Graft Polymerization of Anti-Fouling PEO Surfaces by Liquid-Free

Aug 24, 2012 - XPS confirmed the graft polymerization of PEO to amine anchors on ... iCVD PEO surfaces were effective in minimizing nonspecific protei...
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Graft Polymerization of Anti-Fouling PEO Surfaces by Liquid-Free Initiated Chemical Vapor Deposition Ranjita K. Bose, Siamak Nejati, David R. Stufflet, and Kenneth K. S. Lau* Department of Chemical and Biological Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States ABSTRACT: Poly(ethylene oxide) (PEO), a biomedically important polymer, has been synthesized using a novel liquidfree initiated chemical vapor deposition (iCVD) via a ringopening cationic polymerization mechanism. In addition, a grafting scheme was used to graft polymerize PEO onto amine-terminated surfaces. FTIR, NMR, and XPS showed a stoichiometric match to linear PEO homopolymer. In addition, XPS confirmed the graft polymerization of PEO to amine anchors on silicon with a high graft density. XPS and GPC were used to measure independently the grafted polymer molecular weight that also showed a narrow weight distribution. The grafted iCVD PEO surfaces were effective in minimizing nonspecific protein adsorption using a fluorescently labeled bovine serum albumin assay.

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INTRODUCTION Poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG; low-molecular-weight PEO) is a technologically important polymer with many biomedical applications including tissue engineering,1,2 drug delivery,3,4 spatial patterning of cells,5 and nonfouling surfaces.6−8 Nonfouling surfaces are extremely important in a variety of biomedical devices where the prevention of undesired protein adsorption is critical to device performance. Examples of these applications are biomedical implants where protein adsorption leads to a cascade of events resulting in thrombus and scar tissue formation, ultimately resulting in device failure. One of the most common approaches to reduce protein adsorption on a surface is via PEO coatings.9 PEO is highly hydrophilic10 and has very low interfacial energies in aqueous environments,4 which are factors that make it resistant to protein adsorption.11,12 However, because PEO is water-soluble, it needs to be surface grafted to be useful in any applications involving aqueous conditions. Several approaches have been explored to immobilize PEO onto surfaces including physisorption,13 chemisorption,14 covalent grafting,15 and atom transfer radical polymerization.16 All of these approaches involve deposition of PEO onto the surface from a solution. In many micro- and nanoscale applications, solution-phase surface modification is not appropriate where the solvent may clog pores or cause undesired nonuniform wetting, surface tension effects, damage to fragile substrates, and swelling or shrinking of the substrates.17,18 The preparation of these nonfouling surfaces can be generally classified as “grafting to” and “grafting from” approaches.19 The “grafting to” strategy involves the attachment of prefabricated polymers either via physisorption20 or chemisorption8 and is experimentally straightforward. However, it is difficult to achieve thick and dense polymer brushes due to steric repulsions.21 Additionally, with increased molecular weight, © XXXX American Chemical Society

the reaction between the polymer end group and the functional group on the substrate becomes less efficient.22 The “grafting from” approach results in high polymer densities or large layer thicknesses. However, this method typically requires highly controlled experimental conditions16 and specialized synthesis knowledge involving multistep synthesis of complex precursors and cannot always be applied to large surfaces.19 Therefore, there is a need for an approach combining the high graft densities and molecular weights of the “grafting from” approach with the straightforwardness and elegance of the “grafting to” method. In this work, we present a novel approach known as initiated chemical vapor deposition (iCVD) to simultaneously enable the surface polymerization and grafting of PEO onto surfaces in one single step. This provides a unique “grafting from” approach that enables the anchored growth of polymers through surface immobilized functional groups. iCVD combines a surface polymerization reaction with a vacuumbased chemical vapor deposition (CVD) environment.23,24 iCVD proceeds via a three-step mechanism: first, a polymerization initiator is activated in the gas phase by a resistively heated filament array; second, monomer units and the activated initiator species adsorb onto a cooled substrate; third, the adsorbed activated initiator serve as sites for attaching multiple monomer units to achieve long chain polymers on the surface. iCVD proceeding via a free radical polymerization mechanism has been extensively studied for monomers having vinyl groups.25−27 Previous work on vinyl monomers such as acrylates and methacrylates has found good parallels between the free radical polymerization kinetics of iCVD and conventional liquid-based synthesis.28 It is this similarity that has Received: June 18, 2012 Revised: August 8, 2012

