Unveiling Hidden Crystallinity in Ethylene-Propylene(-Diene) Elastomers with Small-Angle X-ray Scattering and Electron Microscopy

Ethylene-propylene(-diene) (EP(D)M) copolymers and terpolymers are generally amorphous elastomers that may exhibit crystallization at high ethylene concentration and/or during deformation. The EP(D)M elastomers studied appear amorphous by wide-angle X-ray diffraction (WAXS) at 25 °C in the undeformed state but exhibit melting features in calorimetry (DSC) curves, suggesting the presence of some crystallinity invisible to WAXS. We have revealed this hidden crystallinity by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) analysis that unambiguously have proved the existence of crystalline aggregates, whose polyethylene nature has been unveiled by TEM after epitaxial crystallization. This is a unique example of a crystalline phase that is better evidenced by SAXS than WAXS. The “morphological correlation”, due to the aggregation of parallel crystalline lamellae, is more readily detectable in these EP(D)M samples than the “crystalline correlation”. The hidden crystallinity at 25 °C provides an additional contribution to the mechanical performance of EP(D)M rubber, affecting the level of further crystallization occurring during deformation with consequent strengthening of the material. In fact, non-vulcanized EP(D)M rubbers show dramatic changes of mechanical properties even for small changes of ethylene concentration, exhibiting strong strain hardening or viscous flow during deformation depending on the presence or absence of hidden crystallinity, which, indeed, depends on the length of ethylene segments.