PRESENCE OF LOW-MELTING CRYSTALLINITY IN ETHYLENE-PROPYLENE ELASTOMERS

The thermal behaviour of some ethylene-propylene random copolymers (EP Rs) with 55-70 mol% ethylene content was investigated in the temperatu re range from -100 to +200 degrees C. Irrespective of differences in c hemical composition broadness, resulting from different catalytic syst ems, all samples display a low-melting crystallinity of low degree, de veloping rapidly when the material is cooled to low temperatures. The poorly crystalline phase, undetectable by X-rays, melts over a broad r ange of temperature, the lower limit merging with the glass transition and the upper one ending at similar to 60 degrees C. The crystal size distribution is affected by isothermal treatments performed below roo m temperature, but the overall degree of crystallinity is not modified . The low-melting crystalline domains are associated with short ethyle ne segments comprising < 10 CH2 sequences. Experimental evidence sugge sts that 55 mol% is the lower limit of ethylene content in EPR copolym ers required to achieve a detectable crystallinity which melts below r oom temperature. (C) 1997 Elsevier Science Ltd. All rights reserved.