire length and width of the roll. Additionally, the separations on the coated
channels demonstrate that the polymer coating has penetrated through the depth of the paper. Past
work from our group demonstrated that iCVD could be used to pattern microfluidic channels into
cellulose paper to hold solutions within barriers.23,39 The ability to contain liquid within the
patterned channels indicated that the iCVD polymer penetrated through the depth of the paper. The
ability to coat an entire 3 m-long roll of paper in a single deposition allows for efficient fabrication
of 300 microfluidic channels that are 1 cm-wide and 5 cm-long.

3.5 Conclusions
We built a roll-to-roll module to scale up the iCVD process, allowing us to coat 1500 cm2 of
chromatography paper in a single deposition. The PPFDA coverage along the length and the width
of the 3 m-long roll of paper was shown to be uniform using XPS and contact angle goniometry,
and the conformality of the polymer coating around the individual fibers in the paper was verified
using SEM. The hydrophobic paper was used for paper origami and as a SLIPS substrate. We
demonstrated that the module could also be used to apply various polymer functionalities onto the
rolling substrates by depositing xPMAA, which enables the large-scale production of paper-based
51
microfluidic channels in a single deposition. The use of the roll-to-roll module can be extended to
other applications such as large-scale patterning through the use of polymer inhibition40,41 or UV-
responsive polymers.39 Although we demonstrated the process for a roll of several meters, the length
of the substrate can be further increased by increasing the height of the reactor with an extension
collar. Additionally, the substrate can be reciprocated back and forth, allowing for either the
deposition of a thicker polymer layer or for the layering of different polymers. The roll-to-roll
process can be extended to coat other flexible substrates for applications in flexible electronics,
wound dressings, and desalination membranes.

3.6 Acknowledgments
This work was supported by the National Science Foundation under Award 1332394. The
authors declare no competing financial interest.

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Chapter 4. Solvent-Free Synthesis of Selectively Wetting Multilayer and
Janus Membranes

4.1 Abstract
In this work, we systematically studied the deposition of hydrophobic coatings onto porous
materials for the fabrication of membranes with asymmetric and symmetric wetting properties.
Initiated plasma-enhanced chemical vapor deposition was used to deposit an organosilicon polymer
coating onto chromatography paper of different thicknesses. The hydrophobic organosilicon
polymer serves as an environmentally-friendly alternative to fluorinated polymers, and the all-dry
coating process does not use solvents. We demonstrate that the deposition time, thickness of the
paper, and orientation of the paper relative to the stage affect the symmetry of the wettability. The
chemical functionality of the deposited polymer was characterized via Fourier-transform infrared
spectroscopy and X-ray photoelectron spectroscopy. The hydrophobicity and wetting behavior of
the coated paper were characterized with contact angle goniometry and wicking studies, respectively.
We also demonstrate that we can scale up the process by using a roll-to-roll module. Our ability to
systematically tune the wettability of materials allows for the fabrication of multilayer and Janus
membranes that are useful for applications including filtration, smart textiles, flexible sensors, and
wound dressings.

58
4.2 Introduction
The ability to tune the symmetry of wettability for porous materials, such as textiles and
membranes, is important for enhancing their performance and functionality. For example, porous
Janus materials have asymmetric properties on two sides typically caused by different surface
chemistries.1,2,3 Janus membranes with asymmetric wetting can be used for directional transport of
liquids,4 and they have been used as separation membranes,5,6,7 moisture-wicking fabrics,8,9,10 and
medical wound dressings.11,12,13 Janus membranes can be made by either fabricating an asymmetric
material during membrane formation or by post-modification of a membrane to impart asymmetric
functionality.1,2 Vapor-phase techniques are ideal for asymmetric modification of porous materials
because they do not use solvents. For example, Waldman et al. fabricated Janus membranes using
atomic layer deposition of hydrophilic alumina onto hydrophobic polypropylene,14 Tian et al.
reacted hydrophobic perfluorooctyl-trichlorosilane vapor with hydroxy groups on hydrophilic
cellulose,15 and Cheng et al. modified a hydrophilic copper mesh with copper(II) hydroxide
nanowires with a hydrophobic fluorosilane vapor.16 In this paper, we demonstrate a scalable,
versatile, and solvent-free vapor-phase modification technique to apply hydrophobic polymer
coatings onto hydrophilic porous materials. Our fabrication process can be easily extended to other
precursors to deposit hydrophilic polymer coatings17,18 onto hydrophobic porous materials.
Although perfluoroalkyl coatings are very effective for making hydrophobic surfaces, they
have also been shown to degrade into byproducts that resist biodegradation and accumulate in
human and animal tissue.19,20 As a result, many companies have begun to phase out the use of
fluorinated coatings, and the United States Environmental Protection Agency has instituted an
action plan to address the public health impact of perfluoroalkyl substances.21 A potential
replacement for fluorinated polymers are organosilicon polymers, which are hydrophobic and have
59
low toxicity and environmental effects.22,23 Coclite and Gleason recently demonstrated that initiated
plasma-enhanced chemical vapor deposition (iPECVD) can be used to deposit organosilicon
polymers.24,25 iPECVD is an all-dry polymerization technique that utilizes a low power plasma to
preferentially cleave initiator molecules into radicals. These radicals react with monomer molecules
with unsaturated bonds, resulting in the deposition of a thin polymer coating on the substrate
surface. iPECVD has been shown to achieve high deposition rates of 30 nm/min, compared to 2.5
nm/min without the initiator.24 Polymers deposited via iPECVD can achieve high functional group
retention if a low plasma power (20–50 W) is used with the initiator tert-butyl peroxide.24 Because
iPECVD does not use solvents and can be used to deposit organosilicon polymers with low
environmental impacts, it is an attractive process for sustainably fabricating hydrophobic coatings
for Janus membranes.
In this work, we used iPECVD to deposit poly(1,3,5-trivinyl-1,1,3,5,5-
pentamethyltrisiloxane) (PTVTSO) onto porous cellulose paper to fabricate asymmetric and
symmetric porous materials, including Janus membranes and multilayer membranes with selective
wettability. Two different thicknesses of cellulose paper were coated, which affected the depth of
conformal polymer coverage through the thickness of the paper. Increasing the polymer deposition
time resulted in increasing the depth of conformal coating through the paper. The composition of
the deposited polymer coating was characterized using Fourier-transform infrared spectroscopy
(FTIR) and X-ray photoelectron spectroscopy (XPS), and the surface morphology of the coated
paper was imaged using scanning electron microscopy (SEM). The hydrophobicity and depth of
conformal polymer coverage on the paper were characterized using contact angle goniometry and
dyed water wicking. We also demonstrated the scalability of the deposition process using a compact
roll-to-roll module.26 We studied the effects of roll speed and found that the depth of conformal
coverage decreased with increasing roll speed for the thick paper. These membranes fabricated via
60
iPECVD have applications in a variety of fields, including filtration and separation membranes,
microfluidic diagnostic devices, and smart wound dressings. By using other monomers that can be
vaporized, we can use iPECVD to deposit polymers with different functionalities, such as charged
polymers for separations and pH-responsive polymers.

