Manipulating Interfaces through Surface Confinement of Poly(glycidyl

Article pubs.acs.org/Macromolecules

Manipulating Interfaces through Surface Confinement of Poly(glycidyl methacrylate)-block-poly(vinyldimethylazlactone), a Dually Reactive Block Copolymer Bradley S. Lokitz,†,* Jifeng Wei,‡ Juan Pablo Hinestrosa,† Ilia Ivanov,† James F. Browning,§ John F. Ankner,§ S. Michael Kilbey, II,†,⊥ and Jamie M. Messman†,* †

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, One Bethel Valley Road, Oak Ridge, Tennessee 37831, United States ‡ Department of Chemistry, Grinnell College, Grinnell, Iowa 50112, United States § Spallation Neutron Source, Oak Ridge National Laboratory, One Bethel Valley Road, Oak Ridge, Tennessee 37831, United States ⊥ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: The assembly of dually reactive, well-defined diblock copolymers incorporating the chemoselective/functional monomer, 4,4-dimethyl-2-vinylazlactone (VDMA) and the surface-reactive monomer glycidyl methacrylate (GMA) is examined to understand how competition between surface attachment and microphase segregation influences interfacial structure. Reaction of the PGMA block with surface hydroxyl groups not only anchors the copolymer to the surface, but limits chain mobility, creating brush-like structures comprising PVDMA blocks, which contain reactive azlactone groups. The block copolymers are spin coated at various solution concentrations and annealed at elevated temperature to optimize film deposition to achieve a molecularly uniform layer. The thickness and structure of the polymer thin films are investigated by ellipsometry, infrared spectroscopy, and neutron reflectometry. The results show that deposition of PGMA-b-PVDMA provides a useful route to control film thickness while preserving azlactone groups that can be further modified with biotin−poly(ethylene glycol)amine to generate designer surfaces. The method described herein offers guidance for creating highly functional surfaces, films, or coatings through the use of dually reactive block copolymers and postpolymerization modification.

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INTRODUCTION

bioinspired applications, it is our intention to focus mainly on monomers containing active esters that can be polymerized directly.7−14 Furthermore, controlled radical polymerizations (viz. atom transfer radical polymerization, ATRP; nitroxidemediated polymerization, NMP; and reversible addition− fragmentation chain transfer, RAFT) of active ester-containing monomers allows one to tailor the materials and to synthesize block copolymers.10,15−21 In general, the modification of active ester polymers has been investigated in bulk and in solution, while relatively few recent reports describe their confinement on surfaces and in situ functionalization. For instance, Murata et al. have demonstrated the surface initiated polymerization and subsequent modification of N-methacryloyl-β-alanine succinimide ester (MAC2AE) using surface-bound 2,2′-azobis(2methylpropionitirile) initiator followed by reaction with various functional amines.22 Similarly, Schuh and Rühe have evaluated the penetration depth and reactivity of amine-end function-

The ability of block copolymers (BCPs) to spontaneously organize into microphase segregated structures in solution and in thin films is the origin of many useful and potential applications such as nanoporous membranes, nanoscale templates for catalyst formation, memory devices or integrated circuits, vehicles for intravascular delivery of drug molecules, and antireflective coatings.1−4 The self-assembly of BCPs is affected by several parameters, including the volume fraction and interaction parameters of the constituents.5 The ability to manipulate the nature of the blocks and the respective volume fractions through chemistry, and inter-/intramolecular interactions allows one to devise strategies to control or alter morphology not only in the bulk, but also in confined geometries such as thin films. Processing BCPs using techniques such as spin coating, dip coating, or inkjet printing from solution provides a means to fabricate useful materials.1 Reactive polymers contain functional groups that can be readily modified such as epoxides, aldehydes, anhydrides, or active esters.6 While there are many examples of polymers containing active ester groups, which are popular for © 2012 American Chemical Society

Received: May 17, 2012 Revised: July 30, 2012 Published: August 9, 2012 6438

dx.doi.org/10.1021/ma300991p | Macromolecules 2012, 45, 6438−6449

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ponsive nanoparticles based on core-cross-linked VDMAcontaining BCPs, which have strong potential as theranostics.37 In this work we focus on creating polymer-modified surfaces (viz. silicon) using dually reactive BCPs comprised of glycidyl methacrylate (GMA) and VDMA, denoted PGMA-b-PVDMA, in order to generate surface scaffolds in which film thickness, surface grafting density, and functionality can be tailored. While PGMA-b-PVDMA copolymers offer the capacity for both surface tethering as well as the ability to tailor polymer properties by integrating functionality either postpolymerization or after surface attachment, the structure adopted, and therefore, film properties, are likely to be strongly influenced by the competition between surface reactivity of the GMAcontaining blocks and the substrate, and BCP microphase segregation. Herein we demonstrate the controlled synthesis of PGMA-b-PVDMA using RAFT polymerization and thoroughly examine how careful control of processing conditions affects surface attachment and alters assembly on silicon substrates. Surfaces functionalized with the dually reactive polymers afford brush-like polymer scaffolds, which can be readily tailored through subsequent functionalizations to alter surface properties.

