Molecular Mobility in Mixtures of Absorbed Water and Solid Poly(vinylpyrrolidone)

Abstract

Poly(vinylpyrrolidone) (PVP) was used as model system to examine molecular mobility in mixtures of absorbed water with solid amorphous polymers. Water vapor absorption isotherms were determined, along with diffusion and proton NMR relaxation measurements of absorbed water. Concurrently, measurements of glass transition temperatures (T_g) and carbon-13 NMR relaxation times for PVP were determined as a function of water content. Two water contents were used as reference points: W_m, obtained from the fit of water absorption isotherms to the BET equation, corresponding to the first shoulder in the sigmoid isotherm; and W_g, the amount of water necessary to depress T_g to the isotherm temperature. Translational diffusion coefficients of water, along with proton T₁ relaxation time constants, show that both the translational and the rotational mobility of the water is hindered by the presence of the solid polymer and that the absorbed water is most likely represented by two or more populations of water with different modes or time scales of motion. The presence of "tightly bound” or immobilized water at levels corresponding to W_m, however, is unlikely, since water molecules maintain a high degree of mobility, even at the lowest levels of water. Above W_g, water shows an increase in mobility with increasing water content, but it is always less mobile than bulk water. With increasing water content, carbon-13 T₁ relaxation time constants for PVP, measured under the same conditions as above, indicate a major increase in the molecular mobility of carbon atoms associated with the pyrrolidone side chains.