Microstructured Films Formed on Liquid Substrates via Initiated

Jul 15, 2015 - Jyun-Ting Wu , Chih-Yu Wu , Shih-Kang Fan , Chih-Chen Hsieh , Yu-Chih Hou , and Hsien-Yeh Chen. Chemistry of Materials 2015 27 (20), ...
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Microstructured Films Formed on Liquid Substrates via Initiated Chemical Vapor Deposition of Cross-Linked Polymers Laura C. Bradley and Malancha Gupta* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, United States ABSTRACT: We studied the formation of microstructured films at liquid surfaces via vapor phase polymerization of crosslinked polymers. The films were composed of micron-sized coral-like structures that originate at the liquidvapor interface and extend vertically. The growth mechanism of the microstructures was determined to be simultaneous aggregation of the polymer on the liquid surface and wetting of the liquid on the growing aggregates. We demonstrated that we can increase the height of the microstructures and increase the surface roughness of the films by either decreasing the liquid viscosity or decreasing the polymer deposition rate. Our vapor phase method can be extended to synthesize functional, free-standing copolymer microstructured thin films for potential applications in tissue engineering, electrolyte membranes, and separations.

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INTRODUCTION The development of microstructured polymer films is a continuously growing field due to a wide range of applications in tissue engineering,1−3 electrolyte membranes,4−6 and separations.7−9 Recently, Kawano et al. fabricated polymer scaffolds with varying pore sizes to selectively control cell differentiation for tissue engineering.10 Garcia-Ivars et al. demonstrated the synthesis of ultrafiltration membranes with either organic or inorganic additives to improve performance and antifouling properties.11 Cheng et al. controlled the structure and crystallinity of microporous polymers for use as electrolyte membranes by varying the polymer composition to maximize the ionic conductivity of the electrolyte.12 Microstructured polymers are typically made using thermally induced phase separation,13−15 phase inversion,16−18 spinodal decomposition,19−21 or lithography.22−24 These commonly used solution-phase methods rely on tailoring the chemical and physical properties of multicomponent solutions. In this work, we demonstrate the formation of microstructured polymer films by the deposition of cross-linked polymers at liquid vapor interfaces via initiated chemical vapor deposition (iCVD). In the iCVD process, monomer and initiator precursors are flowed in the vapor phase into a vacuum chamber where a heated wire filament decomposes the initiator into radicals. The monomer molecules and initiator radicals adsorb to the substrate surface where polymerization occurs through a free-radical mechanism.25,26 The advantages of using the iCVD method to make microstructured films over solutionphase techniques are that the solventless process produces high-purity polymers and eliminates any issues related to precursor solubility which thereby allows for the synthesis of functional cross-linked and copolymer films. © 2015 American Chemical Society

The iCVD technique is traditionally used to deposit dense, conformal coatings onto solid substrates;27−30 however, recent studies have demonstrated the synthesis of microstructured films. For example, Tao et al. made porous membranes by simultaneous polymerization of cross-linked copolymers and condensation of an inert porogen species followed by the subsequent removal of the porogen.31 Similar to solution-phase methods, the morphology of the films presented by Tao was shown to depend on the phase separation between the polymer and the condensed porogen. Seidel et al. produced microstructured membranes with dual-scale porosity by simultaneous polymerization and deposition of solid monomer.32,33 This technique required that the substrate temperature be maintained below the freezing point of at least one monomer precursor to induce physical monomer deposition which controls the resulting polymer morphology. The methods presented by Tao and Seidel require conditions outside typical iCVD processing parameters in order to facilitate either porogen condensation or solid monomer deposition. We have recently shown that we can deposit polymers via iCVD onto liquid substrates with low vapor pressures.34−40 The deposition of linear polymers leads to the formation of either dense polymer films or discrete polymer particles depending on the polymerliquid surface tension interactions.41,42 In this paper, we demonstrate that we can synthesize unique microstructured films by depositing cross-linked polymers at liquid surfaces under standard iCVD processing conditions. The chemical cross-links result in the formation of films containing micron-sized coral-like features. The growth Received: May 7, 2015 Revised: June 24, 2015 Published: July 15, 2015 7999

