University of Southern California Dissertations and Theses

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

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

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Content i

Fabrication of Functional Porous Membranes via

Polymerization of Solid Monomer by a Vapor-Phase Initiator

by

Golnaz Dianat

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)

December 2019

ii
Committee Members

Dr. Malancha Gupta (Chair)
Dr. Noah Malmstadt
Dr. Aiichiro Nakano

iii
Executive Summary
Porous polymers are useful for applications such as tissue scaffolds, sensors and
separation membranes. Common methods to fabricate porous polymers use liquid phase
processing techniques including phase inversion methods, cryopolymerization, breath-
figure techniques, and solution casting and particulate leaching. Solvent compatibility and
surface tension issues can limit the generality of these methods. In contrast, we have
developed a versatile, solvent-free method to produce tailored porous polymers. In this
dissertation, we will explain the process-structure-property relationships and potential
applications.
Chapter 1 provides a background information on the initiated chemical vapor
deposition process and how the technique was modified to fabricate porous polymers.
Chapter 2 studies a Process-Structure-Property relationship for our membranes that are
formed through polymerization of solid monomer by a vapor phase initiator. We show that
the molecular weight distribution and therefore dissolution behavior of the membranes can
be controllably varied by changing processing parameters such as polymerization time and
polymerization temperature. In Chapter 3 we show the capability of our technique to
produce a free-standing capillary-flow microfluidic device by developing a patterning
technique. Our microfluidic device can be used to separate a mixture of analytes based on
their charge. Chapter 4 demonstrates our novel technique to synthesize asymmetric
polymer membranes. We show that these membranes that are composed of a dense top layer
and porous bottom layer can be fabricated using a combination of conventional and
nonconventional iCVD conditions in a one-pot process. Bottom up nature of our process
allows us to independently control the thickness and chemical functionality of each layer.
iv
Chapter 5 demonstrates our ability to produces all-organic, scratch resistant, and chemical
resistant porous polymer coatings with an excellent adhesion to a variety of substrates
including flexible and complex 3D structures such as cylindrical substrates. Our fabrication
process is a potential solution to lack of success in conformally coating nonpolar substrates
using solution phase techniques. Finally, in Chapter 6 we summarize the conclusion and
discuss the future work.

v
Acknowledgments
I would first like to express my sincere gratitude to my advisor Prof. Malancha Gupta
for the continuous support of my Ph.D study and related research, for all her input,
motivation, and immense knowledge. Her guidance helped me in all the time of research
and I could not have imagined having a better advisor and mentor for my Ph.D study.
Besides my advisor, I would like to thank the rest of my committee members: Prof. Noah
Malmstadt and Prof. Aiichiro Nakano. My sincere thanks also go to Dr. Scott Seidel, who
trained and helped me with my very first research project. I thank my fellow labmates,
specially Nareh Movsesian, for their input and for all the fun we have had in the past few
years.
Last but not the least, I would like to thank my family: my parents, my brothers and
to my husband for supporting me spiritually throughout my Ph.D program and my life in
general. I could have not done it without their encouragement.

vi
Table of Contents
Executive Summary ......................................................................................................... iii
Acknowledgments ............................................................................................................. v
List of Figures ................................................................................................................... ix
List of Tables .................................................................................................................. xiii
Chapter 1 Introduction................................................................................................... 14
1.1 Initiated Chemical Vapor Deposition (iCVD) .................................................... 15
1.2 Porous Polymers.................................................................................................... 17
1.3 References .............................................................................................................. 20
Chapter 2 Process-Structure-Property Relationships for Porous Membranes
Formed by Polymerization of Solid Monomer by a Vapor-Phase Initiator .............. 25
2.1 ABSTRACT ........................................................................................................... 26
2.2 Introduction ........................................................................................................... 26
2.3 Experimental Section ............................................................................................ 28
2.4 Results and Discussion .......................................................................................... 31
2.5 Conclusion ............................................................................................................. 42
2.6 Acknowledgements ............................................................................................... 42
2.7 References .............................................................................................................. 42
Chapter 3 Sequential Deposition of Patterned Porous Polymers using
Poly(dimethylsiloxane) Masks ....................................................................................... 48
3.1 ABSTRACT ........................................................................................................... 49
3.2 Introduction ........................................................................................................... 49
vii
3.3 Experimental ......................................................................................................... 51
3.3.1 Fabrication of PDMS masks ............................................................................ 51
3.3.2 Fabrication of Porous Poly(methacrylic acid) (PMAA) Membranes .............. 51
3.3.3 Cross-linking the Porous Membranes .............................................................. 52
3.3.4 Fabrication of Hydrophobic Coatings .............................................................. 53
3.3.5 Characterization ............................................................................................... 53
3.4 Results and Discussion .......................................................................................... 53
3.5 Conclusions ............................................................................................................ 65
3.6 Acknowledgements ............................................................................................... 66
3.7 References .............................................................................................................. 66
Chapter 4 Vapor Phase Fabrication of Hydrophilic and Hydrophobic
Asymmetric Polymer Membranes ................................................................................. 71
4.1 ABSTRACT ........................................................................................................... 72
4.2 Introduction ........................................................................................................... 72
4.3 Experimental Section ............................................................................................ 74
4.3.1 Fabrication of Membranes with a Hydrophilic Dense Layer and a
Hydrophilic Porous Layer ......................................................................................... 75
4.3.2 Fabrication of Membranes with a Hydrophobic Dense Layer and a
Hydrophilic Porous Layer ......................................................................................... 76
4.3.3 Fabrication of Membranes with a Hydrophobic Dense Layer and a
Hydrophobic Porous Layer ....................................................................................... 77
4.4 Results and Discussion .......................................................................................... 77
4.5 Conclusions ............................................................................................................ 84
viii
4.6 Acknowledgements ............................................................................................... 85
4.7 References .............................................................................................................. 85
Chapter 5 Scratch-Resistant Porous Polymer Coatings with Enhanced
Adhesion to Curved and Planar Substrates ................................................................. 89
5.1 Introduction ........................................................................................................... 90
5.2 Experimental Section ............................................................................................ 90
5.3 Results and Discussion .......................................................................................... 93
5.4 Conclusion ............................................................................................................. 99
5.5 Acknowledgments ................................................................................................. 99
5.6 References ............................................................................................................ 100
Chapter 6 Conclusions and Future Work .................................................................. 102
6.1 Conclusions .......................................................................................................... 103
6.2 Future work ......................................................................................................... 104

ix
List of Figures
Figure 1-1 Schematic of a typical iCVD reactor.............................................................. 16
Figure 1-2. Schematic representation of the a) simultaneous and b) sequential
process for fabricating porous polymer membranes followed by sublimation of
excess solid monomer [32]. ...................................................................................... 18
Figure 1-3 Schematic of porous membrane fabrication showing the growth on a
molecular and macroscopic scale. Cross-sectional SEM images showing the
large-scale pores between (left) and small-scale pores within (right) the
microstructures. [10] ................................................................................................. 19
Figure 1-4 Effect of monomer deposition temperature on the morphology of the
porous membrane. ..................................................................................................... 19
Figure 2-1 Schematic representation of the sequential process for fabricating porous
polymer membranes. ................................................................................................. 31
Figure 2-2 Angled-view and cross-sectional SEM images of porous membranes
polymerized for a) 5min and b) 30 min at a polymerization temperature of -
10C. ......................................................................................................................... 32
Figure 2-3 Cross-sectional and zoomed in SEM images of the membranes formed
at different polymerization temperatures. ................................................................. 35
x
Figure 2-4 Angled-view and cross-sectional SEM images of porous membranes
deposited for a) 2min and b) 5 min and polymerized for 30 min at a temperature
of -10C. .................................................................................................................... 38
Figure 2-5 GPC chromatograms of the porous membrane synthesized by 30 min of
polymerization at 10 C and the dense films deposited on the silicon wafer on
the stage for 30 min, 75 min, and 120 min at a temperature of 10 C. Each
chromatogram is normalized with respect to the area under the corresponding
curve. ......................................................................................................................... 39
Figure 2-6 Dissolution behavior of the intact porous membranes polymerized at
10C for a) 30 min and b) 120 min. .......................................................................... 41
Figure 3-1 Schematic representation of the process to make porous polymer
membranes. There are three steps. In the first step, the solid monomer is
deposited. In the second step, the filament is turned on to begin polymerization.
In the third step, the unreacted monomer is sublimated. .......................................... 55
Figure 3-2 Optical images showing the deposition of porous polymer membranes
onto a silicon wafer that is covered with a poly(dimethylsiloxane) mask. ............... 56
Figure 3-3 Optical and SEM images of porous membrane morphology deposited on
PDMS mask and silicon wafers at TEC temperatures of (a) 0°C, (b) -10°C and,
(c) -20 °C................................................................................................................... 58
Figure 3-4 SEM images of porous membrane morphology at a TEC temperature of
-20˚C on top of (a) a silicon wafer spin-coated with thin PDMS layer, (b) a
xi
reference silicon wafer, (c) a 4mm thick PDMS mask layer, and (d) a silicon
wafer located on top of the 4mm thick PDMS mask. ............................................... 60
Figure 3-5 SEM images of membranes that are approximately 1 mm thick deposited
at TEC temperatures of a) 0°C and b) -20°C and approximately 2 mm thick
deposited at TEC temperatures of c) 0°C and d) -20°C. ........................................... 61
Figure 3-6 Images of pattern shapes and inverse shapes of (a) PDMS mask on top
of a silicon wafer, (b) deposited porous polymer on the masked substrate, and
(c) patterned porous polymer on the silicon wafer after removing the mask. .......... 62
Figure 3-7 (a) A free-standing cross-linked P(MAA-co-EGDA) membrane, (b) dyed
water wicked into the cross-linked membrane by capillary action, and (c)
separation of crystal violet and ponceau S through a cross-linked PMAA
microfluidic channel. ................................................................................................ 64
Figure 3-8 (a) Top-down and (b) angled view of optical images of dyed water
droplets on top of a porous PMAA membrane coated with PDVB and (c) a
SEM image of the sample. ........................................................................................ 65
Figure 4-1 Asymmetric membranes with a) a hydrophilic dense layer and a
hydrophilic porous layer, b) a hydrophobic dense layer and a hydrophilic
porous layer, and c) a hydrophobic dense layer and a hydrophobic porous layer.
................................................................................................................................... 78
Figure 4-2 Contact angle measurements on asymmetric membranes with a) a
hydrophilic dense layer and a hydrophilic porous layer, b) a hydrophobic dense
xii
layer and a hydrophilic porous layer, and c) a hydrophobic dense layer and a
hydrophobic porous layer. ........................................................................................ 80
Figure 4-3 SEM micrographs of dense and porous sides of membranes. A fully
hydrophilic membrane with a) a hydrophilic dense layer and b) a hydrophilic
porous layer, an asymmetric functional membrane with c) a hydrophobic dense
layer and d) a hydrophilic porous layer, and a fully hydrophobic membrane
with e) a hydrophobic dense layer and f) a hydrophobic porous layer. .................... 82
Figure 4-4 SEM micrographs of (a) the dense side and (b) the cross-section of a
membrane with a thin dense PDVB layer. ................................................................ 84
Figure 5-1 Tape test results on a) porous PMAA, b) porous P(MAA-co-GMA), c)
porous P(MAA-co-GMA) with a dense base PGMA layer, and d) porous
P(MAA-co-EGDA-co-GMA) with a dense base PGMA layer. ............................... 95
Figure 5-2 a) SEM image of an annealed porous coating composed of porous
P(MAA-co-EGDA-co-GMA) on a dense base layer of PGMA; tape test results
and SEM images of the coatings after 60 mins of soak in b) IPA, c) methanol,
and d) acetone. .......................................................................................................... 97
Figure 5-4 a) porous polymer conformally coated the curved stainless-steel
substrates and b) tape test results of the coating. ...................................................... 99
Figure 6-1 Recycling monomer waste through downstream monomer capture and
polymerization. ....................................................................................................... 105
xiii
List of Tables
Table 2-1 Effect of polymerization time on the molecular weight distribution and
the total mass of polymer formed at a polymerization temperature of -10 C. ........ 34
Table 2-2 Effect of polymerization temperature on the molecular weight distribution
and the total mass of polymer. .................................................................................. 36
Table 2-3 Effect of monomer concentration on the molecular weight distribution
and the total mass of the polymer. ............................................................................ 37
Table 2-4 Effect of polymerization time on the molecular weight distribution and
the total mass of polymer at 10 C. ............................................................................ 40
Table 2-5 Effect of molecular weight on the dissolution rate of porous polymer. .......... 41
Table 4-1 XPS survey data of the dense and porous layer of each membrane
compared to iCVD reference films. .......................................................................... 81