alized poly(ethylene glycol) (PEG−NH2) into methyl methacrylate/MAC2AE copolymer “brushes”.23 The authors determined that functionalization of the active ester was highly dependent upon the molecular weight of the incoming PEG− NH2, but only weakly influenced by grafting density and molecular weight of the tethered brush. Additionally, Cullen and co-workers used surface initiated ATRP (SI-ATRP) to polymerize 2-vinyl-4,4-dimethylazlactone (VDMA) from an immobilized ATRP initiator on silicon and subsequently modified the polymer brushes with RNase A.24 Orski et al. synthesized functional polymer brushes containing N-hydroxysuccinimide 4-vinyl benzoate (NHS4VB) using SI-ATRP, demonstrated the ability to generate block copolymer brushes by chain extension with a second monomer, and functionalized the NHS4VB moiety to tailor surface properties.25 This work was extended to include surface immobilization of cyclopropenones that were subsequently converted to dibenzocyclooctynes through photoinduced decarbonylation through a photomask, which facilitated creation of functional surfaces (e.g., Lissamine Rhodamine B) with spatially resolved chemical functionality.26 Further exploiting poly(NHS4VB) coatings, Arumugam et al. chemically confined 3-(hydroxymethyl)naphthalene-2-ol moieties to the polymer brushes and subsequently utilized azide−alkyne click chemistry along with light-directed Diels−Alder reactions to generate complex and high-density patterned surfaces.27 Schüwer and co-workers used neutron reflectivity and UV−vis spectroscopy to ascertain the post polymerization modification of p-nitrophenyl chloroformate activated poly(2-hydroxymethyl methacrylate) (NPCPHEMA) brushes.28 In contrast to Schuh and Rühe,23 Schüwer et al. indicated that postpolymerization modification was strongly dependent on both polymer brush thickness and grafting density. Furthermore, these authors concluded that functionalization of the polymer brushes is mediated by a complex interplay of sterics, size and polarity of the modifier, and possibly surface energy differences between the modifier and polymer brush. Günay et al. used poly(pentafluorophenyl methacrylate) (PPFMA) active ester brushes prepared by surface RAFT polymerization of pentafluorophenyl methacrylate (PFMA) to develop extremely stable (hydrolytic) films that were subsequently modified through judicious choice of small molecule amines with near quantitative conversion of active ester groups.29 A very attractive platform to create functional polymers and surfaces makes use of the reactive polymer poly(2-vinyl-4,4dimethylazlactone) (PVDMA); chemical modification of PVDMA was pioneered by Heilmann and Rasmussen and coworkers.30 Subsequently, work in the Lynn group has demonstrated the utility of PVDMA for a variety of layer-bylayer applications.31 Our efforts have taken advantage of the highly reactive azlactone moiety to generate functional surfaces that can be fabricated by inkjet printing,32 manipulate the solution properties of (co)-polymers,33 and produce functional polymer brushes.34,35 Moreover, the enhanced hydrolytic stability of PVDMA,28 as compared to other active esters24 (e.g., NHS), provides a more robust platform to create functional materials and surfaces. Fontaine and co-workers demonstrated the ability to control the polymerization of VDMA via chain extension of macro-chain transfer agents (macro-CTAs) of either polystyrene, poly(methyl acrylate), or poly(methyl methacrylate); however, the authors did not discuss self-assembly properties or the reactivity of the azlactone block.14,36 Fontaine et al. also developed thermores-

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EXPERIMENTAL SECTION

Materials. All reagents, including 2-cyano-2-propyl dodecyl trithiocarbonate (CPDT), were purchased from Aldrich at the highest purity available and used as received unless stated otherwise. Biotin− poly(ethylene glycol)amine (PEG-biotin, MW ∼ 720 Da) was purchased from Aldrich and used as received. 2,2′-Azobis(4methoxy-2,4-dimethyl valeronitrile) (V-70) was purchased from Wako Specialty Chemicals and recrystallized from methanol before use. 2-Vinyl-4,4-dimethyl azlactone (VDMA; Isochem North America, LLC) was fractionally distilled under reduced pressure and the middle fraction (∼70%) was used. Silicon wafers (1 cm ×1.2 cm) and silicon ingots (5 cm diameter) were purchased from Silicon Quest and El-Cat, respectively, and cleaned with piranha acid (Caution!) prior to use. Instrumentation. Nuclear Magnetic Resonance (NMR) Spectroscopy. Solution 1H and 13C NMR spectroscopy was performed on a Varian VNMRS 500 MHz multinuclear spectrometer. Samples were placed in 5 mm o.d. tubes with sample concentrations of 5 and 10% (w/v), respectively. Chloroform-d (CDCl3) was used as the solvent, and residual solvent peaks serve as internal standards. Size Exclusion Chromatography (SEC). Absolute molecular weights and polydispersities were obtained by SEC using a Waters Alliance 2695 Separations Module equipped with three Polymer Laboratories PLgel 5 μm mixed-C columns (300 × 7.5 mm) in series, a Waters Model 2414 Refractive Index detector (λ = 880 nm), a Waters Model 2996 photodiode array detector, a Wyatt Technology miniDAWN multiangle light scattering (MALS) detector (λ = 660 nm), and a Wyatt Technology ViscoStar viscometer. Tetrahydrofuran (THF) was used as the mobile phase at a flow rate of 1 mL/min. Refractive index increment, dn/dc, values were calculated based on 100% mass recovery using Astra V software. Ellipsometry. Film thicknesses were measured using a J.A. Woollam M-2000U variable angle spectroscopic ellipsometer over a wavelength range of 245−999 nm. Thicknesses reported are the average of measurements made from at least three spots on the polymer-modified wafer. To fit the ellipsometric data, each polymer layer was represented as a slab of uniform thickness having sharp interfaces and optical properties described by a Cauchy model (refractive index smoothly decaying with wavelength) assuming that the PGMA and PVDMA layers had refractive indices of 1.50 and 1.52 at 632 nm, respectively. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded using a Bruker Optics Vertex 70 spectrometer. Bulk polymer samples were analyzed via ATR−FTIR using a Harrick Scientific MVP Star accessory equipped with a diamond internal 6439


