Ultimate Tensile Properties of Segmented Polyurethane Elastomers: Factors Leading to Reduced Properties for Polyurethanes Based on Nonpolar Soft Segments

Abstract

Factors that could explain the lower tensile properties of nonpolar soft-segment-based polyurethane block copolymers relative to those of conventional polyether or polyester polyurethanes have been examined. Many of the materials studied in the literature have suffered from the use of soft segments with poor functionality, which has led to low molecular weight, network defects, and an unfavorable amount of crosslinking. Additionally, most of the materials have been based on soft segments whose molecular weight appears to be too high to obtain optimum properties. Polyurethanes based on nonpolar soft segments are also likely to suffer from premature phase separation during polymerization leading to low molecular weight and compositional heterogeneity, especially if the reaction is done in bulk. The lower soft-segment glass-transition temperature of, in particular, the polydimethylsiloxane polyurethanes can also contribute to their lower tensile properties at room temperature. However, if differences in the soft-segment glass-transition temperatures are accounted for by comparing samples at equivalent values of T−Tg and other parameters are optimized, it appears that only a lack of soft-segment crystallizability under strain and possibly an excessively high degree of phase separation are inherently limiting the tensile properties of nonpolar soft-segment-based polyurethane block copolymers. The limited amount of available information concerning the effects of many of these factors suggests a need for additional work in this area.