The effect of thermal history on the phase transitions, morphology, and rheological behavior of six thermoplastic polyurethanes (TPUs) with varying contents of hard segment, each having 4,4'-methylene bis(phenyl isocyanate) and 1,4-butane diol as hard segment and poly(epsilon-caprolactone) diol as soft segment was investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), fourier-transform infrared (FTIR) spectroscopy, and polarizing optical microscopy (POM). Specifically, we conducted dynamic frequency sweep experiments at various temperatures for all six TPUs synthesized, as well as isochronal temperature sweep experiments using the parallel-plate fixture of a cone-and-plate rheometer. In the dynamic frequency sweep experiments, the initial morphology was controlled by heating a specimen to a temperature above the isotropization temperature (Ti) of the hard segment, and then slowly cooled the sample down to a predetermined temperature below the Ti of each TPU specimen. Initial DSC experiments provided information on thermal transition temperatures of each TPU specimen. The rheological measurements are interpreted with the aid of the images from polarized optical microscopy taken under the same thermal histories employed as for the rheological measurements. It was found that thermal history of specimen (L3) had a profound influence on the variations of storage modulus (G ') and loss modulus (G″) with time observed during isothermal annealing, based upon the structural development of spherulitic superstructure of hard segment. The differences in the rheological behavior observed among the different TPUs synthesized in this study are further interpreted with the aid of Fourier transform infrared spectra obtained at various temperatures. It is concluded that hydrogen bonding has a profound influence on the time variations of rheological behavior during measurements. Finally, we have investigated time variations of G ' obtained during isochronal dynamic temperature sweep experiments. It has been observed that values of G' for a TPU specimen having low content of hard segments decrease gradually with increasing temperature, whereas values of G' for a TPU specimen having high content of hard segments decrease rapidly at a critical temperature, which is very close to the melting temperature of the microcrystallites in the hard segment. It is concluded that the determination of the microphase-separation temperature of multi-segmented TPUs is very difficult, if not impossible, owing to the existence of mixed phases consisting of hard and soft segments, making the phase interface much diffused.
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