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reflection element (IRE); a standard deuterated L-alanine doped triglycine sulfate (DLaTGS) detector was used. A background spectrum was collected as the average of 64 scans of the clean ATR crystal while spectra were collected as the average of 64 scans with 4 cm−1 resolution and subsequently baseline corrected. Polymer thin films on silicon were analyzed using a Harrick Scientific VariGATR accessory and a narrow-band (650 cm−1 cutoff), liquid nitrogen-cooled MCT detector. A background spectrum was collected as the average of 128 scans of the clean germanium crystal; GATR spectra were collected as the average of 264 scans with 4 cm−1 resolution and subsequently baseline corrected. Neutron Reflectometry (NR). Measurements were made using the liquids reflectometer (LR) of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, which collects specular reflectivity data in a continuous wavelength band at several different incident angles. For the measurements presented herein, a wavelength band of 2.50 Å < λ < 5.75 Å was used and the reflectivity was measured at seven incident angles (θ = 0.15°, 0.25°, 0.32°, 0.40°, 0.70°, 1.20°, and 2.20°), thereby spanning a total wavevector transfer (Q = 4πλ−1sin θ) range of 0.006 Å−1 < Q < 0.192 Å−1. Data were collected at each angle with incident-beam slits set to maintain a constant wavevector resolution of δQ/Q = 0.05, which allows the data obtained at the seven different angles to be stitched together into a single reflectivity curve. To fit the data, the initial thicknesses measured using spectroscopic ellipsometry were used for reflectivity simulations and then these thicknesses were adjusted to correspond to the features in the neutron reflectometry. The neutron scattering length density, Σ, was determined using the equation Σ = b/V, where b is the monomer scattering length (sum of scattering lengths of constituent atomic nuclei) and V is the monomer volume. The calculated reflectivity curves were optimized for goodness-of-fit.38 Synthetic Procedures. Synthesis of 4,4-dimethyl-d6-2-vinyloxazolone (VDMA-d6). In a typical reaction, acetone-d6 (12.82 g, 0.20 mol) was added to a stirred solution of ammonium chloride (13.37 g, 0.25 mol) and sodium cyanide (12.25 g, 0.25 mol) in CH2Cl2/H2O 1:2 (60 mL). The reaction vessel was sealed and stirred for 48 h, after which time the reaction mixture was extracted with CH2Cl2, dried, and concentrated under reduced pressure. The resulting residue was treated with 12 M aqueous hydrochloric acid (60 mL) at 100 °C for 2 h and then this mixture was concentrated under reduced pressure, dissolved in ethanol, and filtered. 2-Amino3,3,3-trideuterio-2-trideuteriomethyl propionic acid (Aib-d6) was isolated by removal of solvent under reduced pressure (55% yield).39 Next, Aib-d6 (5.99 g, 0.06 mol) and NaOH (4.40 g, 0.11 mol) were dissolved in 50 mL of water, and acryloyl chloride (5.0 g, 0.06 mol) was added dropwise while cooling with an ice bath. The mixture was allowed to warm to room temperature and then stirred for an additional hour before 6.5 mL of HCl (12 M) was added to neutralize the reaction mixture. The product, vinyl Aib-d6 was collected by filtration and recrystallized from a 3:1 ethyl acetate/ methanol mixture (38% yield). Vinyl Aib-d6 (2.76 g, 0.02 mol) and ethyl chloroformate (1.91 g, 0.02 mol) were combined in 100 mL of anhydrous hexane and ring closure was enabled by dropwise addition of triethylamine (3.56 g, 0.04 mol), which was added at a rate that maintained a temperature of 45 °C similar to the procedure by Taylor et al.40 Triethylamine hydrochloride was removed by filtration and hexane was evaporated to yield VDMA-d6 as an oil (33% yield). 1H NMR (500 MHz, CDCl3, δ): 6.20 (m, 2H; CH), 5.85 (d, 1H; CH) (see Supporting Information, Figure S1). PGMA MacroCTA. Reactions were formulated in a single-neck 100 mL Airfree round-bottom reaction flask equipped with a Teflon-coated magnetic stir bar. In a typical reaction, GMA (7.11 g, 5.00 × 10−2 mol) was combined with CPDT (245.71 mg, 7.11 × 10−4 mol; GMA: CPDT = 352), V-70 (43.85 mg; molar ratio of CPDT: V-70 = 5:1), and benzene (50.0 mL). The reaction vessel was capped with a rubber septum and the solution was sparged with dry argon for approximately 30 min. The reaction vessel was then placed in a heated oil bath controlled at 30 °C and allowed to react for 12 h, after which the reaction vessel was immersed in liquid nitrogen to quench the polymerization. PGMA macroCTA was subsequently reconstituted in

THF and precipitated in a 10-fold excess of hexanes (repeated 3×), filtered, and dried in vacuo. PGMA-b-PVDMA or PGMA-b-dPVDMA. Reactions were formulated in a single-neck 50 mL Airfree round-bottom reaction flask equipped with a Teflon-coated magnetic stir bar. In a typical reaction, VDMA (2.50 g, 1.80 × 10−2 mol) or VDMA-d6 was combined with the PGMA-macroCTA (370.96 mg, 3.34 × 10−5 mol; VDMA: PGMAmacroCTA = 540), V-70 (3.43 mg; molar ratio of PGMA-macroCTA: V-70 = 3:1) and benzene (20.0 mL). The reaction vessel was capped with a rubber septum, and the solution was sparged with dry argon for approximately 30 min. The reaction vessel was then placed in a heated oil bath controlled at 30 °C and allowed to react for 18 h, after which the reaction vessel was immersed in liquid nitrogen to quench the polymerization. PGMA-b-PVDMA was subsequently reconstituted in THF and precipitated in a 10-fold excess of hexanes (repeated 3×), filtered, and dried in vacuo. Surface Preparation, Block Copolymer Deposition, and Modification. Silicon samples were cleaned immediately before use by immersion for 90 min in a piranha acid solution warmed to and held at 110 °C (3:1 v/v solution of sulfuric acid (EMD, 95−98%) and 30% hydrogen peroxide (VWR, 29−32%)) followed by rinsing with copious amounts of distilled, deionized water and drying with a stream of dry nitrogen. Caution! Piranha acid is a strong oxidizer and a strong acid; it should be handled with extreme care as it reacts violently with most organic materials. The attachment procedure is similar to the strategy employed by Luzinov et al., where a PGMA “base layer” was first created by reaction of surface hydroxyl groups with epoxy groups and the surface anchored film was subsequently modified by attachment of carboxylic acid terminated polymers.41,42 PGMA73-b-PVDMA174 was characterized using dynamic light scattering in CHCl3 (see Supporting Information) indicating a hydrodynamic radius, Rh, of 3.6 nm. This suggests that the copolymers are present and deposit onto the surface as single chains in solution (as depicted in Scheme 1) and not as micelles, which may

