Article pubs.acs.org/Langmuir
Effect of Colloidal Substrate Curvature on pH-Responsive Polyelectrolyte Brush Growth Benjamin T. Cheesman,† Alexander J. G. Neilson,† Joshua D. Willott,† Grant B. Webber,† Steve Edmondson,‡ and Erica J. Wanless*,† †
Priority Research Centre for Advanced Particle Processing and Transport, University of Newcastle, Callaghan, NSW 2308, Australia Department of Materials, Loughborough University, Loughborough, LE11 3TU, United Kingdom
‡
S Supporting Information *
ABSTRACT: Coatings consisting of polymer brushes are an effective way to modify solid interfaces. Polymer brushmodified hybrid particles have been prepared by surfaceinitiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET ATRP) of 2(diethylamino)ethyl methacrylate (DEA) on silica particles. We have optimized the synthesis with respect to changing the reducing agent, temperature, and reaction solvent from an aqueous ethanol mixture to an aqueous methanol mixture. Our flexible electrostatically adsorbed macroinitiator approach allows for the modification of a variety of surfaces. Polybasic brushes have been grown on silica particles of different sizes, from 120 to 840 nm in diameter, as well as on wafers, and a comparison of the products has allowed the effect of surface curvature to be elucidated. An examination of the thickness of the dry brush and the aqueous hydrodynamic brush at both pH 7 and at 4 demonstrated that growth increased substantially with substrate curvature for particles with a diameter below 450 nm. This is attributed to the increasing separation between active chain ends, reducing the rate of termination. This is believed to be the first time that this effect has been demonstrated experimentally. Furthermore, we have seen that polymer brush growth on planar wafers was significantly reduced when the reaction mixture was stirred.
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INTRODUCTION Polymer brushes, formed by attaching one end of a polymer to an interface, display properties that can be attributed to both the free polymer and to the effects of chain confinement due to tethering at the interface. Surface properties can be tailored depending on the monomer used to form the brush, allowing control over the resulting interfacial hydrophilicity,1 lubricity,2 and response to external stimuli such as temperature or pH.3 The modification of particulate interfaces can produce hybrid particles with dramatically different properties than those of the precursor materials. For example, the stability of particles in suspension may be significantly improved by adding a polymer brush to impart steric stability, making the suspension stable under a broad range of conditions. Polymer brushes may be formed by reacting end-functional polymers with functionality on a surface using a “grafting to” approach; however, there are kinetic, steric, and entropic factors that limit the grafting density that can be achieved by this approach.4,5 Surface-initiated polymerization (SIP) is a “grafting from” technique whereby the polymer is formed in situ from initiator sites that are immobilized on the surface, overcoming many of the limitations of the former approach.6 Surfaceinitiated atom transfer radical polymerization (SI-ATRP) is widely used for SIP because of its controlled nature, the range of potential monomers, and the ability to use relatively mild © 2013 American Chemical Society
reaction conditions. However, potential drawbacks of ATRP are that the copper(I) activator species is oxygen-sensitive and the optimum reaction conditions are often monomer-specific, requiring elucidation if truly controlled growth is desired. Surfaces must be functionalized with initiator moieties before they can be used in SIP, and there are a range of techniques available to achieve this.6−10 In this work we have employed a broadly applicable macroinitiator approach; specifically, we employed the electrostatic adsorption of a cationic copolymer containing the initiator functionality onto negatively charged silica substrates11,12 that requires a single surface treatment, in contrast to the layer-by-layer macroinitiator approach.13,14 Using a macroinitiator removes the requirement to synthesize halogen-containing silane species, which often require a stringently water-free step.6 Our flexible approach also permits the modification of different surfaces, such as planar wafers and particles of different sizes, so that they have equivalent initiatordensity because adsorption from solution readily permits uniform and complete coverage of the substrate with the initiator. Received: January 30, 2013 Revised: April 23, 2013 Published: April 25, 2013 6131
