A Study on the Viscoelastic Properties of polyurethanes

Polyurethane elastomers with almost equal crosslinking density but with different polar segments and different extents of softening were prepared from poly(oxypropylene glycol) by the prepolymer method. Physical properties such as gel fraction, swelling ratio, density, glass transition temperature, extent of softening, stress relaxation, and energy absorption were measured. MOCA-cured polyurethanes (group I) showed softening to a large extent compared with TMP-cured ones (group II). The extent of softening was affected by the structure of diisocyanate. Ten-second modulus vs. temperature relationships obtained from stress relaxation data revealed that group I has fairly broad glass transition region. The other showed nromal temperature dispersions in the relationships. Tg determined through the measurement of linear thermal expansion were in good accordance with the 10-sec modulus curves for group II as in the case of ordinary amorphous polymer networks, but were not true for group I. In rubbery region, moduli of group I were several times higher than those of group II owing to the strong hydrogen bonding ability. In each group, rubbery moduli increased with increasing flexibility of diisocyallate segment. Energy absorption determined by ball rebound method also showed that group I has broad transition regions. Stress relaxation curves were super-imposed successfully and master curves were obtaind. Temperature dependency of log aT obeyed WLF equation. The distribution curves of relaxation times at reference temperature Ts were calculated, which clearly showed that group II behaves as typical amorphous polymer networks and that, in contrast, group I has remarkably gentle slope in transition region. The effect of interchain hydrogen bonding on relaxation mechanisms was discussed