Morphological changes in segmented polyurethane elastomers by varying the NCO/OH ratio

Abstract

SYNOPSISThe 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 ( -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 acceptors: 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. I NTRODUCTIO NT h e domain structure t h a t results from phase segregation of the hard and soft segments in polyurethane-segmented copolymers is well recognized as the principal feature controlling the properties of this class of elastomers.' Considerable effort has been made in understanding the character of the domain structure and some literature reviews 2--5 are available. From these reviews, it is recognized t h a t a wide variety of variables, such as the types of hard and soft segments, molecular weight, and molecular weight distribution of the two components, the chain extender, the stoichiometric ratio between them, the processing history, and the polymerization method, will affect the degree of phase separation and hardsegment domain size and, accordingly, the bulk and surface properties of the polymers.* To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 54, 831-842 (1994) Relatively few data have been published concerning the effect of the NCO/OH ratio on the properties of linear segmented polyurethane elastomers. Redman4 reported the effect of the NCO/ OH ratio on molecular weight and on the rheological, thermal, and mechanical properties of two series of ester-based polyurethanes. Nierzwicki and Wysocka' also studied the effect of the NCO/OH ratio on the microphase separation of ester-based polyurethanes.For NCO/OH > 1, because of the excess diisocyanate, allophanate cross-links are formed. The introduction of chemical cross-links has a detrimental effect on the hard domains, namely, the physical cross-links of the copolymer. In a previous a r t i~l e ,~ it was shown that increased chemical cross-linking leads to reduction in the size and the number of hard domains, changes the morphology, and causes mixing of hard and soft, segments.Two of the most widely used te...

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