dx.doi.org/10.1021/la4004092 | Langmuir 2013, 29, 6131−6140
Langmuir
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Activators regenerated by electron transfer (ARGET) ATRP is a novel, recent variant of ATRP wherein a reducing agent generates the active, oxygen-sensitive Cu(I) catalyst species in situ by reduction of the oxidatively stable Cu(II) deactivator.15,16 Because the reducing agent is present in excess, it imparts a degree of oxygen tolerance to the polymerization, hence requiring less stringently controlled conditions than conventional ATRP. In addition, the copper catalyst concentration can be decreased, even down to parts per million levels,15,16 reducing the requirement for catalyst removal and making it a more industrially attractive technique. We previously reported on the suitability of SI-ARGET ATRP for growing poly(2-(diethylamino)ethyl methacrylate) (poly(DEA)) brushes on 120 nm diameter silica particles17, where we demonstrated their aqueous pH-responsive behavior. We have since expanded on that work by using a range of conditions to synthesize hybrid particles. In the first part of this Article, we report on the optimization of poly(DEA) brush growth kinetics on 120 nm particles when changing the reducing agent, temperature, and reaction solvent from an aqueous ethanol mixture to an aqueous methanol mixture. From these data, we are able to detail a more efficient synthesis methodology than we previously reported for SI-ARGET ATRP of poly(DEA) brushes generated from an electrostatically adsorbed macroinitiator. In the second part of this study, we have investigated the effect of colloidal substrate curvature on polymer brush growth kinetics. The conformation of polymer brushes grown on flat substrates has been well studied following early work by Alexander18 and de Gennes.19 The volume available for chains to occupy linearly increases as the distance increases away from the planar interface, resulting in a linear dependence between the brush thickness and the chain length, assuming a constant grafting density. The conformation of polymer brushes at curved interfaces with fixed brush length has also been the topic of numerous theoretical studies,20−22 and at high grafting densities, the conformational transitions from concentrated to semidilute polymer brush regimes have been identified.23 Chen et al. have grown short brushes using various monomers on silica particles of 30 and 120 nm diameter that were functionalized with a cationic macroinitiator.24 Moreover, Kotsuchibashi et al. recently reported the growth of short poly(DEA) brushes by SI-ATRP on 20 and 128 nm diameter silica particles.25 However, in both of these studies the influence of surface curvature was not reported. We are specifically interested in determining whether the substrate curvature affects polymer brush growth, rather than the earlier, largely theoretical work that has looked at how curvature affects the conformation of solvated brushes of a given length or degree of polymerization.20−22 We are undertaking this work to probe the impact that these geometric constraints impose when the chain end is an active polymerization site. Importantly, in further contrast to the existing theoretical work on brush conformation, we initially focus on the dry, fully collapsed brush in the absence of solvent to concentrate on the total amount of polymer that has been incorporated into the brush, before we look at their solution properties. We propose that brushes of greater thickness will be grown on smaller particles because of a lower rate of termination, as the increased curvature results in a greater distance between active sites. To test this hypothesis, we have synthesized poly(DEA) brushes on silica particles with diameters ranging from 120 to 840 nm and compared them to brush growth on planar wafer substrates.
Article
EXPERIMENTAL SECTION
Materials. A range of sizes of AngstromSphere colloidal silica were supplied by Fiber Optic Center, and used as received. The manufacturer-quoted size and the actual particle diameters (measured by scanning electron microscopy (SEM) and hydrodynamic diameters measured by dynamic light scattering (DLS)) are reported in Table 1. In this Article, the silica particles will be referred to by their measured SEM diameter rounded to two significant figures.
