Monohydric alcohol transport in a rubbery poly(urethan)

Steady-state permeation rates and equilibrium sorption measurements were made as a function of temperature for a series of alcohols in a polybutylene adipate polyurethan. The alcohol series included methanol; ethanol; n-propanol; n-, iso-, sec-, and t-butanol; n-pentanol, n-hexanol, and n-heptanol (all at unit activity). Calculated integral diffusivities of the various penetrants correlated well with effective penetrant size. The solubility coefficients for the various alcohols increased as the difference between the solubility parameters of the polymer and penetrant decreased for the normal alcohol series. The solubility coefficients for the branched alcohol isomers were characterized by values lower than those predicted by the correlation for the normal alcohols; possibly steric hindrance limits sorption on specific polymer sites. A detailed analysis of the system ethanol/poly(butylene adipate-polyurethan) revealed a sigmoidal increase of the diffusion coefficient with increasing penetrant concentration, suggesting a competition between penetrant clustering and polymer plasticization. The diffusion process appears to be time dependent as well as concentration dependent. Anomalies in the diffusion behavior that can reasonably be related to polymer segmental relaxation are present to a much larger degree as temperature is increased. Ethanol sorption was further complicated by positive deviation from Henry's law at activities in excess of 0.2.