LIGHT-SCATTERING FROM POLYMER MIXED-SOLVENT SYSTEMS .1. SELECTIVE ADSORPTION PHENOMENA OF POLY(N-VINYLPYRROLIDONE)

Abstract

The behavior of poly(vinylpyrrolidone) (PVP) in the mixed solvents H2O/THF,H2O/acetone, and ethanol/n-hexane is studied by the method of static light scattering. H2O and ethanol are good solvents for PVP. THF, acetone, and n-hexane are precipitants. A PVP coil first swells as the mass fraction of the precipitant is increased. Above a critical composition of the mixture the coil contracts. The nature of this change is studied on the basis of the excluded-volume theory and the Flory-Huggins theory. We find that the transition is attributed to a change in the selective adsorption. H2O and ethanol are preferentially adsorbed by PVP with regard to the systems PVP H2O/acetone and PVP ethanol/n-hexane, respectively. The system PVP H2O/THF shows both normal and inverse adsorption. The interpenetration function PSI segregates into two branches, one for each type of adsorption. With increasing temperature the number of adsorbed solvent molecules per PVP monomer unit decreases slightly. As a consequence, a PVP molecule precipitates at a higher mass fraction of the precipitant. The adsorption is not affected by a change in the molecular mass of PVP. This implies that adsorption occurs in a uniform manner along the PVP chain. Looking on the THETA state, the radii of gyration are different for each system. Longitudinally acting intramolecular repulsive forces are present, which are distinct in their strength and interaction range. They are caused by selective adsorption of one of the solvent components to the polymer.