Crystallization behavior and morphology of propylene-ethylene copolymers: tailoring properties by molecular catalysis

Random propene-ethylene copolymers were synthesized in the whole range of comonomer composition from 0.4 to 84 wt% of ethylene (C2) with an isospecific metallocene catalyst. For some samples, the catalyst was supported on suitable supports. The crystallization behavior and morphology have been analyzed and correlated with the mechanical properties. Copolymers with low C2 concentration up to 16 wt% crystallize in α or γ forms of isotactic polypropylene (iPP), whereas copolymers with high C2 concentration from 65 to 84 wt% crystallize in the orthorhombic form of polyethylene (PE) and samples with C2 concentration from 26 to 60 wt% are amorphous. The high molecular mass and the uniform comonomer concentration in the whole composition range achieved thanks to the utilized synthetic strategy based on the use of a metallocene catalyst, have allowed production of materials with mechanical properties ranging from high stiffness and strength to high flexibility and ductility, to remarkable elasticity. Elastic properties develop for C2 concentrations from 14 to 16 wt% in samples showing iPP crystallinity and from 60 to 75 wt% in samples showing PE crystallinity. The mechanical properties and elastic behavior have been explained and correlated with the crystallization behavior and crystal morphology. In particular, the experimental values of stress at yielding have been correlated with the values of thickness of crystalline lamellae, estimated from the small-angle X-ray scattering (SAXS) data, whereas the observed elastic properties have been explained by the revealed interwoven crystal morphology.