FRICTION AND THE DYNAMIC-MECHANICAL AND THERMAL-PROPERTIES OF POLYURETHANE ELASTOMERS .1. SOLID POLYURETHANES

The results reported indicate that the properties of polyurethanes ela stomers can be tailored to serve a wide variety of applications, for l ow temperature uses as well. The glass transition temperatures of the soft segments (T(gs)) which control the low temperature properties of polyurethane elastomers, ranged from -79-degrees-C to -27-degrees-C (D SC onset values). The transitions observed in the range 75 to 125-degr ees-C were attributed to hard segment glass transitions. The transitio ns at temperatures from 140 to 170-degrees-C, which were identified es pecially in the samples of over stoichiometric diisocyanate concentrat ion, evidently arose from the dissociation of allophanate crosslinks a nd successive ordering processes. The low temperature beta and gamma t ransitions were observed at temperatures from -70-degrees-C to -125-de grees-C. In addition to the obvious Schatzki crankshaft mechanism, all ophanate crosslinking may influence these transitions. Samples that we re annealed over the crystal melting temperature and subsequently quen ched exhibited higher glass transition temperatures than the samples k ept for a long time at room temperature. These results can be explaine d in terms of better phase separation in annealing and formation of sm all soft segment crystallites in quenching, in contrast to the formati on of large crystalline and amorphous phases in the RT samples.