M. Scoti, F. Auriemma, and C. De Rosa
Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso di Monte Sant’Angelo via Cintia, 80126 Napoli, Italy.
In this communication we report a study of the influence of the presence of crystallinity in the undeformed state and of the occurrence of stress-induced crystallization (SIC) on the mechanical properties and elastic behavior of ethylene-propylene-diene (EPD) random terpolymers, which are the basic material of many commercial rubbers. EPD terpolymers with ethylene concentration in the range 44-78 wt% have been analyzed. EPD samples with high ethylene content of 78 wt% are crystalline in the undeformed state and crystallize from the melt in the pseudohexagonal form of polyethylene. Samples with 71-64 wt% of ethylene show broad diffraction profiles typical of amorphous copolymers but show endothermic peaks in the DSC heating curves and well-defined correlation peaks in the small angle X-ray diffraction (SAXS) patterns. This indicates that presence of a certain amount of crystals that are not visible by wide angle diffraction. Further crystallization of the pseudohexagonal form of PE occurs during deformation (SIC). These results indicates that in EPD terpolymers, even when the concentrations of propene units is high in the range 30-40 wt%, ethylene sequences are long enough to crystallize, due to partial inclusion of propene units in the crystals (1). The crystallinity present in the undeformed state and its improvement during deformation are keys for understanding the remarkable mechanical performances of these rubbers.
 O. Ruiz De Ballesteros, F. Auriemma, G. Guerra, P. Corradini, Macromolecules 29, 7141 (1996). (link)