DOI: 10.1021/acs.langmuir.5b01663 Langmuir 2015, 31, 7999−8005

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(γPV), liquidvapor surface tensions (γLV), and the advancing contact angles of the liquid on the polymers (θ) using a goniometer (raméhart model 290-F1). The liquidvapor surface tensions were determined by the pendent drop method using 3 μL volumes. The pipet tips used for the pendent drop method were coated with trichloro(1H,1H,2H,2H-perfluorooctyl)silane (97%, Sigma-Aldrich) via physical vapor deposition in a desiccator for 30 min prior to the measurements in order to prevent the liquid from wetting the outside of the pipettes. The polymerliquid surface tensions were measured by the acid−base method on polymer-coated glass slides using 5 droplets (5 μL) each of deionized water, glycerol (EMD Chemicals), and diiodomethane (99%, Sigma-Aldrich). The advancing contact angles were measured by the step volume method using an initial drop volume of 3 μL and a step size of 1 μL. The morphology of the films was imaged by scanning electron microscopy (SEM) (JEOL-7001) using a 5 kV acceleration voltage. Prior to imaging, the samples were sputter-coated for 30 s with platinum. The reported film thicknesses were determined from crosssectional SEM images by measuring the thickness at three positions spaced 3 μm apart for three independent samples. The composition of the polymer films was analyzed using FTIR spectroscopy by mounting the films on a silicon wafer and collecting 32 scans between 4000 and 500 cm−1 with a resolution of 4 cm−1. The surface roughness of the films was measured using atomic force microscopy (AFM) (Veeco diInnova DLPCA-200). Images were collected in tapping mode with a scan rate of 0.5 Hz. The reported root-mean squared (RMS) surface roughness was calculated using Gwyddion software on 5 μm × 5 μm areas. The reported values are an average of three total samples from two independent depositions, and the reported error is the standard deviation.

mechanism and polymer morphology are driven by simultaneous aggregation of the polymer at the liquid surface and wetting of the liquid on the growing aggregates. We demonstrate that we can control the morphology of the microstructured films by varying the liquid viscosity and polymer deposition rate. Furthermore, we show that we can tailor the film chemistry by synthesizing copolymer microstructured films for potential applications in tissue engineering, electrolyte membranes, or separations.

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EXPERIMENTAL SECTION

Silicone oil (5, 100, and 500 cSt, Sigma-Aldrich; 5000 cSt, Dow Corning), ethylene glycol diacrylate (EGDA) (97% MonomerPolymer), 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (97% Synquest), 2-hydroxyethyl methacrylate (HEMA) (98% SigmaAldrich), 1-vinyl-2-pyrrolidone (VP) (99% Sigma-Aldrich), tert-butyl peroxide (98% Sigma-Aldrich), and hexane (98% Sigma-Aldrich) were all used as received. The solubility of the monomers (EGDA, PFDA, HEMA, and VP) in 5 cSt silicone oil were independently tested by preparing 1:1 v/v mixtures which were allowed to equilibrate for 48 h. The composition of the silicone oil phase was analyzed using Fourier transform infrared (FTIR) spectroscopy on a Thermo-Scientific Nicolet instrument by collecting spectra between 4000 and 500 cm−1 with a resolution of 4 cm−1. No solution contained the characteristic carbonyl stretching of the monomers in the silicone oil phase, confirming that all the monomers are insoluble in silicone oil. Polymer depositions were carried out in a custom-built deposition chamber (GVD Corporation, 250 mm diameter, 48 mm height). For all depositions, the substrates were maintained at 25 °C, and the nichrome filament array (80% Ni, 20% Cr, Omega Engineering) was heated to 230 °C. The tert-butyl peroxide initiator was maintained at room temperature and flowed into the reactor chamber at a rate of 1.5 standard cubic centimeters per minute (sccm) through a mass flow controller. The polymer deposition rates were monitored on a reference silicon wafer using an in situ 633 nm helium−neon laser interferometer (Industrial Fiber Optics). For PEGDA depositions at rates of 10 and 50 nm/min, the EGDA jar temperatures were 30 and 45 °C, the reactor pressures were 50 and 65 mTorr, and the ratio of the monomer partial pressure to the monomer saturation pressures (PM/PSAT) were 0.13 and 0.17, respectively. For all the copolymer depositions, the deposition rate was kept constant at 10 nm/min using a reactor pressure of 50 mTorr. For the deposition of P(PFDA-coEGDA), the PFDA and EGDA jar temperatures were maintained at 25 and 30 °C to produce a flow rates of 0.5 sccm for each monomer, resulting in PM/PSAT values of 0.18 and 0.11, respectively. For the deposition of P(HEMA-co-EGDA), the HEMA and EGDA jar temperatures were maintained at 25 and 30 °C, to produce a flow rates of 0.5 sccm for each monomer resulting in PM/PSAT values of 0.06 and 0.11, respectively. For the deposition of P(VP-co-EGDA), the VP and EGDA jar temperatures were maintained at 40 and 30 °C to produce a flow rates of 1.0 and 0.5 sccm, resulting in PM/PSAT values of 0.14 and 0.09, respectively. The line temperatures for all the monomers were maintained 15 °C above the jar temperature. The liquid substrates were introduced into the deposition chamber by dispensing 20 μL of 1:1 v/v silicone oilhexane solutions onto 1.5 cm × 1.5 cm silicon wafers. Mixing silicone oil in hexane enabled a consistent volume of silicone oil (10 μL) to be drop-cast for each liquid viscosity (5−5000 cSt). The hexane was removed from the silicone oil by pumping down the samples for 20 min in the deposition chamber prior to the polymer depositions. After deposition, the polymer film was removed from the liquid surface by pressing a piece of carbon tape on a silicon wafer support to the top (vapor) side of the polymer film on the liquid substrate. When the tape was lifted, the polymer film was removed from the liquid surface and the bottom (liquid) side of the film was exposed. The samples were then soaked in hexane for 10 min to remove any residual liquid. The spreading coefficients of the polymer on the liquid substrates were calculated by measuring the polymervapor surface tensions

