Comparative study of the crystalline morphology present in isotropic and uniaxially stretched “conventional” and metallocene polyethylenes

Abstract

A comparative study of the crystalline phase present in conventional and metallocene polyethylenes covering a broad range in density (953–885 kg/m 3) has been carried out using Raman, DSC and WAXS. The materials were compared in the isotropic state, after uniaxial stretching at room temperature, and after annealing of the stretched material. Crystallinity for the isotropic samples and relative molecular orientation for the stretched samples decreased with decreasing density. The Raman crystallinity (using Strobl–Hagedorn's approach) was found to decrease more rapidly with density than crystallinity by the other techniques. Upon cold drawing, the three techniques supported a disruption of the crystalline morphology towards a highly ill-defined and fractured orthorhombic crystalline phase. The disruption of the crystallinity was more dramatic for the lower densities most likely due to branched molecules being pulled through the crystals. The perfection of the orthorhombic crystallinity was further restored upon annealing. Changes in the position of the two components of the –CH 2– bending factor group splitting, i.e. the crystallinity band at 1415 cm −1 and the band at 1440 cm −1, were measured as a function of sample density and physical treatment and were attributed to alterations in crystalline density and perfection. Particular behaviours observed for the materials were attributed to their particular molecular architecture.