Poly(N-isopropylacrylamide) Microgels under Alcoholic Intoxication: When a LCST Polymer Shows Swelling with Increasing Temperature

ACS Macro Lett. 2017 Oct 17;6(10):1042-1046. doi: 10.1021/acsmacrolett.7b00557. Epub 2017 Sep 12.

Abstract

Poly(N-isopropylacrylamide) (PNIPAM) microgel is a smart polymer that shows a volume phase transition temperature (VPTT) at around 32 °C in aqueous solutions, above which it collapses. In this work, combining experiments and molecular simulations, it is shown that PNIPAM microgels do not always exhibit a collapsed structure above the VPTT. Instead, PNIPAM in aqueous alcohol mixtures shows a two-step conformational transition, i.e., a collapse at low temperatures (T < 32 °C) and a reswelling when T > 50 °C. The present analysis indicates that delicate microscopic interaction details, together with the bulk solution properties, play a key role in dictating the reswelling behavior. Even when PNIPAM microgels swell with increasing T, this is not a standard upper critical solution behavior.