Downstream Monomer Capture and Polymerization during Vapor

Jun 4, 2019 - Porous membranes can be formed by the polymerization of solid monomer by a vapor-phase initiator followed by sublimation of the unreacte...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 9908−9914

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Downstream Monomer Capture and Polymerization during Vapor Phase Fabrication of Porous Membranes Nareh Movsesian, Golnaz Dianat, 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: Porous membranes can be formed by the polymerization of solid monomer by a vapor-phase initiator followed by sublimation of the unreacted monomer. This versatile bottom-up process can be used to deposit porous polymer membranes on a variety of substrates including planar, curved, and structured surfaces. In this paper, we incorporate additional thermoelectric coolers (TECs) into the reactor in order to study downstream monomer capture and polymerization. Better uniformity across the TECs is achieved by capturing the unreacted monomer from the preceding surface instead of capturing the monomer onto multiple surfaces at once. We show that the surface temperature of the downstream TEC affects the membrane morphology and the kinetics of polymerization. Lower capture temperatures lead to better surface coverage while higher temperatures promote the polymerization rate. The morphological differences are further confirmed by coating the membranes with a fluoropolymer to study variations in hydrophobicity. Our ability to capture and polymerize monomers across multiple TECs provides a sustainable route for enhancing the throughput of the process which is desirable for practical applications.

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commonly run at continuous flow where the monomer and initiator vapors are constantly fed into the chamber and are exhausted by a vacuum pump.15 CVD systems can also be operated as batch systems to decrease material waste.16−19 For example, Jankowska-Kuchta et al. deposited tungsten in a nonflowing batch reactor to reduce the use of high purity and toxic gases.16 Kukovitsky et al. used a closed batch CVD configuration for the fabrication of carbon nanotubes.17 Petruczok et al. recently developed a closed-batch iCVD method in which the reactor chamber was first filled with reactants and then isolated from the feed and exhaust lines which led to an improved reaction yield and less accumulation of waste.19 In our current study, we run our process under a semibatch processing mode in which the initiator flows during both the monomer deposition and polymerization steps but there is no additional monomer flow during polymerization in order to reduce material waste. The continuous initiator flow provides a steady introduction of free radicals during polymerization. Here we study two different systems for monomer deposition in which the additional TECs serve as downstream cold traps: the deposition of monomer onto multiple TECs at once and the capture of sublimating monomer from the preceding surface.

INTRODUCTION Initiated chemical vapor deposition (iCVD) is a solventless free radical polymerization technique that can be used to fabricate functional polymer films on a variety of substrates.1,2 Monomer and initiator vapors are introduced into a vacuum chamber, and the initiator is cleaved into free radicals with a heated filament array. The monomer adsorbs to the surface of the substrate, which is usually kept at room temperature, and polymerization occurs via a free radical mechanism, leading to the formation of a dense film. Our group has recently modified the iCVD process to fabricate polymer membranes with dual scale porosity by maintaining the monomer partial pressure and the substrate temperature below the triple point pressure and temperature of the monomer using a thermoelectric cooler (TEC).3−6 At these unconventional conditions, the monomer deposits as solid microstructures. Polymerization occurs at both the vapor−solid interface and within the solid structure and the excess unreacted monomer is subsequently sublimated.7−9 There is no unreacted monomer in the final polymer membranes as confirmed by Fourier-transform infrared spectroscopy analysis.3 Compared to solution phase processes for fabricating porous polymer membranes, such as thermally induced phase separation10,11 and solvent casting and particulate leaching,12,13 our bottom up solventless approach allows deposition of membranes onto a variety of substrates including porous membranes for the fabrication of porous-on-porous structures.14 In this paper, we introduce multiple TECs in the reactor to study deposition onto larger areas. The iCVD process is © 2019 American Chemical Society

Received: Revised: Accepted: Published: 9908

March 8, 2019 May 14, 2019 May 21, 2019 June 4, 2019 DOI: 10.1021/acs.iecr.9b01315 Ind. Eng. Chem. Res. 2019, 58, 9908−9914

