Condensation and Polymerization of Supersaturated Monomer Vapor

Nov 13, 2012 - Films grown from supersaturated monomer exhibited distinct surface ... Glycidyl methacrylate (GMA, Sigma Aldrich, 97%), t- butyl peroxi...
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Condensation and Polymerization of Supersaturated Monomer Vapor Ran Tao and Mitchell Anthamatten* Department of Chemical Engineering, University of Rochester, 206 Gavett Hall, Rochester, New York 14627, United States S Supporting Information *

ABSTRACT: Initiated chemical vapor deposition (iCVD) of poly(glycidyl methacrylate) from supersaturated monomer vapor is reported. Rapid film growth rates, up to 600 nm/min, were observed. Films grown from supersaturated monomer exhibited distinct surface undulations. The temporal evolution of surface features during film growth was studied and is explained by monomer condensation followed by droplet coalescence and film growth. High droplet densities were observed at the early times and are attributed to rapid polymerization of monomer within condensed liquid nuclei. Droplet nucleation resulting in surface undulations can be avoided by first depositing a thin, cross-linked film from ethylene glycol diacrylate monomer followed by deposition of supersaturated monomer vapors.

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INTRODUCTION Initiated chemical vapor deposition (iCVD) is a gentle, low energy processing scheme to grow thin polymer films directly from gas phase feeds.1,2 The technique involves feeding low molar mass gaseous initiators and monomers into a low pressure chamber. Gases flow past a hot filament array where initiator species are thermally activated. Activated initiators and monomers then adsorb onto a cooled substrate where polymerization and film growth occurs. Resulting polymer coatings are conformal, pinhole-free, and are of high purity. Due to the absence of surface tension, films can be grown onto micrometer-sized particles,3,4 fibers5 or nanotubes;3 and particle aggregation during processing can be avoided. Over the past decade, the iCVD, photoinitiated CVD (piCVD), and oxidative CVD (oCVD) techniques have been significantly advanced to demonstrate superhydrophobic surfaces and fabrics,5,6 nanoporous membranes for separations,7 bioinert electrical insulating coatings,8 antimicrobial coatings,9 and flexible, electrically conductive thin films.10,11 We are studying how film growth rate can influence morphology by thermodynamically trapping nonequilibrium structures. For example, fast deposition can suppress thin-film dewetting, leading to smaller droplets with greater surface area.12 Similarly, small molecule crystal nucleation and growth can be modulated by deposition rate.13,14 The relatively high capital cost of vacuum process equipment is a major obstacle to widespread iCVD commercialization. Substantially improving coating throughput without sacrificing film quality could offset capital costs and improve commercial viability. The iCVD deposition rate is usually limited by the rate of monomer surface adsorption: lower substrate temperature results in faster deposition. The rate of monomer adsorption is proportional to the monomer’s ideal gas activity which is © 2012 American Chemical Society

quantified by the ratio of the monomer’s partial pressure to its saturated vapor pressure, Pm/Psat.15,16 At low values of Pm/Psat, equilibrium adsorption corresponds to less than a monolayer of monomer; and the equilibrium surface concentration is almost linear with Pm/Psat. At intermediate values of Pm/Psat, multilayers of monomer form, and, at Pm/Psat > 1.0, monomer liquid condensation occurs. At supersaturated conditions, iCVD is less conformal and films are nonuniform. The rule-of-thumb to achieve high-quality, homogeneous films is to run under conditions leading to 1−3 adsorbed monolayers of monomer. For many monomers, this corresponds to Pm/Psat between 0.4 and 0.7.15 The objective of our current investigation is to examine how monomer partial pressure influences deposition rate and film morphology when the monomer is supersaturated (Pm/Psat > 1). We will show that deposition rates can be significantly higher at supersatured conditions, and monomer condensation can form droplets that coalesce into continuous films with thickness undulations. The temporal evolution of surface features during deposition is explained by nucleation, growth, and coalescence of droplets. Finally, a multistage iCVD process involving initial deposition of a thin, cross-linked film is shown to achieve fast deposition while maintaining high surface quality.

