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.
We have investigated the thermal behavior of a set of model linear thermoplastic polyurethanes (TPU) with a relatively high content of hard segments: from 50 to 100 wt %. The soft segment of these samples consists of poly(propylene oxide), end-capped with ethylene oxide (EO-PPO-EO), while the hard segment is composed of a 4,4′-methylenediphenylene isocyanate (4,4′-MDI) chain extended by a short diol chain, 2-methyl-1,3-propanediol (MP-Diol). In the present article, we have investigated the origin of the endotherms observed when samples are annealed below the glass transition of the hard segments T gHS . The work was carried out using mainly differential scanning calorimetry (DSC) and small-angle and wide-angle X-ray scattering (SAXS and WAXS). The so-called "annealing endotherm", T A , was observed 20-30 °C above the annealing temperature. The temperature and enthalpy of T A were found to increase linearly with the logarithm of the annealing time. This endotherm was assigned to the relaxation (physical aging) of the interfacial materials. With increasing annealing temperature a change in the appearance of the hard phase glass transition, T gHP , was observed. For long annealing times T gHP is observed as an endotherm on the DSC thermographs. It is suggested that some of the hard segments undergo relaxation below T gHS resulting in an enthalpy relaxation endotherm being present below or around T gHP . An additional endotherm, T M′ , was observed as a shoulder at high temperature, just below the microphase mixing transition, T MMT . This endotherm is thought to be due to the ordering during the phase separation process of the hard segment present in the mixed phase. Finally T MMT was observed at all annealing temperature used suggesting that even at low annealing temperatures phase separation occurs. The delay time before phase separation starts and the maximum absolute degree of phase separation reached are found to increase with increasing annealing temperature. Our results suggest that a "thermodynamic equilibrium" is reached for each annealing temperature at long enough annealing times.
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