Thermoelasticity and conformational behavior of polyethylene and ethylene‐propylene copolymers

Force‐temperature measurements at constant length and pressure are reported for amorphous, crosslinked networks of polyethylene and some ethylene‐propylene copolymers. The ratio of the energy contribution to the total tension, fe/f , calculated from these measurements and thermal expansion data, was found to depend strongly on the extension ratio as well as on the crosslink density. In current theory fe/fT is equal to the temperature coefficient of the unperturbed mean‐square end‐to‐end distance of the polymer chain in the network ( d In ( r 0 2 )/ dT ). The experimentally found nonconstancy of the fe/f makes it impossible to calculate a single value of d In ( r 2 0 )/ dT ) for comparison with our a priori calculations of this quantity. These findings are at variance with previously published data of Flory, Ciferri, and Hoeve. Possibly, stress‐induced crystallization or, more generally, stress‐dependent intermolecular forces may play a role in thermoelas‐ticity studies under certain circumstances, in spite of the reversibility of the temperature cycles.