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EXPERIMENTAL SECTION

Materials. Glycidyl methacrylate (GMA, Sigma Aldrich, 97%), tbutyl peroxide (TBPO, Sigma Aldrich, 97%), and ethylene glycol diacrylate (EGDA, Polysciences, 98%) were all used as received. Received: August 28, 2012 Revised: November 12, 2012 Published: November 13, 2012 16580

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Initiated Chemical Vapor Deposition. Poly(glycidyl methacrylate) (PGMA) films were deposited using a custom-built deposition chamber shown in Figure 1. The chamber pressure is controlled using

separated from the substrate, and their average thickness was measured using a micrometer. A coating of gold, 6 nm thick, was sputtered on the PGMA films prior to SEM imaging. SEM images contained 1024 × 768 pixels, corresponding to a surface area of about 800 μm2. Selected images were analyzed to determine drop-size distributions. To accomplish this, Adobe Photoshop was first used to locate the edge of condensed droplets. An open source graphics analysis package (ImageJ) was subsequently used to determine the number and size distribution of droplets. Depending on the magnification, droplets with radii beneath about 0.02 μm were rejected from the image analysis. Contact angle measurements of liquid monomers on various substrates, including iCVD films, were made with a contact angle goiniometer (VCA Optima). Molecular weights of as-deposited films were determined using size exclusion chromatography (Viscotek TDA 301). For each sample, the film was dissolved from the substrate using THF (Sigma Aldrich). Polymer solutions (1 mg/mL) were prepared and filtered prior to injecting into the GPC system. The system columns (two Viscotek Mixed Bed Low MW i-series, and one mixed bed medium MW i-series column) were maintained at 60 °C. The software (OmniSEC, Viscotek) calculated the number-average and weight-average molecular weights. Attenuated total reflection Fourier transform infrared spectroscopy (Shimadzu 8000S) was used to confirm the presence of functional groups near the film surface. Spectra were acquired over the range of 400 to 4000 cm−1, averaged over 64 scans.

Figure 1. Schematic of the custom-built iCVD deposition chamber. a downstream throttle valve (MKS Instruments) together with a Baratron capacitance manometer (MKS Instruments). The base pressure of the system is 1 mTorr. As the chamber pressure is a main factor of molecules’ mean free path, the deposition pressure was maintained to be 500 mTorr. Flows of the monomer, GMA, and crosslinker, EGDA, were controlled via needle valves, and the thermal initiator, TBPO, was controlled with a mass flow controller (MKS Type 1479A). A filament array consisting of 14 parallel nickel− chromium (Omega) filaments was heated to about 250 °C by a DC power supply (Gw Instek SPS 606). The substrate temperature was controlled via backside cooling using a recirculating chiller/heater unit (Neslab RTE 740). A thin-wire thermocouple (type E, 0.002″ diameter) was used to measure the surface temperature at the substrate−gas interface. Depositions were conducted on Teflon FEP-coated, removable substrates (Bytac). These substrates provided good thermal contact and enabled substrate temperature to be controlled from 15 to 55 °C. Additional experiments on silicon wafers were conducted to verify that deposition rate depends primarily on substrate temperature and not on substrate type (see the Supporting Information). For a typical deposition, the filament was turned on, and the system was equilibrated at operating pressure for 1 h. Monomer and initiator were then fed into the reactor at 5.8 and 2.2 sccm, respectively. The monomer’s saturated vapor pressure Psat was measured over the range of experimental substrate temperatures, and Antoine coefficients were determined (see the Supporting Information). The monomer partial pressure Pm was determined by the gas feed composition. For multistep deposition experiments, the cross-linker and initiator were first fed at 2.0 and 6.0 sccm, respectively. After 10 min, the cross-linker feed was turned off and the monomer feed was turned on. Following all depositions, the chiller and mass flow controllers were turned off, and the reactor chamber was pumped down for an additional hour to remove unreacted monomers. Characterization of iCVD Films. Scanning electron microscopy (SEM) (Zeiss, AURIGA-CrossBeam) was used to observe surface morphology and to measure film thickness. Films were gently