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Table 1. Reaction Conditions for Studying the Effect of Reactant Availabilities on Deposition Kinetics by Varying (1) Monomer Flow Rate, (2) Initiator Flow Rate, and (3) Substrate Temperaturea iCVD parameter

FM (sccm)

FI (sccm)

FN (sccm)

TS (°C)

PR (torr)

PM/PM,sat

PI/PI,sat

1.

monomer concentration

2.

initiator concentration

3.

substrate temperature

10 9.5 9.0 8.5 10 10 10 10 10 10 10

0.5 0.5 0.5 0.5 0.5 0.25 0.5 0.5 0.5 0.5 0.5

0 0.5 1.0 1.5 0.5 0.75 0 0 0 0 0

−4.3 −4.3 −4.3 −4.3 −4.3 −4.3 −4.3 −1.7 2.2 6.4 10.9

10 10 10 10 10 10 10 10 10 10 10

0.024 0.023 0.022 0.020 0.023 0.023 0.024 0.021 0.018 0.015 0.013

0.71 0.71 0.71 0.71 0.45 0.23 0.71 0.58 0.43 0.32 0.23

FM = flow rate of monomer, FI = flow rate of initiator, FN = flow rate of nitrogen, TS = substrate temperature, PR = reactor pressure. Filament temperature TF was maintained at 350 °C. a

applications in creating nonbiofouling surfaces that are invisible to cells,12,13 increasing blood circulation time,20 guiding cell growth,39 and stabilizing amphiphilic drug and gene delivery agents in water.40

enabled iCVD to utilize the extensive body of knowledge on solution-based polymerization for the heterogeneous synthesis of polymers on surfaces from vapor-phase precursors. Here we report for the first time the use of iCVD in applying a new polymer chemistry and mechanism via the ring-opening cationic polymerization of ethylene oxide with a boron trifluoride complex initiator to synthesize PEO thin films in a solvent-free environment. From literature on solution synthesis of PEO, it is well known that cationic initiators can induce the ring-opening polymerization of ethylene oxide29 and because boron trifluoride compounds are widely used as cationic initiators in conventional solution-based polymerizations,30,31 boron trifluoride-diethyl etherate was chosen as an appropriate initiator for the iCVD synthesis of PEO. Furthermore, because BF3 is a weak Lewis acid, it is expected to initiate polymerization by accepting a free electron pair from the oxygen on the ethylene oxide as well as enable covalent grafting by reacting with primary amines on functionalized surfaces. There are reports of PEO synthesis that have explored vaporbased techniques such as the plasma polymerization of PEO using oligo(ethylene glycol) precursors.32 However, polymers synthesized using plasma processes are known to contain uncontrolled chain cross-linking and dangling bonds and show a loss of chemical functionality due to the presence of UV radiation, ion bombardment, and high-energy electrons.33,34 In previous work with plasma-synthesized PEO, extensive crosslinking could not be avoided.35 There are also reports of the vapor deposition polymerization of ethylene oxide in the presence of BF3.36 However, the role of BF3 as a cationic ringopening polymerization initiator was not discussed. Furthermore, an in-depth study of the kinetics and mechanism of the surface polymerization was not carried out. iCVD with its thermally activated pathways can be carried out under much lower energy conditions and is much more selective, helping to retain full chemical functionality in the resultant polymers.37,38 Furthermore, as will be shown here, thermal activation of BF3 results in faster film growth compared with previous reports of vapor deposition of PEO.36 Additionally, the development of a mechanistic model to describe the kinetic behavior of PEO synthesis during iCVD allows us to systematically control growth rate and polymer molecular weight. Therefore, this work presents an important pathway for the synthesis and surface grafting of PEO by iCVD that addresses many of the challenges associated with current methods. In this way, we can obtain grafted PEO that has wide-ranging