4.3 Experimental Details
1,3,5-trivinyl-1,1,3,5,5-pentamethyltrisiloxane (TVTSO; Gelest, 95%), di-tert-butyl peroxide
(TBPO; Aldrich, 98%), argon gas (99.999%), silicon wafers (Wafer World 119), Whatman 1 CHR (5
cm wide) and 3MM CHR (2 cm wide), gold (Ted Pella, 99.999% Au), and food color (McCormick)
were used as received. The iPECVD process was carried out in a cylindrical reactor chamber (GVD
Corporation; 25 cm diameter, 5 cm height) under vacuum maintained by a rotary vane vacuum
pump (Edwards E2M40). Reactor pressure during depositions was maintained at 130 mTorr using a
throttle valve controller (MKS 153D) connected to a capacitance manometer (MKS 622C01TDE
Baratron). TBPO and TVTSO were loaded into stainless steel jars that were then mounted onto the
reactor, and the TVTSO jar was heated to 40 ºC to achieve a flow rate of 3.2 sccm controlled by a
needle valve. The TBPO jar was kept at 25 ºC, and the flow rate was maintained at 10.5 sccm using a
mass flow controller (MKS Type 1152C). The argon gas flow rate was maintained at 8.2 sccm using
a mass flow controller (MKS Type 1479A). The reactor stage temperature was controlled using a
backside recirculating bath chiller (Thermo Scientific NESLAB RTE 7) and was kept at 60 ºC. The
compact roll-to-roll module was used as previously reported without modification to the reactor
chamber, and all parts for the roll-to-roll module were purchased from Amazon for $70.26 The
distance between the two rolls, termed the primary deposition zone, was 12.5 cm. The motors
controlling the roll speed were connected via copper wires to external adjustable DC power supplies
(VOLTEQ HY3010D).
61
During PTVTSO polymer deposition, an external radio frequency plasma generator (Diener;
13.56 MHz, 100 W) with a manual matchbox (Diener) was connected to the nichrome filament array
(Omega Engineering, 80%/20% Ni/Cr) positioned 4.6 cm above the reactor stage and was operated
at 30 W. The deposition rate on a reference silicon wafer on the reactor stage was monitored in situ
via interferometry with a He-Ne laser (Industrial Fiber Optics, 633 nm) and was approximately 400
nm/min. For depositions onto stationary chromatography paper, polymer was deposited for 1–10
min, corresponding to deposited thicknesses of 0.4–4.0 μm on a reference silicon wafer, and the
paper was in two different orientations: taped down to the stage with all edges sealed using Kapton
tape or elevated to 2.5 cm above the stage taped to a thin PDMS pillar. Roll-to-roll depositions were
carried out on 3 m-long rolls of Whatman 1 at a roll speed of 7.5 cm/min and on 1 m-long rolls of
Whatman 3MM at roll speeds of 2.6, 6.1, 8.0, and 9.8 cm/min. The polymerization was started by
turning on the plasma 1 min before the substrate began rolling and was finished 1 min after rolling
by turning off the plasma.
The chemical functionality of the coatings was characterized using a Fourier transform
infrared spectrometer (Thermo Scientific i510). The atomic compositions of the uncoated and
coated paper were measured using an X-ray photoelectron spectrometer (Kratos Axis Ultra DLD)
with a monochromatic Al K ɑ source. Survey spectra were taken from 800–0 eV in 1 eV steps,
averaged over 5 scans, and referenced to 284.8 eV for the C–C peak. The hydrophobicity of the
polymer coatings was characterized using a contact angle goniometer (ramé-hart 290). The reported
contact angles were averaged over 10 measurements with a drop volume of 5 μL of deionized water,
and the error bars represent one standard deviation above and below the average. The profile images
of the droplets were captured using the goniometer camera. A scanning electron microscope
(Topcon Aquila) was used to image the morphology of the paper surface. The samples were sputter
coated (Ted Pella 108 Auto Sputter Coater) with gold for 40 s at 30 mA prior to imaging to avoid
62
charging. The wicking behavior of the coated paper was analyzed by placing one end of the coated
paper in deionized water with food coloring such that the dyed water wicked vertically up the paper.
The images of paper with dyed water were taken using a stereoscope (National Optical 420 Stereo
Zoom), and the photos were taken using a Nikon D3000 camera. Origami was performed on three 5
× 5 cm squares of the roll-to-roll coated Whatman 1 CHR paper cut from near the center of the
length of the roll. The three squares were folded and joined together to form a single bowl.
63
4.4 Results and Discussion