14

Chapter 1 Introduction

15
1.1 Initiated Chemical Vapor Deposition (iCVD)
Initiated chemical vapor deposition (iCVD) is a solvent-free near room-temperature
polymerization technique that has the ability to produce dense pinhole free polymer films and
coatings with thicknesses of tens of nanometers to few microns. [1-5] Solventless nature of the
iCVD offers environmental benefits and the ability to conformally coat the complex structures such
as trenches through elimination of the surface tension and wetting effects. iCVD is an energy
efficient process that is capable of retaining the full functionality of the precursors. A variety of
functional polymer including hydrophilic, hydrophobic, and responsive films can be produces by
simply varying the monomer precursor. [6,7]
Figure 1-1 shows schematic representation of a typical iCVD reactor. The deposition
process occurs in a custom-built pancake shape chamber of 250 mm diameter and 48 mm height
containing a nichrome filament array (80/20% Ni/Cr). Recirculated water chiller is used to adjust
the temperature of the stage between 10 °C – 40 °C. Monomer and initiator jars are mounted onto
the reactor through separate inlets and the monomer jar can be heated to increase the vapor pressure.
An exhaust line is located on the other side of the reactor and is connected to the vacuum pump to
create a uniform flow in the reactor chamber. Quartz or glass is used to cover the top of the reactor
which allows the use of interferometer for thickness measurement and facilitate the visual
inspection during the experimental procedure. The reactor pressure is kept constant throughout the
deposition, usually in the range of 100 to 500 mTorr, by a throttle valve which is regulated by a
capacitance manometer.
16

Figure 1-1 Schematic of a typical iCVD reactor.
In iCVD process, monomer and initiator molecules are introduced into the reactor at
constant flow rates. The monomer molecules adsorb to the surface of the cooled substrate in vapor
phase and initiator radicals that are thermally decomposed by a hot filament array diffuse to the
surface of the substrate to start the polymerization and formation of the dense polymer film.
Polymerization mostly occurs on the surface of the substrate. [8] The deposition rates and molecular
weights of the polymer films formed by iCVD increase by increasing the monomer concentration
at the surface of the substrate which can be controlled by varying the ratio of monomer partial
pressure over the saturation pressure of monomer (Pm/Psat) at a given substrate temperature.
Pm/Psat values can be increased by either increasing the flow rate of the monomer or by lowering
the temperature of the substrate. In order to have ideal depositions rates Pm/Psat values must be
kept in the range of between 0.4 to 0.7. At Pm/Psat values of greater than one, monomer goes
through condensation phase change which results in the formation of non-conformal polymer film.
[8-13] The temperature at which the initiator decomposes (filament temperature) is independent of
the polymerization temperature (substrate temperature) while in traditional solution phase free-
radical polymerization initiator decomposition and polymerization occur at the same temperature
which may result in degradation of the substrate and chemical functionality of the monomer. [11,12]
17
1.2 Porous Polymers
Porous polymers have many uses as membranes [14,15], sensors [16,17], cell scaffolds
[18,19], and low dielectric materials [20,21]. The production of porous membranes is dominated
by solution-phase techniques including solvent casting and particulate leaching [22,23],
cryopolymerization [24,25], high internal phase emulsion [26,27], and phase separation [28,29].
Although these methods offer a wide range of porosity, the solubility requirements limit the range
of substrates that can be used, and the chemical functionality allowed by these techniques.
We recently developed an all dry technique to synthesize porous polymers using the iCVD
reactor. [30-35] In this process, monomer enters the reactor in vapor phase and undergoes
deposition phase change to form solid monomer structures at the surface of the substrate. Monomer
state changes from vapor to solid because the monomer partial pressure and temperature of the
substrate are kept below the triple point pressure and temperature of the monomer. In our fabrication
process, monomer and initiator radicals can be either introduced simultaneously and result in
concurrent deposition of solid monomer and polymerization (Figure 1-2a) or the monomer can be
introduced before the radicals (Figure 1-2b) to eliminate thermal gradient issue during monomer
deposition and allows for the independent control over processing parameters of each step
independently including temperature, pressure, and duration of each step. Excess unreacted solid
monomer can be removed via sublimation to make a pure polymer membrane with two different
porosity scales.
18

Figure 1-2. Schematic representation of the a) simultaneous and b) sequential process for
fabricating porous polymer membranes followed by sublimation of excess solid monomer [32].

Larger pores in the range of tens of microns form due to the unfilled spaces between the
pillar microstructures and smaller pores in the range of hundreds of nanometer form within the
pillars due to the sublimation of the unreacted monomer. (Figure 1-3) In our process, the shape of
solid monomer, which can be controlled by changing the temperature and pressure during monomer
deposition, defines the porosity of the resulting porous polymer. [30-35] Higher substrate
temperature results in formation of membranes with web-like shape while lower substrate
temperatures result in formation of pillar-like structures. (Figure 1-4)
19

Figure 1-3 Schematic of porous membrane fabrication showing the growth on a molecular and
macroscopic scale. Cross-sectional SEM images showing the large-scale pores between (left) and
small-scale pores within (right) the microstructures. [10]

Figure 1-4 Effect of monomer deposition temperature on the morphology of the porous membrane.

Our process offers a high degree of control over the chemical composition of the membranes
and is “greener” than many current processing techniques due to elimination of organic solvents.
Surface chemistry of the membranes can be tuned by either incorporating another monomer into
the fabrication process or by conformally coating the porous membrane by an additional layer of
20
the polymer using typical iCVD process. [10,30] The membranes can be cross-linked in order to
render them insoluble in aqueous solution and to improve the mechanical strength by incorporating
a crosslinker (ethylene glycol diacrylate) into the process.

1.3 References
[1] Tenhaeff, W. E.; Gleason, K. K. Initiated and Oxidative Chemical Vapor Deposition of
Polymeric Thin Films: iCVD and oCVD. Adv. Funct. Mater. 2008, 18, 979−992.
[2] Kwong, P.; Flowers, C. A.; Gupta, M. Directed Deposition of Functional Polymers onto Porous
Substrates Using Metal Salt Inhibitors. Langmuir 2011, 27, 10634−10641.
[3] Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.;
McKinley, G. H.; Gleason, K. K. Superhydrophobic Carbon Nanotube Forests. Nano Lett. 2012, 3,
1701−1705.
[4] J.L. Yagüe, A.M. Coclite, C. Petruczok, and K.K. Gleason, Chemical Vapor Deposition for
Solvent ‐Free Polymerization at Surfaces, Macromolecular Chemistry and Physics 214 (2013) 302-
312.
[5] S. Seidel, C. Riche, and M. Gupta, Chemical vapor deposition of polymer films, Encyclopedia
of Polymer Science and Technology (2011).
[6] M. Ma, M. Gupta, Z. Li, L. Zhai, K.K. Gleason, R.E. Cohen, M.F. Rubner, and G.C. Rutledge,
Decorated electrospun fibers exhibiting superhydrophobicity, Advanced Materials 19(2007) 255-
259.
[7] M. Ma, Y. Mao, M. Gupta, K. K. Gleason, and G. C. Rutledge, Superhydrophobic fabrics
produced by electrospinning and chemical vapor deposition, Macromolecules 38(2005) 9742–
9748.
21
[8] Chan, K.; Gleason, K. K.; Macromolecules 2006, 39, 3890.
[9] S. J. Limb, K. K. Gleason, D. J. Edell, E. F. Gleason, J. Vac. Sci. Technol. A. 1997, 15, 4.
[10] S. Seidel, P. Kwong, and M. Gupta, Simultaneous polymerization and solid monomer
deposition for the fabrication of polymer membranes with dual-scale porosity,
Macromolecules 46(2013) 2976-2983.
[11] Chan, K.; Gleason, K. K.; Chem. Vap. Deposition 2005, 11, 437.
[12] Lau, K. K. S.; Gleason, K. K.; Macromolecules 2006, 39, 3688.
[13] Mao, Y.; Gleason, K. K.; Langmuir 2006, 22, 1795.

[14] E.E. Nuxoll, M.A. Hillmyer, R. Wang, C. Leighton, and R.A. Siegel, Composite Block
Polymer Microfabricated Silicon Nanoporous Membrane, ACS applied materials &
interfaces 1(2009) 888-893.
[15] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, and A.M. Mayes,
Science and technology for water purification in the coming decades, Nature 452(2008) 301-310.
[16] S.X. Dong, H. Cao, F.M. Bai, L. Yan, J.F. Li, D. Viehland, and Y.K. Gao, Conformal sensor
skin approach to the safety-monitoring of H-2 fuel tanks, (2004).
[17] F. Audouin, M. Fox, R. Larragy, P. Clarke, J. Huang, B. O’Connor, and A. Heise,
Polypeptide-grafted macroporous polyhipe by surface-initiated n-carboxyanhydride (NCA)
polymerization as a platform for bioconjugation, Macromolecules 45(2012) 6127-6135.
[18] M. Tang, M. Purcell, J.A. Steele, K.Y. Lee, S. McCullen, K.M. Shakesheff, A. Bismarck,
M.M. Stevens, S.M., Howdle, and C.K. Williams, Porous copolymers of ε-caprolactone as scaffolds
for tissue engineering, Macromolecules 46(2013) 8136-8143.
22
[19] L. Soletti, Y. Hong, J. Guan, J.J Stankus, M.S. El-Kurdi, W.R. Wagner, and D.A. Vorp, A
bilayered elastomeric scaffold for tissue engineering of small diameter vascular grafts, Acta
biomaterialia 6(2010) 110-122.
[20] K. Taki, K. Hosokawa, S. Takagi, H. Mabuchi, and M. Ohshima, Rapid production of ultralow
dielectric constant porous polyimide films via CO2-tert-amine zwitterion-induced phase separation
and subsequent photopolymerization, Macromolecules 46(2013) 2275-2281.
[21] B. Krause, G.H. Koops, N.F. van der Vegt, M. Wessling, M. Wübbenhorst, and J. van Turnhout,
Ultralow-k dielectrics made by supercritical foaming of thin polymer films, Advanced
materials 14(2002) 1041.
[22] A.G, Mikos, A.J. Thorsen, L.A. Czerwonka, Y. Bao, R. Langer, D.N. Winslow, and J.P.
Vacanti, Preparation and characterization of poly (L-lactic acid) foams, Polymer 35(1994) 1068-
1077.
[23] J. Sa-nguanruksa, R. Rujiravanit, P. Supaphol, S. and Tokura, Porous polyethylene membranes
by template-leaching technique: preparation and characterization, Polymer testing 23(2004) 91-99.
[24] P. Perez, F. Plieva, A. Gallardo, J. San Roman, M.R. Aguilar, I. Morfin, F. Ehrburger-Dolle,
F. Bley, S. Mikhalovsky, I.Y. Galaev, and B. Mattiasson, Bioresorbable and nonresorbable
macroporous thermosensitive hydrogels prepared by cryopolymerization. Role of the cross-linking
agent, Biomacromolecules 9(2007) 66-74.
[25] W. Xue, I.W. Hamley, M.B. and Huglin, Rapid swelling and deswelling of thermoreversible
hydrophobically modified poly (N-isopropylacrylamide) hydrogels prepared by freezing
polymerisation, Polymer 43(2002) 5181-5186.
23
[26] A.S. Hayward, N. Sano, S.A. Przyborski, and N.R. Cameron, Acrylic ‐Acid ‐Functionalized
PolyHIPE Scaffolds for Use in 3D Cell Culture, Macromolecular rapid communications 34(2013)
1844-1849.
[27] I. Pulko, and P. Krajnc, High internal phase emulsion templating–a path to hierarchically
porous functional polymers, Macromolecular rapid communications 33(2012) 1731-1746.
[28] V.O. Ikem, A. Menner, and A. Bismarck, High-porosity macroporous polymers sythesized
from titania-particle-stabilized medium and high internal phase emulsions, Langmuir, 26(2010)
8836-8841.
[29] P. Sukitpaneenit, and T.S. Chung, Molecular elucidation of morphology and mechanical
properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and
rheology, Journal of Membrane Science 340(2009) 192-205.
[30] S. Seidel, G. Dianat, M. Gupta, Macroml. Mater. Eng., DOI: 10.1002/mame.201500340
[31] S. Seidel, and M. Gupta, Systematic study of the growth and morphology of vapor deposited
porous polymer membranes, Journal of Vacuum Science & Technology A, 32(2014) 041514.
[32] Dianat, G.; Movsesian, N.; Gupta, M.; Process–Structure–Property Relationships for Porous
Membranes Formed by Polymerization of Solid Monomer by a Vapor-Phase
Initiator. Macromolecules 2018, 5, 10297-10303
[33] P. Kwong, S. Seidel, S. and M. Gupta, Solventless Fabrication of Porous-on-Porous Materials,
ACS applied materials & interfaces 2013, 5, 9714-9718.
[34] G. Dianat, S. Seidel, M.M. De Luna, and M. Gupta, Vapor Phase Fabrication of Hydrophilic
and Hydrophobic Asymmetric Polymer Membranes, Macromolecular Materials and Engineering
2016, 310, 1037–1043.
24
[35] S. Seidel, G. Dianat, and M. Gupta, Formation of Porous Polymer Coatings on Complex
Substrates Using Vapor Phase Precursors, Macromolecular Materials and Engineering 2016, 301,
371-376.

25

Chapter 2 Process-Structure-Property Relationships for Porous Membranes
Formed by Polymerization of Solid Monomer by a Vapor-Phase Initiator

“Reproduced with permission from [Dianat, G.; Movsesian, N.; Gupta, M.; Process–Structure–
Property Relationships for Porous Membranes Formed by Polymerization of Solid Monomer by a
Vapor-Phase Initiator. Macromolecules 2018, 5, 10297-10303.] Copyright [2018] American
Chemical Society."
26
2.1 ABSTRACT
In this study, we determine the mechanism that governs polymerization during a process in
which solid monomer is polymerized with a vapor-phase initiator. Gel permeation chromatography
data shows a bimodal molecular weight distribution at all processing conditions which can be
attributed to two different polymerization mechanisms. Smaller chains form by polymerization at
the vapor-solid interface and larger chains form by polymerization within the solid. The monomer
mobility and sublimation rate affect the polymerization rate and thereby affect the membrane
structure. The molecular weight of the larger chains can be increased by increasing the
polymerization temperature and the polymerization time. The ability to controllably vary the
molecular weight allows for tuning the solubility of the membranes. The process-structure-property
relationships elucidated in this study can enable the fabrication of porous polymer membranes for
applications in filtration, textiles, and sensors.

