STUDIES ON THE FIRST DSC ENDOTHERM OF POLYURETHANE HARD SEGMENT-BASEDON 4,4'-DIPHENYLMETHANE DIISOCYANATE AND 1,4-BUTANEDIOL

The first high-temperature endotherm (T-1) of polyurethane hard segmen t based on 4,4'-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BD) was studied by differential scanning calorimetry (DSC). The mater ials contain 69% (w/w) of MDI and BD as the hard segment and hydroxyl- terminated cis-polybutadiene with number average molecular weights of 1650 and 2300, respectively, as the soft segment. The hard segment and the soft segment of these polyurethanes appear to be very completely phase-separated, giving rise to very simplified material structures fo r this study. Samples under melt-quenched condition gave rise to a hig hly amorphous phase for the hard phase and a distinct hard-segment gla ss transition behavior, which enabled us to study the T-1 behavior in relation to the amorphous hard-segment T-g (T-gh). Upon annealing belo w the T-gh of the pure amorphous hard phases, both the T-1 temperature and magnitude of the T-1 endotherm increased linearly with the increa se in logarithmic annealing time (log t(a)). On the other hand, if the polyurethane were first annealed to form multiple endotherms at noncr ystalline T-2 region, the annealing above the previous T-gh gave rise to a T-1 which also increased linearly in temperature with the increas e in log t(a). These phenomena are typical of enthalpy relaxations res ulting from the physical aging of the amorphous hard segment. Thus, we suggest the long-term confusing nature of T-1 is due to an enthalpy r elaxation of the amorphous hard segment. On the other hand, we also su ggest that T-2, which was previously associated with a long-range orde r of unspecified nature, would disturb the amorphous hard segment and cause a rise in the T-g in different degrees to a higher temperature n ear T-2.