Scheme 1. Surface Attachment of PGMA-b-PVDMA on Silicon

complicate the formation of stable films on the substrates. In this work, thin films were made by spin-coating (Laurell WS-400B-6NPP/LITE) the PGMA-b-PVDMA block copolymers onto silicon substrates (at 2500 rpm for 15 s) from dilute CHCl3 solutions in concentrations ranging from 0.25 to 1.0 wt %. After spin coating, the films were immediately annealed for 18 h in a preheated oven (80, 90, 100, or 110 °C). After annealing, the modified wafers were allowed to cool to room temperature, immersed in CHCl3 and bath sonicated using a Branson Model 5510 ultrasonic cleaner operating at 40 kHz for 1 h to remove any physisorbed polymer chains from the surface, and then dried with a stream of dry, filtered N2. While deposition from CHCl3 was found to yield films that uniformly covered the surface, PGMA 6440


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and PGMA-b-PVDMA films deposited from THF and methyl ethyl ketone (MEK) were not uniform, showing evidence of dewetting from the silicon substrates. PGMA-b-PVDMA-modified silicon surfaces were functionalized in 20 mL scintillation vials by submerging the surfaces for 18 h in a 0.5 mg/mL solution of biotin−poly(ethylene glycol)amine (PEG−biotin, MW ∼ 720 Da) in THF. The surfaces were subsequently washed with copious amounts of solvent, sonicated in THF for at least 30 min, and then dried under vacuum. The biotin−PEGylated films were analyzed using ellipsometry, GATR FTIR spectroscopy, and water contact angle measurements.

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RESULTS AND DISCUSSION Synthesis of VDMA Block Copolymers via RAFT (PGMA-b-PVDMA and PGMA-b-dPVDMA). We have devised a method to generate PVDMA surfaces based on selective reactivity of well-defined block copolymers29 to afford uniform polymer scaffolds that are covalently bound to silicon substrates, which is illustrated in an ideal sense in Scheme 1. A PGMA macro-chain transfer agent (macro-CTA) (Mw = 13.3 kg/mol; PDI 1.24) was synthesized by RAFT polymerization and chain extended using different formulations of VDMA in order to systematically vary the VDMA block length and target PVDMA:PGMA ratios; these results are summarized in Table 1.

Figure 1. Partial ATR−FTIR spectra of bulk PGMA-b-PVDMA block copolymers highlighting the portion of the spectra where modes associated with functional groups appear.

solution.43,44 It is widely known that the grafting density controls interchain interactions; therefore, it is a key parameter affecting the range and strength of interactions across a brushmodified interface. The PGMA block contains epoxy moieties that react with surface silanol groups, anchoring the copolymers to the silicon substrates through chemical bonds while the VDMA block, provides a facile route to highly tailorable surfaces upon subsequent reaction(s) with nucleophiles. Not only does PGMA bind the block copolymer to the surface at multiple points, it leads to high tethering densities relative to block copolymers deposited from solution35,45−47 and provides, as supported by the results described below, a variety of ways to use processing conditions to manipulate the attachment and, therefore, layer structure. Thus, not only is this approach useful because it overcomes the critical slowing down due to steric hindrance observed in preferential adsorption of block copolymers,45,46 and “grafting to” approaches using endfunctional chains47,48 but also the main advantage in the context of this study is that layers of high grafting density greatly facilitate characterizations, especially by neutron reflectometry. In order to assess the layer structure, several methods were used to investigate the surfaces including ellipsometry, GATR−FTIR, RAMAN microscopy, and neutron reflectometry. PGMA has previously been used as an anchor layer to attach a variety of end-functionalized polymers.30,36,37 Not only can the epoxy groups of PGMA react with functional groups (i.e., SiOH, −NH2, COOH), but self-cross-linking can occur, resulting in loss of epoxy groups and ultimately a thicker base layer.40 This issue is relevant to the deposition of PGMA-bPVDMA copolymers and it is necessary to evaluate the impact of processing conditions on surface attachment. This is accomplished by depositing the PGMA macroCTA onto freshly cleaned silicon wafers at various concentrations (0.06−1.00 wt % in CHCl3) and annealing at different temperatures (25−110 °C); the glass transition temperature (Tg) of PGMA as measured by differential scanning calorimetry (DSC) is 57 °C (see Supporting Information). The thickness of the PGMA layer determined by ellipsometry increases linearly, from approximately 2 to 75 nm, with increasing polymer solution concentration as shown in Figure 2A. The general trend illustrated in Figure 2B shows that film thicknesses

Table 1. Composition and Molecular Weight Characteristics of PGMA macro-CTA and PGMA-b-PVDMA samplea PGMA macroCTA-1 PGMA73-bPVDMA23 PGMA73-bPVDMA59 PGMA73-bPVDMA174 PGMA macroCTA-2 PGMA59-bdPVDMA152

mol % PVDMAb

Mwc (kg/mol)

PDI

dn/dcd (mL/g)

− 20

13.3 16.5

1.24 1.22

0.088 0.084

50

21.0

1.13

0.087

70

38.4

1.11

0.080

− 70

12.5 35.4

1.48 1.16

0.089 0.086

a

Subscripts indicate the degree of polymerization for each block. Determined by 1H NMR (Supporting Information, Figure S1). c Absolute weight-average molecular weight, Mw, obtained from SECMALS. dDetermined by 100% mass recovery using Astra V software. b

The polymerizations were conducted in benzene at 30 °C to ensure retention of the reactive epoxide and azlactone functionalities during polymerization and subsequent precipitation/recovery. To this end, Figure 1 shows partial ATR− FTIR spectra of PGMA-b-PVDMA that illustrate preservation of functionality (COazlactone, 1820 cm−1; C−Oepoxide, 906, 841, 754 cm−1) of the dried/isolated block copolymers. Moreover, the peak height ratio of the COazlactone (1820 cm−1) to the C−Nazlactone (1670 cm−1) remains relatively constant (∼1.5) for each copolymer while the peak height ratios of the COazlactone (1820 cm−1) to the COPGMA (1725 cm−1) reflect copolymer composition. Surface Attachment and Characterization. Creating polymer brushes by tethering pre-made polymer chains at one end to a surface is advantageous because it allows the polymer chains to be fully characterized prior to surface attachment. This allows the grafting density to be readily calculated from the measured molecular weight, rather than relying on estimates that are based on free polymer chains grown in 6441


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Figure 2. Thickness of PGMA films versus concentration (A) before annealing and (B) after annealing and sonication to removed nonbonded chains, as a function of solution concentration and annealing temperature.

