Thermoelasticity of stretched elastomers under small torsions: Effects of crosslinking, extension, and swelling

By using a simple version of Treloar's torsion method the internal energy contribution Me to the restoring couple M has been determined for stretched testpieces subjected to small torsions. Values obtained for Me/M decreased significantly with increasing extension. Corresponding decreases were observed on swelling, but no specific effect of swelling liquids could be detected. The degree of crosslinking had no effect on Me/M. Thus, Me/M at small strains appears to reflect a characteristic molecular property, largely independent of local environment, network density, or type of deformation, as Flory has maintained. The present values are generally consistent with previously published data, except in the case of polyethylene, for which a much larger negative value is obtained, i.e., −1.6 compared to −0.5. The reason for this discrepancy is not known.