64
Figure 4-1. (a) Image of the plasma in the iPECVD reactor chamber. (b) Schematic of the
free-radical polymerization mechanism. (c) FTIR spectra of the TVTSO monomer (bottom)
and PTVTSO polymer deposited using iPECVD (top). The dashed lines correspond to the
peaks associated with the vinyl bonds. SEM images of Whatman 1 paper (d) before and (e)
after coating with PTVTSO.
In the iPECVD process (Figure 4-1a), the monomer undergoes free-radical polymerization
(Figure 4-1b), resulting in a thin polymer coating on the substrate surface. We chose TVTSO as the
monomer to fabricate organosilicon polymer coatings because the multiple vinyl groups in the
structure allow for crosslinking and thereby stronger coatings. To study the polymerization
mechanism, we characterized the chemical functionality of the polymer using FTIR spectroscopy
(Figure 4-1c). In the iPECVD polymer spectrum, the decrease in the peaks corresponding to
monomer vinyl bonds (3052, 3014, 1407, 857, 706 cm-1) and the appearance of a broad peak at
2936–2800 cm-1 corresponding to the carbon backbone of the polymer confirm that the
polymerization is predominantly occurring through the vinyl bonds as opposed to monomer
fragmentation. The retention of peaks corresponding to methyl groups (2960, 1259 cm-1), Si–O–Si
bonds (1054, 796 cm-1), and Si–C bonds (840 cm-1) demonstrate that iPECVD results in good
preservation of the functional moieties of the monomer.24 XPS was used to further characterize the
iPECVD polymer. PTVTSO deposited via iPECVD had a measured atomic composition of 55.9%
C, 26.1% O, 18% Si whereas the TVTSO monomer has a theoretical atomic composition of 68.75%
C, 12.5% O, 18.75% Si. The carbon content of the polymer is lower than that expected from the
monomer, indicating that some monomer fragmentation may be occurring. However, the structural
retention in the FTIR spectrum and the percent of silicon in the XPS spectrum indicate that the
primary polymerization mechanism is through the saturation of vinyl bonds.
To test the suitability of using PTVTSO deposited via iPECVD as a hydrophobic coating to
replace conventional fluorinated coatings, we characterized the hydrophobicity of the polymer
coating via contact angle goniometry. The contact angle of PTVTSO deposited onto a flat silicon
65
wafer is 90.9° ± 1.2º, with advancing and receding contact angles of 92.9° ± 1.7° and 76.5° ± 1.4°,
respectively. We also coated cellulose chromatography paper of two different thicknesses, Whatman
1 (180 μm thick) and 3MM (340 μm thick). The uncoated paper is hydrophilic and readily absorbs
water. After deposition of PTVTSO, the paper is rendered hydrophobic, with contact angles of 125–
130°. These contact angles are higher than that of PTVTSO on a silicon wafer because the
roughness of the cellulose paper (Figure 4-1d–e) enhances the surface hydrophobicity, which is also
known as the Lotus effect.27,28 The SEM images demonstrate that the surface morphology and
roughness of the cellulose paper (Figure 4-1d) are preserved after polymer coating (Figure 4-1e).

Figure 4-2. (a) Chromatography paper taped down on the reactor stage during PTVTSO
deposition. (b) To test the wetting behavior of paper coated with PTVTSO, strips are cut
from the coated paper samples and dipped in dyed water. (c) Wicking behavior of Whatman
1 CHR (thin) and 3MM CHR (thick) chromatography paper taped to the stage and coated
with PTVTSO at varying deposition times.
66
The fabrication of Janus membranes requires one side of the chromatography paper to be
conformally coated with PTVTSO while the other side remains hydrophilic. We tested the ability of
iPECVD to coat only one side of the paper as a function of deposition time and paper thickness.
We taped the samples onto the stage before coating such that the bottom side of the paper would
only be coated if the polymer penetrated through the thickness of the paper from the top (Figure
4-2a). To characterize the wetting properties of the coated paper, we cut a strip of the coated paper
and dipped it into dyed water to test whether the water absorbed into the paper and then wicked up
the strip (Figure 4-2b). Dyed water will not wick up fully hydrophobic paper, which would indicate
full polymer coverage through the thickness of the paper. For both the thin and thick paper, the top
of the samples was conformally coated for all deposition times as demonstrated by the
hydrophobicity. At short deposition times (1 and 2 min), the thin paper remained hydrophilic on the
bottom side, demonstrating that we can make Janus membranes using iPECVD (Figure 4-2c). At
deposition times of 5 min and longer, the thin coated paper became fully hydrophobic and did not
wick the dyed water. For the thick paper, the coated paper exhibited asymmetric wetting for all
deposition times up to 10 min, resulting in Janus membranes.

