Sorbed water on polymers near 0 °C

Abstract

Sorption of water has been measured for polyacrylonitrile, polymethylmethacrylate, and atactic and isotactic polystyrene over a range of temperatures from negative to positive degrees centigrade and for polyvinylpyrrolidone at −2 °C, +7 °C and +10 °C. The isosteric heats and differential molar entropies as function of coverage show in the cases of polyacrylonitrile and polymethylmethacrylate that water below monolayer coverage is less strongly bound and of less order then condensed water. At about one monolayer coverage, the sorption goes over to condensation of water. Parameters derived from the BET equation are in agreement with the conclusions derived from isosteric heats and entropies of adsorption. Atactic and isotactic polystyrene do not sorb water at all up to a relative pressure of 0.85 but adsorb some water at higher relative pressures. Polyvinylpyrrolidone is a special case, as it is water soluble. The sorption isotherms have a zero temperature coefficient. They are similar to the “isotherm” previously obtained for “bound” water in freezing PVP solutions. The conclusion is reached that water sorbed on or “bound” to PVP has an ice-like structure.