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Industrial & Engineering Chemistry Research We systematically study how the temperature of the downstream cold traps affects the morphology of the membranes and the kinetics of polymerization. An advantage of our fabrication process is the ability to modify the surface properties of the porous membranes with a top coating by transitioning from the unconventional iCVD process to the conventional iCVD process.20 We used this technique to apply a dense fluorinated coating on the membranes which allowed us to study morphological variations as a function of processing temperature by assessing water repellency. Combinatorial methods for the synthesis and screening of a vast range of materials have long attracted interest because they can accelerate discovery and lead to high throughput production and optimization of new materials.21−23 The series of cold traps in our system allows for combinatorial synthesis of porous polymer coatings in a single run. Our platform adds to previously reported combinatorial vapor-phase deposition methods for the fabrication of organic and inorganic material libraries with compositional gradients such as antimicrobial polymers,24 amorphous and microcrystalline silicon,25,26 and thin films of magnesium and aluminum.27

Figure 1. Schematic representation of multiple TECs in the iCVD reactor.

−10 °C or −20 °C during the 5 min of monomer deposition, and subsequently the temperature of all TECs was increased to 10 °C during 30 min of polymerization. After polymerization, the filament was turned off, the TBPO flow was halted, and the TECs used during each of the experiments were set to 5 °C to allow for sublimation of the remaining unreacted monomer which was confirmed by the reactor returning to a base pressure of 16−18 mTorr. Cracks form when the monomer sublimation is too fast; therefore, we decreased the sublimation rate by decreasing the temperature of the TEC from 10 to 5 °C. We also studied a system in which the monomer is captured from the preceding TEC surface instead of being captured onto all TECs at once. Figure 2 shows the schematic of a

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EXPERIMENTAL SECTION Methacrylic acid (MAA) (Aldrich, 99%), tert-butyl peroxide (TBPO) (Aldrich, 98%), and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (SynQuest Laboratories, 97%) were used as received. Poly(methacrylic acid) (PMAA) membranes were fabricated in a custom designed initiated chemical vapor deposition (iCVD) chamber of 250 mm diameter and 48 mm height (GVD Corporation) containing a nichrome filament array (Omega Engineering, 80/20% Ni/Cr). The stage temperature was maintained at 20 °C using a recirculating chiller (Thermo Scientific Haake A25). A rotary vane vacuum pump (Edwards E2M40) was used to achieve vacuum and the reactor pressure was controlled by a throttle valve (MKS 153D) which was regulated by a capacitance manometer (MKS Baratron 622A01TDE). MAA and TBPO feed jars were mounted onto the reactor and maintained at 35 and 28 °C, respectively. A needle valve was used to meter the MAA flow, and a mass flow controller (MKS 1479 A) was used to meter the TBPO flow. The 4 × 4 cm2 thermoelectric coolers (TECs) (TE Technology) were placed onto the reactor stage approximately 1 cm apart from each other with TEC1 closest to the feed lines. Silicon wafers (Wafer World 119) were cut into 3 × 3 cm2 pieces and placed on top of the TECs. The temperature of each TEC was regulated using an adjustable DC power supply (Volteq HY3010D). Since methacrylic acid freezes at approximately 16 ± 1 °C, all TEC temperatures were kept at 10 °C or below for all experiments. Figure 1 shows a schematic of the iCVD reactor with multiple TECs. Silicon wafers were placed on the TECs for sample collection. For deposition and polymerization on only TEC1, the temperature of TEC1 was first maintained at −10 °C. TEC2 and TEC3 were kept off the whole time. TBPO was introduced into the chamber at 0.7 sccm to achieve a total pressure of 650 mTorr. Then MAA was introduced into the chamber at a flow rate of 0.13 sccm for 5 min to cause monomer deposition as solid microstructures. The temperature of TEC1 was then increased to 10 °C and the filament array was heated to 240 °C to start polymerization for 30 min. For the experiments with deposition onto multiples TECs at once, monomer deposition on TEC1 and TEC2 and on TEC1, TEC2, and TEC3 were performed with all TECs set to either

