New Developments in Thermoplastic Elastomers: The Hard Segment Inelastic Effects On The Mechanical Performance of Polyurethane Elastomers

摘要

A study was made of a family of polyurethane copolymers, in which the chemical components were: a hard segment (giving, on phase separation, hard nano-scale reinforcing particles); a soft segment (giving, on phase separation, an elastomeric matrix), and a diol chain extender. The chemical compositions of all three components were varied systematically and independently, and their mechanical responses were measured in cyclic tensile tests at room temperature, up to stretches in the range 5-6. Particular attention was paid to characterizing the inelastic features - hysteresis, and stress relaxation in interrupted tests - and their variations between the materials. The same materials were also studied by wide-angle X-ray scattering (WAXS), to determine levels of crystallinity. Results showed that hysteresis was increased by increasing hard phase crystallinity. This was the case for polyurethanes based on the novel diisocyanate 4,4'-dibenzyl diiscyanate (DBDI). The extent of stress relaxation in interrupted tests was found to increase with hysteresis and hence with hard segment crystallinity, reflecting the higher flow stress of the reinforcing hard domains. Polymers based on DBDI hard segments, displayed higher stiffness and strength than did the conventional materials. Both features of the response were attributed to differences in hard phase plastic flow stress, resulting from crystallinity in the DBDI phase.