Molecular mobility and hydration properties of segmented polyurethanes with varying structure of soft- and hard-chain segments

The molecular mobility and hydration properties of model segmented polyuret hanes from either poly(propylene glycol) (PPG) or poly(butylene adipate) (P BAD), both of molecular weight 2000 (soft segments), and three different di isocyanates (all-trans 4,4'-dicyclohexylmethane diisocyanate, 100% t,t HMDI ; HMDI with 20% of trans isomers, 20% t,t HMDI; and 4,4'-diphenylmethane di isocyanate, MDI) (hard segments) were investigated using differential scann ing calorimetry (DSC), thermally stimulated depolarization currents (TSDC) measurements, ac dielectric relaxation spectroscopy (ac DRS), equilibrium w ater-sorption isotherms (ESI), and dynamic water-sorption isotherms (DSI). No effects of the structure and of the amount of the soft segments on the o verall degree of microphase separation (DMS) into microphases rich in soft and hard segments, respectively, were observed. On the contrary, DMS depend s on the composition of the diisocyanates used and systematically increases in the order MDI, 20% t,t HMDI, 100% t,t HMDI as indicated by DSC, TSDC, a nd ac DRS. The PPG-based polyurethanes are characterized by larger values o f water content at saturation, h, and smaller values of the diffusion coeff icient of water, D. h increases with temperature, indicating that the sorpt ion process is endothermic. (C) 1999 John Wiley & Sons, Inc.