2.2 Introduction
Porous polymer membranes have many important uses as biomaterials,[1,2] sensors,[3,4]
optical devices,[5,6] microreactors,[7,8] and filters.[9,10] Porous membranes are typically
produced by solution-phase methods including phase separation,[11,12] cryopolymerization,[13,14]
solvent casting and particulate leaching,[15,16] and copolymer self-assembly.[17,18] We have
recently shown that we can produce porous polymer membranes by polymerization of solid
monomer by a vapor-phase initiator.[19,20] In contrast to other fabrication methods, our process
is solventless and bottom-up which eliminates solvent compatibility and surface tension issues and
offers a green alternative to forming membranes.[19] In this paper, we study the process-structure-
27
property relationships that govern our fabrication process which is important for determining
polymer properties such as mechanical strength,[21,22] solubility,[23,24] and crystallization.[25,26]
In our fabrication process, monomer precursors enter the reactor in the vapor phase in
conjunction with initiator molecules. The substrate temperature and monomer pressure are kept
below the triple point temperature and pressure of the monomer, respectively, to allow the monomer
vapor to deposit as solid pillar-like microstructures. Free radicals that are formed by thermally
cleaving the initiator molecules react with the monomer to start polymerization. Unreacted solid
monomer is then removed through sublimation and the porous polymer membrane
remains.[20,27,28] Larger pores on the order of tens to hundreds of microns in magnitude form due
to empty spaces between the solid microstructures and smaller pores form on the order of hundreds
of nanometers inside the microstructures. Using this method, we have previously shown the
fabrication of porous-on-porous membranes,[29] asymmetric membranes,[28] and patterned porous
polymer membranes.[30]
In this work, we systematically study the molecular weight distribution of these membranes
for the first time. Formation of the porous polymer membranes can occur by simultaneous or
sequential polymerization. In the simultaneous process, the monomer and the initiator free radicals
are introduced together in order for the monomer to deposit and polymerize concurrently[19,20],
while in the sequential process the monomer is first deposited and subsequently polymerized by the
introduction of initiator free radicals[30,31]. Our work focuses on the sequential method because
of the independent control offered over temperature, pressure, and the duration of each step. We
show that the sequential process leads to a bimodal polymer distribution due to polymerization at
the vapor-solid interface and polymerization within the solid monomer. Polymers with bimodal
28
molecular weight distributions have been shown to balance the mechanical strength and rheological
properties of materials. [32,33]
Methods for initiating solid monomers have been generally limited to using high energy
initiators such as X-rays and electrons, however activated species can form during these processes
resulting in considerable uncertainties in the mechanistic understanding of these polymerization
reactions.[34] In addition, polymerization of solid state monomers is reported to be typically slow
due to lack of mobility in the solid phase.[35,36] For example, Bamford et al. polymerized solid
methacrylic acid monomer in solid (4°C) and near solid (13°C) states by ultraviolet irradiation and
found that conversion reaches 10% after 65 hours and 30 hours in the solid and near solid states,
respectively.[35] In contrast, we utilize a low energy process using a thermally cleavable vapor
phase initiator that can easily diffuse in the solid monomer microstructures. This process operates
at mild reactor conditions with the ability to retain the full chemical functionality of the
precursors.[37-39] Here we demonstrate that the molecular weight distribution of the polymer
membranes formed by our technique can be tuned by changing the polymerization temperature and
time, which can lead to changes in the membrane dissolution rate.

2.3 Experimental Section
Methacrylic acid (MAA) (Aldrich, 99%), tert-butyl peroxide (TBPO) (Aldrich, 98%),
sodium phosphate dibasic (Aldrich,  99.0%), and sodium azide (Aldrich,  99.5%) were used as
received without further purification. Silicon wafers with 3 3 cm2 dimension were placed on top of
a thermoelectric cooler (TE Technology) which was located on the stage of a custom designed
initiated chemical vapor deposition (iCVD) vacuum reactor (GVD Corporation, 250 mm diameter,
48 mm height). The temperature of the thermoelectric cooler (TEC) was regulated using an
29
adjustable DC power supply (Volteq HY3010D). The stage temperature was maintained at 10 °C
using a recirculating chiller (Thermo Scientific Haake A25). A rotary vane vacuum pump (Edwards
E2M40) and a throttle valve (MKS 153D) was used to control the pressure of the reactor.
To synthesize the porous poly (methacrylic acid) PMAA membranes, the TEC was maintained at a
temperature of -10 °C during monomer deposition. TBPO was introduced into the chamber at 0.6
sccm to maintain the pressure at 650 mTorr. The MAA monomer was then introduced into the
chamber at a flow rate of 0.3 sccm for 5 minutes for all the experiments. Next, MAA flow was
stopped and TBPO continued flowing into the reactor at a reactor pressure of 650 mTorr at 0.6
sccm. To study the effect of the polymerization temperature, the TEC temperature was set to -20°C,
-10°C, 0°C, and 10°C immediately after monomer deposition and before starting the polymerization.
Polymerization was started by resistively heating the filament array to 240 °C to decompose the
initiator molecules into free radicals. The filament was then turned off after polymerization and the
flow of TBPO was halted. The TEC temperature was then kept at 5°C and slowly increased to 14°C
to allow the unreacted MAA to sublimate and leave the system, which was confirmed by the reactor
returning to a base pressure of 18 mTorr. During the porous PMAA fabrication process, a silicon
wafer was placed on the stage next to the TEC and the thickness of the polymer deposited onto it
was monitored by an in situ 633 nm helium-neon laser interferometer (Industrial Fiber Optics).
A gel permeation chromatography (GPC) system equipped with a HPLC pump (Agilent
1200 series), a refractive index detector (Wyatt Optilab rEX), a multi-angle light scattering detector
(Wyatt DAWN HELEOS), and two Agilent columns (PL aquagel-OH MIXED-M) in series was
used to measure the molecular weight distribution of the polymer membranes. The GPC eluent was
made by dissolving 0.1 molar sodium phosphate dibasic and 200 ppm sodium azide in deionized
water to make pH 9 buffer.[40] Membranes were dissolved in the GPC eluent and filtered through
30
0.45 m filters prior to injection. The injected volume was 100 L and the flow rate was 0.5 mL/min.
Differential refractive index chromatograms were deconvoluted into two peaks and the area under
each peak was calculated by PeakFit v4.12 software using exponentially modified Guassian
distribution. The number-averaged molecular weight (Mn) and polydispersity index (PDI) were
calculated by a calibration curve equation based on five poly(methacrylic acid), sodium salt
analytical standards with molecular weights of 7000 Da (Polysciences, Inc.), 18700 Da (PSS
Polymer Standards Service GmbH), 25000 Da (Polysciences, Inc.), 70000 Da (Polysciences, Inc.),
and 350000 Da (Polysciences, Inc.). The membranes formed at a temperature of 10°C for more than
30 min were found to have high molecular weights (above 450000 Da) which is outside the range
of the calibration curve, therefore the Mn and PDI values were based on a combination of
differential refractive index with multi-angle light scattering analysis. Three membranes were
fabricated for each data point and the error bars were found from calculating the standard deviation.
Scanning electron microscopy (SEM Topcon Aquila) with an accelerating voltage of 20 kV was
used to visualize the membranes. A layer of gold was sputter coated onto the membranes prior to
SEM imaging to prevent charging. In order to study the effect of molecular weight on the solubility,
membranes were ground to limit the effect of morphology and same mass (10 mg) of the ground
membranes were dissolved in 10 mL of deionized water and 10 mL of pH 9 buffer. To count for
morphology effect on solubility the intact membranes were immersed in a constant volume (25 mL)
of the pH 9 buffer.

31
2.4 Results and Discussion
Figure 2-1 shows a schematic of the sequential deposition process to fabricate porous
polymer membranes. A wafer is placed on a thermoelectric cooler (TEC) that is set at -10C. The
TEC is located on the reactor stage which is cooled to 10 C. First, the initiator is introduced into
the reactor in order to build up and maintain the reactor pressure. Then the monomer is flown into
the reactor for 5 min for all the experiments in order to keep the initial monomer concentration
(approximately 300 mg) constant. We use methacrylic acid (MAA) as the monomer due to its
relatively high freezing point (15 C) and the useful properties of the resulting poly(methacrylic
acid) (PMAA) membranes such as pH-responsiveness.[41,42] The flow of the monomer is then
halted and the filament is turned on to cleave the initiator molecules into free radicals to start the
polymerization process. In this study, the polymerization temperature is systematically varied by
changing the TEC temperature to -20, -10, 0, or 10 C right before turning on the filament. The
polymerization time is varied by keeping the filament on from 5 to 240 min. After the
polymerization process, unreacted solid monomer is sublimated.

Figure 2-1 Schematic representation of the sequential process for fabricating porous polymer
membranes.
32
We first investigated the effect of polymerization time on the morphology of the porous
membranes. Figure 2-2a shows that the structures formed after 5 minutes of polymerization at a
temperature of -10 C are tilted which is likely due to their lack of mechanical strength resulting in
structural collapse during the sublimation process. Figure 2-2b shows that a densified porous
structure formed at the silicon wafer boundary as the polymerization time was increased from 5
min to 30 min. This densified porous structure adds to the mechanical strength of the membranes,
and therefore our remaining experiments focused on membranes formed at polymerization times
above 30 min.

Figure 2-2 Angled-view and cross-sectional SEM images of porous membranes polymerized for a)
5min and b) 30 min at a polymerization temperature of -10C.

In order to systematically understand the effect of polymerization time on the molecular
weight distribution, we formed membranes at polymerization times of 30 min, 60 min, 120 min,
and 240 min at a polymerization temperature of -10C. The membranes were dissolved in pH 9
buffer and the molecular weight distributions were measured using GPC. The GPC data in Table
33
2-1 shows that the membranes have a bimodal molecular weight distribution for all polymerization
times which indicates the presence of two types of polymerization mechanisms. Our hypothesis is
that the polymerization starts at the vapor-solid interface and forms lower molecular weight chains
because the higher initiator concentration at the interface results in a faster termination rate. The
higher molecular weight chains are likely formed by diffusion of the initiator free radicals inside
the solid monomer pillars . The GPC data shows that the Mn of the shorter chains does not
significantly vary with time while the Mn of the longer chains increases. The data also shows that
the total mass of polymer increases with time. The mass of the lower molecular weight fraction
increases more drastically than the higher molecular weight fraction. Comparisons among the
estimated number of polymer chains produced at each condition indicate that the number of
polymer chains increases with increasing time for the low molecular weight fraction since initiator
radicals are constantly introduced at the vapor-solid interface, whereas the number of polymer
chains for the high molecular weight fraction stays relatively constant which suggests the
consumption of solid-state monomer into the growth of existing polymer chains. The formation of
longer chains with lower PDIs can be attributed to the lower initiator concentration inside the pillars
resulting in lower termination rates. Our hypothesis is that the polymer formed at the vapor-solid
interface acts as a diffusion barrier for the free radical initiators. Therefore, termination of the
growing radical chains within the solid is slowed due to a lower initiator concentration.

34
Table 2-1 Effect of polymerization time on the molecular weight distribution and the total mass of
polymer formed at a polymerization temperature of -10 C.

In order to understand the effect of polymerization temperature on the polymer morphology
and the molecular weight distribution, we set the TEC temperature to -20 C, -10 C, 0 C, and 10 C
during the polymerization step while keeping all other processing parameters constant. The SEM
images in Figure 2-3 show that the densified porous structures that form at the silicon wafer
boundary thicken as the temperature is increased even though the total membrane thickness remains
unchanged (approximately 650 µm). The increase in the thickness of the densified porous structure
is likely due to higher sublimation rates at higher polymerization temperatures. In order to confirm
our hypothesis, we placed a silicon wafer on the stage at 10 C next to the TEC in order to monitor
the sublimation process. During the polymerization on the TEC, monomer sublimating from the
porous membrane adsorbs onto the silicon wafer in the vapor phase and polymerizes to form a
dense pinhole-free polymer film. The thickness of this dense film was measured by in situ
interferometry at all polymerization temperatures (-20 C, -10C, 0 C, and 10 C) for a constant
polymerization time of 30 min. The thickness of the dense polymer films increased from
approximately 50 nm to 500 nm as the temperature increased from -20 C to 10 C, confirming that
the higher polymerization temperature increases the sublimation rate of the solid monomer and thus
results in higher monomer adsorption and polymerization on the silicon wafer placed on the stage.
As shown in Figure 2-3, the pillar structure of the membranes formed at -20C does not have visible
35
micron scale pores while the morphology of the pillars formed at -10 C and 0 C are similar, and
sponge like with small-scale pores within the pillars. The higher sublimation rate at higher
polymerization temperatures likely leads to the formation of coarser pores within the pillars. This
data is consistent with recent studies on the effect of sublimation rate on the pore size of porous
particles formed by vapor deposition of poly-para-xylylenes on sublimating ice particles.[43] The
pillars formed at 10 C are partially covered with a dense film which is likely due to condensation
and polymerization of the monomer at the surface of the pillars because of the higher local monomer
partial pressure and polymerization temperature close to the freezing point of MAA.