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RESULTS AND DISCUSSION

iCVD Deposition. Films were deposited from glycidyl methacrylate (GMA) and t-butyl peroxide (TBPO) using the conditions listed in Table 1. The film’s composition was confirmed to be predominately poly(glycidyl methacrylate) (PGMA) using FTIR (see Supporting Information), and observations were highly consistent with prior studies pertaining to iCVD of PGMA.17 As-deposited films could be easily dissolved using solvents, such as tetrahydrofuran, indicating that deposited films are not cross-linked or highly branched. The measured molecular weight of deposited PGMA increased with increasing Pm/Psat, and the observed polydispersity index (PDI) is typical for conventional radical polymerization. Film growth rates are plotted against monomer saturation in Figure 2, and SEM micrographs of deposited films are shown in Figure 3. For depositions conducted beneath monomer saturation (Pm/Psat < 1), deposition rates agree well with a prior reports of iCVD-grown poly(glycidyl methacylate)17,18 and smooth films were observed. There, film growth is limited by monomer adsorption onto a growing polymer film.12,13 Experiments conducted at or beyond monomer saturation (Pm/ Psat > 1) showed significantly higher deposition rates, and the deposition rate increased nearly linearly with Pm/Psat. The maximum deposition rate observed was about 600 nm/min (Pm/Psat = 2.18). The net molar flux of monomer into the growing film is given by

Table 1. Reactor Conditions and Deposition Rates of iCVD Experimentsa substrate temperature [°C]

Pm/Psat

deposition rate [nm/min]

Mn [g/mol]

PDI

JM × 108 [mol/m2·s]

JI × 108 [mol/m2·s]

50.7 33.1 27.9 22.5 17.2

0.28 0.72 0.99 1.45 2.18

32 82 320 423 600

7000 80 000 210 000 290 000 420 000

1.5 1.5 1.6 1.7 2.4

302 774 3020 3992 5663

6.1 1.4 2.0 2.0 1.9

For all experiments, the monomer (GMA) and initiator (TBPO) flow rates were 5.8 and 2.2 sccm, respectively, and the chamber pressure was fixed at 0.5 Torr. a

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JI =

Figure 3. Cross-sectional SEM micrographs of vapor-deposited poly(glycidyl methacrylate) films: (a) Pm/Psat = 0.72, (b) Pm/Psat = 0.99, and (c) Pm/Psat = 1.45.

⎛ dh ⎞ ρf ⎜ ⎟ ⎝ dt ⎠ M m

(2)

where Mn is the number-average molecular weight of the deposited polymer. Taking the film density to be that of poly(glycidyl methacrylate) (ρf = 0.805 g/mL), both fluxes were calculated and are included in Table 1. The monomer flux increases with increasing monomer vapor activity; however, the initiator flux is far less variable and is nearly constant at supersaturated conditions. This is expected, since the same filament temperature and chamber pressure were used for each experiment. Thus, iCVD of GMA at supersaturated conditions is limited by monomer flux into the growing film. The film growth rate may also be limited by initiator flux;17 however, this is not the subject of the present study. All films grown under supersaturated conditions exhibited surface undulations. The period and amplitude of the undulations is greater at higher levels of supersaturation. Surface undulations observed at Pm/Psat >1 may be explained by monomer condensation followed by droplet coalescence and film growth.19,20 Depending on the wetting properties of a surface, vapors may condense to form an assembly of droplets (dropwise condensation), or vapors may condense into a uniform film (filmwise condensation). The contact angle of liquid glycidyl methacrylate on the Teflon-coated substrate was measured to be 52°, suggesting that dropwise condensation initially occurs. Since the amplitude of surface undulations is less than the thickness of the film, droplet nucleation and coalescence is believed to occur at early stages of film growth, and the majority of film growth involves filmwise condensation. For the case of complete wetting, Beysens and Knobler showed that the rate of filmwise condensation should be proportional to the difference in saturation pressures between the hot vapor phase and the cooled surface (i.e., proportional to Pm/Psat).19 The data in Figure 2 at high monomer activity qualitatively agree with this trend. Nucleation and Growth at Supersaturated Conditions. The evolution of surface morphology during the early stages of film growth was studied using SEM. Images were acquired on films grown for different times (5, 10, 15, and 20 min). Results are shown in Figure 4. Droplets are irregularly shaped and often deviate from ideal spherical caps. This may be due to pinning of the solid−liquid−vapor contact line or to vitrification of bulk PGMA as a polymer glass forms. In several cases, partially fused drops were observed that appear not to have completely coalesced (e.g., Figure 4a). A distribution of droplet sizes is apparent at each deposition time. The images suggest that droplets are coalescingthe size of large droplets increases while the number of droplets decreases with increasing deposition timeas one would expect for heterogeneous nucleation and growth. Figure 5 shows how the drop density depends on deposition time at the early stages of deposition for three different levels of monomer vapor supersaturation. The initial drop densities of ∼5 × 108 cm−2 for all samples are rather high compared to other reported values of liquid nucleation on cooled surfaces (104−108 cm−2).20−24 High drop densities are attributed to rapid polymerization of monomer within condensed liquid nuclei; polymerization kinetics will be discussed later. Polymerization stabilizes droplet nuclei by preventing evaporation of condensed monomer. The observed decay of drop density with deposition time is attributed to coalescence. For experiments conducted well above saturation, the drop-density decay

