Thermoelastic properties of crosslinked polyurethanes

Thermoelastic (stress–temperature) measurements in elongation have been carried out on polyurethane elastomers obtained from poly(ethylene adipate), poly(ethylene maleate), poly(ethylene glycol), and 4,4′-diphenylmethane diisocyanate. The elastomers were crosslinked by an excess of diisocynate and some of them were additionally crosslinked by dicumyl peroxide. Values of the temperature coefficient d ln r /dT of the unperturbed dimensions of the network chains and the fraction f_e/f of the stress, which is due to energetic effects, were calculated from measurements of unswollen samples and samples swollen in benzene to a constant volume fraction ν₂ of about 0.6. The coefficient d ln r /dT and the ratio f_e/f for unswollen samples were positive but dependent on the extension ratio α and crosslinking density. The f_e/f ratio displayed a tendency to decrease with increasing α and this decrease was smaller for elastomers of higher crosslinking density. Additional crosslinking due to dicumyl peroxide reduced the dependence of f_e/f on α. Swollen samples had lower positive values of f_e/f; which were somewhat independent of α and crosslinking density. Departures from the molecular theories of rubberlike elasticity are explained by intermolecular effects, which, in general, are influenced by crosslinking.