Roll-to-Roll Surface Modification of Cellulose Paper via Initiated

Aug 20, 2018 - In this work, we demonstrate the ability to use roll-to-roll processing to continuously modify the surface properties of large areas of...
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Roll-to-Roll Surface Modification of Cellulose Paper via Initiated Chemical Vapor Deposition Christine Cheng and Malancha Gupta*

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Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, 925 Bloom Walk, Los Angeles, California 90089, United States ABSTRACT: Tuning surface properties of flexible materials enhances the versatility of existing materials, giving them new functions for applications in textiles, filtration, flexible electronics, and sensors. However, traditional surface modification methods are typically solvent-based, which limits the range of substrates that can be coated. In this work, we demonstrate the ability to use roll-to-roll processing to continuously modify the surface properties of large areas of flexible substrates using initiated chemical vapor deposition, which is an all-dry process. We designed and built a roll-to-roll module that can be used to uniformly coat 1500 cm2 of chromatography paper in a single deposition. Rolls of paper were coated with a fluoropolymer and an ionizable polymer, and the coated paper was used for origami, nonstick surfaces, and paper-based microfluidic devices.

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flexible electronics. The scale-up of vapor phase coating techniques including atomic layer deposition (ALD),24 PECVD, 25 and oxidative chemical vapor deposition (oCVD)26 has been previously demonstrated. For example, Parsons and colleagues demonstrated high-throughput ALD of alumina coatings on textiles using spatial ALD, in which substrates were moved through zones of precursor exposure.24 PECVD has been used on the commercial scale since the 1980s, expanding from its origins in the semiconductor industry to other applications in soft materials. Wertheimer and coworkers designed a roll-to-roll process for coating polymer substrates.27 The PECVD of polymers compared to iCVD has disadvantages that include damage to the substrate via etching, poor retention of deposited functional groups, and nonconformal coatings.28 Gleason and coworkers reported custom modifications to a large roll-to-roll PECVD reactor to scale up oCVD of a conductive polymer26 and iCVD of poly(glycidyl methacrylate).29 These modifications involved redesigning several reactor components and resulted in significant changes to the iCVD process, including moving the substrate vertically, which gives rise to a convective flow pattern different than that in a conventional horizontal iCVD reactor. In this paper, we demonstrate the ability to coat large areas of flexible materials in a nonmodified iCVD chamber by designing a compact roll-to-roll module from low-cost, readily available materials. With our compact roll-to-roll module, we

INTRODUCTION The surface modification of flexible materials is important for a variety of applications, including flexible electronics,1−3 wound dressings,4,5 paper-based microfluidics,6,7 and desalination membranes.8,9 Typical methods for modifying flexible materials are solution-based;2−9 however, solvent effects can limit the efficacy of these processes. Finding a common solvent that is compatible with both the substrate and the coating is not always possible, and surface-tension effects prevent conformal coating of micro- or nanoscale features. As a result, vapor-phase processes such as initiated chemical vapor deposition (iCVD)10,11 and plasma enhanced chemical vapor deposition (PECVD)12,13 are preferred for coating structured surfaces. In the iCVD process,10,11 monomer vapor and initiator ditert-butyl peroxide (TBPO) vapor are introduced into a reactor chamber under vacuum. The peroxide bond in TBPO is thermally cleaved by a heated filament array, initiating free radical polymerization with the desired monomer. The polymer is deposited onto substrates at ambient temperatures which prevents degradation of the substrates during the coating process. Because iCVD is a vapor-phase process, surface tension effects are avoided, resulting in conformal coatings on a variety of structured surfaces including microtrenches14,15 and microporous membranes.16,17 A variety of functional polymers can be deposited to fabricate nonstick surfaces,18,19 antibiofouling materials,20,21 and paper-based microfluidics.22,23 Scaling up the iCVD process can enable the high-throughput manufacturing of coated substrates in large quantities for realistic commercial applications such as scalable production of paper-based microfluidic analytical devices, bandages, and © XXXX American Chemical Society