Figure 2. Poly(glycidyl methacrylate) deposition rate versus Pm/Psat. Errors in deposition rate correspond to the standard deviation of multiple micrometer measurements (n = 3). Error bars are not provided for SEM measurements on thinner films.

JM =

⎛ dh ⎞ ρf ⎜ ⎟ ⎝ dt ⎠ M n

(1)

where dh/dt is the average film growth rate, ρf is the film density, and Mm is the monomer molecular weight. The net molar flux of initiator into the film is similarly given by 16582

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Figure 4. SEM images of poly(glycidyl methacrylate) nucleation (Pm/Psat = 1.45) in iCVD; each figure is 5 μm × 5 μm: (a) 5 min, (b) 10 min, (c) 15 min, and (d) 20 min.

corresponds to newly formed droplets that have experienced fewer coalescence events than the population at larger radii. The distribution slightly shifts rightward with increasing deposition time, reflecting some coalescence. However, coalescence in this sample (Pm/Psat = 0.99) appeared limited, and the maximum drop size observed, ∼0.4 μm, was quite small. Films grown at higher levels of saturation (Pm/Psat = 1.19 and Pm/Psat = 1.45) exhibited more pronounced coalescence. There, the population of small droplets diminishes and nearly disappears as substantially larger droplets, with radii as high as 4.3 μm, form. Coalescence is further quantified by examining how drop-size varies with deposition time. Since volume is conserved during coalescence, droplets, modeled as spherical caps, are weighted by employing the volume-average drop radius:

Figure 5. Plot of drop density versus time during iCVD growth of PGMA films. Each data set corresponds to a different level of monomer saturation.

rV =

exhibited power-law behavior that agrees qualitatively with others.19,20,25 On the other hand, the experiment conducted near the saturation point (Pm/Psat = 0.99) showed a much slower drop-density decay, indicating significantly lower rates of monomer coalescence. Drop-size distributions, determined from image analysis, during early stages of deposition are shown in Figure 6. Distributions are plotted on a logarithmic scale to better observe the formation of large droplets. For experiments conducted near monomer saturation (Pm/Psat = 0.99), a bimodal distribution was observed. The peak at small radii

∑ Nri i 4 ∑ Nri i 3

(3)

Figure 7 shows a plot of rV against time. Experiments performed at higher levels of saturation show significantly faster droplet growth. For Pm/Psat = 1.45, droplets steadily grow at a rate of about 2 × 10−3 μm/s. Interestingly, this rate is about 3 orders of magnitude lower than the growth rate of condensed water droplets on solid surfaces.19,20 The sluggish droplet-growth rates observed here are mainly attributed to polymerization that occurs during drop growth. Polymerization increases the film’s viscosity, preventing coalescence. To a first order approximation, the polymerization 16583

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Figure 6. Time evolution of drop-radius distribution, plotted on a logarithmic scale, during iCVD growth of PGMA films: (a) Pm/Psat = 0.99; (b) Pm/Psat = 1.19; (c) Pm/Psat = 1.45.