Figure 2. Stepwise process of capturing and polymerizing monomer from the preceding TEC.

system of two TECs. In step 1, monomer deposition on TEC1 followed the same reactor conditions described above while TEC2 and TEC3 were kept off. After this step, no additional monomer was introduced into the process. In step 2, the temperature of TEC1 was increased to 10 °C, the temperature of TEC2 was set to −20 °C, −10 °C, or 0 °C, and the filament array was heated to 240 °C to cleave the initiator into free radicals. Polymerization occurred on both TECs for 30 min and TEC2 served as a cold trap to partially capture and polymerize monomer that sublimated from TEC1. In step 3, polymerization was stopped, and sublimation of the unreacted monomer occurred at 5 °C. For the experiments with an additional TEC (TEC3), steps 1 and 2 were identical to those shown in Figure 2. Monomer capture and polymerization on TEC2 occurred at −20 °C for 30 min. Next, the temperature of 9909

DOI: 10.1021/acs.iecr.9b01315 Ind. Eng. Chem. Res. 2019, 58, 9908−9914

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Industrial & Engineering Chemistry Research TEC2 was increased to 10 °C and the temperature of TEC3 was turned to −20 °C for an additional 30 min of monomer capture and polymerization. In step 4, polymerization was stopped and all the TECs were set to 5 °C to sublimate the remaining unreacted monomer. The reactor pressure, the initiator flow rate, the monomer flow rate during deposition, and the temperature of TEC 1 during deposition and polymerization are fixed for all the experiments. All polymer masses were measured on a Metter AE 160 scale and averaged over three samples. The poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) depositions onto the PMAA membranes were performed using conventional iCVD parameters. The reactor stage was kept at 30 °C and the PFDA jar was maintained at 50 °C. PFDA and TBPO were introduced into the reactor at 0.2 and 1.5 sccm, respectively. The reactor pressure was maintained at 70 mTorr, and the nichrome filament array was resistively heated to 240 °C for 45 min. The deposition rate was 2 nm/min which was monitored on a reference silicon wafer by in situ interferometry using He−Ne laser (Industrial Fiber Optics, 633 nm). The surface properties of the PPFDA-coated porous membranes were characterized using contact angle goniometry (ramé-hart model 290). The reported static contact angles were averaged over 10 measurements with a droplet volume of 10 μL of deionized water, and the advancing and receding contact angle measurements had a starting droplet volume of 5 μL with a step size of 1 μL and a maximum droplet volume of 10 μL. Hysteresis was measured as the difference between the maximum and minimum contact angles and was averaged over 3 measurements. Error bars were found by calculating the standard deviation. All samples were imaged by scanning electron microscopy (SEM; Topcon Aquila) at 20 kV accelerating voltage. Prior to imaging, gold was sputtered onto the polymer samples for 30 s at 20 mA to avoid charging.

Figure 3. (a) In-situ optical images of the monomer deposited on TEC1, TEC2, and TEC3 at −10 °C at once and the corresponding (b) optical and (c) SEM images of the porous membranes after polymerization and sublimation of the unreacted monomer.

pillar-like morphology on TEC2 and TEC3. To determine if the deposition temperature affects the uniformity of the membranes, we also deposited monomer onto all three TECs at −20 °C for 5 min and polymerized at 10 °C for 30 min. Figure 4a shows that the monomer deposits more uniformly on all the