Received: July 4, 2018 Revised: July 31, 2018 Accepted: August 4, 2018

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DOI: 10.1021/acs.iecr.8b03030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

He−Ne laser (Industrial Fiber Optics, 633 nm) and was 4 nm/ min for PPFDA and 8 nm/min for xPMAA. For roll-to-roll depositions, polymerization was carried out for 3 min on the substrate before starting the rolling, for 2 h onto the moving substrate, and for an additional 3 min on the substrate after rolling. For the deposition of PPFDA onto stationary paper, polymer was deposited for 1 h. For the deposition of xPMAA onto stationary paper, polymer was deposited for 10 min. The temperature of the paper during a deposition was measured by mounting a thermocouple to the paper in the middle of the primary deposition zone and depositing PPFDA for 30 min. Distances along the length of the paper were defined such that the part of the roll that was first coated was 0 m, and distances across the width of the paper were defined such that the edge of the paper closest to the gas feed was 0 cm. Atomic compositions of deposited PPFDA coatings and uncoated paper were characterized using an X-ray photoelectron spectrometer (Kratox Axis Ultra DLD) with a monochromatic Al Kα source. Survey spectra were taken from 800 to 0 eV in 1 eV steps and averaged over 5 scans. Spectra were referenced to 284.8 eV for the C−C peak. Morphology of paper surfaces were imaged using a scanning electron microscope (JSM 7001F) at an operation voltage of 20 kV. Deposited PPFDA coatings were also characterized using a contact angle goniometer (ramé-hart 290). Reported contact angles were averaged over 20 measurements with a drop volume of 5 μL of deionized water, and error bars represent one standard deviation below and above the average. Droplet profile images were captured using the goniometer camera. Camera images were taken using a Nikon D3000. Origami was performed on three 5 × 5 cm squares of paper taken from near the center of the roll-to-roll paper coated with PPFDA. The three squares were folded and joined together to form a single bowl. The SLIPS material was made from paper cut from near the center of the roll-to-roll paper coated with PPFDA. Krytox was added to the fluorinated paper via pipet to form a SLIPS material, which was placed on a glass slide and tilted at an angle of 15° using a tilting-base goniometer. Solvents were dispensed in 30 μL droplets. Paper-based microfluidic devices were made from roll-to-roll paper coated with xPMAA by cutting 1 cm-wide strips along the length of the paper (the length of the strip was 5 cm, spanning the width of the roll). The ability of uncoated and coated paper to separate dyes was analyzed by applying 1 μL droplets of a buffered pH 10 solution of 2.5 mg/mL crystal violet and 0.25 mg/mL Ponceau S onto the paper. One end of the channel was placed in buffered pH 10 solution such that the eluent wicked vertically up the paper.

can coat several meters of substrate in a single deposition without modifying the iCVD process. We demonstrate the versatility of our system by conformally coating 3 m-long rolls of 5 cm-wide chromatography paper with a hydrophobic polymer, poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA), and a cross-linked ionizable polymer, poly(methacrylic acid-co-ethylene glycol diacrylate) (xPMAA). The uniformity of the coating along the length and the width of the substrate was confirmed using X-ray photoelectron spectroscopy (XPS) and contact angle goniometry, and the conformality of the polymer on the cellulose fibers was verified with scanning electron microscopy (SEM). We demonstrate the functionality of the coated substrates by fabricating slippery liquid-infused surfaces (SLIPS), microfluidic channels, and origami structures.