Figure 4-3. Atomic compositions from XPS survey spectra of PTVTSO coatings on
Whatman 3MM CHR paper.
67
XPS, which has a probe depth of 5–10 nm, was used to further confirm the asymmetry of
the polymer coating on the thick chromatography paper (Figure 4-3). The uncoated paper, which is
composed of cellulose fibers, had a measured composition of 63.7% C and 36.3% O. For the thick
paper coated at both short (2 min) and long (10 min) deposition times, the atomic compositions on
the top of the samples agree well with the theoretical composition of PTVTSO (68.75% C, 12.5%
O, 18.75% Si ), which indicates that there is at least 5–10 nm of coating. The amount of Si on the
bottom of the paper (2–3%) indicates that there is some polymer growth on the bottom of the
membranes. However, the polymer coverage is very low and therefore water is still able to absorb
into the paper and wick up, resulting in Janus wettability. The decreasing conformality is consistent
with the previous study by Coclite and Gleason where they showed that a surface covered with 1 μm
diameter microspheres can achieve a degree of planarization of 99% by depositing 1.8 μm of
PTVTSO coating using iPECVD.25

Figure 4-4. Wicking behavior of chromatography paper elevated to 2.5 cm above the stage
and coated with PTVTSO at varying deposition times.
68
For fabrication of next-generation textiles, absorbents, and diagnostic assay strips, the ability
to tune the wettability of each layer can lead to enhanced capabilities. To fabricate multilayer
membranes with selective wettability, we elevated the paper to 2.5 cm above the stage, allowing the
polymer to coat the paper from both the top and bottom sides. The thin paper did not wick water
for all deposition times (Figure 4-4), indicating that the paper becomes fully hydrophobic even at
short deposition times. Because the polymer coating can penetrate the paper from both the top and
bottom sides, the elevated thin paper becomes fully hydrophobic at shorter deposition times
compared to the paper that was taped down which required at least 5 min to become fully
hydrophobic. It is interesting to note that the paper was coated with polymer on both the top and
bottom even though the cathode is facing the top side of the paper. For the thick paper at short
deposition times of 1–5 min, the middle of the paper selectively wicks water. The XPS spectra of the
thick paper coated for 2 min showed that the atomic compositions on both sides of the coated
paper (top: 66.8% C, 18.2% O, 15% Si; bottom: 66.9% C, 17.6% O, 15.5% Si) agree well with the
theoretical composition of PTVTSO even though the middle of the paper is hydrophilic because the
XPS probe depth is less than the depth of conformal polymer coverage. At a deposition time of 10
min, the thick paper is fully hydrophobic, indicating that the longer deposition time results in
conformal polymer coverage through the thickness of the paper. Our data shows that the depth of
conformal polymer coverage and subsequently the wetting properties of the membrane can be
controlled by varying the polymer deposition time.
69

Figure 4-5. (a) Schematic of the compact roll-to-roll module in the iPECVD system.
Contact angle goniometry measurements (b) along the length (inset: representative contact
angle image) and (c) across the width of Whatman 1 CHR paper coated with PTVTSO via
70
roll-to-roll iPECVD. (d) Origami bowl folded from roll-to-roll coated hydrophobic paper
and filled with dyed water.
Roll-to-roll processing is important for process scalability. We therefore studied the scale-up
of the iPECVD process by using a compact roll-to-roll module to coat 3 m of the thin paper with
PTVTSO (Figure 4-5a). During the rolling process, the paper is elevated to 2.5 cm above the stage,
which is the same height of the elevated samples in the stationary studies shown in Figure 4-4,
allowing the trends for the moving and stationary samples to be compared. The primary deposition
zone is 12.5 cm long. The thin paper was rolled at 7.5 cm/min and therefore the paper was coated
for 1.7 min. To characterize the depth of conformal polymer coverage through the thickness of the
paper, we tested the wicking of a strip of paper from the middle of the length of the roll. The coated
paper was fully hydrophobic, indicating full polymer coverage through the thickness of the paper
even at a relatively short coating time, which is consistent with the stationary elevated samples which
were fully coated even at 1 min. The contact angle measurements on both sides of the paper along
the entire length (Figure 4-5b) and width (Figure 4-5c) of the roll show no significant variation,
indicating that the paper is uniformly coated along the length and width on both sides throughout
the roll-to-roll process. Additionally, the coated paper can be cut and shaped into an origami bowl
(Figure 4-5d) since the thickness of the PTVTSO coating (approximately 700 nm) is much less than
the thickness of the paper and thus does not affect the macro-scale properties of the paper. The
origami bowl is able to hold dyed water because of surface tension, illustrating the efficacy of roll-to-
roll iPECVD for fabricating conformal hydrophobic coatings.
71