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RESULTS AND DISCUSSION To understand the effect of adding multiple TECs into the reactor, we compared the morphology and mass of the membranes for a system of one, two, and three TECs. The TEC temperature was kept at −10 °C during 5 min of monomer deposition and was increased to 10 °C during 30 min of polymerization. An initiator was introduced at a constant flow rate during both the deposition and polymerization steps; however, there was no additional monomer introduced during the polymerization step. Unreacted monomer was sublimated after polymerization, and the masses of the membranes were measured. For the case of deposition and polymerization on only one TEC, the mass was 29.4 ± 3.3 mg. For the case of deposition and polymerization onto two TECs at once, the mass was 20 ± 0.3 mg and 22 ± 4.1 mg on TEC1 and TEC2, respectively. For the case of deposition and polymerization onto three TECs at once, the mass was 17.2 ± 2.5 mg, 14.8 ± 2.2 mg, and 8.4 ± 2.3 mg for TEC1, TEC2, and TEC3, respectively. On the basis of these results, we can conclude that the mass per area decreases as more TECs are added into the system. In our process, the deposited monomer serves as the template for polymerization and therefore affects the morphology of the resulting membrane.3,4 Figure 3a shows the in situ optical images of the monomer prior to polymerization on the three TECs. It can be seen that the deposition of monomer onto TEC3 is not uniform. The corresponding optical and SEM images of the final porous membranes in Figure 3b,c confirm the loss of uniformity and

Figure 4. (a) In-situ optical images of the monomer deposited on TEC1, TEC2, and TEC3 at −20 °C at once and the corresponding (b) optical and (c) SEM images of the porous membranes after polymerization and sublimation of the unreacted monomer.

TECs compared to Figure 3a and therefore the resulting membranes in Figure 4b are also more uniform than in Figure 3b. As shown in the SEM images in Figure 4c, these membranes have pillar-like structures due to greater surface nucleation at lower temperatures and dual-scale porosity, consistent with our previous studies.3,4,7 The larger pores form due to the void spaces between the solid microstructures and smaller pores form within the microstructures due to the 9910

DOI: 10.1021/acs.iecr.9b01315 Ind. Eng. Chem. Res. 2019, 58, 9908−9914

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Figure 5. (a) Top down in situ optical images of monomer capture and polymerization on TEC2 as a function of time at temperatures of −20, −10, and 0 °C, while polymerization on TEC1 occurs at 10 °C in all cases. (b) Images of polymer membranes formed on TEC2 after sublimation of the unreacted monomer.

The SEM images in Figure 6a show that the pillar microstructures formed on TEC1 are not affected by the

sublimation of the monomer. At both deposition temperatures (−10 °C and −20 °C), the membrane formed on TEC3 has significant structural loss. Since all three TECs are kept at the same temperature during monomer deposition and polymerization, the nonuniformity is likely caused by the monomer flow pattern across the reactor. To test whether we could achieve better uniformity across multiple TECs by changing the flow pattern of the monomer, we studied a system in which the unreacted monomer is captured from the preceding surface as shown in Figure 2 instead of capturing the monomer onto multiple surfaces at once. For our preliminary studies, we focused on a system of two TECs. Monomer was deposited onto TEC1 in the first step. After this step, no additional monomer was introduced into the process. Our motivation for this set up was that our data indicated that the use of only one TEC resulted in the highest polymer mass per area. In the second step, the temperature of TEC1 was increased to 10 °C and the temperature of TEC2 was set to −20 °C, −10 °C, or 0 °C. The monomer that sublimated from TEC1 was partially captured as solid microstructures on TEC2. During this step, the filament array was heated to cleave the initiator into free radicals and polymerization occurred on both TECs for 30 min. The temperature of TEC1 was increased from −10 to 10 °C to enable a faster polymerization rate and faster monomer sublimation, both of which increase the conversion of the process.8 The in situ optical images in Figure 5a show monomer transfer and polymerization on TEC2 within the reactor as a function of time. A lower TEC2 temperature (−20 °C) leads to more nucleation spots on the silicon surface resulting in uniform coverage, whereas a higher TEC2 temperature (0 °C) results in larger patterns and less coverage. After sublimation, the membranes were removed from the reactor and imaged (Figure 5b). The macroscopic structures of the final polymer membranes are consistent with the in situ images inside the reactor. The membranes formed at −20 °C have better surface coverage than the membranes formed at 0 °C which have cracks and voids.