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EXPERIMENTAL DETAILS 1H,1H,2H,2H-Perfluorodecyl acrylate (PFDA) (SynQuest Laboratories, 97%), methacrylic acid (MAA) (Aldrich, 99%), ethylene glycol diacrylate (EGDA) (PolySciences, Inc.), TBPO (Aldrich, 98%), silicon wafers (Wafer World 119), Krytox 1525 (Aldrich), food color (Kroger), ethyl violet (Sigma), crystal violet (Aldrich, 90%), Ponceau S (Aldrich, 75%), pH 10 buffer solution (BDH, ACS grade), and grade 1 chromatography paper (Whatman) were used as received. To build the roll-to-roll module, two motors (Uxcell DC gear motor 12 V 2 rpm) were mounted onto L-shaped brackets (Uxcell 37 mm motor mounting bracket holder). A strip of plexiglass (Source One Premium, 1/16 in.) was laser cut, and the two brackets were then mounted onto the ends, approximately 12.5 cm apart. Metal rods (3 mm diameter, 6 cm length) were connected to the motors with an aluminum alloy shaft coupler (Uxcell), and the rods served as spools for the flexible substrate. All parts for the roll-to-roll module were purchased from Amazon for $70. The motors were connected with copper wire to external power supplies with variable voltages (VOLTEQ HY3010D). The iCVD process was carried out in a cylindrical reactor (25 cm diameter, 5 cm height) (GVD Corporation) under vacuum maintained by a rotary vane vacuum pump (Edwards E2M18). Reactor pressure during depositions was controlled using a throttle valve (MKS 153D) controlled by a capacitance manometer (MKS 622C01TDE Baratron). Monomers were loaded into stainless steel jars that were then mounted onto the reactor, and the jars were heated to achieve appropriate vapor pressure for deposition. PFDA was heated to 50 °C; MAA was kept at 25 °C, and EGDA was heated to 45 °C. The initiator TBPO was kept at 25 °C, and its flow rate was controlled using a mass flow controller (MKS Type 1152C). The reactor stage temperature was controlled using a backside recirculating chiller (Thermo Scientific NESLAB RTE 7) and was kept at 30 °C. During depositions, the nichrome filament array (Omega Engineering, 80%/20% Ni/Cr) held at 4.6 cm above the stage was resistively heated to 230 °C to thermally cleave TBPO, initiating free radical polymerization. For depositions of PPFDA, monomer and initiator were introduced into the reactor at 0.3 and 2 sccm, respectively, and the reactor pressure was maintained at 70 mTorr. For depositions of xPMAA, MAA, EGDA and initiator were introduced into the reactor at 5, 0.1, and 2 sccm respectively, and the reactor pressure was kept at 250 mTorr. The deposition rate was monitored on a reference silicon wafer piece in situ using interferometry with a

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RESULTS AND DISCUSSION We designed a compact roll-to-roll module that could be inserted into an iCVD reactor without any modification to the reactor chamber (Figure 1a). Two motors are used to move the substrate during the deposition. The substrate unwinds from the feed roll, passes through the primary deposition zone (12.5 cm long) with the top of the substrate facing the filament array and the bottom of the substrate facing the stage, and winds onto the receiver roll (Figure 1b). The velocity at which the substrate travels through the primary deposition zone can be varied between 2.5−6 cm/min for our roll-to-roll module. In the primary deposition zone, the substrate is suspended 1 cm above the stage to maintain sufficient tension in the paper for rolling. The heated filament array is placed above the rollB

DOI: 10.1021/acs.iecr.8b03030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Atomic compositions from XPS survey spectra for uncoated paper, PPFDA (theoretical), and on the top and bottom of roll-to-roll paper coated with PPFDA at multiple points along the length of the coated paper. Figure 1. (a) Photograph of the roll-to-roll module in an iCVD reactor chamber. A 3 m-long roll of chromatography paper is mounted on the module. (b) Schematic showing that the substrate rolls from the feed roll, through the primary deposition zone (shown in blue), and onto the receiver roll.

