pH-Responsive Nanostructures Assembled from Amphiphilic Block Copolymers Chen Xu, ² Bradford B. Wayland, ‡ Michael Fryd, ‡ Karen I. Winey, ² and Russell J. Composto* ,² Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6272, and Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323 ReceiVed April 19, 2006; ReVised Manuscript ReceiVed June 27, 2006 ABSTRACT: We present a novel route to assemble thin films containing pH-responsive nanostructures of hydrophilic cylindrical domains oriented perpendicular to a silicon substrate. The amphiphilic block copolymer, poly(styrene-b-acrylic acid) (PS-b-PAA), is prepared from the precursor, poly(styrene-b-tert-butyl acrylate) (PS- b-PtBA), by an autocatalytic reaction involving surface hydroxyl groups. The surface morphology and evolution of the nanostructures in aqueous solutions over a pH range of 2.6-9.1 are captured by in-situ atomic force microscopy (AFM). The ordered PS-b-PAA films exhibit unique surface morphologies across three pH regimes. At low pH (pH < 4.0) PAA chains collapse within the cylindrical domains, resulting in a hexagonal packed array of holes. At intermediate pH (4.0 < pH < 6.0) the PAA cylinders swell and transform into mushrooms with swollen caps. The height of these caps is pH-dependent, and dynamics are described by a two-stage swelling mechanism. At high pH (pH > 6.0) PAA chains stretch strongly to cover the entire surface, leading to a continuous PAA wetting layer decorated by hexagonally packed depressions. The equilibrium film thickness increases as pH increases and is reversibly recovered upon decreasing pH. The water contact angle decreases by 30° as pH increases from 2.6 to 9.1, demonstrating that wettability can be tuned by varying the pH of the surrounding medium. Because of their pH-responsive behavior and small feature size, nanostructured devices designed from amphiphilic block copolymers have potential applications including sensors and membranes. Introduction Soft materials that respond to external stimuli are of great interest for applications ranging from artificial muscle to drug delivery. 1-4 Polymer gels are particularly attractive as reversibly responsive materials because they can swell or collapse by several hundred times in response to subtle variations in external stimuli such as temperature, pH, and electric field. 5 Because this responsive behavior can transform chemical energy directly into mechanical work, stimuli-responsive materials can be used to build macro- or nanoscale machines such as those that mimic living organisms. 6 For example, artificial muscle and biomimetic actuators have been fabricated from polymer gels that respond to an electric field. For instance, a gel of poly(vinyl alcohol) containing free poly(acrylic acid) chains undergoes rapid bending deformation due to an applied electric field. 6 Besides bulk properties, surface properties such as wetting and biocom- patibility can spontaneously adapt to environmental stimuli. 7-9 Tunable surfaces have recently been employed to create sensors, chemical gates, and protein adsorption devices. 10-14 Surface response can be imparted by structural reorganization or compositional rearrangements. 7,15 For example, layers of mixed polymer brushes or Y-shaped molecules demonstrate high sensitivity and selectivity upon exposure to good and bad solvents. 16-20 Because they self-assemble into periodic domains with complementary properties, block copolymers are promising candidates for preparing responsive soft materials. For example, upon exposure to acetic acid, poly(styrene-b-methyl methacryl- ate) (PS-b-PMMA) films exhibit extensive swelling of the PMMA domains, resulting in a nanoporous structure. 21,22 Whereas a majority of copolymer studies involve hydrophobic blocks, 23-30 amphiphilic block copolymers are receiving growing interest because they can behave as nanoreactors and stimuli- responsive materials. 31 In solution, amphiphilic block copoly- mers self-assemble into micellar structures, such as spheres, which can be used as nanoreactors to synthesize nanoparticles or nanoclusters. 32-35 Amphiphilic block copolymers are also attractive as pH-responsive materials because domains can be tuned to respond to aqueous environments. For example, a copolymer containing a structural block that assembles into physical cross-links with a polyacid block has been used to create a chemically driven synthetic muscle. 36 Consisting of hydrophobic, glassy PMMA spheres embedded in a poly- (methacrylic acid) matrix, this nanostructured gel responds in a reversible, affine manner upon exposure to cyclic pH variations. As another example, Armes et al. prepared biocom- patible, pH-responsive micelles and vesicles with pH-tunable permeability. 37-39 In a novel approach to control nanostructure formation and eliminate micelle formation in solution, Krausch et al. spin-coated a triblock copolymer containing a tert-butyl methacrylate block which was made hydrophilic by acid- catalyzed deprotection. 40 This copolymer of poly(styrene-b-2- vinylpyridine-b-methacrylic acid) (PS-b-P2VP-b-PMAA) dis- played reversible swelling/shrinking of film thickness while retaining a perforated lamella structure. Previously, we have shown that amphiphilic diblock copoly- mer films of PS-b-PAA can self-assemble into reversible, stimuli-responsive nanostructures. 4 The nanostructure consists of perpendicular, hydrophilic, cylindrical PAA domains embed- ded in a glassy, hydrophobic PS matrix. Upon exposure to water, ² Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter. ‡ Department of Chemistry. * Corresponding author: phone (215) 898-4451; fax (215) 573-2128; e-mail composto@seas.upenn.edu. 6063 Macromolecules 2006, 39, 6063-6070 10.1021/ma060881f CCC: $33.50 © 2006 American Chemical Society Published on Web 08/04/2006