Figure 4-6. (a) Whatman 3MM CHR paper coated with PTVTSO via roll-to-roll iPECVD
at varying roll speeds. Contact angle measurements (b) along the length (inset: representative
72
contact angle image) and (c) across the width of thick paper coated at a roll speed of 8.0
cm/min.
To study the depth of conformal coverage at various roll speeds, the thick paper was also
coated with PTVTSO using roll-to-roll iPECVD, and strips were cut from the middle of the length
of the roll and dipped in dyed water (Figure 4-6a). For faster roll speeds of 9.8 cm/min and 8.0
cm/min, the middle of the paper remains hydrophilic. At slower roll speeds of 6.1 cm/min and 2.6
cm/min, the paper is fully hydrophobic, demonstrating conformal polymer coverage through the
thickness of the paper. Roll speeds of 9.8, 8.0, 6.1, and 2.6 cm/min correspond to coating times in
the primary deposition zone of 1.3, 1.6, 2.1, and 4.8 min, respectively. The depth of conformal
polymer coverage as a function of time in the primary deposition zone follows the same trend as
that for deposition time on stationary elevated paper: at short coating times of 1.3 and 1.6 min, the
middle of the paper remains hydrophilic, whereas the entire depth of the paper is rendered
hydrophobic at longer coating times of 2.1 and 4.8 min. The time to conformally coat the entire
depth of the paper is shorter for the rolling sample (2.1 min) than for the stationary sample (10 min)
since the rolling samples are continuously exposed to the iPECVD process during the unwinding
and winding of the roll. For the multilayer membrane that remains hydrophilic in the middle at the
roll speed of 8.0 cm/min, the contact angle measurements show that the coating deposited on both
sides of the paper are uniform across the length (Figure 4-6b) and the width of the paper (Figure
4-6c). The measured hydrophobicity is consistent with the hydrophobicity of PTVTSO on paper
despite the hydrophilicity of the middle of the paper, because the water droplet during contact angle
goniometry measurements only contacts the outer surface of the paper.

4.5 Conclusions
We fabricated asymmetric and symmetric membranes by using iPECVD to deposit
hydrophobic PTVTSO onto hydrophilic cellulose paper. Asymmetric Janus membranes that were
73
hydrophilic on one side and hydrophobic on the other side and symmetric membranes that
remained hydrophilic in the center were fabricated by changing the paper thickness, varying the
orientation of the paper on the reactor stage, and tuning the polymer deposition time. Coating the
paper from only one side resulted in Janus membranes for the thin paper at short deposition times
of 1 and 2 min and for the thick paper at deposition times of 1–10 min. Coating the thick paper
from both sides resulted in multilayer membranes with hydrophobicity at both surfaces and
hydrophilicity in the middle for deposition times of 1–5 min. XPS and wicking studies confirmed
these results. We demonstrated that the iPECVD process could be scaled up using a roll-to-roll
module which allows for high-throughput fabrication of asymmetric and symmetric materials for use
as separation membranes, wound dressings, and diagnostic assay strips.