Figure 3. Thickness of PGMA-b-PVDMA films (after thermal annealing, rinsing, and sonication) as a function of annealing temperature and block copolymer molecular weight at deposition concentrations: (A) 0.25, (B) 0.50, (C) 0.75, or (D) 1.00 wt % in CHCl3.

50 °C have a nominal thickness of ∼2 nm regardless of polymer concentration. Films annealed at 80, 90, and 100 °C have an

decrease after annealing and sonication due to the removal of unreacted (physisorbed) polymer. Surfaces annealed at 25 and 6442


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molecular weight determined by SEC-MALS; the results are plotted in Figure 4.

average and uniform thicknesses of 4, 5, and 9 nm, respectively. Surfaces annealed at 110 °C retain their pre-sonicated thicknesses upon annealing/sonication. Moreover, film thickness is directly proportional to polymer solution concentration as shown in Figure 2B as the thickest film (74 nm) is produced from the highest deposition concentration (1.0 wt %), which is consistent with previous results.49 Attempts to quantify the amount of self-cross-linking using GATR−FTIR spectroscopy were unsuccessful because the spectra obtained from thin (≤2 nm) films had low intensities, which hindered the analysis. For thicker films (Tanneal ≥ 80 °C), it was observed that the peak height at 908 cm−1 (C−O epoxy) decreased slightly with respect to the PGMA carbonyl peak (1730 cm−1) as the annealing temperature increased, suggesting that some epoxy groups were consumed via self-cross-linking. Results from the PGMA attachment studies indicate that 80 °C is the lowest annealing temperature that creates a uniform film (in a reasonable amount of time) and that annealing at 110 °C leads to thicker, partially cross-linked films. Consequently, PGMA-b-PVDMA copolymers (Table 1) were deposited onto silicon substrates to investigate the ability to control film thickness, surface coverage, and morphology. To this end, PGMA-b-PVDMA films were prepared from polymer solutions of 0.25, 0.50, 0.75, or 1 wt % in CHCl3 and annealed at 80, 90, or 110 °C. Film thickness as a function of PGMA-b-PVDMA molecular weight and annealing temperature is shown in Figure 3, which separates the data based on concentration. (Note that the scales on the y-axes vary.) Similar to the behavior exhibited by the PGMA films, increasing the copolymer solution concentration results in a linear increase in film thickness prior to annealing and sonication, with films ranging from ca. 20 to 91 nm. There is a slight decrease in thickness for all films annealed at 80 and 90 °C after sonication, while those annealed at 110 °C retain their as-deposited thickness after sonication. This suggests that when the films are annealed at 80 and 90 °C, only the PGMA blocks close to the surface react, thus anchoring the block copolymer to the surface while unbound polymer is washed away during rinsing and sonication steps. SEC-MALS of PGMA73-bPVDMA174 before and after sonication (c = 3.54 mg/mL in THF, 1 h, 40 kHz) showed no changes in concentration, molecular weight, or PDI suggesting that sonication does not cleave chains, but instead facilitates removal of unbound polymer from the surface (see Supporting Information). It is postulated that at 110 °C, PGMA self-cross-links, which allows chains farther from the silicon/polymer interface to be incorporated into the film, resulting in thicker films. Furthermore, at 110 °C there should be sufficient mobility of the block copolymers to phase separate because the annealing temperature (Tanneal) ≥ Tg of PVDMA homopolymer (Tg,PVDMA ∼ 92 °C - 104 °C as measured by DSC).30,33 DSC analysis (i.e., typical heat−cool-heat methods) of PGMA-b-PVDMA produces complex results due to reversing (glass transition) and nonreversing (e.g., epoxy curing, enthalpic recovery) heat flow; these materials are being investigated using modulated DSC experiments. The measured thicknesses were used along with eq 1 to determine the areal chain density, σ, on the surface. σ = (hρNa)/M n

Figure 4. Areal chain density, σ, of PGMA-b-PVDMA films versus concentration as a function PVDMA block length and annealing temperature.

In general, increasing the annealing temperature leads to higher σ, indicating that a greater number of epoxides react with the surface. The amount of polymer grafted to the surface at 80 °C is nearly independent of concentration but seems to depend on molecular weight: the highest σ values are observed for PGMA73-b-PVDMA23, which is the copolymer with the shortest PVDMA block and the lowest overall molecular weight (Table 1, entry 2). Larger values of σ are observed at 90 °C, and they increase with solution concentration, except for concentrations of 0.75 and 1.00 wt % where PGMA73-bPVDMA59 (Table 1, entry 3) exhibits slightly larger σ. When the annealing temperature is increased to 110 °C, the areal chain density increases with the deposition solution concentration and is inversely proportional to polymer molecular weight. Thus, selecting the polymer molecular weight, deposition solution concentration, and annealing temperature allows a wide range of areal chain densities (0.06−3.32 chains/ nm2) to be accessed, which should play an important role in subsequent scaffold functionalization. It should also be noted that the method for anchoring reactive block copolymers reported herein is capable of producing a scaffold layer roughly six times as dense as when carboxyl-terminated PVDMA chains are grafted onto PGMA functionalized surfaces.29 This result is perhaps not unexpected, as attaching pre-made chains to an interface is known to suffer from a critical rate reduction as σ increases (i.e., surface crowding), thus preventing access of the single, reactive end-group to the surface. GATR−FTIR and Raman micro-spectroscopy indicate that the PGMA block reacts with surface hydroxyls on the silicon substrate forming a covalently bound anchor layer and that the azlactone functionality of the PVDMA block is retained. Figure 5 shows the partial GATR−FTIR spectra of a series of PGMAb-PVDMA copolymers attached to silicon substrates in which the polymers were deposited from 0.25 wt % solutions in CHCl3 and subsequently annealed at 110 °C for 18 h. Comparison of the spectra illustrates that, in all cases, azlactone groups are available for further modification, as evidenced by

(1)

In this expression, h is the dry brush layer thickness, ρ is the bulk density of the polymer layer (taken to be 1.08 g/cm3), Na is Avogadro’s number, and Mn (g/mol) is the number-average 6443