53.1% F) since the uncoated paper is composed of cellulose which had a measured composition of 60.5% C and 39.5% O and did not contain fluorine. The fluorine content along the length of the coated paper ranged from 45−52% F, which is slightly lower than that expected for PPFDA likely because the thickness of the PPFDA coating on the paper is less than the probe depth of XPS (about 5−10 nm), and therefore, the spectrometer is also sampling the underlying paper. Comparisons of the atomic compositions along the length of the coated paper on both sides show no significant variation, which indicates the uniformity of the PPFDA coating on the paper during the roll-to-roll process. The topography of the paper after roll-to-roll coating was characterized via SEM (Figure 3). Coated paper cut from the middle of the roll was imaged, and the morphology of the paper was found to be unchanged compared to uncoated paper, confirming that the individual fibers in the paper are conformally coated.

to-roll module at 4.6 cm above the stage. The filaments directly above the motors were removed to prevent touching the motors which have a height of 4.8 cm above the stage. The addition of the roll-to-roll module did not require changes to the iCVD process itself, beyond the modification of the filament array, which makes the module an attractive way to scale up existing iCVD processes. We chose to coat Whatman #1 chromatography paper as a model substrate because it has several properties that make it attractive for roll-to-roll processing: (i) it is flexible and can be shaped into rolls, (ii) it is porous and therefore we can study whether the polymer coating penetrates through the entire thickness of the paper (180 μm), (iii) it is foldable and can be shaped after coating, and (iv) it can be used for applications such as paper-based analytical devices and filtration. We placed a 3 m-long roll of 5 cm-wide paper into the reactor and continuously coated it with PPFDA in a single deposition. In the iCVD process, the deposition rate of the polymer can be tuned by changing the ratio of the monomer partial pressure to the monomer saturation pressure (PM/Psat), which is proportional to the amount of monomer adsorbed to the substrate surface.30 In our process, the temperature of the suspended paper substrate (68 °C) was higher than that of the reference silicon wafer (30 °C) placed on the stage. The value of PM/Psat was 0.1 on the silicon wafer, therefore we rolled the substrate at the slowest velocity (2.5 cm/min) to ensure that the substrate was coated. The entire roll of paper was coated in 2 h. The processing time could be decreased by increasing substrate velocity while increasing the deposition rate via increasing the flow rate of the monomer or increasing the reactor pressure. We chose PPFDA as a model polymer because the presence of polymer coating along the length and on both sides of the paper could be verified by XPS analysis. The fluorine content along the length of the roll-to-roll coated paper (Figure 2) was attributed to the presence of PPFDA (40.6% C, 6.3% O, and

Figure 3. SEM images of paper (a and b) before and (c and d) after roll-to-roll coating with PPFDA. C

DOI: 10.1021/acs.iecr.8b03030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research The uniformity of the PPFDA coating along the length of the roll-to roll coated paper was further verified using contact angle measurements. Whereas uncoated chromatography paper wets immediately (Figure 4a), the PPFDA coating on the roll-

Figure 5. (a) Atomic compositions from XPS survey spectra across the width of the coated paper at the center of the roll (1.5 m). (b) Contact angle measurements across the width of the top (triangles) and bottom (squares) of the coated paper at various lengths (0.1, 1.5, and 2.9 m).

Figure 4. (a) Images of water droplets on uncoated chromatography paper, stationary paper that was coated with PPFDA, and paper that was coated with PPFDA in a roll-to-roll deposition. (b) Contact angle measurements of the top (triangles) and bottom (squares) of the rollto-roll paper coated with PPFDA as a function of distance along the length of the paper.