Figure 6. SEM images of the porous membranes formed on (a) TEC1 and (b) TEC2 as a result of monomer capture and polymerization from the preceding surface.

temperature variations on TEC2. The SEM images in Figure 6b show that the membranes formed at higher TEC2 temperatures have denser structures than the membranes formed at lower temperatures due to lack of nucleation, which is consistent with the macroscopic images shown in Figure 5 panels a and b. An advantage of our fabrication process is that we can vary processing parameters in situ to apply a thin polymer coating onto the membranes without affecting their morphology by transitioning from unconventional iCVD parameters to conventional iCVD parameters.20 The fluorinated polymer PPFDA was deposited onto the hydrophilic PMAA membranes and the resulting hydrophobicity was studied using contact angle goniometry. PPFDA is a hydrophobic polymer that has a water contact angle of approximately 120° on a flat 9911

DOI: 10.1021/acs.iecr.9b01315 Ind. Eng. Chem. Res. 2019, 58, 9908−9914

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Industrial & Engineering Chemistry Research surface.28 High surface roughness enhances hydrophobicity and therefore contact angle measurements provide information about the roughness of the membranes.29 The SEM images after PPFDA coating in Figure 7a show a similar morphology

studies that show that increasing the polymerization temperature increases the sublimation rate of the monomer, leading to increased polymer mass at the vapor−solid interface. However, there is no significant change from −10 to 0 °C. Although the polymerization rate is higher at 0 °C, the membrane formed at −10 °C has a larger surface area which allows for more polymerization at the vapor−solid interface.8 We further studied the capability to capture and polymerize monomer from the preceding surfaces by adding a third TEC. After monomer capture and polymerization on TEC2 at −20 °C for 30 min as described in Figure 2, the temperature of TEC2 was increased to 10 °C and TEC3 was set to −20 °C for an additional 30 min of capture and polymerization. The remaining unreacted monomer in the system was then sublimated. We performed the monomer capture and polymerization on TEC2 and TEC3 at −20 °C in order to obtain the best surface coverage. Figure 8 shows that the

Figure 7. (a) SEM images of the membranes formed on TEC2 after PPFDA coating and (b) their corresponding static water contact angle.

to those in Figure 6b, indicating no significant change in structure after coating. The membranes formed on TEC1 are very rough due to the pillar microstructures and therefore water droplets immediately roll off after coating with PPFDA, indicating superhydrophobicity similar to the Lotus Leaf effect.30 The membranes formed on TEC2 were less hydrophobic, indicating less roughness as shown in Figure 6a. The water contact angle decreases with increasing TEC2 temperature due to the denser structures and the hysteresis (differences in the maximum advancing and minimum receding contact angles) increases with increasing substrate temperature and was measured to be 10.2 ± 2.5°, 20.2 ± 2.1°, and 27.2 ± 2.9° for membranes formed at −20 °C, −10 °C, and 0 °C, respectively (Figure 7b). Table 1 shows that the polymer mass on TEC1 is relatively constant at varying TEC2 temperatures likely due to the high

Figure 8. (a) Optical images and (b) the corresponding SEM images of the porous membranes formed on TEC1, TEC2, and TEC3 as a result of monomer capture and polymerization from the preceding surface.

optical and SEM images of the membranes have uniform patterns across each silicon wafer. The membranes formed on TEC1 and TEC2 have similar morphologies as those formed in the two TEC systems for the case of TEC2 at −20 °C in Figure 6b; however, changing the temperature of TEC2 from −20 to 10 °C promotes a faster sublimation rate which leads to enlargement of the smaller scale pores within the microstructures. Monomer capture and polymerization on TEC3 extends the polymerization time to 60 min for the membranes formed on TEC1 and TEC2 which results in polymer masses of 35.6 ± 2.7 mg and 37.7 ± 1.4 mg, respectively. The polymer mass on TEC3 as a result of 30 min of monomer capture and polymerization at −20 °C is 4.9 ± 0.6 mg which is consistent with the polymer mass obtained on TEC2 at a capture and polymerization temperature of −20 °C in the system of two TECs (Table 1). The morphology across each TEC was more uniform compared to deposition onto all three TECs at once, indicating that downstream capture and polymerization from the preceding surface is a possible route toward achieving uniformity across multiple TECs.