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MATERIALS AND METHODS

PEO Synthesis. For the iCVD synthesis of PEO, ethylene oxide gas (EO; AmSpec Gases) and boron trifluoride-diethyl etherate (BF3dee; Alfa Aesar) were used as the monomer and initiator, respectively. The stable diethyl etherate complex of BF3 was employed because it is comparatively safer to handle than BF3 gas itself. iCVD was performed in a custom-designed CVD flow reactor.27,41 Typical iCVD conditions made use of 10 and 0.5 sccm flow rates of EO and BF3-dee, respectively. Reactor pressure was maintained at 10 Torr using a downstream throttle valve and pressure controller (MKS Instruments) together with a dry vacuum pump (iH80, Edwards Vacuum). From preliminary experiments, we found that the choice of filament material is important for PEO polymerization, with phosphor bronze being the most effective for activating the initiator. The phosphor bronze filament wire array was heated to 350 °C using a Sorensen DLM 60-10 DC power supply. A 50:50 mixture of ethylene glycol and water set at −15 °C using a recirculating chiller (Thermo Scientific) was used for cooling the substrate. At this coolant temperature and with heating from the heated filament during iCVD, the actual substrate surface temperature was predetermined to be −4.3 °C using a thermocouple attached to a silicon wafer. The deposition rate (polymer growth rate) was monitored in situ using a laser interferometry system equipped with a 633 nm HeNe laser (JDS Uniphase) passing through a glass window positioned on top of the reactor. Kinetics of PEO iCVD. A series of iCVD runs were systematically carried out to explore the iCVD synthesis space and understand the effect of iCVD process parameters on reaction behavior. We hypothesized that the concentration of the monomer and initiator at the surface governs polymerization kinetics, directly controlling deposition rate and kinetic chain length. In gas-to-solid adsorption, surface concentration or availability of a reactant species can be related to the ratio of the partial pressure of the reactant in the gas phase to its saturation vapor pressure at the substrate temperature in the Henry’s law limit (Pr/Pr,sat).42 Therefore, Pr/Pr,sat is analogous to the reactant concentration in solution polymerization. As shown in Table 1, a set of experiments was first performed to study the effect of monomer surface availability. This was done by changing monomer flow rate and adding a patch flow of inert nitrogen to keep the total flow rate and residence time constant with all other iCVD variables remaining the same. A second set of experiments was then performed to study the effect of initiator surface availability by varying the initiator flow rate while again adding a nitrogen patch flow to keep the total flow rate and residence time constant, all other iCVD conditions remaining unchanged. Finally, a third set of experiments was carried out in B

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created had 200 μm wide lines with 200 μm spacing between consecutive rows. All of the amine-treated substrates were then rinsed in ethanol, followed by APTMS cross-linking at 100 °C for 4 h. Because primary amines are known to conjugate easily with epoxy moieties, EO was directly grafted and polymerized by iCVD onto the amine surfaces using typical deposition conditions described above. Any superficial, ungrafted polymer that may have been formed was removed by sonicating the iCVD coated substrates in DI water for 30 s prior to XPS measurements. XPS spectra were collected on the grafted films, as described for the bulk PEO films. Protein Adsorption. The PEO-grafted surfaces were evaluated for its resistance to nonspecific protein binding. Fluorescein isothiocyanate-labeled bovine serum albumin (FITC-BSA, Sigma) was dissolved in 10 mM phosphate buffer saline (PBS) solution (pH 7.4) at a concentration of 250 μg/mL. A few drops of the protein solution were evenly distributed onto the substrates and stored at room temperature for 30 min based on an established protocol.46,47 The substrates were then rinsed thoroughly with PBS solution and water, blown dry in a stream of nitrogen, and directly analyzed under a fluorescence microscope (Zeiss Axioskop 2) using a 495 nm filter. Fluorescence intensities were quantified using ImageJ software (NIH, Bethesda, MD) as described previously,48,49 and the average intensity of five different samples was taken to quantify the amount of protein adsorbed. Control surfaces of bare silicon, amine-terminated silicon, and hydroxyl-terminated silicon were used as comparisons. The hydroxyl-covered surfaces were created by treating the silicon wafers in a piranha solution and then rinsing in DI water.