rate can be estimated from the propagation rate constants for glycidyl methacrylate (EA = 21.9 kJ/mol, A = 4.41 × 106 L/ mol·s, kp (60 °C) = 1620 L/mol·s).26 Considering the arrival of a single activated radical initiator into a condensed liquid monomer film, then, about 1 s is required for the chain to grow to 104 g/mol. Thus, the time scale for polymerization is a few orders of magnitude smaller than that of droplet growth, and polymerization is likely influencing droplet coalescence and growth. Multistage Film Growth. Heterogeneous nucleation theory suggests that nucleation can be avoided altogether by deposition onto a wettable substrate. Furthermore, for deposition onto nonwettable substrates, the nucleation rate should increase with decreasing contact angle,22,27 leading to smoother surfaces. Multistage iCVD experiments were conducted to minimize droplet nucleation while maintaining rapid condensation of monomer to form continuous polymer films with high quality surface finish. The idea is to first convert

Figure 7. Plot of volume-average drops radius rV versus time during supersaturated iCVD growth of PGMA films.

Table 2. Reactor Conditions and Observed Deposition Rates for Two-Stage iCVD Experimentsa monomer

substrate temperature [°C]

Pm/Psat

stage time [min]

average deposition rate [nm/min]

a bb

GMA GMA

50.7 22.5

0.28 1.45

20 80

32 350

ac bb

EGDA GMA

50.7 22.5

0.27d 1.45

10 120

20 310

experiment

stage

1

b

2

a For all the experiments, the chamber pressure was fixed at 0.5 Torr. bThe monomer (GMA) and initiator (TBPO) flow rates were 5.8 and 2.2 sccm, respectively. cThe cross-linker (EGDA) and initiator (TBPO) were 2.0 and 6.0 sccm, respectively. dThe Psat of EGDA was estimated from the SIMPOL.1 group contribution method.28

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Figure 8. SEM images of iCVD films grown by multistage iCVD deposition: (a) arial and (b) cross-sectional views of supersaturated PGMA deposition on a smooth PGMA base layer (experiment 1); and (c) arial and (d) cross-sectional views of supersaturated PGMA deposition on a smooth polymerized PEGDA base layer (experiment 2).

surface (polymerized EGDA) results in filmwise condensation. Unreacted acrylate groups present on the polymerized EDGA surface are believed to react with adsorbed monomer to form chemical linkages between the two layers. To examine this possibility, a bilayer film was carefully removed, and ATR-FTIR spectra were obtained on both sides. Results are shown in Figure 9. Unreacted vinyl groups, indicated by absorption around 1630 cm−1, were detected on the polymerized EGDA side, but not on the PGMA side. The use of reactive surface vinyl groups to form covalent linkages with deposited material is similar to the surface modification of photolithographically patterned substrates demonstrated by Li et al.29 Chemical bond formation between the EGDA and PGMA layers should lower the interfacial surface energy, thereby promoting wetting and stabilizing the film from droplet nucleation. Moreover, condensed GMA monomer liquid may swell the PEGDA thin film, also lowering its surface energy. To test this possibility, a deposited PEGDA film was swollen with GMA monomer and wiped dry. The resulting contact angle was measured to be 5° which is considerably lower than the measurement (25.1°) prior to swelling. The growth of a smooth film is attributed to a combination of monomer swelling and chemical reaction across the interface.

the low energy Teflon-substrate to a higher energy one, followed by filmwise deposition at supersaturated conditions. Two multistage iCVD experiments are summarized in Table 2. In the first experiment, a thin, continuous PGMA film was grown at subsaturated conditions followed by PGMA deposition at supersaturated conditions. In a second experiment, a cross-linked film was grown from a bifunctional monomer, ethylene glycol diacrylate (EGDA), followed by PGMA deposition at supersaturated conditions. The contact angles of GMA on PGMA and polymerized EGDA were measured to be 11.7° and 25.1°; and both are significantly lower than the contact angle of GMA on Teflon (52°). Figure 8 shows SEM images of films taken following different stages of growth. For the experiment involving only GMA monomer, smooth films were obtained after initial growth at subsaturated conditions. However, subsequent deposition of GMA at supersaturated conditions resulted in heterogeneous condensation. The SEM image of this film’s cross-section shows an interface between the two deposition stages. The experiment involving a thin EGDA base coating showed high quality surface finish after each stage, and the interface between the two stages could not be discerned. Note, also from Table 2, the rate of filmwise growth of PGMA during experiment 2b is somewhat lower than the dropwise growth observed during experiment 1b. This is expected because the small drops (r < 10 μm), which can strongly influence the total condensation rate, contribute about 15 times more condensation per unit surface area than the large drops (r > 50 μm).20 The difference in surface finish between the two experiments is attributed, in part, to the presence of unreacted acrylate groups near the surface of the EGDA coating. Heterogeneous nucleation theory alone cannot explain why the more wettable surface (PGMA) leads to nucleation, and the less wettable

