MORPHOLOGY OF HDPE/LDPE BLENDS WITH DIFFERENT THERMAL HISTORIES

High-density and low-density polyethylene (HDPE and LDPE) were melt-blended with the intent of building a morphological model from the unit cell level as a function of weight content and heat treatment. The compression-moulded sheets were slowly and rapidly cooled to compare the extremes in crystallization conditions. Based on the d-spacings, there is no apparent distortion on the unit cell level for either heat treatment. The addition of LDPE caused the crystallite size to decrease from 265 Angstrom for the HDPE homopolymer to 130 Angstrom for the LDPE. This is indicative of separate crystallite formation since a co-crystal would necessarily have the same size regardless of LDPE weight content. The high branch content of the LDPE seems to suppress crystallite growth of the rapidly cooled blends as the 25 wt% HDPE blend and LDPE have the same crystallite dimensions. Although two phases are present, there is a noticeable shift of both the HDPE and LDPE peaks towards one another, indicating some interaction on the molecular level. The lamellae that comprise the crystalline phase decrease in population and thickness as the LDPE content increases.