which the substrate temperature was changed that resulted in a change in the monomer as well as initiator surface availabilities because this altered the vapor pressures of both the monomer and initiator. For each run, polymerization kinetics was captured by measuring polymer deposition rate and the resulting polymer molecular weight. Deposition rate was calculated from knowing the film thickness deposited over the total run time. Film thickness was determined by using a ZygoNewView 6000 optical profilometer with a 10× objective. In addition, real-time deposition was monitored using the reactor laser interferometry system, which served as a secondary means in estimating film thickness. Polymer molecular weight was determined by gel permeation chromatography (GPC) in a Waters column (Styragel HR 4) with a refractive index detector (Waters 2414) using tetrahydrofuran (THF; Aldrich) as the eluting solvent and a set of narrow polystyrene standards for calibration. Polymer Characterization. The chemical composition and structure of iCVD PEO films were studied by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (NMR) spectroscopy. FTIR spectra were acquired on a Nicolet 6700 spectrometer in transmission mode using a DTGS detector over the range of 450−4000 cm−1 at a resolution of 4 cm−1 and averaged over 32 scans. XPS was performed on a Physical Electronics PHI 5000 VersaProbe with a scanning monochromatic source from an Al anode and with dual beam charge neutralization. Survey XPS spectra were acquired at 100 W with a pass energy of 117 eV over the range of 0−1100 eV at a resolution of 0.5 eV and a dwell time of 50 ms and averaged over five scans. Highresolution XPS spectra of C1s, N1s, Si2p, and O1s core electrons were acquired at 100 W with a pass energy of 11.75 eV using different acquisition times chosen based on the observed intensities of the elements from the surveys to obtain similar signal-to-noise ratio. 1H NMR spectra were obtained on a 500 MHz Varian Unity INOVA system with a detection limit of 1 μmol per mL of solvent. For NMR analysis, 1 to 2 mg of iCVD PEO films was dissolved off silicon wafer substrates in 0.9 mL of chloroform-D (99.9 atom % D; Aldrich), and the spectra were obtained at 25 °C for eight scans with an optimized pulse width of 16.5 μs and a recycle delay of 5 s. Differential scanning calorimetry (DSC) was performed on a TA Instruments Q2000 series calorimeter to determine the thermal transitions of iCVD PEO films. 10 mg of sample hermetically sealed in an Al pan was heated at a ramp rate of 10 K/min in nitrogen. Film samples for all of the experiments were deposited on 100 mm single-crystalline silicon wafers. A PEO standard (Mn = 10 000 g/mol; Aldrich) was used for comparison with the iCVD PEO films. Contact angle measurements were carried out on a KSV Instruments CAM 200 goniometer using 20 μL drops of distilled water. Surface Grafting. To achieve ring-opening graft polymerization of ethylene oxide onto substrate surfaces using our iCVD chemistry, we first amine-terminated the surfaces were to provide the grafting point for starting PEO polymerization. The amine functionalization was carried out using aminopropyltrimethoxysilane (APTMS; Alfa Aesar) according to an established protocol.43 In brief, silicon or glass substrates were treated with a piranha solution (3:1 concentrated sulfuric acid:30% hydrogen peroxide) and rinsed in deionized (DI) water prior to soaking in a 5% APTMS solution in 1,4-dioxane for 5 min. A cross-linking step at 100 °C for 4 h was followed by an ethanol rinse. In addition to fully amine-terminated surfaces, a microcontact printing protocol44,45 was used to create patterned regions of amine functionality. In brief, a negative photoresist (SU-8 2000; Microchem) and transparency sheets printed with the desired patterns were used to create masters of the pattern. A PDMS polymer base and a curing agent were mixed together (Silastic T-2; Dow), molded onto the master, and cured at room temperature to obtain PDMS stamps with the relief pattern. After curing, the PDMS stamps were demolded and exposed to 5 min of oxygen plasma at 15 W and 300 mtorr to render the surfaces hydrophilic. A 1% solution of APTMS in DI water was then used as the “ink”, and 100 μL of the ink solution was placed on these PDMS stamps. After excess ink was removed using a nitrogen gas stream, the stamps were placed on the substrates and gently pressed using a pair of forceps. The amine-functionalized patterns

