Structural evolution of flow-oriented high density polyethylene upon heating: In situ SAXS and WAXD studies

Abstract

The structural evolution of micro-injection molded high density polyethylene (HDPE) at the molecular and lamellar levels was investigated as a function of temperature during heating process using wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques. The molded sample exhibits a peculiar multilayer structure of skin and shear layers, and is marked by the existence of shish-kebab-like structure with two populations of lamellar stacks. The variations of the long period as well as the average thicknesses of lamellar and amorphous regions with temperature were determined from one-dimensional correlation functions for the respective lamellar stacks. The average length and the misorientation of shish-like formation were calculated using Ruland equation, and the results indicate that the shish-like structure grows in the longitudinal direction with increasing temperature via incorporation of several partially relaxed chains as a result of autocatalytic process. In addition, the degree of orientation of the chains in the crystalline phase was found to increase slightly with increasing temperature below 100 °C, which could be traced back to the formation of additional crystallites with a low degree of orientation after injection molding. These crystalline lamellae built up during the secondary crystallization are less thermally stable than the primary ones, and therefore are melted selectively upon annealing at relatively lower temperatures yielding an overall increase in the molecular orientation.