Table 1. Details of the As-Received AngstromSphere Colloidal Silica Particles Used in This Study silica sample name (nm)
manufacturer quoted diameter (μm)
120 200 450 840
0.1 0.25 0.5 1.0
SEM diameter (nm) 120 202 448 839
± ± ± ±
7 11 14 18
hydrodynamic diameter (DLS) (nm) 124 236 486 957
± ± ± ±
4 4 36 49
The inhibitor was removed from 2-(diethylamino)ethyl methacrylate monomer (DEA, Aldrich, 99%) using a short alumina column (activated, basic), and inhibitor-free monomer was stored at −20 °C and warmed to room temperature immediately before use. Copper(II) bromide (99.999%), 2,2′-bipyridine (≥99%), L-ascorbic acid (≥99%), and (+)-sodium L-ascorbate (≥98%) were purchased from Aldrich and used as received. Ethanol (absolute, Ajax Finechem) was distilled before use, and Milli-Q water was used throughout. The cationic poly[2-(dimethylamino)ethyl methacrylate-stat-glycerol monomethacrylate] (PDMA-PGMA)-based water-soluble macroinitiator (MI), the structure of which is shown in Figure 1, was previously reported.17 The PDMA65-PGMA29 statistical copolymer precursor (Mn = 14 880; Mw/Mn = 1.27) was confirmed to contain a molar ratio of 2:0.9 DMA/GMA units by 1H NMR. The copolymer precursor was sequentially derivatized to produce the macroinitiator by esterifying the dihydroxy-functional glycerol residues with 2bromoisobutyryl bromide to provide the ATRP initiator sites and then by quaternizing the tertiary amine groups with methyl iodide, making the copolymer water-soluble and cationic to promote adsorption on the negatively charged silica substrates.7,24 The resulting macroinitiator contained approximately a 1:1 ratio of initiator to positively charged sites. Macroinitiator Adsorption onto Silica Particles. The cationic macroinitiator was adsorbed onto the silica particles (2.0 g) by the dropwise addition of a well-dispersed 10 wt % aqueous suspension of silica to an equal volume of 4 mg/mL macroinitiator. The resulting 5 wt % silica, 2 mg/mL macroinitiator suspension was stirred overnight to allow the adsorption to reach equilibrium. The concentration of macroinitiator was high enough to achieve a maximum adsorbed amount on the silica, although not so high as to cause flocculation. Macroinitiator-modified silica particles were recovered by centrifugation, and washed by 2 cycles of redispersion in water and centrifugation. Poly(DEA) Brush Growth on Particles by SI-ARGET ATRP. DEA monomer (20.00 g) was dissolved in an equal volume of solvent (typically 4:1 v/v ethanol (16 mL)/water (4 mL)), and the resulting mixture was divided in half. One half was used to disperse the macroinitiator-modified silica particles and the resulting suspension was deoxygenated by bubbling with nitrogen for 15 min. The second half was separately deoxygenated before copper (II) bromide (CuBr2, 0.0096 g), 2,2′-bipyridine (bpy, 0.067 g), and sodium ascorbate (Na asc., 0.086 g) was added (DEA/CuBr2/bpy/Na asc. molar ratio of 2500:1:10:10 were present in the final polymerization mixture). The polymerization was initiated by combining the macroinitiator-modified silica particle suspension with the reaction mixture in a separate deoxygenated 100 mL round-bottomed flask, and the resulting polymerization mixture contained ∼4 wt % silica. The polymerization was carried out under a positive pressure of nitrogen, typically at room temperature (∼22 °C), and samples were withdrawn at timed 6132
dx.doi.org/10.1021/la4004092 | Langmuir 2013, 29, 6131−6140
Langmuir
Article
Figure 1. Reaction scheme depicting the electrostatic adsorption of the cationic macroinitiator onto colloidal silica particles and subsequent poly(DEA) brush growth by SI-ARGET ATRP. intervals. Samples were diluted with cold (4 °C) 1:1 ethanol/water (oxygenated) to slow the reaction and were centrifuged to separate the poly(DEA)-brush particles from the reaction mixture, quenching the reaction. Brush-modified particles were further washed with 1:1 ethanol/water by successive redispersion and centrifugation cycles before analysis. For the hybrid particles that were synthesized to investigate the effect of surface curvature, the maximum monomer incorporation into the hybrid products corresponded to less than 14% monomer depletion for the synthesis of the 120 nm hybrids,
