Sorption and diffusion of water in a rubbery polyurethane

Unsteady‐state water sorption measurements have been carried out over the temperature range 27–40°C. on a polyurethane elastomer composed of MDI, 1,4‐butanediol, and polybutylene adipate (MW 2000) in the mole ratios 3.2:2:1. The sorption isotherm is concave upward above an activity of 0.6, reaching a saturation water concentration of 1.6 g./100 cc., the heat of mixing is zero, and the diffusion constant decreases slowly and linearly with increasing water concentration. The overall behavior is attributed to the immobilization of sorbed water in clusters. Analysis of the sorption process in terms of the cluster integral reveals that the interaction of water with specific polymer sites occurs at the lowest concentrations but that clustering dominates for water activities greater than 0.45. Since the activation energy for diffusion decreases with increasing water concentration, from 11,000 cal./mole at zero concentration to about half that value at saturation, plasticization of the polymer also affects the transport process.