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RESULTS AND DISCUSSION To study the growth of microstructured films on liquid substrates, we used poly(ethylene glycol diacrylate) (PEGDA) deposited onto silicone oil as a model system. The EGDA monomer is a cross-linker and is not soluble in silicone oil as verified by FTIR spectroscopy; therefore, polymerization only occurs at the liquid surface. SEM images of films formed by 90 min depositions at a deposition rate of 10 nm/min (as measured on a reference silicon wafer) showed that the films contained micron-sized coral-like microstructures that were composed of spherical aggregates. The microstructures originate at the bottom (liquid) side and extend vertically through the film cross section to the top (vapor) side of the films (Figure 1a−c). The resulting polymer films have void space between the microstructures and between the spherical aggregates, and therefore the total thickness of the PEGDA microstructured films formed after 90 min (9.1 ± 0.8 μm) was significantly greater than the deposition thickness of the dense PEGDA film formed on a reference silicon wafer (∼0.9 μm) (Figure 1d). We examined the chemical composition by FTIR spectroscopy which showed that the spectra of the films removed from the surface of the silicone oil and the film formed on a reference silicon wafer were identical and displayed the characteristic carbonyl stretching43 at 1735 cm −1 . Furthermore, the spectrum of the microstructured PEGDA film does not contain the characteristic signals of silicone oil44 for Si−C stretching between 900 and 800 cm−1, confirming that the microstructured films are composed of homopolymer PEGDA and that silicone oil is not integrated into the film (Figure 1e). We studied the growth mechanism of the microstructured films by varying the deposition time of PEGDA onto silicone oil (5 cSt) between 10 and 90 min (Figure 2). SEM images of the bottom side at 10 and 20 min show that the films are not complete, containing large gaps which decrease in size between 8000

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Figure 2. SEM images of the bottom sides of PEGDA films removed from the surface of 5 cSt silicone oil after 10, 20, 40, and 90 min depositions. The insets are cross-sectional images of the complete films at 40 and 90 min.