Figure 2-3 Cross-sectional and zoomed in SEM images of the membranes formed at different
polymerization temperatures.

As shown in Table 2-2, there is a bimodal distribution of molecular weights at each
polymerization temperature. The Mn of the lower molecular weight chains has a moderate increase
while the Mn of the higher molecular weight chains increases drastically with increasing
36
polymerization temperature due to the increased mobility of the monomer molecules leading to
higher propagation rates. The mass of the higher molecular weight chains stays constant, while the
mass of the shorter chains increases with increasing temperature due to the high concentration of
the free radicals at the vapor-solid interface and the higher sublimation rates at higher temperatures.
This results in an increased monomer conversion from approximately 2% (5.9 mg) at a
polymerization temperature of -20 C to approximately 6% (18.3 mg) at 10 C. (Table 2-2) This data
is consistent with the results shown by Bamford et al. on the polymerization of solid MAA by
ultraviolet irradiation which shows slower polymerization rates at lower temperatures.[35] Based
on the data, the polymerization of the solid monomer is dominated by the reaction kinetics governed
by temperature while the monomer sublimation rate is the dominant factor in the polymerization at
the vapor-solid interface.

Table 2-2 Effect of polymerization temperature on the molecular weight distribution and the total
mass of polymer.

To further study the polymerization mechanism, we synthesized membranes at a lower
initial monomer concentration where monomer was deposited for 2 minutes instead of 5 minutes
as in all other cases and polymerized for 30 minutes at -10C. The GPC data for the condition with
2 minutes of monomer deposition shows similar molecular weights as in the 5minute monomer
deposition data which is due to presence of high monomer concentration for both deposition times.
37
(Table 2-3) Therefore it can be concluded that the polymerization rate is not limited by initial
monomer concentration under these conditions and is dependent on the polymerization time and
temperature. As shown in the SEM images in Figure 2-4, the membranes that were deposited with
a higher initial monomer concentration (5 minutes) are thicker and therefore have a higher vapor-
solid interfacial area. This higher vapor-solid interfacial area results in a greater mass of shorter
polymer chains.

Table 2-3 Effect of monomer concentration on the molecular weight distribution and the total mass
of the polymer.

38

Figure 2-4 Angled-view and cross-sectional SEM images of porous membranes deposited for a)
2min and b) 5 min and polymerized for 30 min at a temperature of -10 C.

In order to test our hypothesis that the lower molecular weight chains form by
polymerization at the vapor-solid interface, we studied the molecular weight of the dense films that
formed on the silicon wafer placed on the stage next to the TEC during a polymerization
temperature of 10 C. The high polymerization temperature resulted in a high monomer sublimation
rate from the TEC and the sublimated monomer that adsorbed onto the silicon wafer on the stage
polymerized to form a dense transparent film. The GPC data in Figure 2-5 demonstrates that the
molecular weight of the dense film, polymerized for 30 min, has one peak with a Mn of 11.6103
g/mol, which is similar in magnitude to the Mn of the lower molecular weight fraction (17.5 103
g/mol) found in the porous membrane fabricated on the TEC in the same deposition. This
experiment was repeated for polymerization times of 75 min and 120 min and the Mn of the dense
39
films found to be 9.5 103 and 11.8 103 g/mol, respectively. Increasing time did not increase the
molecular weight, which is consistent with the trends observed for the low molecular weight
fraction of the porous membranes.

Figure 2-5 GPC chromatograms of the porous membrane synthesized by 30 min of polymerization
at 10 C and the dense films deposited on the silicon wafer on the stage for 30 min, 75 min, and 120
min at a temperature of 10 C. Each chromatogram is normalized with respect to the area under the
corresponding curve.

In order to further confirm the effect of time and temperature on the molecular weight
distribution, we formed membranes at 10 C for 30 min, 75 min, 120 min and 240 min. The mass
of the low molecular weight fraction increases more rapidly than the mass of the high molecular
weight fraction, which is consistent with the data in Table 2-1. For each of the polymerization times,
the Mn of the higher molecular weight fraction and the total polymer yield are greater at 10 C than
-10C, due to the higher mobility and sublimation rate, respectively. (Table 2-4) The Mn of the low
molecular weight fraction is slightly higher at 10 C (Table 2-4) compared to the Mn of the low
40
molecular weight fraction at -10C (Table 2-1) due to faster kinetics and a higher monomer
concentration likely caused by a higher sublimation rate at this temperature. We further observe
that the monomer sublimation rate during the polymerization step reaches steady state between 75
to 90 min of polymerization by monitoring the dense film deposition via interferometry. Thus, there
is a decrease in the mass growth rate of the low molecular weight fraction over time which indicates
that the sublimation rate is the dominant factor in the polymerization at the vapor-solid interface.
Mn of the higher molecular weight fraction for polymerization times of 75 min and above are on
the order of 1000 kDa and multi-angle light scattering was used to confirm these high molecular
weights.
Table 2-4 Effect of polymerization time on the molecular weight distribution and the total mass of
polymer at 10 C.

The ability to tune the solubility of the porous membranes is important for applications such
as water filtration and drug delivery. As shown above, increasing the time and temperature of
polymerization increases the molecular weight of the polymer, which can be used to decrease
solubility.[23] To study the effect of molecular weight on the dissolution behavior of the membranes,
three different porous membranes were fabricated at a polymerization time of 30, 75, and 120 min
and a polymerization temperature of 10 C. The membranes were first ground in order to minimize
the effect of the membrane morphology on the dissolution rate and then 10 mg of each membrane
was immersed in 10 mL of deionized water. As shown in Table 2-5, the dissolution rate of the
41
porous membranes decreased by increasing the Mn of the high molecular weight fraction. In order
to further investigate the dissolution behavior, the ground membranes were also dissolved in pH 9
buffer and the same trend was observed with increasing polymerization time. To elucidate the effect
of morphology on the dissolution rate, the intact porous membranes that were polymerized for 30
min and 120 min at a polymerization temperature of 10 C were each immersed in a same volume
(25 mL) of pH 9 buffer. The membrane formed at shorter time dissolved quickly in the solvent
within 1 min while the membrane formed at longer time required over 15 min to dissolve
completely. (Figure 2-6)

Table 2-5 Effect of molecular weight on the dissolution rate of porous polymer.

Figure 2-6 Dissolution behavior of the intact porous membranes polymerized at 10 C for a) 30 min
and b) 120 min.
42
2.5 Conclusion
We have shown that polymerization of solid monomer with a vapor phase initiator results
in a bimodal molecular weight distribution. Polymerization at the vapor-solid interface leads to the
formation of shorter chains and polymerization within the solid monomer structures leads to higher
molecular weight chains. The molecular weight of the longer chains increases with time, while the
molecular weight of the shorter chains stays relatively constant due to the high concentration of
initiator radicals at the vapor-solid interface. Higher polymerization temperatures affect the
molecular weight distribution of the membranes by altering the mobility of the solid monomer
inside the pillars resulting in an increase in the molecular weight. A densified porous structure forms
at the silicon wafer boundary and thickens as a result of the higher sublimation rate at higher
temperatures. We show that we can control the molecular weight distribution with time and
temperature in order to tune the dissolution rate of the membranes. These findings set important
guidelines for the fabrication of next-generation porous materials with tailored structures and
properties for practical applications in separations, textiles, and sensors.

2.6 Acknowledgements
This work was supported by the National Science Foundation CAREER Award CMMI-1252651.
We thank Dr. Shuxing Li and the USC NanoBiophysics Core Facility for help with the gel
permeation chromatography.

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48

Chapter 3 Sequential Deposition of Patterned Porous Polymers using
Poly(dimethylsiloxane) Masks

Publication Citation: Dianat, G.; Gupta, M.; Sequential deposition of patterned porous polymers
using poly (dimethylsiloxane) masks. Polymer 2017, 126,463-469.

49
3.1 ABSTRACT
The deposition of porous polymer membranes was patterned using poly(dimethylsiloxane)
(PDMS) masks. The porous polymer was deposited by introducing the monomer and initiator
sequentially to allow for the temperature, pressure, and duration of each step to be controlled
independently. The porosity and thickness of the membranes was controlled by varying the
substrate temperature during monomer deposition. The addition of a cross-linker during
polymerization allowed for the fabrication of robust free-standing shaped hydrophilic membranes
that are insoluble in aqueous solutions. Our ability to control the shape, thickness, porosity, and
functionality of the porous membranes allows for the design of new surfaces for a variety of
applications in sensors, filtration, and microfluidics.

3.2 Introduction
Initiated chemical vapor deposition (iCVD) is a solventless polymerization technique that
is typically used to deposit thin polymer films and coatings. [1,2] Monomer and initiator molecules
are introduced in the vapor phase and a hot filament array is used to cleave the initiator molecules
into free radicals. The initiator radicals and monomer molecules adsorb to the surface of the cooled
substrate and polymerization occurs via a free radical polymerization mechanism. A variety of
functional polymer films can be formed by varying the monomer including hydrophilic,
hydrophobic, and responsive films. [3,4]
Porous polymers have many uses as membranes [5,6], sensors [7,8], cell scaffolds [9,10],
and low dielectric materials [11,12]. The production of porous membranes is dominated by
solution-phase techniques including solvent casting and particulate leaching [13,14],
cryopolymerization [15,16], high internal phase emulsion [17,18], and phase separation [19,20].
50
Although these methods offer a wide range of porosity, the solubility requirements limit the range
of substrates that can be used and the chemical functionality allowed by these techniques.
We recently showed that we can modify the iCVD process to deposit porous polymer membranes
by keeping the monomer partial pressure above its saturation pressure and the substrate temperature
below the freezing point of the monomer. These conditions result in the simultaneous deposition of
solid monomer and polymerization. Excess unreacted solid monomer can then be removed via
sublimation and a porous polymer membrane with dual-scale porosity remains. Large-scale pores
in the range of tens to hundreds of microns form due to the void spaces between the solid monomer
microstructures and small-scale pores in the range of hundreds of nanometer form within the
microstructures due to the sublimation of the unreacted monomer. Compared to other methods to
make porous polymers, the solventless nature of our method eliminates organic solvent waste and
surface tension effects and also provides control over the chemical composition of the membranes.
[21-26]
In this paper, we demonstrate our ability to control the location of porous polymer
deposition using poly(dimethylsiloxane) (PDMS) masks. The masks were made of PDMS since its
elastic behavior allows for easy attachment and detachment from the silicon substrate and PDMS
can also be easily cut or molded into different shapes. We also modified our deposition process to
utilize sequential iCVD conditions for the first time to make the porous polymer. The solid
monomer is deposited in the first step, the filament is turned on to begin polymerization in the
second step, and the unreacted monomer is sublimated in the third step. Our method allows for
deposition of the patterned porous membranes directly onto a substrate. We show that the
morphology and thickness of the membranes can be tailored by varying the substrate temperature.
We demonstrate that we can add a cross-linker to form free-standing membranes. These free-
51
standing membranes are useful for a range of applications including paper-based microfluidic
devices. [27-30] Paper-based microfluidics are cost-effective, lightweight, and portable diagnostic
platforms in which fluid flow is driven by capillary action through a hydrophilic porous medium.
Our patterned free-standing cross-linked membranes are robust and hydrophilic and therefore can
be used for capillary-flow microfluidics. We also demonstrate that we can change the surface
properties from hydrophilic to hydrophobic by conformally coating the patterned porous membrane
with a hydrophobic polymer.

