The chemical structure of the amorphous phase of propylene–ethylene random copolymers in relation to their stress–strain properties

A better understanding of structure-property relations is necessary to design novel materials. In this study, we investigate the morphology and chemical structure of five commercial grades of propylene-based polymers in relation to the change in yield- stress as a function of strain-rate. Substantial emphasis has been laid on understanding the chain microstructure in the relation to chain dynamics in the amorphous phase. Heterogeneous Ziegler–Natta catalysis was used to prepare the samples with differing ratios of propylene and ethylene units. Various analytical techniques such as WAXS, SAXS, solution- and solid-state NMR were employed to characterize their structure. The results indicate a reduction in crystallinity, melting temperature, long-period and crystal thickness with increasing ethylene content. Solid-state NMR data reveal the presence of four components in these samples, which is an extension of the traditional three phase model found in most semi-crystalline polymers. The additional fourth phase is attributed to a rubber-like component that is primarily composed of chain segments rich in ethylene units and shows an increase in chain dynamics with increasing ethylene content in the samples. Mechanical experiments show that yield stress decreases with increase in the amount ethylene which can be correlated to the observed increase in chain dynamics in the amorphous phase.