Grafting polymer chains at a sufficiently high density to realize polymer brushes is a powerful surface modification strategy. Polymer brush coatings are instrumental in imparting desirable properties such as biocompatibility, antifouling, and stimuli-responsiveness. Of particular interest, are solvent-responsive polymer brushes that readily reconfigure their wettability in both oil and water phases. While previous work has demonstrated the ability of polymer brushes to undergo conformational changes to mediate favorable interactions with the environment, our fundamental understanding of how the spatial arrangement of hydrophilic and lipophilic groups along polymer brushes shapes the kinetics of conformational changes is largely incomplete. I investigate the solvent responsivity of amphiphilic polymer brushes by copolymerizing a lipophilic monomer, 2-(diisopropylamino)ethyl methacrylate (DiPAEMA) and a hydrophilic monomer 2-(dimethylamino)ethyl methacrylate (DMAEMA. Selective methylation of DMAEMA dramatically increases hydrophilicity by placing a hard charge on the amine. Statistical copolymerization of hydrophilic and hydrophobic monomers typically presents a solubility challenge; thus, this post-polymerization methylation is essential. I tested five ratios of hydrophilic-to-hydrophobic incorporation along with two polymer microstsuctures: statistical and block. Surface-initiated atom transfer radical polymerization afforded precise control over chemical composition and polymer microstructure. The swelling response was monitored via ellipsometry equipped with a liquid cell where favorable solvent interactions result in brush swelling and unfavorable interactions leads to brush collapse and deswelling. Swelling measurements from in situ ellipsometry were confirmed via quartz crystal microbalance. By measuring the swelling response in a hydrophobic and hydrophilic solvent, wecompared polymer brushes’ solvent responsivity as a function of composition and spatial distritbution of hydrophobic/hydrophilic monomers. Copolymers of DiPAEMA and quaternized DMAEMA (QDMAEMA) imbibed the hydrophilic solvent, water (good solvent for QDMAEMA, but poor solvent for DiPAEMA), following a rule-of-mixtures type trend between the two homopolymers. Interestingly, polymer microstructure did not impact solvent responsivity; both block and statistical copolymers followed a rule-of-mixtures trend. Differences between the two architectures were noted when considering the swelling of DiPAEMA-co-QDMAEMA in the hydrophobic solvent, isopropanol (good solvent for DiPAEMA, but poor solvent for QDMAEMA), where the statistical copolymer would swell above the rule-of-mixture line, whereas the block copolymer fell below this line. While surface-sensitive techniques, such as contact angle, are often employed in the literature to comparebrush hydrophobicity/hydrophilicity, these measurements fail to capture the more nuanced solvent-responsive behavior of the brushes revealed by in situ ellipsometry and QCM. We thoroughly characterized the solvent-responsive behavior of these amphiphilic polymer brushes. Our results will inform the design of surface modification strategies to augment nanoparticle transport at oil-water interfaces.