3.3 Experimental
Methacrylic acid (MAA) (Aldrich, 99%), ethylene glycol diacrylate (EGDA) (Polysciences,
Inc), divinylbenzene (DVB) (Aldrich, 80%), tert-butyl peroxide (TBPO) (Aldrich, 98%), crystal
violet (Aldrich, 90%), ponceau S (Aldrich, 75%), and pH 7 buffer (BDH, ACS grade) were used as
received without further purification.
3.3.1 Fabrication of PDMS masks
Sylgard 184 base (Aldrich) was mixed with cross-linker at a ratio of 10:1 and then the
mixture was thermally cured in a Petri dish at 60 °C for 24 h to make 4mm slabs. Patterns were cut
or molded into the PDMS slabs. The Sylgard 184 mixture was also spin-coated onto a silicon wafer
at 800 rpm for 30 seconds and then placed onto a hot plate at 60 °C for 4 hours to create a thin
PDMS layer on silicon wafers.
3.3.2 Fabrication of Porous Poly(methacrylic acid) (PMAA) Membranes
The masked samples were placed on a thermoelectric cooler (TE Technology), which was
located on the stage of a custom designed iCVD vacuum reactor (GVD Corporation, 250 mm
52
diameter, 48 mm height). The temperature of the thermoelectric cooler (TEC) was controlled using
an adjustable DC power supply (Volteq HY3010D). The pressure in the reactor was controlled by
a rotary vane vacuum pump (Edwards E2M40) and a throttle valve (MKS 153D).
For the deposition of the porous PMAA membrane, the TEC was maintained at temperatures
of 0 °C, -10 °C, or -20 °C during monomer deposition. TBPO was first introduced into the chamber
at 0.6 sccm to maintain the pressure at 550 mTorr. The MAA monomer was then introduced into
the chamber at a flow rate of 3.5 sccm for 2 to 5 minutes. The filament array was then resistively
heated to 220 °C to cleave the initiator into free radicals to start the polymerization process which
lasted for 20 to 25 minutes. The TEC was kept at -10 °C during this polymerization step for all
fabricated membranes. The filament was then turned off and the flow of TBPO was stopped. The
TEC temperature was then increased to 5 °C to allow for unreacted MAA to sublimate. The reactor
pressure returned to the base pressure of 16 mTorr in approximately 45 minutes. The sample was
removed from the reactor chamber and the PDMS mask was peeled off the silicon wafer. In order
to determine whether the porosity and the thickness of the membranes could be independently
controlled, a thin porous membrane was fabricated by decreasing the MAA deposition time to 160
seconds at a TEC temperature of -20ºC and a thick membrane was fabricated by increasing the
MAA deposition time to 20 min at a TEC temperature of 0ºC.
3.3.3 Cross-linking the Porous Membranes
To synthesize cross-linked patterned PMAA membranes, first MAA was deposited and
polymerized as described in the previous section. Before the sublimation process, the TEC
temperature was increased to 0 °C and the reactor pressure was decreased to 200 mTorr with the
TBPO flowing at 0.6 sccm flow rate and the filament temperature at 220 °C. EGDA was then
introduced into the reactor chamber at a flow rate of 0.2 sccm. Polymerization lasted for 30 minutes.
53
The EGDA and TBPO flow rates were then stopped and the filament was turned off. The unreacted
MAA monomer was then sublimated as described in the previous section. Separation was
performed in the cross-linked PMAA channel by pipetting 10𝜇𝑙 of pH 7 buffer containing 0.22
mg/mL ponceau S and 0.20 mg/mL crystal violet into the inlet.
3.3.4 Fabrication of Hydrophobic Coatings
In order to add a hydrophobic coating onto the porous PMAA, first the porous PMAA was
fabricated as described in the previous section. After the sublimation of the unreacted MAA, the
porous PMAA membrane was coated with poly(divinylbenzene) (PDVB) using the traditional
iCVD process. Initiator and DVB were introduced with flow rates of 0.6 sccm and 0.6 sccm,
respectively, the pressure was kept at 150 mTorr, the filament temperature was set to 220 °C, and
the TEC temperature was kept at 15 °C for 30 minutes.
3.3.5 Characterization
Scanning electron microscopy (SEM; JEOL-7001; Topcon Aquila) at a 20 kV accelerating
voltage was used to visualize the samples. A gold layer was deposited onto the samples prior to
SEM analysis to prevent charging. The thicknesses of the patterned porous PMAA layers were
estimated by processing the SEM images in ImageJ and averaging over at least 3 different
thicknesses per sample. Contact angle goniometry (Ramé-hart 290-F1) with droplet volumes of 10
µL was used to measure the static contact angle of water droplets on the porous membrane coated
with the PDVB layer.
3.4 Results and Discussion
Figure 3-1 shows a schematic of our process to deposit porous polymer membranes using a
sequential process. We chose methacrylic acid (MAA) as our monomer because the relatively high
54
freezing point of MAA (16 ± 1˚C) allows the use of moderately cooled substrate temperatures.
[Error! Bookmark not defined.] A thermoelectric cooler (TEC) is used to cool the silicon wafer t
o temperatures below 0 ˚C. We chose to introduce our monomer and initiator sequentially because
it allows control over the temperature, pressure, and duration of each step and eliminates a thermal
gradient during monomer deposition because the filament is off during this step. In our sequential
process, first the MAA monomer was introduced into the reactor without the filament on so there
was no initiation. The gaseous MAA goes through a phase change and forms solid microstructures
on the surface of the silicon wafer. This phase transition occurs because the monomer partial
pressure and substrate temperature are below the triple point pressure and temperature of the
monomer, respectively. In the second step, initiator molecules are cleaved into free radicals using
a hot filament array set to 220 ˚C. These free radicals attack the solid monomer and polymerization
begins. In the last step, the unreacted solid monomer is removed through sublimation and only the
porous polymer membrane remains. The temperature of the substrate and the pressure of the reactor
chamber must still remain below the triple point temperature and pressure of MAA in this step to
fully sublimate the excess solid MAA without going through condensation since MAA
condensation will affect the morphology.
55

Figure 3-1 Schematic representation of the process to make porous polymer membranes. There are
three steps. In the first step, the solid monomer is deposited. In the second step, the filament is
turned on to begin polymerization. In the third step, the unreacted monomer is sublimated.

In order to fabricate patterned porous polymer membranes, first a shape was cut into a PDMS
slab to make a mask. The mask was then placed on top of the silicon wafer and then the porous
polymer was deposited as described in Figure 3-1 by first introducing the monomer, then turning
56
the filament on to begin polymerization, and then finally sublimating the unreacted monomer. The
PDMS mask was then removed from the substrate and a patterned porous membrane remained as
shown in Figure 3-2. The chosen shape for our systematic studies is a channel with an inlet which
is a common geometry for paper-based microfluidic devices.

Figure 3-2 Optical images showing the deposition of porous polymer membranes onto a silicon
wafer that is covered with a poly(dimethylsiloxane) mask.

The ability to control the porosity of the membranes is important for biological, sensing,
and microfluidic applications. [31-33] To show our ability to tune the morphology, we deposited
the porous polymer onto masked silicon wafers at different substrate temperatures while keeping
all other processing parameters constant. The monomer was introduced into the chamber for 5
minutes. The solid monomer acts as the template for polymerization and therefore any parameter
that can change the shape of the solid monomer can thereby affect the porosity of the resulting
membrane. Figure 3-3 shows that the membranes deposited at a substrate temperature of 0 °C are
57
thinner and denser than the membranes deposited at lower temperatures. The thicknesses were
measured by SEM and were found to be approximately 1.15 ± 0.06 mm, 1.65 ± 0.05 mm, and 2.07
± 0.03 mm at TEC temperatures of 0 °C, -10 °C, and -20°C, respectively. This increase in thickness
with decreasing temperature is consistent with the data shown in our previous work that showed
that the monomer forms three-dimensional pillar-like microstructures at lower substrate
temperatures whereas the monomer forms two-dimensional web-like microstructures at higher
substrate temperatures. [21,22] Our data demonstrates that substrate temperature can be used to
tailor the thickness and porosity of the membranes.

58

Figure 3-3 Optical and SEM images of porous membrane morphology deposited on PDMS mask
and silicon wafers at TEC temperatures of (a) 0°C, (b) -10°C and, (c) -20 °C.

The porous polymer membranes that formed on top of the PDMS masks were found to be
thinner and less conformal than the porous polymer that formed on the silicon wafer because the
temperature on top of the PDMS is higher due to its lower thermal conductivity (0.18 W/mK) [34]
59
as compared to the relatively high thermal conductivity of the silicon wafer (148 W/mK at room
temperature) [35]. The thickness of the PDMS masks were optimized to be approximately 4
millimeters thick in order to be thin enough to avoid monomer depletion inside the patterned holes
while also being thick enough to reduce the amount of polymer that forms on top of the PDMS
mask. The temperature on top of the PDMS was measured with a thermocouple and was found to
be approximately 4 °C when the TEC temperature was at 0 °C which results in porous polymer
thicknesses ranging from 20 to 80 µm, -6 °C when the TEC temperature was at -10 °C which results
in porous polymer thicknesses ranging from 80 to 110 µm, and -12 °C when the TEC temperature
was at -20 °C which results in a conformal porous polymer thickness of approximately 260 µm.
(Figure 3-3) The higher temperature on top of the PDMS during deposition causes a reduction of
monomer adsorption on top of the mask leading to thinner and less conformal porous polymer
compared to the silicon wafer. The silicon wafer temperatures were found to be only 1°C warmer
than the TEC temperature at all three temperatures due to its high thermal conductivity resulting in
thicker and more conformal films compared to the PDMS. The higher temperature on top of the
PDMS during the sublimation of unreacted monomer results in cracking of the porous membranes
likely because the higher substrate temperature accelerates the sublimation process.
To confirm our hypothesis that the substrate temperature plays an important role in the
formation and morphology of our porous membranes irrespective of the type of material we used
as a substrate, we deposited the porous membranes using the same conditions onto different types
of substrates at a TEC temperature of -20˚C. First, we compared the deposition onto an uncoated
silicon wafer and a silicon wafer that had a thin layer of approximately 200 µm of PDMS spin-
coated onto it. The temperature on top of the thin PDMS layer was found to be approximately -
17 °C which is only slightly higher than the temperature on top of silicon wafer (-19°C). SEM
60
images show a similar morphology on both substrates which indicates that the thin PDMS layer
does not significantly alter membrane formation. (Figure 3-4a-b) We also compared the deposition
onto a 4mm thick mask and a silicon wafer that was placed on top of this mask. The temperature
on top of both these substrates was found to be -12 °C. SEM images show similar morphology on
both substrates and support the hypothesis that the morphology of the porous membranes is
primarily dependent on the substrate temperature. (Figure 3-4c-d)

Figure 3-4 SEM images of porous membrane morphology at a TEC temperature of -20˚C on top
of (a) a silicon wafer spin-coated with thin PDMS layer, (b) a reference silicon wafer, (c) a 4mm
thick PDMS mask layer, and (d) a silicon wafer located on top of the 4mm thick PDMS mask.

In order to determine whether the porosity and thickness of the membranes could be
independently controlled, we compared thin and thick membranes formed on the silicon wafer
surface at two different substrate temperatures. Figure 3-5 shows that the thin membrane
61
(approximately 1 mm) deposited at -20ºC is more porous than the membrane formed at 0ºC.
Similarly, the thick membrane (approximately 2 mm) deposited at -20ºC is more porous than the
membrane formed at 0ºC. For some applications such as optical displays and sensors, it is useful to
pattern thin porous materials directly onto substrates. [36,37] Figure 3-6 shows our ability to use
PDMS masks to pattern shapes and inverse shapes. Monomer was deposited for only 2 minutes at
-10°C to make thinner membranes. SEM analysis showed that the thickness of the porous
membrane was 540 ± 20µm for the circles and 550 ± 20µm for the inverse pattern.

Figure 3-5 SEM images of membranes that are approximately 1 mm thick deposited at TEC
temperatures of a) 0°C and b) -20°C and approximately 2 mm thick deposited at TEC temperatures
of c) 0°C and d) -20°C.
62

Figure 3-6 Images of pattern shapes and inverse shapes of (a) PDMS mask on top of a silicon wafer,
(b) deposited porous polymer on the masked substrate, and (c) patterned porous polymer on the
silicon wafer after removing the mask.

For practical applications that require aqueous solutions such as microfluidics and water
filtration, it is important to make sure that the PMAA membrane can be free-standing and will not
dissolve in aqueous solutions while remaining hydrophilic. Membranes with thicknesses greater
than 500µm are largely prone to delamination from the silicon wafer surfaces. Thin membranes
typically do not delaminate from the silicon wafer surfaces. In order to make very thin free-standing
membranes, the membranes can be deposited onto a sacrificial layer such as alginate [25] which
can then be removed by dissolution to delaminate the membrane from the substrate. In order to
make the PMAA membranes insoluble we can add a cross-linking step into our sequential process.
First, vapor MAA was deposited for 5 minutes onto the masked substrate and subsequently
polymerized by turning on the filament for 25 minutes to form 1.5 mm thick porous membranes.
The filament was kept on as vapor EGDA monomer was introduced as a cross-linker in conjunction
with initiator molecules. The EGDA molecules and initiator radicals diffuse to the substrate, react
63
with unreacted solid MAA, and form P(MAA-co-EGDA). In typical iCVD conditions, all
monomers must be introduced into the reactor chamber at the same time to make copolymers which
may cause limitations on operating conditions. Here, we introduced each monomer separately with
suitable operating conditions such as a lower reactor pressure (200 mTorr) and higher TEC
temperature (0 °C) for the EGDA deposition in order to avoid EGDA condensation. Finally, excess
unreacted MAA was sublimated. Figure 3-7a shows a free-standing cross-linked sample that is
robust and flexible enough that it can held with tweezers. A liquid solution can be wicked into the
sample without dissolution allowing for potential applications in microfluidics. (Figure 3-7b) In
order to demonstrate the enhanced capabilities of our microfluidic devices, we showed that we
could selectively separate a mixture of analytes by pipetting a solution containing cationic crystal
violet and anionic ponceau onto our PMAA channel. Figure 3-7c shows that the negatively charged
cross-linked PMAA membrane successfully separates positively charged crystal violet from
negatively charged ponceau S.

64

Figure 3-7 (a) A free-standing cross-linked P(MAA-co-EGDA) membrane, (b) dyed water wicked
into the cross-linked membrane by capillary action, and (c) separation of crystal violet and ponceau
S through a cross-linked PMAA microfluidic channel.

