Glass Transition Behaviors of a Polyurethane Hard Segment based on 4,4‘-Diisocyanatodiphenylmethane and 1,4-Butanediol and the Calculation of Microdomain Composition

The long-term confusing glass transition temperature, T g, and specific heat capacity, Δ C p, of the 4,4‘-diisocyanatodiphenylmethane (MDI) and 1,4-butanediol (BD) hard segment in segment polyurethanes have been clearly evidenced by using a new series of completely phase-mixed polyurethanes while being extrapolated to 100% hard segment content. The hard segment T g and Δ C p thus obtained are 108 °C and 0.38 J/g °C, respectively, which are equivalent to the reported Tg and Δ C p of a high molecular weight MDI−BD homopolymer. In addition, if a single homogeneous phase is present, the observed Δ C p and T g are found to be given by the linear weighted combination of the pure constituent values. This provides a simple relationship to access the composition of the individual microdomain in segment polyurethanes, providing that the phase-separate morphology does not affect the microdomain glass transition behavior.