4.6 Acknowledgements
C. C. is supported by the Alfred E. Mann Innovation in Engineering Doctoral Fellowship.

4.7 References

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Comparative Study. ACS Materials Lett. 2020, 2, 336–357, DOI:
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(3) Yang, H.-C.; Hou, J.; Chen, V.; Xu, Z.-K. Janus Membranes: Exploring Duality for Advanced
Separation. Angew. Chem. Int. Ed. 2016, 55, 13398–13407, DOI: 10.1002/anie.201601589
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(4) Wang, H.; Zhou, H.; Yang, W.; Zhao, Y.; Fang, J.; Lin, T. Selective, Spontaneous One-Way Oil-
Transport Fabrics and Their Novel Use for Gauging Liquid Surface Tension. ACS Appl. Mater.
Interfaces 2015, 7, 22874–22880, DOI: 10.1021/acsami.5b05678
(5) Li, C.; Li, X.; Du, X.; Tong, T.; Cath, T. Y.; Lee, J. Antiwetting and Antifouling Janus
Membrane for Desalination of Saline Oily Wastewater by Membrane Distillation. ACS Appl.
Mater. Interfaces 2019, 11, 18456–18465, DOI: 10.1021/acsami.9b04212
(6) Gu, J.; Xiao, P.; Chen, J.; Zhang, J.; Huang, Y.; Chen, T. Janus Polymer/Carbon Nanotube
Hybrid Membranes for Oil/Water Separation. ACS Appl. Mater. Interfaces 2014, 6, 16204–16209,
DOI: 10.1021/am504326m
(7) Yang, X.; Yan, L.; Ran, F.; Pal, A.; Long, J.; Shao, L. Interface-Confined Surface Engineering
Constructing Water-Unidirectional Janus Membrane. J. Membrane Sci. 2019, 576, 9–16, DOI:
10.1016/j.memsci.2019.01.014
(8) Babar, A. A.; Miao, D.; Ali, N..; Zhao, J.; Wang, X.; Yu, J.; Ding, B. Breathable and Colorful
Cellulose Acetate-Based Nanofibrous Membranes for Directional Moisture Transport. ACS
Appl. Mater. Interfaces 2018, 10, 22866–22875, DOI: 10.1021/acsami.8b07393
(9) Wang, X.; Huang, Z.; Miao, D.; Zhao, J.; Yu, J.; Ding, B. Biomimetic Fibrous Murray
Membranes with Ultrafast Water Transport and Evaporation for Smart Moisture-Wicking
Fabrics. ACS Nano 2019, 13, 1060–1070, DOI: 10.1021/acsnano.8b08242
(10) Miao, D.; Huang, Z.; Wang, X.; Yu, J.; Ding, B. Continuous, Spontaneous, and Directional
Water Transport in the Trilayered Fibrous Membranes for Functional Moisture Wicking
Textiles. Small 2018, 14, 1801527, DOI: 10.1002/smll.201801527
(11) An, Y.-H.; Yu, S. J.; Kim, I. S.; Kim, S.-H., Moon, J.-M; Kim, S. L.; Choi, Y. H.; Choi, J. S.; Im,
S. G.; Lee, K. E.; Hwang, N. S. Hydrogel Functionalized Janus Membranes for Skin
Regeneration. Adv. Healthcare Mater. 2017, 6, 1600795, DOI: 10.1002/adhm.201600795
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(12) Shi, L.; Liu, X.; Wang, W.; Jiang, L.; Wang, S. A Self-Pumping Dressing for Draining Excessive
Biofluid around Wounds. Adv. Mater. 2019, 31, 1804187, DOI: 10.1002/adma.201804187
(13) Soz, C. K.; Trosien, S.; Biesalski, M. Superhydrophobic Hybrid Paper Sheets with Janus-Type
Wettability. ACS Appl. Mater. Interfaces 2018, 10, 37478–37488, DOI: 10.1021/acsami.8b12116
(14) Waldman, R. Z.; Yang, H.-C.; Mandia, D. J.; Nealey, P. F.; Elam, J. W.; Darling, S. B. Janus
Membranes via Diffusion-Controlled Atomic Layer Deposition. Adv. Mater. Interfaces 2018, 5,
1800658, DOI: 10.1002/admi.201800658
(15) Tian, X.; Jin, H.; Sainio, J.; Ras, R. H. A.; Ikkala, O. Droplet and Fluid Gating by Biomimetic
Janus Membranes. Adv. Funct. Mater. 2014, 24, 6023–6028, DOI: 10.1002/adfm.201400714
(16) Cheng, Z.; Wang, B.; Lai, H.; Liu, P.; Zhang, D.; Tian, D.; Liu, H.; Yu, X.; Sun, B.; Sun, K.
Janus Copper Mesh Film with Unidirectional Water Transportation Ability toward High
Efficiency Oil/Water Separation. Chem. Asian J. 2017, 12, 2085–2092, DOI:
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(17) Pfluger, C. A.; Carrier, R. L.; Sun, B.; Ziemer, K. S.; Burkey, D. D. Cross-Linking and
Degradation Properties of Plasma Enhanced Chemical Vapor Deposited Poly(2-hydroxyethyl
methacrylate). Macromol. Rapid Commun. 2009, 30, 126–132, DOI: 10.1002/marc.200800647
(18) Gursoy, M.; Ucar, T.; Tosun, Z.; Karaman, M. Initiation of 2-Hydroxyethyl Methacrylate
Polymerization by Tert-Butyl Peroxide in a Planar PECVD System. Plasma Process. Polym. 2016,
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(19) Conder, J. M.; Hoke, R. A.; de Wolf, W.; Russell, M. H.; Buck, R. C. Are PFCAs
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(20) Haukas, M.; Berger, U.; Hop, H.; Gulliksen, B.; Gabrielsen, G. W. Bioaccumulation of Per- and
Polyfluorinated Alkyl Substances (PFAS) in Selected Species from the Barents Sea Food Web.
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(21) EPA’s Per- and Polyfluoroalkyl Substances (PFAS) Action Plan, 2019. United States
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with OH and NO3 Radicals and O3 at 297 ± 2 K. Environ. Sci. Technol. 1991, 25, 863–866, DOI:
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Enhance Monomer Structure Retention. Plasma Process. Polym. 2012, 9, 425–434, DOI:
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Chapter 5. Future Directions
5.1 Grafted Functional Coatings for Soft Material Implants
Future work on coating soft material implants can investigate grafting (covalently attaching)
functional iCVD polymer coatings onto soft material medical implants to improve the
biocompatibility and decrease implant failure. Preliminary work has demonstrated that 3D printed
poly(lactic acid) and acrylonitrile-butadiene-styrene lattice structures can be coated with cross-linked
PHEMA.1 After iCVD, the substrate surface demonstrated an increase in hydrophilicity, indicating
that the substrates were effectively coated with polymer. Based on these successful results, iCVD
can be used to coat soft material medical implants and improve their longevity in situ.
Multiple different grafting techniques can be studied and compared. Three methods for
initiating solvent-free polymer grafting include photoinitiation,2 oxygen plasma initiation,3 and
methyl radical initiation.4 For each grafting technique, multiple processing parameters, such as
precursor flow rates and substrate temperature, can be tuned to improve the polymer coverage. Full
polymer coverage on a medical implant is desirable to achieve the most effective coating for
antibiofouling. These parameters either change the density of radicals generated on the surface or
the rate of polymerization.
Studying monomer and initiator adsorption, absorption, and depletion effects in iCVD on
soft materials is important for being able to coat substrates with complex geometries. Currently,
there is little fundamental understanding of these competing effects in the iCVD system, particularly
because the three effects compete with each other and iCVD onto macro-scale soft material
substrates is not yet well studied. In a typical iCVD process onto a non-porous substrate, monomer
and initiator will adsorb to the surface and polymerize. However, soft materials have porosity that
allows small molecules like monomers and initiators to absorb into the subsurface of the substrate
79
through the pores. For medical implants, monomer absorption into the bulk substrate is not
desirable as it may alter bulk properties or eventually leach back out of the implant. Additionally,
plastics have low thermal conductivity, so the presence of a significant heat source nearby (such as
the heated filament array) can result in a thermal gradient over the length of the substrate. This
thermal gradient can lead to nonuniform coatings5 and monomer depletion6 effects, both of which
are undesirable for coating medical implants.

