Issue 13, 2023

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 (xD = 0.10), which decreased with increasing xD; DMSO is a chaotropic cosolvent. This region was adjacent to a cononsolvency region. Interestingly, reentrant swelling was observed for above approximately xD = 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 xD. 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.

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

Supplementary files

Article information

Article type
Paper
Submitted
28 Nov 2022
Accepted
12 Feb 2023
First published
10 Mar 2023
This article is Open Access
Creative Commons BY-NC license

Polym. Chem., 2023,14, 1526-1535

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

H. Robertson, A. R. J. Nelson, S. W. Prescott, G. B. Webber and E. J. Wanless, Polym. Chem., 2023, 14, 1526 DOI: 10.1039/D2PY01487D

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