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quartz wafers and annealed at room temperature (23 °C) and at 80, 90, and 110 °C. Unfortunately, the typical deposition strategy used to attach PGMA73-b-PVDMA23 onto silicon resulted in films that gave weak Raman signals. As a result, films were drop cast using 5.0 wt % PGMA73-b-PVDMA23 in chloroform and then annealed at the aforementioned temperatures; the data are shown in the Supporting Information (Figure S4). Qualitatively, the Raman spectra indicate that increasing the annealing temperature results in a reduction in the intensity of the epoxy peaks (1260, 906, 841 cm−1) relative to the carbonyl modes attributed to PGMA (1725 cm−1) and PVDMA (1820 cm−1). This suggests that a greater amount of epoxy groups react at elevated temperatures, while preserving azlactone groups. In order to investigate the layer structure, water contact angle was measured for films made by deposition of the PGMA macroCTA and the three block copolymers. The PGMA macroCTA had an average contact angle of 60° ± 2° which is in good agreement with values previously reported by Luzinov and co-workers.50 The block copolymers had contact angles of 73° ± 2°, 67° ± 2°, and 71° ± 1° for PGMA73-b-PVDMA23, PGMA73-b-PVDMA59, and PGMA73-b-PVDMA174, respectively. These values are consistent with those reported by Lynn and co-workers for a PVDMA surface51 and indicate that PVDMA is present at the air/polymer interface regardless of the VDMA block length. Additionally, atomic force microscopy (AFM) was performed on a photolithographically patterned surface allowing lines of PGMA-b-PVDMA having 1000 μm widths to be created in order to compare the thicknesses measured by ellipsometry and to examine film topography and roughness. AFM height measurements at the edge of the PGMA73-b-

Figure 5. GATR−FTIR analysis of PGMA-b-PVDMA block copolymers attached to silicon substrates (PGMA-b-PVDMA c = 0.25 wt %; Tanneal = 110 °C).

the mode at 1820 cm−1 that is due to the carbonyl of the azlactone ring. Moreover, the spectra mimic those of the bulk copolymers (see Figure 1) and thus demonstrate that azlactone groups are not affected by the processing conditions. For clarity, only the 0.25 wt % data are shown in Figure 5; however, thicker films (i.e., those prepared at higher concentrations and a higher annealing temperature) show increased absorbance in GATR−FTIR spectra, indicating that more polymer has been attached. In an effort to examine the extent of PGMA cross-linking, PGMA73-b-PVDMA23 was spin-coated onto Piranha-cleaned

Figure 6. Reflectivity as a function of wave-vector transfer, Q for (A) PGMA/PVDMA-d6 mixed surface annealed at 25 °C and (B) PGMA/PVDMAd6 mixed surface annealed at 80 °C. The data are shown as solid squares and the models as solid lines. The insets in A and B show the RQ4 versus Q profiles. (C) The SLD profiles for the films annealed 25 °C (red) and 80 °C (blue) as a function of thickness, Z, used to generate the model curves. 6444


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Table 2. Summary of NR Fitting Parameters and Layer Compositions for PGMA-b-dPVDMA surface

strata

Σ x 10‑4 (nm‑2)

thickness (nm)

roughness (nm)

% PVDMA-d6a

% PGMAa

mixed 25 °C

top middle bottom top middle bottom top middle bottom

4.02 2.24 1.77 4.36 2.11 1.54 3.62 4.01 2.27

6.6 5.0 4.3 6.7 5.1 4.0 8.4 8.4 2.2

3.9 2.6 1.5 3.6 3.0 2.2 2.5 2.0 1.8

88 38 25 96 34 18 76 86 38

12 62 75 4 66 82 24 14 62

mixed 80 °C

block 80 °C

a

On the basis of ΣPVDMA = 5.00 × 10−4 nm−2 and ΣPGMA = 0.90 × 10−4 nm−2.30,33

layer (closest to the silicon substrate) to be determined using eqs 2 and 3:

PVDMA174 microchannels (see Supporting Information Figure S3) are in good agreement with the thickness values obtained from ellipsometry. Moreover, examination of the layer reveals a smooth topography without any indication of lateral or longrange order, a finding that is consistent with the notion that PGMA serves as an anchor layer, tethering the PVDMA chains to the silicon substrates, creating a brush-like structure. Furthermore, neutron reflectometry measurements were performed on various films helping to elucidate the internal organization of the films. Neutron reflectometry (NR) experiments provide a unique opportunity for noninvasive monitoring of the layer segment density profile and were performed to gain further insight into the structure of the reactive polymer layers. To determine the layer structure, VDMA-d6 was synthesized and used in these studies to provide scattering contrast between the two blocks. The isotopic substitution increases the SLD of PVDMA providing contrast that distinguishes PVDMA from PGMA, which enhances the information that can be gained from the measurement by allowing the interfacial profile to be determined more easily. Mixed Homopolymer Surfaces. To probe the interaction between PGMA and PVDMA and their bulk miscibility, a 0.25 wt % solution of a 50/50 mixture by mass of PGMA and PVDMA-d6 homopolymers was codeposited on a silicon substrate. NR experiments were done on films annealed under vacuum at 25 and 80 °C for 18 h. Figure 6A shows the reflectivity and SLD profile for the mixed surface annealed at 25 °C. The reflectivity data were fit using a slab model to represent the polymer layers.33 The reflectivity, which is multiplied by Q4 to enhance the visibility of features in the data, reveals two periodicities, indicating that at least two distinct layers are present on the surface. Initially, a two-layer model of the film was constructed and the best fit was obtained when the PGMA layer was located at the film/silicon oxide interface and the PVDMA-d6 layer was located at the film/air interface. Because uniqueness of fit is not guaranteed, it is necessary to constrain the fits with known information, such as film thickness and SLDs based on the polymer composition and our previous results on PGMA layers,34,35 which results in more accurate and representative models. Assuming that the densities of the polymer layers are similar allows the models to be further constrained by relating the SLD and thickness of each layer to the polymer(s) mass ratio. In general, VDMA and GMA monomer units are distributed in each layer and the SLDs of each layer are weighted sums of the SLDs of the component polymers based on their respective volume fractions (ϕ). This allows the SLDs of the top layer (closest to air) and the bottom