5.2 Roll-to-Roll Deposition of Patterned Coatings
For applications such as paper-based microfluidic devices, patterning the deposited polymer
coating is important for device fabrication. Large-scale patterning can be achieved through multiple
techniques. One such method is using a UV-switchable polymer is poly(ortho-nitrobenzyl
methacrylate) (PONBMA), which switches from a hydrophobic to hydrophilic state upon UV
exposure.2 Although iCVD is not a line-of-sight process, UV exposure of the polymer is. Thus, for
complex geometries, there may be regions where the UV exposure is limited, and studies can
examine the UV-responsive behaviors in environments that may have partial masking due to the
substrate geometry. PONBMA is useful for applications in which it is desirable to pattern the
surface of a medical implant (Figure 5-1. Patterned PONBMA channels filled with dyed liquid. The
polymer was deposited in our lab using iCVD.), or the polymer can act as a UV-dosage indicator.

Figure 5-1. Patterned PONBMA channels filled with dyed liquid. The polymer was
deposited in our lab using iCVD.7
Large-scale patterning can also be achieved through the use of polymer inhibition, in which
application of metal salts locally inhibits polymerization.8,9 These salts can be applied in solution
80
onto large surface areas of paper using inject printing, which allows the salts to be applied in
complex shapes with high resolution. This patterned paper can be used for microfluidic barrier
channels or chromatographic separations.

5.3 Mechanistic Studies of Initiated Plasma-Enhanced Chemical Vapor Deposition
This work demonstrated that using iPECVD is promising for coating porous materials,
which raises interest in further understanding the process from a mechanistic standpoint. From
previous work studying iPECVD of organosilicon polymers, the monomer functionality retention
has been shown to be improved compared to PECVD. However, FTIR spectra show that there is
some peak broadening in the polymer compared to the monomer, and atomic compositions from
XPS of the polymer deviate from theoretical values from the monomer. These changes indicate that
although polymerization is occurring predominantly by saturation of vinyl bonds, some monomer
fragmentation may be occurring during the process. Multiple processing parameters, including
precursor flow rates and reactor pressure, can be tuned to optimize the functionality retention of the
deposited film. In addition, control of coating conformality through the thickness of porous
substrates can be studied by also varying process parameters beyond deposition time, which could
allow for increased processing speeds.

5.4 Hybrid Metal and Polymer Films
Because the reactor chamber is now equipped with an RF-plasma generator, it will be
possible to use the plasma to sputter deposit metal thin films within the same reactor as the iCVD
and iPECVD processes. The ability to deposit metal films can allow for either simultaneous or
alternating deposition of metal and polymer layers. Hybrid inorganic/organic materials have
applications in electronics, catalysis, and drug delivery.10
81

5.5 References

(1) Cheng, C.; Gupta, M. Surface Functionalization of 3D-Printed Plastics via Initiated Chemical
Vapor Deposition. Beilstein J. Nanotechnol. 2017, 8, 1629–1636.
(2) De Luna, M. M.; Chen, B.; Bradley, L. C.; Bhandia, R.; Gupta, M. Solventless Grafting of
Functional Polymer Coatings onto Parylene C. J. Vac. Sci. Technol. A 2016, 34, 041403.
(3) Shearn, M.; Sun, X.; Henry; M. D.; Yariv, A.; Sherer, A. Advanced Plasma Processing. In
Semiconductor Technologies; Grym, J., Ed.; 2010.
(4) Sojoudi, H.; McKinley, G. H.; Gleason, K. K. Linker-Free Grafting of Fluorinated Polymeric
Cross-Linked Network Bilayers for Durable Reduction of Ice Adhesion. Mater. Horiz. 2015, 2,
91–99.
(5) Lau, K. K. S.; Gleason, K. K. Initiated Chemical Vapor Deposition (iCVD) of Poly(alkyl
acrylates): An Experimental Study. Macromolecules 2006, 39, 3688–3694.
(6) Seidel, S.; Dianat, G.; Gupta, M. Formation of Porous Polymer Coatings on Complex
Substrates Using Vapor Phase Precursors. Macromol. Mater. Eng. 2016, 301, 371–376.
(7) Haller, P. D.; Flowers, C. A.; Gupta, M. Three-Dimensional Patterning of Porous Materials
Using Vapor Phase Polymerization. Soft Matter 2011, 7, 2428–2432.
(8) Kwong, P.; Flowers, C. A.; Gupta, M. Directed Deposition of Functional Polymers onto
Porous Substrates Using Metal Salt Inhibitors. Langmuir 2011, 27, 10634–10641.
(9) Kwong, P.; Seidel, S.; Gupta, M. Effect of Transition Metal Salts on the Initiated Chemical
Vapor Deposition of Polymer Thin Films. J. Vac. Sci. Technol. A 2015, 33, 031504.
(10) De Luna, M. M.; Karandikar, P.; Gupta, M. Synthesis of Inorganic/Organic Hybrid Materials
via Vapor Deposition onto Liquid Surfaces. ACS Appl. Nano Mater. 2018, 1, 6575–6579.