hydrophobic paper and the water, whereas water seeps out of a bowl composed of uncoated paper. Additionally, the coated paper was used to make SLIPS materials. The Aizenberg group has shown that porous substrates with low surface energy can be infused with perfluorinated liquids to form SLIPS.31,32 Whereas the PPFDA-coated paper is hydrophobic (repels water), the SLIPS material is omniphobic (repels polar and nonpolar liquids). We made SLIPS materials by taking a section of coated paper from the center of the roll and infusing it with perfluorinated liquid (Krytox). For our SLIPS material at a 15° tilt, water (Figure 6b), isopropanol (Figure 6c), and chloroform (Figure 6d) all rolled off within seconds, whereas the latter two solvents do not roll off paper coated with PPFDA. To demonstrate that a variety of polymers can be deposited using our roll-to-roll module, we studied the deposition of xPMAA which can be used for the fabrication of functional paper-based microfluidic devices. Paper-based microfluidic devices are useful for point-of-care diagnostics,33−35 and the carboxylic acid group of MAA allows for separation of analytes because it is deprotonated in solution.36−38 During the iCVD process, the reactor pressure and the monomer flow rates were selected such that the PM/Psat matched that of PFDA. After coating the roll, microfluidic channels were cut at three distances along the length of the roll. The xPMAA coating

to-roll coated paper modifies the paper to become hydrophobic (Figure 4b). There are no significant trends or variations among the contact angles along the length of coated paper, confirming that the entire roll is coated uniformly, and the contact angles on the top and bottom sides of the paper are comparable, demonstrating that the coating has penetrated through the thickness of the paper. The contact angles are comparable to those on stationary coated paper (Figure 4a), indicating that rolling the substrate does not affect the polymerization process. The uniformity of the polymer coating across the width of the paper was also studied. Figure 5a shows that the atomic compositions across the width of the coated paper at the center of the roll ranged from 48−56% F and showed no trend, which demonstrates that the paper is coated uniformly across the width. Additionally, contact angles across the width of the paper were taken at multiple points along the length of the roll (Figure 5b) and there is no significant trend, further demonstrating the uniformity of the coating across the width. To demonstrate the hydrophobic properties of the coated paper, a section from the center of the roll was folded into a bowl shape using origami (Figure 6a). The bowl was able to hold water because of the surface interaction between the D

DOI: 10.1021/acs.iecr.8b03030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. Images of a mixture of Ponceau S (pink) and crystal violet (violet) on paper. Separations are shown on uncoated paper, stationary paper coated with xPMAA via iCVD, and roll-to-roll paper coated with xPMAA at different distances along the length of the paper. The arrow indicates the direction of eluent flow.

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

Figure 6. (a) Image of an origami bowl made with paper that was coated with PPFDA in a roll-to-roll deposition. The bowl is filled with water that is dyed with blue food coloring. (b−d) Time series images of dyed solvents rolling off of a SLIPS material at a 15° tilt. The solvents used were (b) water dyed with red food coloring, (c) isopropanol dyed with green food coloring, and (d) chloroform dyed with ethyl violet.

separated a mixture of crystal violet and Ponceau S in pH 10 buffer solution because the anionic xPMAA coating trapped the cationic crystal violet as the anionic Ponceau S eluted upward, whereas the uncoated paper was unable to separate the dyes (Figure 7). The separation on the three roll-to-roll coated channels was comparable to that on stationary paper coated via iCVD, indicating that the paper was coated uniformly along the entire length and width of the roll. Additionally, the separations on the coated channels demonstrate that the polymer coating has penetrated through the depth of the paper. Past work from our group demonstrated that iCVD could be used to pattern microfluidic channels into cellulose paper to hold solutions within barriers.23,39 The ability to contain liquid within the patterned channels indicated that the iCVD polymer penetrated through the depth of the paper. The ability to coat an entire 3 m-long roll of paper in a single deposition allows for efficient fabrication of 300 microfluidic channels that are 1 cm-wide and 5 cm-long.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Malancha Gupta: 0000-0002-6828-7445 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Award 1332394.

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CONCLUSIONS We built a roll-to-roll module to scale up the iCVD process, allowing us to coat 1500 cm2 of chromatography paper in a single deposition. The PPFDA coverage along the length and

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DOI: 10.1021/acs.iecr.8b03030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX