Temperature- and pH-Responsive Schizophrenic Copolymer Brush Coatings with Enhanced Temperature Response in Pure Water

Novel brush coatings were fabricated with glass surface-grafted chains copolymerized using surface-initiated atom transfer radical polymerization (SI-ATRP) from 2-(2-methoxyethoxy)­ethyl methacrylate (OEGMA188) and acrylamide (AAm), taken in different proportions. P­(OEGMA188-<i>co</i>-AAm) brushes with AAm mole fraction >44% (determined with XPS and TOF-SIMS spectroscopy) and nearly constant with the depth copolymer composition (TOF-SIMS profiling) exhibit unusual temperature-induced transformations: The contact angle of water droplets on P­(OEGMA188-<i>co</i>-AAm) coatings increases by ∼45° with temperature, compared to 17–18° for POEGMA188 and PAAm. The thickness of coatings immersed in water and the morphology of coatings imaged in air show a temperature response for POEGMA188 (using reflectance spectroscopy and AFM, respectively), but this response is weak for P­(OEGMA188-<i>co</i>-AAm) and absent for PAAm. This suggests mechanisms more complex than a simple transition between hydrated loose coils and hydrophobic collapsed chains. For POEGMA188, the hydrogen bonds between the ether oxygens of poly­(ethylene glycol) and water hydrogens are formed below the transition temperature <i>T</i>_c and disrupted above <i>T</i>_c when polymer–polymer interactions are favored. Different hydrogen bond structures of PAAm include free amide groups, <i>cis-trans</i>-multimers, and <i>trans</i>-multimers of amide groups. Here, hydrogen bonds between free amide groups and water dominate at <i>T</i> < <i>T</i>_c but structures favored at <i>T</i> > <i>T</i>_c, such as <i>cis-trans</i>-multimers and <i>trans</i>-multimers of amide groups, can still be hydrated. The enhanced temperature-dependent response of wettability for P­(OEGMA188-<i>co</i>-AAm) with a high mole fraction of AAm suggests the formation at <i>T</i>_c of more hydrophobic structures, realized by hydrogen bonding between the ether oxygens of OEGMA188 and the amide fragments of AAm, where water molecules are caged. Furthermore, P­(OEGMA188-<i>co</i>-AAm) coatings immersed in pH buffer solutions exhibit a ‘schizophrenic’ behavior in wettability, with transitions that mimic LCST and UCST for pH = 3, LCST for pH = 5 and 7, and any transition blocked for pH = 9.