MULTIPHASE STRUCTURE OF SEGMENTED POLYURETHANES - EFFECTS OF HARD-SEGMENT FLEXIBILITY

Synchrotron small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) were used to investigate the structure of a few series of segmented polyurethanes with different hard-segment flexibilities. The segmented polyurethanes based on 1,6-hexamethylene diisocyanate (HDI) and 1,4-butanediol (BD) as the hard segment showed a folded-chain conformation. The phase structure was found to be insensitive to the increasing hard-segment content and thermal treatment. Phase separation was very fast in these systems as the hard-segment mobility was relatively high and the system viscosity was low. DSC results showed a soft-segment glass transition temperature which was only about 5-degrees-C above that of the pure soft segment, indicating that the separation between soft and hard segments was nearly complete. The segmented polyurethanes based on 4,4'-methylenebis(phenyl isocyanate) (MDI) and 4,4'-diaminodiphenyl ether (DDE) probably did not exist in the folded-chain conformation. DSC results showed a soft-segment glass transition temperature which was about 15-degrees-C above that of the pure soft segment. Both systems showed very strong interactions among the hard segments. Results were discussed based on the viscosity-mobility-interaction argument. In addition, a long-time controversy about the dependence of T(g,s) upon the chemical structure of the soft and hard segments and the soft- and hard-segment lengths could be explained by the viscosity-mobility-interaction argument. The present study once again suggested the importance of kinetic effects in formulating a better understanding of the structure-property relationships of segmented polyurethanes.