ΣT = φT ΣPGMA + (1 − φT )ΣPVDMA

(2)

ΣB = φBΣPGMA + (1 − φB)ΣPVDMA

(3)

However, because these fits were deemed to be insufficient over 0.005 Å−1 < Q < 0.16 Å−1, a third layer or stratum (middle) was added to account for a broad polymer/polymer interface, resulting in a three-stratum model with SLD and thickness values of 4.07 × 10−4 nm−2 and 6.6 nm, 2.26 × 10−4 nm−2 and 5.0 nm, and 1.78 × 10−4 nm−2 and 4.3 nm for top (T), middle (M), and bottom strata (B), respectively. For the three stratum model the SLDs were determined using eqs 4−6 and full descriptions of the constrained models and comparisons between two-layer and three-layer fits are given in the Supporting Information. ΣT = φT ΣPGMA + (1 − φT )ΣPVDMA

(4)

ΣM = φM ΣPGMA + (1 − φM )ΣPVDMA

(5)

ΣB = φBΣPGMA + (1 − φB)ΣPVDMA

(6)

The model suggests that the layer at the air/film interface consists of 88% PVDMA-d6, the mixed layer consists of 38% PVDMA-d6 and 62% PGMA, and the layer at the film/silicon oxide interface consists of 75% PGMA. The NR results indicate that PGMA prefers the silicon surface, enriching the substrate/ film interface with PGMA while the film/air interface is enriched with PVDMA-d6 during film preparation. A second film was annealed above the glass transition temperature of PGMA to provide increased chain mobility. Once again, as shown in Figure 6B, a three stratum model was needed to capture the measured reflectivity. The model is described by SLD and thickness values of 4.36 × 10−4 nm−2 and 6.7 nm; 2.11 × 10−4 nm−2 and 5.1 nm; and 1.54 × 10−4 nm−2 and 4.0 nm for the top stratum, the middle stratum, and the bottom stratum, respectively. The results indicate that the layer at the air/film interface is 96% PVDMA-d6, the mixed interface layer is 34% PVDMA-d6 and 66% PGMA and the layer at the film/silicon oxide layer is 82% PGMA. This shows that increasing chain mobility through increased temperature allows the PVDMA chains to migrate to the air/film interface, while the PGMA chain preferentially moves to the film/SiO2 interface. Moreover, this implies that there is minimal interaction/reaction between PGMA and PVDMA, as elevated temperatures effectively separate the layers into nearly pure components at the top and bottom strata. 6445


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Figure 7. (A) Reflectivity as a function of wave-vector transfer, Q for PGMA59-b-dPVDMA152 annealed at 80 °C. The data are shown as solid squares and the best fit three-layer model is shown as a solid red line. (B) Scattering length density profile for the three-layer model as a function of thickness, Z, used to generate the model curves.

PGMA-b-PVDMA Surfaces Studied by NR. PGMA59-bdPVDMA152 (Table 1, entry 6) was spin-coated onto silicon, annealed, and measured by NR to determine its layer structure and composition. Using the information gained from the mixed surfaces and the polymer’s mass ratio, a constrained model was used to fit the data and the results are summarized in Table 2. Like the models that are used for the mixed homopolymer surfaces, a three stratum model is needed to best capture the features in the reflectometry data. A key difference for the block copolymer layer is that the middle stratum has a higher percentage of PVDMA-d6, 86%, and thus a higher SLD than the polymer layer at the air/film interface, which is roughly 76% PVDMA, as illustrated in Figure 7. The lower volume fraction of PVDMA at the air/film interface could be attributed to polymer chains that are trapped and possibly inverted, leaving PGMA in the top stratum and supporting a PVDMA-d6enriched mixed-layer structure. Scheme 2 shows a cartoon representation of the polymer film structure on the silicon substrate with the SLD profile based on neutron reflectivity

superimposed. The thicknesses of the three polymer layers are drawn to scale relative to one another. From the cartoon and the NR data, it is implied that because the PVDMA and PGMA blocks are chemically bound to one another, chain mobility is hindered by macromolecular size, linking of PGMA to the silicon substrate through the epoxy moiety, and the possibility of PGMA self-cross-linking. Collectively, these phenomena function to form a polymer film that, although is not the ideal case as depicted in Scheme 1, allows for control of layer thickness and density through synthesis (e.g., MW and copolymer composition) and processing conditions providing a high concentration of active ester for post-assembly modification. Biotinylation/PEGylation of PGMA-b-PVDMA. PEG containing a primary amine on one chain-end and biotin on the opposite end was reacted with PGMA-b-PVDMA films as a proof of concept. Figure 8 shows partial GATR−FTIR spectra

Scheme 2. Cartoon Representation of the Layer Structure PGMA-b-dPVDMA Copolymer Film on a Silicon Substrate with the SLD Profile Based on Neutron Reflectivity Superimposed

Figure 8. Partial GATR−FTIR spectra of PGMA73-b-PVDMA174 on silicon (green line) and PEG-biotin functionalized PGMA-b-PVDMA (dark blue line).

comparing the parent PGMA73-b-PVDMA174 (0.25 wt % deposition; green spectrum is also shown in Figure 5) surface and its PEGylated analogue. The spectrum obtained from the PEG-biotin-functionalized (dark blue spectrum) surface shows a marked decrease in azlactone functionality (1820 cm−1) and a substantial increase in both amide I (1650 cm−1) and amide II (1546 cm−1) indicating reaction of the azlactone group. 6446


