The effect of comonomer content on structure and property relationship of propylene-1-octene copolymer during uniaxial stretching

The structure and property relationships of propylene-1-octene random copolymer having different octene comonomer concentrations were investigated. In specific, the crystal structure evolution in these copolymers during uniaxial stretching at 60 C was characterized by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. With high octene content, the copolymer behaved like an elastomer with small elastic modulus and yield stress, low crystallinity and low crystal orientation under stretching. Furthermore, step-cycle tensile test showed that the copolymer with high octene content had a high recovery ratio. With low octene content, the copolymer behaved like a plastomer with large elastic modulus and yield stress, high crystallinity and high crystal orientation under stretching. From 2D SAXS results, it was found that in low octene content sample, lamellar fragmentation occurred resulting in a significant decrease in lamellar lateral size. In contrast, in high octene content sample, stress might be mainly concentrated on the amorphous matrix, leading to an inter-lamellar slip and a small decrease in lamellar lateral size. Schematic structural changes of propylene-1-octene copolymer under tensile deformation were illustrated to explain the different elasticity behavior in these copolymers.