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.

The structure and morphology of a set of high hard block content (50% to 100% hard segment by weight) linear thermoplastic polyurethanes has been investigated mainly via small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). All the results obtained in this investigation have been correlated to our previous work on the thermodynamic properties of the same set of samples (Saiani et al. Macromolecules 2001, 34, 9059). The soft segment was based on a poly(propylene oxide) polyol end-capped with ethylene oxide and the hard segment on a 4,4‘-methylenediphenyleneisocyanate (MDI) chain extended by 2-methyl-1,3-propanediol (MP−Diol). Samples with a hard segment volume fraction higher than 65% are shown to have a morphology consisting in soft phase domains embedded in a hard phase matrix. Alignment of the soft phase domains could be observed under specific preparation conditions. From our SAXS results, the same average interdomain distance was found for all the samples (d i ≈ 15 nm). These results confirm our DSC results and suggest a two-phase structure for the melt-quenched samples:  “pure” hard segment phase + mixed phase (soft + hard segments) with a hard segment content of 65 wt %. The mixed phase then undergoes phase separation during the annealing at 120 °C. The dynamics of the phase separation was also investigated showing a strong correlation between the peak observed in the scattering curves and the so-called T MMT melting endotherm. The results confirm the assignment of this endotherm to the microphase mixing of the soft and hard segments. The rate of phase separation was found to be a function of the hard segment content of the samples, and a delay time was found before the start of the phase separation process for the high hard block content samples. The degree of phase separation was calculated and found to be the same for all the samples except the PU-50%HS sample, which showed a higher degree of phase separation. The interface thickness is found to increase with increasing hard segment content.

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