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RESULTS AND DISCUSSION PEO Synthesis. Preliminary experiments were performed using various initiator systems such as diethyl zinc, aluminum isopropoxide, trifluoromethanesulfonic anhydride, and stannous chloride, which are commonly used in cationic solution polymerization.50 Because of insufficient vapor pressure leading to low initiator flow rates, and likely degradation on contact with the heated filament, these initiators did not translate well to the vapor deposition process. BF3 conjugated with tetrahydrofuran, diethyl ether, and butyl methyl ether were all successful, and BF3-diethyl etherate was chosen due to its suitable vapor pressure and stability. Typically for iCVD, substrates are maintained at room temperature. In this case, because ethylene oxide monomer is highly volatile, substrate temperatures below 0 °C were used to enhance surface monomer availability. PEO Characterization. As shown in Figure 1, the FTIR spectrum of iCVD PEO shows a sharp peak centered at 1150

Figure 1. FTIR spectra of (a) solvent-cast PEO standard (Mn = 10,000 g/mol) and (b) PEO by iCVD. The sharp peak centered at 1150 cm−1 shows stretching of ether groups and stretching modes of alkyl groups are seen from 2850−3000 cm−1. C

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cm−1, which is associated with stretching of ether groups while characteristic alkyl (R−CH2) stretching modes from 2850− 3000 cm−1 were observed.51−53 Figure 1 also compares the iCVD PEO to a commercially available standard PEO, which shows good agreement. In previously reported FTIR spectra of plasma-polymerized PEO-like films, the polymers were shown to have a higher C/O ratio than theoretical values.54,55 The proton NMR spectrum of iCVD PEO, as given in Figure 2, shows a proton peak at a shift of 3.6 ppm, which has been

Figure 4. High-resolution XPS spectra of iCVD PEO. The (a) carbon C1s and (b) oxygen O1s spectra suggest that one dominant environment is present that is indicative of linear PEO. The minor peaks are attributed to adsorbed surface contaminants. Figure 2. 1H NMR spectra of (a) PEO standard (Mn = 10 000 g/mol) and (b) iCVD PEO. The CH2O peak marked by * at a shift of 3.6 ppm shows a good match between iCVD and standard PEO.

attributed to atmospheric carbon impurities, and it is observed in the standard PEO XPS signal (not shown) and has been reported before for PEG coupled to a silicon surface.58 The very insignificant peak at ∼289 eV which is also observed on both bare silicon and amine functionalized substrates without PEO, can be attributed to CO2 complexed to the surface.59 The O1s signal as shown in Figure 4b is centered at 532.6 eV, which is assigned to the oxygen bonded to the carbon in the repeat unit of PEO.57 FTIR, NMR, and XPS confirm that iCVD PEO polymer retains full chemical functionality. These spectroscopic results therefore suggest that iCVD is a suitable vapor-based technique to synthesize stoichiometric PEO similar to that made via conventional solution-based methods. PEO is known to be a semicrystalline polymer in which the degree of crystallinity depends on the method of synthesis and processing.60 Figure 5 shows the DSC trace of PEO having a melting temperature of 50 °C, which is similar to that previously reported for standard PEO.56 On the basis of the

assigned to the hydrogens of CH2O in the PEO polymer chain (denoted by *). This compares well with the standard PEO spectrum. Because no end groups are observed, it is likely that sufficiently long polymer chains are formed. PEO is a hygroscopic polymer that swells in the presence of water and tends to retain the absorbed water in its hydrogel network,56 resulting in the water peak observed in both the standard and iCVD PEO spectra. The XPS survey of the deposited iCVD PEO film on silicon as seen in Figure 3 shows only two characteristic peaks

Figure 3. XPS survey of iCVD PEO deposited on silicon. The ratio of carbon to oxygen of ∼2 is in agreement with the stoichiometry of linear PEO homopolymer.

