Cosolvent effects on the structure and thermoresponse of a polymer brush: PNIPAM in DMSO–water mixtures

Abstract

Cosolvents play an integral role in polymer solubility, with myriad applications in drug delivery and energy storage. In particular, dimethyl sulfoxide (DMSO) has received substantial attention to date due to its cryoprotective properties and interesting nonideal mixing behaviour. Here, for the first time, we probe the fundamentals of DMSO–water solvent structuring using a thermoresponsive poly(N-isopropoylacrylamide) (PNIPAM) brush as an exemplar. Spectroscopic ellipsometry and neutron reflectometry were employed to monitor changes in brush swelling and conformation as a function of temperature and solvent composition, whereby changes in solvent structure can be deduced. Importantly, unlike free polymer, grafted polymers permit measurements across the entire solvent composition space, including ‘poor’ solvent conditions, permitting the characterisation of polymers in complex media for future technologies. In the water-rich regime, the prevalent hydrogen-bond network resulted in the PNIPAM brush exhibiting a lower critical solution temperature (LCST) up to DMSO mole fractions of 0.10 (x_D = 0.10), which decreased with increasing x_D; DMSO is a chaotropic cosolvent. This region was adjacent to a cononsolvency region. Interestingly, reentrant swelling was observed for above approximately x_D = 0.2. In DMSO-rich regimes, non-site-specific dipole–dipole interactions resulted in the PNIPAM brush exhibiting an uppercritical solution temperature (UCST), whereby the periphery of the swollen brush was more diffuse than at low x_D. At all temperatures, pure DMSO is a good solvent for PNIPAM and no thermoresponse was observed. Herein we demonstrate how the structure and swelling of a polymer brush film can be modulated by tuning solvent composition by mixing two ‘good’ solvents.