Presence of low-melting crystallinity in ethylene-propylene elastomers

Abstract

The thermal behaviour of some ethylene—propylene random copolymers (EPRs) with 55–70 mol% ethylene content was investigated in the temperature range from − 100 to + 200°C. Irrespective of differences in chemical composition broadness, resulting from different catalytic systems, all samples display a low-melting crystallinity of low degree, developing rapidly when the material is cooled to low temperatures. The poorly crystalline phase, undetectable by X-rays, melts over a broad range of temperature, the lower limit merging with the glass transition and the upper one ending at ∼60°C. The crystal size distribution is affected by isothermal treatments performed below room temperature, but the overall degree of crystallinity is not modified. The low-melting crystalline domains are associated with short ethylene segments comprising < 10 CH 2 sequences. Experimental evidence suggests that 55 mol% is the lower limit of ethylene content in EPR copolymers required to achieve a detectable crystallinity which melts below room temperature.