aggregation at the liquid surface and wetting of the liquid on the growing aggregates. The presence of gaps in the films at short deposition times demonstrates that the polymer diffuses and aggregates on the liquid surface as predicted by the negative spreading coefficient. Since cross-linked polymers have extremely limited mobility,46 we expect the aggregation occurs by the diffusion of nucleated chains before they become crosslinked with the larger polymer network. We confirmed that there was enough polymer deposited in a 10 min deposition to form a complete film over the entire liquid surface by observing the formation of smooth, continuous films on a reference silicon wafer as well as on high-viscosity (5000 cSt) silicone oil due to slower rates of polymer diffusion on the liquid surface.41 To verify that the silicone oil wets between the microstructures, we performed depositions of 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) cross-linked with EGDA (P(PFDA-coEGDA)) on a PEGDA microstructured film that was removed from the liquid surface as well as a microstructured film that remained on the silicone oil. The iCVD technique characteristically deposits conformal coatings on substrates with complex geometries; therefore, coating the microstructured film that was removed from the liquid surface with P(PFDA-co-EGDA) resulted in a uniform increase in the width of the individual microstructures (Figure 3a). For the deposition onto the microstructured film that remained on the silicone oil, we observed the formation of a dense layer on the top side (Figure 3b). The spreading coefficient of P(PFDA-co-EGDA) is positive (S = 18 mN/m), which leads to the formation of dense films at the liquid surface; therefore, the dense layer formed on the top side of the microstructured film demonstrates that the liquid wets between the microstructures to the top side, preserving the morphology of the PEGDA microstructures. We examined how we could influence the morphology of the PEGDA microstructured films by increasing the liquid viscosity which has been shown to slow the diffusion of polymers at liquid surfaces.47−49 We performed 60 min depositions onto 5, 100, 500, and 5000 cSt silicone oil at a rate of 10 nm/min, and SEM images of the bottom side show that the lateral feature

Figure 1. SEM images of the (a) bottom (liquid) side, (b) top (vapor) side, and (c) cross section of PEGDA microstructured films formed on 5 cSt silicone oil by a 90 min deposition at a rate of 10 nm/min. (d) Zoomed-in image of the outlined region in part c. (e) FTIR spectra of reference silicone oil and reference PEGDA deposited on a silicon wafer compared to PEGDA film removed from the surface of silicone oil.

10 and 40 min. Complete microstructured films that cover the entire liquid surface form for depositions longer than 40 min. Between 40 and 90 min, the topography of the bottom side did not change as shown by similar RMS surface roughness measured by AFM of 412 ± 25 and 417 ± 6 nm, respectively, while the thickness of the microstructured films continued to increase from 3.8 ± 0.5 to 9.1 ± 0.8 μm, respectively (Figure 2, insets). We have previously demonstrated that the morphology of linear polymers deposited at liquidvapor interfaces depends on the spreading coefficient of the polymer on the liquid (S = γLV(1 + cos θ) − 2γPV),45 in which systems with positive spreading coefficients form films and systems with negative spreading coefficients form particles.41 The calculated spreading coefficient for PEGDA on silicone oil (5 cSt) is approximately −56 mN/m which predicts the formation of polymer particles; however, discrete particles do not form due to chemical cross-links forming long-range polymer networks as observed in Figure 2. We hypothesized that the cross-linked microstructured PEGDA films grow by simultaneous polymer 8001

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size decreases with increasing liquid viscosity (Figure 4a). This trend was quantified by measuring a decrease in the RMS surface roughness of the bottom side of the films from 428 ± 6 nm on 5 cSt silicone oil to 15 ± 7 nm on 5000 cSt silicone oil. The decrease in the RMS surface roughness is due to slower rates of polymer diffusion, resulting in less aggregation on the liquid surface. This also leads to less void space between the microstructures and therefore a decrease in the thickness of the microstructured films from 5.4 ± 0.3 to 4.0 ± 0.3 to 2.7 ± 0.3 μm with increasing liquid viscosity from 5 to 100 to 500 cSt, respectively. The films formed on the high viscosity (5000 cSt) silicone oil were smooth and dense due to very slow diffusion of the polymer at the liquid surface.41 While increasing the liquid viscosity will decrease the diffusion rate of the polymer on the liquid surface, it will also decrease the wetting rate of the liquid on the polymer.50 Therefore, we evaluated the effect of the polymer deposition rate on the film morphology in order to determine which effect dominates the growth mechanism of the films. The total thickness of polymer deposited onto a reference silicon wafer was kept constant at 600 nm for each deposition rate. Increasing the polymer deposition rate from 10 to 50 nm/ min will result in larger molecular weights of nucleated chains51 that will have slower diffusion on the liquid surface.52−54 We found that for films formed on 5, 100, and 500 cSt silicone oil, increasing the deposition rate decreased the lateral feature size on the bottom side and decreased the total film thickness (Figure 4b). These qualitative changes in morphology were reflected in a measured decrease in the RMS surface roughness on the bottom side of the films with increasing deposition rate as well as an observed decrease in the lateral feature size in the AFM topographical images (Figure 5). The surface roughness of all the films was greater than the surface roughness of polymer deposited onto reference silicon wafers which was 2 ± 1 and 3 ± 1 nm for deposition rates of 10 and 50 nm/min, respectively. Since the wetting rate of the liquid on the polymer is constant at the same liquid viscosity, the significant changes in the film morphology on the same viscosity liquid with

Figure 3. SEM cross-sectional images of films formed by P(PFDA-coEGDA) depositions onto (a) a PEGDA microstructured film removed from the liquid surface and mounted on carbon tape and (b) a PEGDA microstructured film that remained on the silicone oil surface (5 cSt).

