Thermally induced time dependence of mechanical properties in biomedical grade polyurethanes.

The time dependence of the mechanical properties of segmented urethanes as well as urethane-urea systems were monitored after the materials had been given a short thermal treatment followed by rapid cooling. Both linear and crosslinked materials were studied but the major focus was on many of the common biomedical grade urethanes. As had been noted in earlier studies on nonmedical segmented urethanes from this laboratory, many of the biomedical grade materials also showed time-dependent changes in mechanical properties that can be directly related to time-dependent changes in the degree of domain structure (microphase separation) that may occur in these segmented copolymers. Interestingly, those systems possessing significant amounts of urea linkage show little or no significant time-dependent changes in structure or properties following thermal treatment. The effect of chemical cross-linking can also influence the domain formation process and its thermal stability. The ramifications of these time dependent effects may have bearing on the biomaterial applications of segmented urethane polymers.