Another benefit of our fabrication technique is the ability to modify the surface properties of
the porous membranes by coating with another polymer without affecting the structure. Figure 3-8
shows our ability to add a hydrophobic polymer layer of PDVB onto the hydrophilic patterned
porous PMAA membrane. The process involves first making the patterned membrane as described
above. A star shaped mask was used. The MAA monomer was deposited for 2 minutes deposition
at a TEC temperature of -10°C and then polymerized and the unreacted monomer was sublimated.
The PDVB was then deposited onto the patterned porous layer using traditional iCVD conditions.
The ability to conformally coat the microstructured porous membrane by stacking another polymer
on top is an unique feature of the traditional iCVD technique because of the long mean free path of
the monomer and the initiator. After the PVDB layer was deposited, the PDMS mask was removed
65
and a hydrophobic patterned porous membrane remained on the substrate (Figure 3-8a and 8b). The
water contact angle on the PMAA membrane coated with a PDVB layer was found to be 130° while
the contact angle on a PDVB layer on a flat surface is 100°. [25] This increase in the contact angle
is expected due to the surface roughness caused by the microstructures. The SEM image confirms
that the porous membrane morphology remains intact after depositing PDVB on top of the porous
membrane (Figure 3-8c). The thickness of this membrane was found to be 420 ± 30 µm.

Figure 3-8 (a) Top-down and (b) angled view of optical images of dyed water droplets on top of a
porous PMAA membrane coated with PDVB and (c) a SEM image of the sample.

3.5 Conclusions
We have demonstrated the fabrication of patterned porous polymer membranes via
sequential iCVD onto a substrate covered with a PDMS mask. The PDMS mask can be easily cut
or molded into a variety of shapes and patterns. The porosity and thickness of the patterned porous
membrane can be controlled by varying the substrate temperature during the monomer deposition.
Free-standing patterned hydrophilic membranes that are insoluble in aqueous solutions can be
fabricated by using a cross-linker. The surface chemistry of the membrane can be tuned by stacking
another polymer layer on top of the porous membrane. Our process allows for the control of the
66
shape, thickness, porosity, and functionality of patterned porous membranes for applications in
sensors and microfluidics.
3.6 Acknowledgements
This work was supported by the National Science Foundation CAREER Award CMMI-1252651.
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Letters 86(2005) 3502.
[36] Z. Nie, and E. Kumacheva, Patterning surfaces with functional polymers, Nature
materials 7(2008) 277-290.
[37] T. Shimoda, K. Morii, S. Seki, and H. Kiguchi, Inkjet printing of light-emitting polymer
displays, Mrs Bulletin, 28(2003) 821-827.

71

Chapter 4 Vapor Phase Fabrication of Hydrophilic and Hydrophobic
Asymmetric Polymer Membranes

Publication Citation: Dianat, G.; Seidel, S.; De Luna, M.M.; Gupta, M.; Vapor Phase Fabrication
of Hydrophilic and Hydrophobic Asymmetric Polymer Membranes. Macromolecular Materials
and Engineering 2016, 301,1037-1043. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
Reproduced with permission.
72
4.1 ABSTRACT
The fabrication of asymmetric polymer membranes via vapor phase deposition is
demonstrated. In this solventless process, the dense layer is deposited first and then the porous layer
is subsequently deposited onto the dense layer. A variety of functional polymer membranes can be
produced by varying the precursor molecules. The functionality of the dense and porous layers can
be independently tailored to be either hydrophobic or hydrophilic, resulting in membranes that are
fully hydrophilic, fully hydrophobic, or asymmetric in both structure and chemical functionality.
The thickness of both the porous and dense layers can be separately tuned by controlling the
deposition time.

4.2 Introduction
Asymmetric polymer membranes are composed of a dense top layer that selectively
separates molecules and a porous bottom layer that offers mechanical strength.[1-4] Asymmetric
polymer membranes allow for the separation of small molecules based on chemical affinity[5-10]
for various applications such as pharmaceutical purification,[11] kidney dialysis,[12] and water
desalination.[13,14] The performance of asymmetric membranes is dependent on many properties
of the dense layer including its thickness,[12] chemical composition,[15] and mechanical and
chemical stability.[12,16]
Interfacial polymerization, solution-casting, and phase inversion are the most widely used
techniques to synthesize asymmetric membranes.[5] In the interfacial polymerization method, the
dense layer is formed on top of the microporous structure by polymerization of monomers at the
aqueous-organic liquid-liquid interface.[17,18] In the solution-casting method, asymmetric
membranes are formed by casting a polymer solution directly on top of the microporous support
73
structure.[5] Phase inversion via immersion precipitation or the Loeb-Sourirajan process is the most
common method to prepare asymmetric membranes.[5,19] In this process, the initially
homogeneous polymer solution is first immersed, and then precipitated, in a non-solvent
coagulation bath.[20] The morphology and thickness of the dense membrane obtained by this
method is limited to the choice of materials for the polymer, the solvent, and the non-solvent.[21]
In the phase inversion method, there is poor control over the chemical composition because the
whole asymmetric membrane structure is composed of a single material.[5]
The most important considerations for fabricating asymmetric membranes are controlling
their selectivity and permeability by varying the chemistry and thickness, respectively, of the dense
layer.[22] However, synthesizing dense layers with thicknesses of less than 1 µm is difficult using
conventional techniques of asymmetric membrane fabrication.[12] Therefore it is important to
explore alternate methods to fabricate asymmetric membranes with dense layers that have varying
thicknesses and chemical functionality.
In this work, we present a novel technique to synthesize asymmetric membranes by the
combination of both conventional and nonconventional conditions of the initiated chemical vapor
deposition process (iCVD). The iCVD process is a solventless technique that has the ability to
produce dense films with thicknesses ranging from tens of nanometers to several microns.[23] In
the conventional iCVD process, the monomer partial pressure is kept below its saturation pressure
at a given substrate temperature to achieve surface adsorption of monomer.[24,25] Initiator
molecules are cleaved by a hot filament array and polymerization of the monomer occurs via a free-
radical mechanism which leads to the formation of dense polymer films. Our group recently
demonstrated that the iCVD process can also be used to fabricate membranes by increasing the
monomer partial pressure above its saturation pressure and lowering the substrate temperature
74
below the freezing point of the monomer. These nonconventional conditions lead to the
simultaneous deposition of solid monomer and polymerization. [25,26] The unreacted solid
monomer is removed after the deposition which leads to the formation of membranes with dual-
scale porosity.
In this paper, we utilize both sets of iCVD conditions for the first time to synthesize
asymmetric membranes. The dense layer is deposited first and then the porous layer is subsequently
deposited onto the dense layer. Due to the bottom-up and solventless nature of this process, the
dense layer can be formed with a broad range of thicknesses from the polymerization of any
monomer that is available in the iCVD literature irrespective of the type of material that is used for
the porous layer. This allows for the formation of membranes that are both asymmetric in structure
and also asymmetric in chemistry. In addition, the chemical functionality of the porous layer can
be further tuned by depositing another functional polymer on top because the iCVD process is able
to conformally coat structured porous materials.[27,28] We show that we can make dense and
porous layers that are either hydrophobic or hydrophilic, resulting in membranes that are fully
hydrophilic, fully hydrophobic, or asymmetric in both structure and chemical functionality.

4.3 Experimental Section
Methacrylic acid (MAA) (Aldrich, 99%), tert-butyl peroxide (TBPO) (Aldrich, 98%), 1,3-
diaminopropane (DAP) (Aldrich, 99%), and divinylbenzene (DVB) (Aldrich, 80%) were used as
received without further purification. For the fabrication of asymmetric membranes, first alginic
acid sodium salts (Sigma Aldrich) were dissolved in water (2.0 wt%) and the solution was
spincoated onto a silicon wafer (Wafer World) at 500 rpm for 30 seconds to create a sacrificial
75
layer. These coated wafers were then placed on a hot plate at 75 °C for 2 min before being placed
on top of a thermoelectric cooler (TE Technology) located on the stage of a custom designed iCVD
vacuum reactor (GVD Corporation, 250 mm diameter, 48 mm height). The thermoelectric cooler
was maintained at temperatures of 15 °C or -10 °C using an adjustable DC power supply (Volteq
HY3010D) during the formation of the dense and porous layers, respectively. The pressure in the
reactor was achieved by a rotary vane vacuum pump (Edwards E2M40) and maintained by a throttle
valve (MKS 153D).
A contact angle goniometer (Ramé-hart 290-F1) with drop sizes of 10 µL was used to
measure static water contact angles on the porous layers of the membranes and on dense layers that
were deposited onto the alginate layers. The reported results are the average for three different
samples for each layer. The samples were visualized using a scanning electron microscope (SEM;
JEOL-7001; Topcon Aquila) at a 20 kV accelerating voltage. A platinum layer with a thickness of
3 nm was deposited prior to SEM analysis to prevent charging. The porous layers were imaged
before soaking in water and the dense layers were imaged by soaking the samples in water for a
few minutes and then placing them on double-sided tape. A Kratos Axis Ultra DLD X-ray
photoelectron spectrometer (XPS) equipped with a monochromatic Al K 𝛼 source was used to
analyze the chemical composition of the samples. Survey spectra were collected from 0 to 800 eV
a total of 3 times per sample. The thickness of the dense poly(divinylbenzene) (PVDB) layers were
measured on a reference silicon wafer using ellipsometry (stokes waferskan™ ellipsometer
L115S300) using a refractive index of 1.6150 for PDVB.[29]
4.3.1 Fabrication of Membranes with a Hydrophilic Dense Layer and a Hydrophilic Porous Layer
For the fabrication of membranes with a hydrophilic dense layer and a hydrophilic porous
layer, first a dense poly (methacrylic acid) (PMAA) layer was deposited onto the sacrificial alginate
76
using conventional iCVD. The MAA monomer was introduced into the chamber at a flow rate of
3.5 standard cubic centimeter per minute (sccm) in conjunction with the TBPO initiator at 0.6 sccm
and the pressure was maintained at 300 mTorr. A nichrome filament array (80% Ni, 20% Cr, Omega
Engineering) placed 32 mm above the stage was resistively heated to 240 °C to cleave the initiator
into free radicals to start the polymerization process. After 20 minutes, the filament was turned off
and the flow of TBPO and MAA were stopped. The TEC was kept at 15 °C during the formation
of this dense layer. The deposition of the porous PMAA layer was started after the reactor returned
to base pressure and the TEC was cooled down to -10°C. In order to form the porous layer, TBPO
was introduced into the chamber at 0.6 sccm and the pressure was set to 300 mTorr and then the
filament was heated to 240 °C. Then MAA was introduced into the chamber at 3.5 sccm for 10
minutes. The flow of precursors was stopped, and the filament was cooled down to room
temperature. The solid MAA was further polymerized by keeping the TEC at -10 °C and keeping
the reactor pressure at base pressure for two hours. The TEC temperature was then increased to
+5 °C to allow for unreacted MAA to leave the system which took approximately 30 minutes. The
sample was removed from the reactor chamber and placed in an oven at 180 ± 2 °C for 60 minutes
to anneal the PMAA and convert it to poly (methacrylic acid-co-methacrylic anhydride) P(MAA-
co-MAN). These samples were exposed to 1,3-diaminopropane (DAP) vapor in a closed container
for 24 hours at room temperature to permanently cross-link the polymer. The samples were finally
placed on top of a hot plate at a temperature of 75 °C for one hour to remove any unreacted DAP.
4.3.2 Fabrication of Membranes with a Hydrophobic Dense Layer and a Hydrophilic Porous
Layer
For the fabrication of membranes with a dense hydrophobic layer and a porous hydrophilic
layer, first a dense PDVB layer was deposited onto the sacrificial alginate using conventional iCVD.
TBPO and DVB were introduced into the chamber with flow rates of 0.6 sccm and 0.6 sccm,
77
respectively, and the pressure was maintained at 150 mTorr. The filament was then heated to 240 °C
and kept at this temperature for 20 minutes during the polymerization process. After the reactor
pressure returned to base pressure, the porous PMAA layer was synthesized on top of the dense
layer and the sample was annealed and exposed to DAP as described in section 2.1.
4.3.3 Fabrication of Membranes with a Hydrophobic Dense Layer and a Hydrophobic Porous
Layer
For the fabrication of membranes with a hydrophobic dense layer and a hydrophobic porous
layer, the dense PDVB layer and porous PMAA layers were synthesized using the same conditions
as mentioned in sections 2.2 and 2.1, respectively. After sublimation of the unreacted MAA, the
porous PMAA layer was coated with PDVB using the conventional iCVD process. Initiator and
DVB were introduced into the chamber with flow rates of 0.6 sccm and 0.6 sccm and the pressure
was maintained at 150 mTorr. The filament temperature was set to 240 °C and the TEC temperature
was kept at 15 °C for 10 minutes. For the fabrication of the hydrophobic membrane with an ultrathin
dense layer, the deposition time of the dense PDVB layer was decreased to 5 minutes.
4.4 Results and Discussion
The asymmetric membranes were fabricated by first depositing the dense layer and then
depositing the porous layer. Due to the bottom-up nature of this fabrication technique, the chemical
functionality of the dense and porous layers can be independently tailored, and the thicknesses of
each layer can be separately tuned. To demonstrate this capability, three different types of
asymmetric membranes with varying combinations of surface chemistries were fabricated: (1) a
hydrophilic dense layer and a hydrophilic porous layer (which results in a fully hydrophilic
membrane), (2) a hydrophobic dense layer and a hydrophilic porous layer (which results in a
membrane which is asymmetric in chemical functionality), and (3) a hydrophobic dense layer and
78
a hydrophobic porous layer (which results in a fully hydrophobic membrane) (Figure 4-1).

Figure 4-1 Asymmetric membranes with a) a hydrophilic dense layer and a hydrophilic porous
layer, b) a hydrophobic dense layer and a hydrophilic porous layer, and c) a hydrophobic dense
layer and a hydrophobic porous layer.