attributed to carbon and oxygen. The atomic ratio of C to O is ∼2.05, which is close to the theoretical value for linear −(CH2CH2O)n− PEO polymer. The high-resolution carbon signal shows a main carbon environment with two very small shoulders, as seen in Figure 4a. The main peak located at 286.4 eV is attributed to the C−O species, which has been reported to be in the same range for poly(ethylene glycol) cast from chloroform.57 The very small shoulder centered at 285 eV is

Figure 5. DSC curve of iCVD PEO shows crystallization and melting peaks at 24 and 50 °C, respectively. D

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vacuum in the absence of impurities and moisture, it is likely that conditions are favorable for self-initiation. On the same note, chain-transfer reactions, which are otherwise likely in cationic polymerizations, are minimal due to the vacuum environment. The initiation reactions most likely occur in the gas phase with the aid of thermal heating from the filaments. Subsequent adsorption of the initiated groups lead to propagating species with the addition of more monomer units resulting in long polymer chains. Termination then occurs by reacting with another initiator molecule with the evolution of BF3 and F2. The presence of the additional initiator molecule for termination is believed to be necessary to disrupt the active ionic propagating chain end. Additionally, because of the much lower volatility of the BF3-dee initiator compared with the ethylene oxide monomer, surface availability of the initiator is typically an order of magnitude higher than the monomer. (See Table 1.) Therefore, there is a greater likelihood that the initiator would also be involved with termination as well. The role of BF3 in the proposed mechanism is supported by the presence of boron and fluorine in the PEO XPS survey spectra (not shown). On the basis of the proposed reaction scheme, the rate of polymerization (Rp) and the kinetic chain length (Xn) can be derived by applying the pseudo-steady-state approximation. This leads to Rp being first-order with respect to the initiator and second-order with respect to the monomer and Xn being related directly to the ratio of monomer-toinitiator concentration. Equivalently, this means that the iCVD polymer deposition rate should be first-order with respect to PI/PI,sat and second-order with respect to PM/PM,sat and that the polymer molecular weight should be related directly to the ratio of PM/PM,sat to PI/PI,sat. Figure 7a,b plots out these relationships, and the goodness of the fits strongly support the validity of the proposed reaction mechanism and kinetic model. Grafted PEO. To measure the graft chain length, the grafted PEO chains were first detached from the substrate prior to GPC analysis by hydrolyzing the Si−O bond on the wafer with 0.1 M sulfuric acid for 10 min, as described elsewhere.64 To establish the proper calibration curve of PEO molecular weight with elution time, we added 0.1 M aqueous H2SO4 to PEO standards in THF to ensure similar ionic conditions of the calibration standards during GPC measurements as the detached iCVD samples. On the basis of this calibration, the grafted iCVD PEO has a molecular weight Mn of 2425 g/mol and a polydispersity of 1.3. This suggests that iCVD graft polymerization produces narrowly distributed polymer of sufficient grafting chain length. Additionally, a separate GPC calibration was made to relate the GPC peak area to the solute concentration. On the basis of the total mass of grafted PEO and the surface area of the silicon wafers, we estimate the grafting density of PEO to be 1308 ng/cm2. Using Mn = 2425 g/mol from GPC, the grafting density can be expressed as 539.3 pmol/cm2. On the basis of the idealized helical PEO chain having a cross-sectional area of 21.3 Å2,65 the theoretical maximum grafting density of PEO is 780 pmol/cm2.65−67 Thus based on this maxima, in this work we achieve a grafting density of 69.2% for Mn = 2425 g/mol PEO chains. A previous work by Sofia et al. using a “grafting to” approach experimentally achieved a maximum value of 100 ± 10 ng/cm2 for PEO (Mn = 3400 g/mol) on silicon,59 which translates to a grafting density of 2%. Another study by Papra et al. reports 92% PEO grafting for Mn = 300, 60% PEO grafting for Mn = 1000, and 35% PEO grafting for Mn = 2000 g/mol,67 which are comparable to other reported values as well.54,68 Therefore, compared with previous