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CONCLUSIONS In summary, we have studied the morphology and growth kinetics of poly(glycidyl methacrylate) grown by iCVD from supersaturated monomer vapors. Film growth rates were found to increase with monomer partial pressure, and film growth rates as high as 600 nm/min were observed. Deposited material consists of high molecular weight polymer (>100 kDa), and the molecular weight increases with monomer vapor pressure. Deposited films grown over 1 h were continuous and exhibited 16585

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Laboratory for Laser Energetics. The authors also acknowledge Yuan Zhang for assistance in acquiring SEM images.

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(1) Alf, M. E.; Asatekin, A.; Barr, M. C.; Baxamusa, S. H.; Chelawat, H.; Ozaydin-Ince, G.; Petruczok, C. D.; Sreenivasan, R.; Tenhaeff, W. E.; Trujillo, N. J.; Vaddiraju, S.; Xu, J. J.; Gleason, K. K. Chemical Vapor Deposition of Conformal, Functional, and Responsive Polymer Films. Adv. Mater. 2010, 22, 1993−2027. (2) 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. (3) Lau, K. K. S.; Gleason, K. K. All-dry synthesis and coating of methacrylic acid copolymers for controlled release. Macromol. Biosci. 2007, 7, 429−434. (4) Baxamusa, S. H.; Montero, L.; Dubach, J. M.; Clark, H. A.; Borros, S.; Gleason, K. K. Protection of Sensors for Biological Applications by Photoinitiated Chemical Vapor Deposition of Hydrogel Thin Films. Biomacromolecules 2008, 9, 2857−2862. (5) Ma, M. L.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Superhydrophobic fabrics produced by electrospinning and chemical vapor deposition. Macromolecules 2005, 38, 9742−9748. (6) Ma, M. L.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.; Rutledge, G. C. Decorated electrospun fibers exhibiting superhydrophobicity. Adv. Mater. 2007, 19, 255−259. (7) Asatekin, A.; Gleason, K. K. Polymeric Nanopore Membranes for Hydrophobicity-Based Separations by Conformal Initiated Chemical Vapor Deposition. Nano Lett. 2011, 11, 677−686. (8) O’Shaughnessy, W. S.; Murthy, S. K.; Edell, D. J.; Gleason, K. K. Stable biopassive insulation synthesized by initiated chemical vapor deposition of poly(1,3,5-trivinyltrimethylcyclotrisiloxane). Biomacromolecules 2007, 8, 2564−2570. (9) Martin, T. P.; Kooi, S. E.; Chang, S. H.; Sedransk, K. L.; Gleason, K. K. Initiated chemical vapor deposition of antimicrobial polymer coatings. Biomaterials 2007, 28, 909−915. (10) Winther-Jensen, B.; West, K. Vapor-phase polymerization of 3,4ethylenedioxythiophene: A route to highly conducting polymer surface layers. Macromolecules 2004, 37, 4538−4543. (11) Im, S. G.; Gleason, K. K. Systematic control of the electrical conductivity of poly(3,4-ethylenedioxythiophene) via oxidative chemical vapor deposition. Macromolecules 2007, 40, 6552−6556. (12) Chen, X. C.; Anthamatten, M. Solvent-Assisted Dewetting during Chemical Vapor Deposition. Langmuir 2009, 25, 11555− 11562. (13) Green, Z. I.; Chen, X. C.; Papastrat, A. G.; Zou, L. J.; Anthamatten, M. Morphology of Vapor Deposited Polyimides Containing Copper Phthalocyanine. Chem. Vap. Deposition 2009, 15, 106−111. (14) Papastrat, A. G.; Chu, T.; Anthamatten, M. Monomer Crystallization During Vapor-Deposition Polymerization. Chem. Vap. Deposition 2011, 17, 141−148. (15) Lau, K. K. S.; Gleason, K. K. Initiated chemical vapor deposition (iCVD) of poly(alkyl acrylates): An experimental study. Macromolecules 2006, 39, 3688−3694. (16) Lau, K. K. S.; Gleason, K. K. Initiated chemical vapor deposition (iCVD) of poly(alkyl acrylates): A kinetic model. Macromolecules 2006, 39, 3695−3703. (17) Mao, Y.; Gleason, K. K. Hot filament chemical vapor deposition of poly(glycidyl methacrylate) thin films using tert-butyl peroxide as an initiator. Langmuir 2004, 20, 2484−2488. (18) Gupta, M.; Gleason, K. K. Large-scale initiated chemical vapor deposition of poly(glycidyl methacrylate) thin films. Thin Solid Films 2006, 515, 1579−1584. (19) Beysens, D.; Knobler, C. M. Growth of Breath Figures. Phys. Rev. Lett. 1986, 57, 1433−1436. (20) Leach, R. N.; Stevens, F.; Langford, S. C.; Dickinson, J. T. Dropwise condensation: Experiments and simulations of nucleation and growth of water drops in a cooling system. Langmuir 2006, 22, 8864−8872.