PMAA was used as the hydrophilic polymer while PDVB was used as the hydrophobic
polymer. PMAA was selected for the formation of the porous layer for all three types of membranes
because the methacrylic acid monomer forms porous polymer at modest temperatures due to the
relatively high freezing point temperature of the monomer. The membranes were deposited onto a
sacrificial layer of alginate in order to allow for easy removal from the silicon substrate. Therefore,
the membranes must be insoluble in water to survive the sacrificial layer removal process. Another
advantage of PMAA is that it can be cross-linked to render it insoluble via annealing followed by
79
DAP vapor treatment.
For many practical applications, it is important to design membranes with hydrophilic
surfaces to prevent fouling by biological contaminants. [13,20] To make fully hydrophilic
membranes, first a dense PMAA layer was deposited on top of the alginate sacrificial layer using
typical iCVD conditions. Next, a porous PMAA layer was deposited on top of the dense layer. After
the depositions, the membranes were placed in an oven to convert the PMAA to P(MAA-co-MAN).
Some of the methacrylic acid moieties convert into methacrylic anhydride moieties through an
annealing process that causes the elimination of water, which results in the formation of anhydrides
between adjacent pairs of carboxylic acid groups.[30] In order to increase the long-term stability of
P(MAA-co-MAN), the membranes were further exposed to DAP vapor. DAP is a difunctional
amine molecule which can react with the methacrylic anhydride linkages to permanently cross-link
the polymer in order to make the membranes insoluble. We used contact angle goniometry (Figure
4-2) to verify that the dense layer and the porous layer remain hydrophilic as indicated by a fully
wetting water drop on both surfaces. The chemical composition of each layer was quantified by an
XPS survey scan. The data shows that the atomic percentages of carbon, oxygen and nitrogen for
the dense and porous layers are similar to the atomic percentages of a reference film of P(MAA-
co-MAN) cross-linked with DAP (Table 4-1). SEM images confirmed the asymmetric structure of
the membranes (Figure 4-3).
80

Figure 4-2 Contact angle measurements on asymmetric membranes with a) a hydrophilic dense
layer and a hydrophilic porous layer, b) a hydrophobic dense layer and a hydrophilic porous layer,
and c) a hydrophobic dense layer and a hydrophobic porous layer.

81
Table 4-1 XPS survey data of the dense and porous layer of each membrane compared to iCVD
reference films.

Atomic Composition
Samples %C %O %N
Reference P(MAA-co-MAN) film cross-linked with DAP 69.7 21 9.3
Reference PDVB film 93.0 7 0
Reference PDVB film after DAP exposure 86.3 9.5 4.2
Reference PDVB film on alginate layer 83.7 16.3 0
Hydrophilic dense layer of fully hydrophilic membrane 67.6 20.9 11.5
Hydrophilic porous layer of fully hydrophilic membrane 70.3 21.4 8.3
Hydrophobic dense layer of asymmetric functional membrane 79.0 16.7 4.3
Hydrophilic porous layer of asymmetric functional membrane 71.9 19.9 8.2
Hydrophobic dense layer of fully hydrophobic membrane 84.1 15.9 0
Hydrophobic porous layer of fully hydrophobic membrane 90.5 9.5 0

82

Figure 4-3 SEM micrographs of dense and porous sides of membranes. A fully hydrophilic
membrane with a) a hydrophilic dense layer and b) a hydrophilic porous layer, an asymmetric
functional membrane with c) a hydrophobic dense layer and d) a hydrophilic porous layer, and a
fully hydrophobic membrane with e) a hydrophobic dense layer and f) a hydrophobic porous layer.

Since a single material might not have the optimal membrane properties, it might be useful
to independently vary the chemical composition of the dense and porous layers.[12] We
demonstrated our ability to make a membrane that is asymmetric in chemical functionality by first
depositing a dense hydrophobic layer and then a porous hydrophilic layer. PDVB was used as the
hydrophobic polymer because it is mechanically and thermally stable. [31,32] PMAA was used to
form the porous layer. The membrane was then annealed and exposed to DAP in order to prevent
the porous PMAA layer from dissolving. Contact angle measurements (Figure 4-2) confirm that the
83
asymmetric membrane is hydrophobic on the dense side (99.5 ± 2 ) and hydrophilic on the porous
side (fully wetting). The XPS survey scan of the porous layer showed similar atomic percentages
as the reference film of P(MAA-co-MAN) cross-linked with DAP, as expected. However, the dense
layer had increased nitrogen and increased oxygen concentrations relative to the reference PDVB
film deposited onto a silicon wafer. The increased nitrogen is due to the exposure of the PDVB
layer to DAP which was confirmed by XPS analysis of the reference PDVB film after DAP
exposure. The increased oxygen is due to the diffusion of the alginate into the PDVB layer which
was confirmed by XPS analysis of a reference PDVB film deposited onto an alginate layer.
To make fully hydrophobic membranes, first a dense PDVB layer was deposited and then a
porous PMAA layer was deposited. The porous PMAA layer was then coated with an additional
layer of PDVB in order to modify the surface chemistry of the porous layer. This additional coating
was deposited using traditional iCVD conditions and is a benefit of using a bottom-up processing
method. Contact angles were found to be 99.5 ± 2 and 149 ± 2 degrees for the dense PDVB layer
and the porous layer composed of PMAA coated with PDVB, respectively (Figure 4-2). The
difference between the contact angles are due to the surface roughness imparted by the structure of
the porous layer. [33,34] The XPS survey scan of the porous layer showed similar atomic
percentages as the reference film of PDVB deposited onto a silicon wafer, as expected. The XPS
data of the dense layer showed similar atomic percentages as the reference PDVB film deposited
onto an alginate layer, as expected.
The ability to control the thickness of the dense layer is essential for tailoring the efficiency
and throughput of membranes in separation applications. We demonstrated our ability to control
the thickness of the dense layer by modifying the deposition time of PDVB from 20 minutes to 5
minutes. Ellipsometry was used to measure the thickness of the dense layer on a reference silicon
84
wafer. For the shorter deposition time, the dense layer was 28 ± 5 nm, while the thickness was
found to be 130 ± 5 nm for the longer deposition. Figure 4-4 shows the images of the membrane
with a thin dense PDVB layer.

Figure 4-4 SEM micrographs of (a) the dense side and (b) the cross-section of a membrane with a
thin dense PDVB layer.

4.5 Conclusions
We have demonstrated the fabrication of asymmetric membranes through the combination
of conventional and nonconventional iCVD processes. This method allows for a high degree of
control over the dense layer thickness and paves the way to synthesize ultrathin dense layers with
sub-50nm thickness that is difficult to attain using other methods. The dense layers can be formed
with chemistries independent of the porous layer which has the potential to create more tailored and
efficient membranes for a range of applications

85
4.6 Acknowledgements
This work was supported by the National Science Foundation CAREER Award CMMI-1252651
and the National Science Foundation Graduate Research Fellowship under Grant DGE-0937362
(S.S.).

4.7 References
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acid)‐Based Membranes with Controlled Dissolution Behavior. Macromolecular Materials and
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89

Chapter 5 Scratch-Resistant Porous Polymer Coatings with Enhanced
Adhesion to Curved and Planar Substrates

90
5.1 Introduction
Porous polymers have applications in drug delivery devices [1,2], biosensors [3,4], and
separations [5,6]. Porous polymers are commonly synthesized using liquid phase processing
techniques such as phase inversion [7,8], cryopolymerization [9,10], and solvent casting and
particulate leaching [11,12]. The presence of solvents in these techniques can cause swelling and
degradation of substrates and also limits the conformality of the coatings onto curved surfaces. [13]
We recently developed a solvent-free method to grow porous polymers from vapor phase precursors.
[14-17] The all dry and bottom-up nature of our fabrication process eliminates surface tension
issues and provides a high degree of control over the thickness. In this study, we demonstrate that
we can create robust coatings with enhanced adhesion to the substrate by co-polymerization with
an epoxide-containing monomer (glycidyl methacrylate) (GMA) and a cross-linker (ethylene glycol
diacrylate) (EGDA). These porous coatings are resistant to scratches and organic solvents We
demonstrate that these coatings be applied to a variety of inorganic, organic, planar, and curved
surfaces.
5.2 Experimental Section
Glycidyl methacrylate (GMA) (Aldrich,  97.0%), methacrylic acid (MAA) (Aldrich, 99%),
ethylene glycol diacrylate (EGDA) (Polysciences, Inc), and tert-butyl peroxide (TBPO) (Aldrich,
98%) were used as received without further purification. Porous polymer coatings were deposited
onto silicon wafers (Wafer World) and polynorbornene (Norsorex, M040924-12 from D-Nov
Austria) substrates with 11 cm2 dimension and stainless-steel tube. Norsorex substrates were
prepared by compression molding a flat plaque of Norsorex material. The material was first
consolidated to remove entrapped air by pressing it to a thickness of 0.5 mm at a temperature of
100 C for 30 seconds. The film was then vulcanized by curing it in an oven at 150 C for 30 mins.
91
Prior to the deposition of porous polymer, the Norsorex substrates were treated with a laboratory
corona treater (I’ll add the model info later) for 10 minutes. The substrates were placed on top of a
thermoelectric cooler (TE Technology) which was located on the stage of a custom built initiated
chemical vapor deposition (iCVD) vacuum reactor (GVD Corporation, 250 mm diameter, 48 mm
height). The temperature of the TEC was controlled using an adjustable DC power supply (Volteq
HY3010D). The stage temperature was set to 10 °C using a recirculating chiller (Thermo Scientific
Haake A25). The pressure of the reactor was controlled by a rotary vane vacuum pump (Edwards
E2M40) and a throttle valve (MKS 153D).
For the fabrication of the porous polymer coating composed of a dense base layer of the
PGMA and the porous P(MAA-co-EGDA-co-GMA), first a dense layer of poly (glycidyl
methacrylate) (PGMA) was deposited onto the substrates by introducing TBPO and GMA into the
reactor chamber at flow rates of 0.6 sccm and 0.9 sccm, respectively. The TEC temperature was
kept at 5 °C, the pressure was maintained at 40 mTorr, and the nichrome filament array (80% Ni,
20% Cr, Omega Engineering) was resistively heated to 230 °C to start the polymerization which
lasted for 3 minutes. The flow of GMA was then stopped. To form the porous layer of P(MAA-co-
EGDA-co-GMA), the TEC temperature was decreased to -10 °C and reactor pressure was increased
to 650 mTorr. The MAA monomer was then introduced into the chamber at a flow rate of 0.3 sccm
for 1 minute. During this step, the MAA deposits as a solid with an approximate thickness of 100
m and the solid begins to polymerize. The flow of MAA flow was then halted and polymerization
of the solid MAA was continued for 30 minutes with continuous flow of TBPO. To incorporate
EGDA into the porous layer by copolymerization, the TEC temperature was increased to 0 °C, the
reactor pressure was maintained at 200 mTorr, and the EGDA monomer was introduced at a flow
rate of 0.2 sccm for 30 minutes. To incorporate GMA into the porous layer, the flow of EGDA was
92
stopped, the TEC temperature was set to 5 °C, the reactor pressure was maintained at 40 mTorr,
and GMA was flowed into the reactor at a flow rate of 0.9 sccm for 30 minutes. The flow of both
GMA and TBPO were then halted and filament was turned off. The remaining unreacted solid MAA
was then sublimated until the reactor returned to base pressure of 15 mTorr. The samples were then
annealed on a hot plate outside the reactor in air at a temperature of 120 °C for 30 mins. For the
fabrication of the control coatings, we used the exact same conditions except for the introduction
of the monomers that are not in the final coating.
Scanning electron microscopy (SEM Topcon Aquila) with an accelerating voltage of 10 kV
was used to characterize the morphology of the coatings. A gold layer was sputtered onto the
coatings prior to SEM imaging to prevent charging. Adhesion between the substrate and the
coatings was examined by tape test. Scotch tape was placed on the sample and an index finger was
used to apply gentle pressure in order to allow for contact between the sample and the tape. The
tape was then manually removed as fast as possible. This process was repeated using a new piece
of tape for each subsequent tape test. The scratch/mar resistance of the coatings was characterized
according to the ASTM standard F3300-18 [18] and performed on a Taber 5750 Linear Abraser
(Taber Industries, NY, USA) with a standard mar tip. The total weight (mar tip and tip arm) applied
on the coatings was 170g. During testing, the samples were firmly attached to the testing platform
using double sided tape (3M, MN, USA) in between two shims that have the same height as the
film. This was done to ensure smooth transition of the mar tip from the platform to the sample and
eliminate cracking at the edges. The tip oscillates on top of the sample to generate any potential
scratches, which were then characterized under a microscope. Samples were scratched until failed,
which is defined as the exposure of the substrate. The number of passes when failed is reported as
93
the scratch/mar resistance of the coating. The chemical resistance of the coatings was checked by
soaking the samples in isopropyl alcohol, acetone, and methanol for 60 mins.