integrated area of the melting transition and the enthalpy of fusion value of fully crystalline PEO from literature, the crystallinity of iCVD PEO is estimated to be 56%. This is within the range of reported degrees of crystallinity of PEO from 23 to 89%.60 Kinetics of Deposition. Pr/Pr,sat is a measure of the surface availability of the reacting species and is analogous to the concentration of reactants in liquid-phase polymerization. To vary the surface availability of the reactant, we employed two strategies: changing the reactant flow rates, which will change Pr, and changing the substrate temperature, which will alter Pr,sat. Similar experiments reported previously have been performed to study iCVD reaction kinetics in free radical initiated polymerization of acrylates.28 Here we have applied similar studies to investigate the cationic polymerization of PEO. The first and second series of experiments were carried out by changing the monomer and initiator flow rates, respectively, whereas the third set of experiments was performed with different substrate temperatures. Table 1 shows detailed deposition conditions for each run. It is worthwhile to note here that compared with previous vapor deposition reports of PEO with BF3 using no thermal activation36 iCVD yields faster deposition rates for PEO films while retaining full chemical functionality, as seen in the FTIR and NMR spectra. To understand the kinetic behavior with variations in monomer and initiator concentrations, we propose the following reaction mechanism to describe the cationic polymerization of PEO, as shown in Figure 6. The mechanism suggests

Figure 6. Proposed reaction mechanism for iCVD of PEO. BF3 undergoes self-initiation, followed by propagation with the addition of monomer before termination with additional BF3 initiator that is present due to the lower volatility of the initiator.

a two-step reaction involving the self-initiation of BF3. For liquid-phase polymerization, there have been previous reports of BF3-diethyl etherate being used as an initiator61 and of Lewis acids acting as self-initiators.62,63 Here it is proposed that two molecules of BF3 from BF3-dee undergo bimolecular ionization along with reaction with a monomer moiety. Most of the previous evidence to support self-ionization process is indirect, but there is a consensus that it is achieved in systems that are pure and moisture free.50 Because iCVD occurs under high E

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Figure 8. High-resolution XPS spectra of (a) C1s and (b) N1s for grafted PEO on amine-terminated substrates. The insets show the high-resolution signal before iCVD graft polymerization.

from the amine graft anchors. The highest binding energy peak located at 401.6 eV observed on surfaces both before and after grafting is assigned to protonated amine groups due to the possible hydrogen bonding of the amine groups.71 With the XPS information, an independent estimate of the molecular weight of the iCVD grafted PEO can be made. This was done by determining the number of grafted PEO carbons at each amine anchor site by taking the ratio of the C−O peak area in the C1s spectrum to the C−N peak area in the N1s spectrum, after accounting for the elemental sensitivity factors and subtracting out the small C−N contribution that is within the C−O peak. From this ratio, a molecular weight of the grafted PEO was derived to be 2441 g/mol, which is in remarkably good agreement with the value obtained independently through GPC (2425 g/mol). Protein Adsorption. The PEO grafted substrates were used to study resistance to a FITC-BSA protein based on protocols described elsewhere.47,71 Fluorescence micrographs in Figure 9 show a strong FITC signal from the bare silicon as well as amine- and hydroxyl-covered substrates. For the substrates patterned with APTMS, there is no noticeable difference in the APTMS-stamped regions containing amines and the bare regions containing hydroxyl surface groups. A previous study on BSA adsorption for different surface functionalities reported similar levels of adsorption for amine and hydroxyl groups.72 For the fully PEO grafted substrates, a distinct lack of fluorescence is observed, whereas with the patterned substrates where PEO selectively grafted to the amine regions, fluorescence is observed only in the PEO-free regions. More quantitatively, the fluorescence intensity can be related to the amount of protein adsorption, normalized to bare silicon, and based on averaging the signal intensity over five samples, Figure 10 shows that PEO surfaces are able to reduce significantly nonspecific protein adsorption compared with monolayer-functionalized surfaces. This can be attributed to the

