MORPHOLOGICAL-CHANGES IN SEGMENTED POLYURETHANE ELASTOMERS BY VARYING THE NCO/OH RATIO

The morphological changes induced to segmented polyurethane elastomers by varying the NCO/OH ratio during the second step of polymerization, while keeping constant the total hard-segment content (similar to 30%), were studied by means of Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and the thermally stimulated depolarization current method (TSDC). The DSC results gave evidence for the existence of phase separation in thermoplastic polyurethanes, whereas the elastomeric ones appeared to form a more homogeneous network. The FTIR results gave additional evidence regarding phase separation by detecting the strength of the hydrogen bonds. Moreover, an attempt was made to assess the relative contribution made to the hydrogen bonding in such systems by the two potential accepters: ester and urethane carbonyls. The TSDC results gave a relaxation mechanism that is due to interfacial (Maxwell-Wagner-Sillars) polarization, providing evidence of the existence of an interfacial phase. The introduced secondary chemical cross-links was found to affect mainly the irregularly packed hard domains and to form a more homogeneous network at higher values of the NCO/OH ratio. (C) 1994 John Wiley and Sons, Inc.