Table 1. Polymer Mass Obtained on TEC1 and TEC2 as a Result of Monomer Capture and Polymerization from the Preceding Surface at Varying TEC2 Temperatures TEC1 temp (°C)

TEC2 temp (°C)

deposition polymerization

simultaneous deposition and polymerization

TEC1

TEC2

−20 −10 0

20.4 ± 4.9 22.4 ± 1.3 23.7 ± 3.0

5.2 ± 1.6 10.2 ± 0.5 9.4 ± 0.9

−10 −10 −10

10 10 10

polymer mass (mg)

initial mass of the deposited monomer on TEC1 (∼600 mg) in addition to the slow solid phase polymerization kinetics (approximately 10 wt % or less of monomer is converted)8 which is consistent with previously reported solid phase polymerization techniques.31−33 The polymer mass on TEC2 increases from −20 °C to −10 °C due to faster polymerization rates at higher temperatures. This is consistent with our recent

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CONCLUSION We have shown that multiple TECs can be incorporated in an iCVD reactor. We showed that deposition of the monomer 9912

DOI: 10.1021/acs.iecr.9b01315 Ind. Eng. Chem. Res. 2019, 58, 9908−9914

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Industrial & Engineering Chemistry Research onto multiple TECs at once results in less uniformity than capturing and polymerizing the monomer from the preceding surface. We studied the effect of temperature on the reaction kinetics and the morphology of the membranes formed as a result of downstream capture. Lower temperatures lead to better surface coverage and increased surface roughness. Higher temperatures lead to more mass; however, there is less nucleation leading to poor surface coverage. On the basis of our results, it is beneficial to capture the monomer from the preceding surfaces at lower temperatures to achieve better uniformity across multiple surfaces. Since there was structural loss in the pillar morphology across the length of the reactor, future work will focus on developing new reactor configurations that can enable the production of more consistent morphologies.

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Golnaz Dianat is currently a doctoral student in the Mork Family Department of Chemical Engineering and Materials Science at the University of Southern California. She received her B.S. and M.S. in mechanical engineering from Eastern Mediterranean University in 2008 and 2011. Her thesis focuses on developing novel synthetic routes to fabricate asymmetric membranes and studying process− structure−property relationships that govern the deposition of porous polymer membranes.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Malancha Gupta: 0000-0002-6828-7445 Notes

The authors declare no competing financial interest.

Dr. Malancha Gupta is currently an associate professor in the Mork Family Department of Chemical Engineering and Materials Science at the University of Southern California. She received her BS in chemical engineering from the Cooper Union in 2002 and her Ph.D. in chemical engineering from MIT in 2007 under the guidance of Prof. Karen Gleason. She was a postdoctoral fellow under the guidance of Prof. George Whitesides at Harvard University from 2007 to 2009. She received the Jack Munushian Early Career Chair in 2013 and the NSF CAREER award in 2013. Her research is focused on studying the chemical vapor deposition of functional polymers onto structured materials and liquid surfaces. She has mentored 14 doctoral students, and she has published 59 peer-reviewed manuscripts and holds 3 patents.

Biographies

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Science Foundation CAREER Award CMMI-1252651.

Nareh Movsesian is currently a doctoral student in the Mork Family Department of Chemical Engineering and Materials Science at the University of Southern California. She received her B.S. in chemistry from the University of California, Los Angeles, in 2013 and her M.S. in chemical engineering from the University of Southern California in 2016. Her thesis focuses on developing sustainable processes for vapor-phase deposition of porous polymer membranes for high throughput coating applications.

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DOI: 10.1021/acs.iecr.9b01315 Ind. Eng. Chem. Res. 2019, 58, 9908−9914