Optical properties and orientation in polyethylene blown films

We report structural factors affecting the optical properties of blown polyethylene films. Two types of blown polyethylene films of similar degrees of crystallinity were made from (1) single-site-catalyst high-density polyethylene (HDPE; STAR alpha) and (2) Ziegler-Natta-catalyst HDPE (ZN) resins. The STAR alpha film exhibited high clarity and gloss, whereas the ZN film was turbid. Small-angle X-ray scattering (SAXS), small-angle light scattering (SALS), and optical microscopy gave quantitative and qualitative information regarding structure and orientation in the films. A new approach is described for determining the three-dimensional lamellar normal orientation from SAXS. Both the dear STAR alpha and turbid ZN films had similar lamellar crystalline structures and long periods but displayed different degrees of orientation. It is demonstrated that optical haze is related to surface features that seem to be linked to the bulk morphology. The relationship between haze and structural orientation is described. The lamellar orientation is linked to rodlike structures seen in optical microscopy and SALS through a stacked lamellar or cylindrite morphology on a nanometer scale and through a fiberlike morphology on a micrometer scale. The micrometer-scale, rodlike structures seem directly related to surface roughness in a comparison of index-matched immersion and surface micrographs. The higher haze and lower gloss of the ZN film was caused by extensive surface roughness not observed in the STAR alpha film. (C) 2001 John Wiley & Sons, Inc.*.