Temperature Dependence of Tensile Properties and Deformation Behavior in Highly Strong Heat-Elongated Polypropylene

We investigated the tensile properties and deformation behavior at various temperatures of highly strong heat-elongated polypropylene (PP), in which stacks of crystalline lamellae are macroscopically arranged in the elongated direction and lamellae are connected by thin fibrils. The elastic modulus <i>E</i>' and the αc-relaxation temperature for the onset of crystalline chain motion, obtained through dynamic mechanical analysis, were higher in the heat-elongated than the unelongated PP, indicating the suppression of crystalline chain motion. The heat-elongated PP deformed beyond the yield point at high temperatures above the αc-relaxation point, and it exhibited high tensile stress; e.g., the yield stress was 60 MPa at 120 °C, which was 7.5 times higher than that of the unelongated PP. Small-angle X-ray scattering intensity patterns changed from layered to diffuse, and DSC thermograms showed that melting peak position shifted to lower temperatures when stretching at small strains at various temperatures. The results suggest that lamella fragmentation occurs under small strains at various temperatures. Thus, the good high-temperature strength of the heat-elongated PP is due to the fragmentation of lamellae during small-strain stretching and the suppression of crystalline chain motion by thin crystalline fibrils connected to the lamellae.