Friction and the Dynamic Mechanical and Thermal Properties of Polyurethane Elastomers. 1. Solid Polyurethanes

The results reported indicate that the properties of polyurethanes elastomers can be tailored to serve a wide variety of applications, for low 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°C to -27°C (DSC onset values). The transitions observed in the range 75 to 125°C were attributed to hard segment glass transitions. The transitions at temperatures from 140 to 170°C, which were identified especially in the samples of over stoichiometric diisocyanate concentration, evidently arose from the dissociation of allophanate crosslinks and successive ordering processes. The low temperature β” and γ transitions were observed at temperatures from -70°C to -125°C. In addition to the obvious Schatzki crankshaft mechanism, allophanate crosslinking may influence these transitions. Samples that were annealed over the crystal melting temperature and subsequently quenched exhibited higher glass transition temperatures than the samples kept for a long time at room temperature. These results can be explained in terms of better phase separation in annealing and formation of small soft segment crystallites in quenching, in contrast to the formation of large crystalline and amorphous phases in the RT samples.