Temperature- and Tension-Induced Coil−Globule Transition of Poly(<i>N</i>-isopropylacrylamide) Chains in Water and Mixed Solvent of Water/Methanol

Abstract

The concept of cooperative dehydration, defined as the simultaneous dissociation of the water molecules bound in correlated sequences to a polymer chain, has been applied to study the collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating in aqueous solutions. We examined the applicability of this concept in three situations: (i) PNIPAM in water (in the absence of added force), (ii) PNIPAM in water subjected to a tension applied to the chain ends, and (iii) PNIPAM in a mixed solvent of water and a second water-miscible solvent. The transition becomes sharper as the cooperativity parameter of hydration increases. The tension−elongation curve of a hydrated chain at various temperatures, calculated following an approach similar to the classical theory of coil−helix transition, presents a flat plateau corresponding to the tension for which collapsed segments reel out of the globules. The reeled-out segments are hydrated immediately upon exposure to water. The calculations suggest a possible shift to higher temperatures of the cloud points of aqueous PNIPAM solutions under shear flow. The reentrant coil−globule−coil transition in mixed solvent of water and methanol is studied from the viewpoint of competitive hydrogen bonds between polymer−water and polymer−methanol.