Tensile deformation mechanism of propylene-1-butene random copolymer: The role of initial crystalline morphology

Abstract

Structural evolution of two propylene 1-butene random copolymer (iPPBu) films with different initial crystalline morphology during uniaxial deformation were investigated by synchrotron wide-angle and small-angle X-ray scattering (WAXS/SAXS), Fourier-transform infrared (FTIR) spectroscopy. The iPPBu with α crystal crystallized via mesophase exhibits a nodular morphology, while melt-crystallized iPPBu exhibits a characteristic spherulitic structure. The initial crystalline morphology has been found to exert influence on the structural evolution of the samples, including cavitation, orientation behavior of molecular chains in crystalline and amorphous phases, the occurrence of mesophase, and reduction in crystallinity, etc. Compared with the iPPBu with spherulitic morphology, no obvious cavitation was observed around the yield point in iPPBu with nodular morphology. Moreover, the variation rate of crystallinity with respect to draw ratio is smaller in the iPPBu with nodular morphology. A plausible reason for the phenomenon is that the iPPBu with nodular morphology has the lower lateral extension of crystallites. The onset draw ratio (λonset) of α-crystal-to-mesophase transition in sample with nodular morphology is larger than that in the sample with spherulitic morphology. The molecular chains tend to be aligned perpendicular to the stretching direction in iPPBu with spherulitic morphology around yield point. This is attributed to the bending and separation of the lamellae with the normal parallel to the tensile direction. However, this phenomenon is not observed in the iPPBu with nodular morphology. In the strain-hardening regime, the strain hardening modulus Gp of iPPBu with nodular morphology is higher than that of iPPBu with spherulitic morphology, which results from the more cross-linking points and more tie chains in highly oriented the amorphous phase of the former. A deformation mechanism is proposed to understand the role played by the initial morphology of iPPBu based on the crystal transition, variation in crystallinity, cavitation, and orientation.