5.3 Results and Discussion
A modified initiated chemical vapor deposition (iCVD) process was used to fabricate the
porous polymer coatings. In the conventional iCVD process, monomer and initiator molecules are
introduced into the reactor in gas phase. In our modified process, we use operating conditions such
that the monomer partial pressure and substrate temperature are below the triple point pressure and
temperature of the monomer. [19-22] The monomer vapor therefore undergoes a phase change and
deposits as solid pillar-like microstructures at the surface of the cold substrate. The initiator
molecules are cleaved by a hot filament array and the resulting radicals react with the solid
monomer to begin polymerization. Unreacted solid monomer is removed by sublimation. We
typically use methacrylic acid (MAA) as the monomer since it has a high freezing point temperature
(16 C). Figure 5-1a shows porous poly (methacrylic acid) (PMAA) deposited onto a silicon wafer.
A tape test shows that there is no adhesion between the porous PMAA layer and the silicon wafer.
For practical applications, it is critical to have good adhesion between the substrate and the
porous polymer coating. To enhance adhesion, MAA was copolymerized with GMA to form porous
poly (methacrylic acid-co-glycidyl methacrylate) (P(MAA-co-GMA)). These P(MAA-co-GMA)
coatings were then annealed at 120 C for 30 mins to allow the reactive pendant epoxide rings of
the GMA monomer to undergo a ring-opening reaction which enables self-crosslinking. [23,24]
The tape test shown in Figure 5-1b shows that the adhesion of the coating to the silicon wafer is
significantly improved. The first tape test shows good adhesion to the silicon wafer except at the
edges which is due to a slightly higher thickness of the porous layer at the edges. The tape test was
94
repeated ten times and the adhesion began to fail after the first few tape tests. Therefore, to enhance
the adhesion further, we deposited a dense base layer of poly (glycidyl methacrylate) (PGMA) onto
the silicon wafer using the conventional iCVD process prior to depositing porous P(MAA-co-
GMA). This dense base layer was anchored to the substrate through a reaction between the epoxy
groups of GMA and the hydroxyls groups on the surface of the silicon wafer [23,25] and was
adhered to the porous P(MAA-co-GMA) through diffusion and entanglement of PGMA chains
across the interface since the annealing temperature is above the glass transition temperature of
PGMA (61.3 C) [23]. The samples were then annealed at 120 C for 30 mins. The images in Figure
5-1 show excellent adhesion to the silicon wafer even up to ten tape tests. The scratch resistance of
the coatings was then tested according to ASTM standard F3300-18. [18] The results showed that
although the coating with the base PGMA layer has excellent adhesion to the substrate, the sample
failed the scratch test after just a few passes of the tip.

95

Figure 5-1 Tape test results on a) porous PMAA, b) porous P(MAA-co-GMA), c) porous P(MAA-
co-GMA) with a dense base PGMA layer, and d) porous P(MAA-co-EGDA-co-GMA) with a dense
base PGMA layer.

In order to fabricate porous coatings that have good adhesion to the substrate and also pass
the scratch test, we introduced the crosslinker EGDA into the copolymerization step to form a
coating composed of the dense base PGMA layer and a porous poly (methacrylic acid-co-ethylene
glycol diacrylate-co-glycidyl methacrylate) (P(MAA-co-EGDA-co-GMA)) layer. EGDA was
added to improve the mechanical properties of the coatings. [26] Samples were annealed at 120 C
for 30 mins and there was excellent adhesion between the coating and the substrate after 10 tape
tests (Figure 5-1d). Scratch test results show that the scratch resistance of the coatings is highly
improved since the samples can withstand an average of 50 passes before failing. We fabricated a
96
control sample composed of porous poly (methacrylic acid-co-ethylene glycol diacrylate) (P(MAA-
co-EGDA)) with a dense base layer of PGMA (i.e. there was no GMA in the porous layer). The
scratch test showed that this coating could withstand only one pass which confirms that it is
essential to copolymerize MAA with both GMA and EGDA to improve the scratch resistance of
the coating. For applications such as filtration and for post-processing modification, it is important
to have a coating that can preserve its adhesion to the substrate following immersion in organic
solvents. In order to measure the chemical resistance of the coatings, the coated wafers were first
soaked in common laboratory solvents including isopropyl alcohol, methanol, and acetone for 60
mins and then tape tests and structural characterization were performed. (Figure 5-2) The tape test
confirm that the coatings retain their adhesion to the substrate after soaking in solvents. The images
of the coatings show that the coating dimension and morphology did not change after soaking in
solvent. The SEM images confirm that the coatings retain their pillar structures after soaking in
solvent.

97

Figure 5-2 a) SEM image of an annealed porous coating composed of porous P(MAA-co-EGDA-
co-GMA) on a dense base layer of PGMA; tape test results and SEM images of the coatings after
60 mins of soak in b) IPA, c) methanol, and d) acetone.
It is difficult to fabricate adherent coating onto flexible materials because the possible
mismatch in the coefficient of thermal expansion can result in crack and delamination of the coating
from the substrate during the cooling and heating processes. [26] For example, porous P(MAA-co-
GMA) with a dense base layer of PGMA (there was no EGDA) was deposited onto norsorex and
annealed at 120 for 30 min. As shown in Figure 5-3a, the coating cracked during the post-
polymerization annealing process. EGDA was added during copolymerization to create a more
robust coating. As shown in Figure 5-3b, porous P(MAA-co-EGDA-co-GMA) with a dense base
layer of PGMA that was deposited onto norsorex and then annealed did not crack. Figure 5-2c
shows excellent adhesion of the coating onto the surface of norsorex as measured by repeated tape
tests. Figure 5-3d shows that the coatings are flexible enough to bend without cracking or
delamination.

98

Figure 5-3 Effect of the annealing step on porous polymer coatings deposited onto norsorex
substrates a) without EGDA in the porous layer and b) with EGDA in the porous layer. c) Tape test
results of porous P(MAA-co-EGDA-co-GMA) with a dense base PGMA layer on norsorex and d)
images showing flexibility of this sample.

It is useful to be able to apply porous polymer coatings onto non-planar and complex
structures such as biomedical implants, membranes, and textiles. To show the ability of our
technique to coat non-planar surfaces, the porous P(MAA-co-EGDA-co-GMA) with a dense base
PGMA layer was deposited onto the outer and inner surfaces of a stainless-steel tube. Figure 5-4a
shows that the coatings are conformal. The tape tests in figure 5-4b shows good adhesion between
the coating and the substrate.

99

Figure 5-4 a) porous polymer conformally coated the curved stainless-steel substrates and b) tape
test results of the coating.

5.4 Conclusion
In summary, we used a modified iCVD process to synthesize a porous polymer coating that
has excellent adhesion to the substrate and is resistant to scratches and organic solvents.
Copolymerization of MAA with EGDA and GMA and a base layer of dense PGMA was necessary
to pass the tape test and scratch test. The pillared structure of the coating and the adhesion was
retained after soaking in IPA, methanol. and acetone. The porous polymer coatings were deposited
onto silicon, norsorex, and curved stainless steel which shows the generality of our technique for
coating a variety of substrates.

5.5 Acknowledgments
We thank Bose Corporation for funding this project.
100
5.6 References
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Bley, S. Mikhalovsky, I.Y. Galaev, and B. Mattiasson, Biomacromolecules 2007, 9, 66.
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Petruczok, R. Sreenivasan, W.E. Tenhaeff, N.J. Trujillo, S. Vaddiraju, Adv. Mater. 2010, 22, 1993.
[14] S. Seidel, P. Kwong, M. Gupta, Macromolecules 2013, 46, 2976.
[15] S. Seidel, M. Gupta, J. Vac. Sci. Technol. A 2014, 32, 041514.
101
[16] G. Dianat, M. Gupta, Polymer 2017, 126, 463.
[17] G. Dianat, S. Seidel, M.M. De Luna, and M. Gupta, Macromol. Mater. Eng. 2016, 310, 1037.
[18] Standard Test Method for Abrasion Resistance of Flexible Packaging Films Using a
Reciprocating Weighted Stylus (F3300-18 ASTM standard).
[19] S. Seidel, G. Dianat, and M. Gupta, Macromol. Mater. Eng. 2016, 301, 371.
[20] N. Movsesian, M. Tittensor, G. Dianat, M. Gupta, N. Malmstadt, Langmuir 2018, 34, 9025.
[21] G. Dianat, N. Movsesian, M. Gupta, Macromolecules 2018, 51, 10297.
[22] N. Movsesian, G. Dianat, M. Gupta, Ind. Eng. Chem. Res. 2019, 58, 9908.
[23] V.J.B. Jeevendrakumar, D.N. Pascual, M. Bergkvist, Adv. Mater. Interfaces 2015, 2, 1500076.
[24] S.G. Im, K.W. Bong, C.H. Lee, P.S. Doyle, K.K.Gleason, Lab on a Chip 2009, 9, 411.
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Eychmüller, J. Mater. Chem. 2008, 18, 214.
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102

Chapter 6 Conclusions and Future Work

103
6.1 Conclusions
In conclusion, we present a green approach to synthesize functional porous polymer
coatings and membranes, with dual-scale porosity, from vapor phase precursors. In our fabrication
process monomer vapor goes through phase change and form solid microstructures. Vapor initiator
that is thermally cleaved attack the solid monomer to start the polymerization. After the
polymerization, excessive solid monomer undergoes sublimation phase change and porous polymer
remains. General morphology of the resulting membrane is defined by the shape of deposited solid
monomer and therefore can be tuned by modifying the parameters affecting the phase change
process including temperature. Size of the small-scale pores that are formed within the solid
microstructures as a result of sublimation of the unreacted monomer can be tuned by changing the
temperature during the polymerization step. We showed our ability to control the molecular weight
distribution of our membranes by modifying the processing parameters such as polymerization time
and polymerization temperature. Physical properties of our membranes such as dissolution rate can
be controlled by changing the duration of the polymerization. One of the capabilities of our process
is that we can pattern the deposition of the porous polymer membranes by using PDMS masks. The
addition of a cross-linker during polymerization allows for the fabrication of robust free-standing
shaped membranes. Our ability to control the shape, thickness, porosity, and functionality of the
porous membranes allows for the design of new surfaces for a variety of applications in sensors,
filtration, and microfluidics. One potential application for our process is the fabrication of
asymmetric polymer membranes that are composed of a dense top layer and porous bottom layer.
These membranes can be synthesized using a combination of conventional and nonconventional
iCVD conditions in a one-pot process. Our method allows for a high degree of control over the
thickness of the dense layer by varying the deposition time and paves the way to synthesize ultrathin
104
dense layers with sub-50nm thickness that is challenging to achieve using other methods. The
functionality of the dense and porous layers can be independently tailored to be either hydrophobic
or hydrophilic, resulting in membranes that are fully hydrophilic, fully hydrophobic, or asymmetric
in both structure and chemical functionality. Finally, we demonstrated our ability to apply porous
polymer coating onto a wide range of substrates, including planar, non-planar, and flexible. Here,
we showed that all dry fabrication process allows us to incorporate different monomers with varying
functionalities into the structure of porous polymer coating in order to make a polymer coating with
desired optimal properties.
6.2 Future work
Polymerization of solid monomer by a vapor phase initiator is usually slow due limited
mobility of the solid monomer. Development of the sequential fabrication process enhanced the
polymer yield by 12% but there is still over 80% monomer loss. Therefore, it is important to make
modifications into the process to further improve the percent monomer conversion. One way to
resolve this issue is to recycle the monomer that is sublimating from the porous polymer during the
polymerization step. As it was shown in chapter 2, increasing the polymerization temperature
results in faster sublimation rate of unreacted monomer during the polymerization step. We have
shown that the sublimating monomer from porous layer can be adsorb onto the downstream
substrate and form dense polymer film. Therefore, additional TECs can be incorporated into the
reactor downstream in order to facilitate the solid monomer deposition and polymerization onto the
second TEC. Third TEC can be added to further decrease the monomer waste by capturing the
monomer from the preceding surface. (Figure 6-1) Temperatures of the downstream TECs can
control both the morphology of the membranes depositing onto them by controlling the phase
change process and the polymerization rate. This combinatorial approach is powerful for practical
105
coating applications in which the throughput of the process is enhanced by enabling the fabrication
of multiple morphologically controlled porous polymer coatings in a single run.1 As it was shown
in this manuscript our technology has many capabilities and the solvent-free nature of this process
offers many benefits over common techniques. Therefore, we would like to scale-up the process
and make it commercially available.

Figure 6-1 Recycling monomer waste through downstream monomer capture and polymerization.

[1] Movsesian, N.; Dianat, G.; Gupta, M.; Downstream Monomer Capture and Polymerization
during Vapor Phase Fabrication of Porous Membranes, Industrial & Engineering Chemistry
Research 2019, 58, 9908-9914.

Abstract (if available)

Abstract Porous polymers are used in a wide range of applications such as tissue scaffolds, sensors and separation membranes. Common methods to fabricate porous polymers use liquid phase processing techniques which can limit the generality of these methods due to solvent compatibility and surface tension issues. Here we used a versatile all dry fabrication process to synthesize functional porous polymer membranes through polymerization of solid monomer by a thermally cleavable vapor-phase initiator. This process offers a high degree of control over the morphology and chemical composition of the membranes and is “greener” than many current processing techniques due to elimination of organic solvents. In this dissertation, we will explain the process-structure-property relationships and potential applications.

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Asset Metadata

Creator Dianat, Golnaz (author)

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

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 12/17/2020

Defense Date 12/17/2019

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

Tag initiated chemical vapor deposition,OAI-PMH Harvest,porous polymers,process-structure-property relationship

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Gupta, Malancha (committee chair), Malmstadt, Noah (committee member), Nakano, Aiichiro (committee member)

Creator Email gdianat@usc.edu,golnaz.dianat@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-258823

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