Pressure-induced reentrant phase behavior in the poly(N-vinyl-2-pyrrolidone)-water system.

Abstract

Phase separation and dynamic light scattering experiments were performed on aqueous solutions of poly(N-vinyl-2-pyrrolidone) under high pressure conditions to help elucidate the role that hydrogen bonding plays in the solution behavior of water-soluble polymers. From the pressure-induced phase separation experiments we observe reentrant phase behavior in the pressure-temperature plane at fixed composition. Variation of the phase separation pressure with both composition and molecular weight is also investigated. The concentration dependence on the phase separation pressure is weak and the molecular weight dependence exhibits an approximately ${\mathit{M}}_{\mathit{w}}^{\mathrm{\ensuremath{-}}1/2}$ scaling. All this behavior can be explained through a modified Flory-Huggins theory that includes the effect of pressure and temperature on hydrogen bonds and the hydrophobic interactions. We use our data to derive fit parameters for this model and the results suggest that not only does the hydrogen bond weaken at high pressure but hydrophobic interactions increase. Dynamic light scattering (DLS) experiments on solutions within the one-phase region of the phase diagram quantify the influence of changing solvent quality on the polymer's conformation. For dilute solutions, which at ambient temperature and pressure exhibit classic good-solvent behavior, a pressure-induced crossover from good to poor solvent behavior is found. Such measurements are shown to give a measure of ${\mathit{A}}_{2}$, an otherwise difficult to measure property at elevated pressure. Above the overlap concentration, this polymer system exhibits aggregate formation. The autocorrelation spectrum contains two relaxations: a fast relaxation mode associated with the usual cooperative diffusion and a slow relaxation mode associated with internal dynamics of the aggregates. Through DLS experiments in the semidilute regime we show that these two modes merge upon increasing pressure, an effect that is completely reversible. The dispersal of the aggregates under pressure suggests that the aggregates depend strongly on water-mediated hydrogen bonds between monomers. \textcopyright{} 1996 The American Physical Society.