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Furthermore, the appearance of the strong peak at 1110 cm−1, attributed to the C−O−C antisymmetric stretch of aliphatic esters, indicates incorporation of PEG. Peak integration of the azlactone before and after functionalization reveals that 75% of the azlactone moieties are consumed via the primary amine chain-end on the PEG-biotin. Moreover, the ellipsometric thickness of the PGMA73-b-PVDMA174 film increases from an initial thickness of 21 to 68 nm upon addition of PEG−biotin. Additionally, the water contact angle of the two surfaces differs considerably: the PEGylated surface has a water contact angle 41° ± 1°, while the unmodified PGMA73-b-PVDMA174 has a water contact angle 71° ± 1°. Furthermore, it is possible to control the amount of PEGylation. For the three different PGMA-b-PVDMA copolymers attached to silicon, an increased consumption of azlactone (i.e., PEG-biotin attachment) is observed in the following order: PGMA73-b-PVDMA174 > PGMA73-b-PVDMA59 > PGMA73-b-PVDMA23 at a fixed reaction time (18 h) and film thickness increases accordingly, as summarized in Table 3. Thus, the extent of functionalization

of conversion of active ester groups.22 Assuming that “(a) the chain length of the surface-attached molecules does not change during the reaction, (b) the aminolysis proceeds quantitatively, and (c) the grafting density of the layers is unaffected by the reaction,”22 the reaction of PEG-biotin with the PGMA73-bPVDMA174 film should yield a thickness of 155 nm. It is believed that the PGMA73-b-PVDMA174 film is packed sufficiently on the surface to allow full penetration of PEGbiotin. Similarly, the diffusion barrier is potentially overcome by the new functional groups created upon azlactone ring-opening, which may enhance the swelling of the modified brushes in comparison to the original PVDMA brushes.52 The hydrolytic stability of these PGMA-b-PVDMA films is expected to be good at or near neutral pH, but it would be expected that exposure to more basic or acidic environments will open the azlactone33 far more readily than cleaving PFMA esters.29 Nevertheless, hydrolysis of the azlactone results in a pH-responsive system.53 Additionally, the nature of the azlactone modification is attractive because upon reaction with a nucleophile or a base, there is no leaving group as compared to pentafluorophenol or N-hydroxysuccinimide, which could interact with a peptide or protein (i.e., modifier).29,54 It is not the intention of this discussion to suggest that one active ester is preferred over another, but rather to delineate that different active esters work in different ways, and that using those different compounds (azlactone, NHS, pentafluorophenyl) there is potential for modification strategies that either exploit in sequence or through orthogonal chemical reactions and imbue polymer brushes with a wide variety of properties through different functional groups.

Table 3. Percent Conversion of Azlactone Groups in PGMAb-PVDMA and Film Thicknesses before and after PEGBiotin Functionalizationa ellipsometric thickness (nm)

PGMA73-bPVDMA23 PGMA73-bPVDMA59 PGMA73-bPVDMA174 c PGMA73-bPVDMA174

percent conversion of azlactone (%)b

before PEGylation

after PEGylation

areal chain density (chains/nm2)

29

20

28

0.96

35

22

36

0.82

73

21

62

0.44

100

25

76

0.48

■

CONCLUSIONS The work presented herein demonstrates the ability to use dually reactive block copolymers to generate thin films of controlled thickness with chemoselectivity for subsequent chemical modification. The interfacial structure of the films appears to be governed more strongly by the interaction and surface attachment of GMA rather than BCP phase separation, which leads to the observed mixed layer structure. Furthermore, the annealing temperature plays a significant role, dictating the extent to which different stratified structures are preferred. The role of the PGMA block is complex insofar that it binds the BCP to the surface, but can also undergo a cross-linking reaction, which increases the surface thickness and may trap chains resulting in the copolymer film as depicted in Scheme 2. NR requires a three-layer model containing a mixed interface layer to accurately describe the films, rather than a simple twostrata slab-like model. This finding provides strong evidence that the polymer films are PVDMA-rich at the air/film interface and PGMA-rich at the film/silicon interface, but do not completely phase separate even after annealing at temperatures above Tg for both components. While the focus of this manuscript is the formation of a covalently bound reactive polymeric scaffold, PGMA-b-PVDMA block copolymers can be used to tailor the surface properties of a wide array of substrates with varying topologies that can be controllably manipulated through judicious choice of nucleophilic modifiers. These materials represent a platform for a variety of applications including purification membranes, drug delivery, and mimics for biological membranes.

a PGMA-b-PVDMA deposited via 0.25 wt % solution in chloroform. All surfaces were submerged in 0.5 mg/mL PEG-biotin solutions for 18 h. bDetermined from peak heights of the azlactone CO stretch at 1820 cm−1 before and after functionalization. cFunctionalization reaction allowed to proceed for 48 h.

increases as σ decreases or as the PVDMA chain length is increased. This dual interpretation is consistent with eq 1, which shows that σ is inversely proportional to Mn and is reflected in the data shown in Figure 4. These results are in agreement with the findings of Schüwer et al. for NPC-PHEMA brushes in which grafting density was varied by mixing active and inactive ATRP surface initiator.28 For longer reaction times, it is possible to achieve quantitative conversion of azlactone groups, as evaluated by the complete disappearance of the azlactone peak (≈ 1820 cm−1) using GATR−FTIR spectroscopy. In this case, we take advantage of the dually reactive nature of PGMA-b-PVDMA to tether the PGMA blocks to the substrate, leaving the PVDMA blocks available (unreacted), which allows the surface properties to be tailored. For the complete aminolysis of PGMA73-b-PVDMA174 (Table 3), the “expected” film thickness (155 nm) based on Murata et al.,22 does not agree well with what is observed (76 nm) although GATR−FTIR spectroscopy indicates complete disappearance of the azlactone peak. Murata et al. showed that amino-terminated PEG functionalization of active ester brushes is dependent on the chain length of the incoming modifier where increasing the MW of PEG-amine decreases the degree 6447


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ASSOCIATED CONTENT

S Supporting Information *

Discussion of the instrumentation used, 1H NMR spectra, AFM of 1000 μm channels, TGA analysis, Raman microscopy, SEC− MALS comparison of PGMA73-b-PVDMA174 before and after sonication, and the mass balance model used to fit the neutron reflectometry data sets are included. This material is free of charge via Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (B.S.L.) [email protected]; (J.M.M.) messmanjm@ ornl.gov. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was conducted at the Center for Nanophase Materials Sciences and Spallation Neutron Source, which are sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy, and enabled through ORNL’s Laboratory Directed Research and Development Program, Project No. D07-138. Candice Halbert and Chaitra Deodhar are kindly acknowledged for help with reflectometry measurements.

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Manipulating Interfaces through Surface Confinement of Poly(glycidyl

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