Competitive Hydrogen Bonds and Cononsolvency of Poly(N-isopropylacrylamide)s in Mixed Solvents of Water/Methanol

Abstract

Collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating in aqueous solutions is theoretically studied on the basis of cooperative dehydration (simultaneous dissociation of bound water molecules in a group of correlated sequence), and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods. The transition becomes sharper with the cooperativity parameter σ of hydration. Phase diagrams with very flat LCST phase separation line for aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions are theoretically derived on the basis of sequential hydrogen bond formation between polymer chains and water molecules (cooperative hydration), and compared with experimental spinodal curves. The two-phase region systematically changes its shape with the cooperativity parameter σ, and the spinodals turned out to be almost independent of the polymer molecular weight for strongly cooperative hydration (small σ) as observed in PNIPAM solutions. Reentrant coil-globule-coil transition in mixed solvent of water and methanol is also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves (LCST cononsolvency) with the mol fraction of methanol due to the competition is calculated and compared with the experimental data.