Abstract (if available)

Abstract Two vapor-phase deposition techniques for fabricating polymer coatings are studied in this work. Chapter 1 introduces the initiated chemical vapor deposition (iCVD) and initiated plasma-enhanced chemical vapor deposition (iPECVD) processes and the motivation behind coating large-scale soft material substrates. ❧ Chapter 2 demonstrates deposition of polymers onto 3D printed plastic substrates. 3D printing is a useful fabrication technique because it offers design flexibility and rapid prototyping. The ability to functionalize the surfaces of 3D printed objects allows the bulk properties, such as material strength or printability, to be chosen separately from surface properties, which is critical to expanding the breadth of 3D printing applications. In this work, we studied the ability of iCVD to coat 3D printed shapes composed of poly(lactic acid) and acrylonitrile butadiene styrene. The thermally insulating properties of 3D printed plastics pose a challenge to the iCVD process due to large thermal gradients along the structures during processing. In this study, processing parameters such as the substrate temperature and the filament temperature were systematically varied to understand how these parameters affect the uniformity of the coatings along the 3D printed objects. The 3D printed objects were coated with both hydrophobic and hydrophilic polymers. Contact angle goniometry and X-ray photoelectron spectroscopy were used to characterize the functionalized surfaces. These results can enable the use of iCVD to functionalize 3D printed materials for a range of applications such as tissue scaffolds and microfluidics. ❧ Chapter 3 investigates the use of iCVD for coating large areas of flexible materials. Tuning surface properties of flexible materials enhances the versatility of existing materials, giving them new functions for applications in textiles, filtration, flexible electronics, and sensors. However, traditional surface modification methods are typically solvent-based, which limits the range of substrates that can be coated. In this work, we demonstrate the ability to use roll-to-roll processing to continuously modify the surface properties of large areas of flexible substrates using iCVD, which is an all-dry process. We designed and built a roll-to-roll module that can be used to uniformly coat 1500 cm² of chromatography paper in a single deposition. Rolls of paper were coated with a fluoropolymer and an ionizable polymer, and the coated paper was used for origami, nonstick surfaces, and paper-based microfluidic devices. ❧ Chapter 4 studies the deposition of hydrophobic coatings onto porous materials for the fabrication of membranes with asymmetric and symmetric wetting properties. iPECVD was used to deposit an organosilicon polymer coating onto chromatography paper of different thicknesses. The hydrophobic organosilicon polymer serves as an environmentally-friendly alternative to fluorinated polymers, and the all-dry coating process does not use solvents. We demonstrate that the deposition time, thickness of the paper, and orientation of the paper relative to the stage affect the symmetry of the wettability. The chemical functionality of the deposited polymer was characterized via Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. The hydrophobicity and wetting behavior of the coated paper were characterized with contact angle goniometry and wicking studies, respectively. We also demonstrate that we can scale up the process by using a roll-to-roll module. Our ability to systematically tune the wettability of materials allows for the fabrication of multilayer and Janus membranes that are useful for applications including filtration, smart textiles, flexible sensors, and wound dressings. ❧ Chapter 5 discusses future directions for this work, including grafting polymer coatings onto soft material implants, investigating roll-to-roll patterned coatings, studying the reaction mechanisms in iPECVD to enhance functionality retention and tune coating conformality, and depositing hybrid inorganic/organic films.

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University of Southern California Dissertations and Theses

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Asset Metadata

Creator Cheng, Christine (author)

Core Title Scale-up of vapor-phase deposition of polymers: towards large-scale processing

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 07/06/2021

Defense Date 06/10/2020

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

Tag 3D printing,coating materials,deposition,Janus membranes,OAI-PMH Harvest,polymers,thin films,wettability

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Gupta, Malancha (committee chair), Malmstadt, Noah (committee member), Meng, Ellis (committee member)

Creator Email chengchr@usc.edu,christinecheng@alumni.usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-325476

Unique identifier UC11665383

Identifier etd-ChengChris-8643.pdf (filename),usctheses-c89-325476 (legacy record id)

Legacy Identifier etd-ChengChris-8643.pdf

Dmrecord 325476

Document Type Dissertation

Rights Cheng, Christine

Type texts

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

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

Repository Name University of Southern California Digital Library

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