Figure 7. Effect of monomer and initiator concentration on (a) PEO deposition rate and (b) PEO kinetic chain length. In both cases, the data show a good fit to a straight line (R2 = 0.997 and 0.988) that suggests that the iCVD polymerization of PEO is similar to that of liquid-phase ring-opening polymerization and follows the proposed mechanism in Figure 6.

reports where there is a trade-off between % grafting and chain length, iCVD graft polymerization yields a high grafting density of 69% for PEO chains with Mn = 2425 g/mol. To the best of our knowledge, there have been no reports of measuring molecular weight and grafting density using “grafting from” techniques. We have further confirmed the grafting of PEO on the amine-terminated substrates by comparing XPS spectra of the substrate before and after iCVD graft polymerization. The spectra were collected on sonicated and rinsed wafers to ensure removal of any ungrafted PEO chains and were corrected to the proper binding energy scale by setting the observable silicon substrate peak to be 99.3 eV reported for Si.69 As shown in Figure 8a, the high-resolution C1s spectrum has a distinct peak at 286.45 eV, which in comparison with the same peak from the APTMS-functionalized surface (inset) indicates that the iCVD process significantly increased the amount of C−O as a result of the PEO polymer. The presence of this strong C−O peak is concomitant with the appearance of a new nitrogen peak in the N1s spectrum, as shown in Figure 8b. Comparison of the APTMS substrate before (inset) and after iCVD grafting clearly shows that the free amine of APTMS at 399.6 eV70 is significantly reduced while a new peak at 400.6 eV is formed. This new peak can be attributed to C−N moieties, which have been reported to be in the range of 400.3 to 400.7 eV,70 and provides evidence for the covalent graft attachment of PEO F

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CONCLUSIONS PEO, a biomedically important polymer, has been successfully synthesized using novel liquid-free iCVD via a ring-opening cationic polymerization mechanism that was supported by a detailed study of deposition kinetics. In addition, a grafting scheme involving the use of silanized amine surfaces was successful in achieving the iCVD graft polymerization of PEO. FTIR, NMR, and XPS showed a stoichiometric match to linear PEO homopolymer. In addition, XPS confirmed the graft polymerization of PEO through the amine anchors that yielded a high grafting density. XPS and GPC independently corroborated the grafted polymer molecular weight that also showed a narrow distribution. The grafted iCVD PEO surfaces were effective in minimizing nonspecific protein adsorption. As a further extension to the grafting technique, microcontact printing was successful in enabling selective PEO grafting on amine-patterned regions and selective suppression of protein adsorption in only the PEO-grafted regions.

9. FITC-BSA adsorption on bare silicon, amine-terminated hydroxyl-terminated silicon, amine-patterned silicon, fully iCVD PEO on amine-terminated silicon, and iCVD PEO on amine patterned silicon. Scale bar: 50 μm, inset: 500 μm.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by an NSF CAREER Award (CBET-0846245). In addition, the XPS instrumentation system was made possible through an NSF MRI Award (CBET0959361).

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REFERENCES

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Figure 10. Protein adsorption of iCVD-grafted PEO compared with other hydrophilic surfaces. iCVD PEO films show good protein resistance compared with controls of bare silicon as well as amine- and hydroxyl-terminated surfaces. The values are measured from fluorescent intensities of BSA-FITC using ImageJ and normalized to bare silicon.

greater wettability and hydrophilicity that has been found to be inversely related to the level of protein adsorption.73 This is in good agreement with water contact-angle experiments that showed PEG grafted films (13°) were more hydrophilic than bare silicon (38.5°) or amine-functionalized (30°) surfaces. Additionally, there are reports of PEO polymer brushes offering higher protein resistance for longer polymer chains74 because the longer polymer chains exclude proteins from the surface more effectively. Furthermore, an increase in grafting density is also shown to reduce protein adsorption.74 Because iCVD is capable of producing long stoichiometric PEO chains with a high grafting density using a grafting from approach, it is expected to be a valuable solvent free technique for PEO synthesis. The resulting protein-resistant surfaces are foreseen to be particularly useful in applications involving substrates that are solvent sensitive or require low-temperature processing. G

dx.doi.org/10.1021/ma301234z | Macromolecules XXXX, XXX, XXX−XXX

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