Cononsolvency in mixed aqueous solutions of poly(N-isopropylacrylamide)

Abstract

The lower critical solution temperature (LCST) of poly(N-isopropylacrylamide)(PNIPAAM), prepared by an aqueous redox polymerization, was studied in the cononsolvent mixture of water and methanol by means of cloud-point and microcalorimetric measurements. The observed solution behavior was quite distinct from that of poly(vinylmethyl ether) in the same binary solvent. Other cononsolvents, including dioxane and tetrahydrofuran, yielded phase diagrams similar to those of PNIPAAM in water-methanol mixtures. Flory-Huggins ternary solution theory can explain the observed results only in terms of a change in the solvent-solvent interaction parameter (χ _(12)) in PNIPAAM solutions. We believe this to be physically implausible in the dilute (0.04%) solutions used in this work. Furthermore, we find in comparing our work with that of Hirotsu (Hirotsu, S. J . Chem. Phys. 1988,88, 427) that the behavior of PNIPAAM in water-methanol mixtures is strikingly insensitive to a 200-fold variation in polymer concentration. We suggest that perturbation of is not the origin of cononsolvency or of the gel collapse transitions reported by Hirotsu and by Tanaka and co-workers (Amiya,T.; Hirokawa, Y.; Hirose, Y.; Li, Y.; Tanaka, T. J . Chem. Phys. 1987, 86,2375)