Figure 9. ATR-FTIR spectrum of supersaturated PGMA deposition on a smooth polymerized PEGDA base layer (experiment 2). (a) is the PGMA side, and (b) is the PEGDA side. The downward arrow indicates the presence of vinyl functional groups.

surface undulations in thickness. SEM image analysis of films grown over short deposition times (5−20 min) indicate that surface undulations form following nucleation, growth, and coalescence of droplets. Polymerization of monomer within condensed liquid nuclei leads to higher drop density and inhibits further droplet coalescence. This explains why surface undulations are observed after long deposition times, when the surface is completely covered. High deposition rates with uniform thickness and high quality surface finish were obtained by a multistage iCVD process involving a thin coating of a cross-linked EGDA network followed by deposition of glycidyl methacrylate at supersaturated conditions. Covalent bond formation between the two layers likely plays an important role in maintaining filmwise growth throughout the multistage iCVD process. The interplay between polymerization kinetics and surface morphology highlights new possibilities to thermodynamically trap nonequilibrium structures in films grown by iCVD.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: 585-273-5526. Fax: 585-273-1348. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding provided by National Science Foundation under Grant CBET-0828437. R.T. appreciates support from a Horton Fellowship administered through 16586

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(21) Twomey, S. Experimental Test of the Volmer Theory of Heterogeneous Nucleation. J. Chem. Phys. 1959, 30, 941−942. (22) Zhao, H.; Beysens, D. From Droplet Growth to Film Growth on a Heterogeneous Surface: Condensation Associated with a Wettability Gradient. Langmuir 1995, 11, 627−634. (23) Mu, C. F.; Pang, J. J.; Lu, Q. Y.; Liu, T. Q. Effects of surface topography of material on nucleation site density of dropwise condensation. Chem. Eng. Sci. 2008, 63, 874−880. (24) Chen, X. M.; Wu, J.; Ma, R. Y.; Hua, M.; Koratkar, N.; Yao, S. H.; Wang, Z. K. Nanograssed Micropyramidal Architectures for Continuous Dropwise Condensation. Adv. Funct. Mater. 2011, 21, 4617−4623. (25) Meakin, P. Droplet Deposition Growth and Coalescence. Rep. Prog. Phys. 1992, 55, 157−240. (26) Van Herk, A. M. Pulsed initiation polymerization as a means of obtaining propagation rate coefficients in free-radical polymerizations. II Review up to 2000. Macromol. Theory Simul. 2000, 9, 433−441. (27) Beysens, D. Dew nucleation and growth. C. R. Phys. 2006, 7, 1082−1100. (28) Pankow, J. F.; Asher, W. E. SIMPOL.1: a simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds. Atmos. Chem. Phys. 2008, 8, 2773−2796. (29) Li, L.; Driscoll, M.; Kumi, G.; Hernandez, R.; Gaskell, K. J.; Losert, W.; Fourkas, J. T. Binary and gray-scale patterning of chemical functionality on polymer films. J. Am. Chem. Soc. 2008, 130, 13512− 13513.

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