Origin of Multiple Melting Endotherms in a High Hard Block Content Polyurethane. 1. Thermodynamic Investigation

The phase behavior of a set of high hard block content (50% to 100% hard segment by weight) linear thermoplastic polyurethanes has been investigated mainly via differential scanning calorimetry (DSC). The soft segment was based on a polypropylene oxide polyol end-capped with ethylene oxide and the hard segment on a 4,4‘-methylene diphenyldiisocyanate (MDI) chain extended by 2-methyl-1,3-propanediol (MP-Diol). By investigating thermal behavior of the samples, it has been possible to assign the observed high-temperature endothermic transitions to the disruption of an ordered structure appearing in the hard phase under certain annealing conditions and to the microphase mixing of the soft and hard segments. These results suggest a two-step melting process: (1) melting of the ordered structure present in the hard phase; (2) microphase mixing of the soft and hard segments. Wide-angle X-ray scattering experiments gave further support to this assignment. In addition, investigation of the melt-quenched samples has shown that for a hard segment concentration lower than 65% a homogeneous mixed phase is obtained while for a hard segment concentration higher than 65% a two-phase system is obtained, one pure hard segment phase coexisting with a mixed phase with a hard/soft segment weight ratio of ∼1.8 corresponding to 65% hard segment concentration.