Figure 4. SEM images of the bottom sides and cross sections of PEGDA films removed from the silicone oil as a function of viscosity for (a) a 60 min deposition at 10 nm/min and (b) a 12 min deposition at 50 nm/min. 8002

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Figure 5. (a) RMS surface roughness measured by AFM of the bottom sides of PEGDA films as a function of silicone oil viscosity and polymer deposition rate. AFM tapping mode topographic images (5 × 5 μm frames) of films formed at a 50 nm/min deposition rate on (b) 5 cSt and (c) 500 cSt silicone oil.

increasing deposition rate demonstrates that the growth of the microstructured films is dominated by the diffusion and aggregation of polymer at the liquid surface. To demonstrate the generality of our technique for the synthesis of functional microstructured films, we examined the morphology of cross-linked copolymer films by depositing 2hydroxyethyl methacrylate (HEMA) cross-linked with EGDA (P(HEMA-co-EGDA)) and 1-vinyl-2-pyrrolidone (VP) crosslinked with EGDA (P(VP-co-EGDA)) onto 500 cSt silicone oil. The film morphology was examined for 60 min depositions at a rate of 10 nm/min. We used FTIR spectroscopy to confirm the incorporation of HEMA into the P(HEMA-co-EGDA) film by the presence of O−H stretching55 between 3700 and 3050 cm−1 and the incorporation of VP into the P(VP-co-EGDA) films by the carbonyl stretching of VP56 centered at 1684 cm−1 (Figure 6a). The spreading coefficients for both copolymers are negative (−63 and −58 mN/m for P(HEMA-co-EGDA) and P(VP-co-EGDA), respectively), dictating that it is energetically favorable for the polymer to aggregate on the liquid surface. The copolymer films were observed to contain microstructures which resulted in a RMS surface roughness on the bottom side of 245 ± 15 and 193 ± 4 nm for P(HEMA-co-EGDA) and P(VP-co-EGDA), respectively (Figure 6b). These results demonstrate that the iCVD technique can be used to synthesize functional microstructured films with tailored compositions by the deposition of copolymers onto liquid surfaces for potential applications in tissue engineering, electrolyte membranes, and separations. We would also like to note that the mechanical properties of the films could be optimized by extending the deposition time to increase the total film thickness or by forming the films on wire supports39 placed at the liquid surface.

Figure 6. (a) FTIR spectra and (b) SEM images of copolymer P(HEMA-co-EGDA) and P(VP-co-EGDA) films removed from the surface of 500 cSt silicone oil.

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CONCLUSIONS In conclusion, we have demonstrated the formation of microstructured cross-linked polymer films on liquid substrates via iCVD. The films were composed of micron-sized coral-like structures that originate at the liquidvapor interface and extend vertically. We determined that the microstructures form by simultaneous diffusion and aggregation of the polymer at the liquid surface and wetting of the liquid on the growing aggregates. Increasing the liquid viscosity and increasing the polymer deposition rate were found to decrease the thickness of the microstructured films and decrease the RMS surface

roughness of the bottom side due to decreased aggregation of the polymer on the liquid surface. This work demonstrates that microstructured films form in cross-linked systems with negative spreading coefficients and that the morphology of the films can be controlled to fabricate functional polymer materials for potential applications in tissue engineering, electrolyte membranes, and separations. Furthermore, the liquid substrate can allow for easy fabrication of free-standing thin films, and our synthesis method operates under standard 8003

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iCVD processing conditions which can allow for a wide range of functionalities, such as thermoresponsive or pH-responsive, to be incorporated into copolymer microstructured films.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award #DESC0012407. L.C.B. is supported by a National Science Foundation Graduate Research Fellowship under Grant DGE-0937362.

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DOI: 10.1021/acs.langmuir.5b01663 Langmuir 2015, 31, 7999−8005

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DOI: 10.1021/acs.langmuir.5b01663 Langmuir 2015, 31, 7999−8005