Evidence is presented showing how the long relaxation times in the melt affect the subsequent morphology of blown films made from medium molecular weight (MMW) HDPE homopolymers. The blown film morphology was studied using the small-angle X-ray scattering (SAXS) technique, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SAXS data were used to obtain the lamellae orientation functions using the Herman's orientation function equation commonly used for wide-angle X-ray diffraction (WAXD). The blown film results showed an increase in orientation function with increase in E-R, a normalized melt elasticity parameter that reflects an increase in the longest melt relaxation times. In our study, such an increase in the longest relaxation times results from an increase in long-chain branching (LCB). As expected, compression molded samples show random orientation by SAXS. Considerable differences were observed in the lamella organization between the blown films of low and high E-R resins. TEM micrographs show the presence of fibril nuclei several microns long. Lamellae stack perpendicular to the machine direction (MD) in films made from higher E-R resins. In contrast, stacks of lamellae arranged randomly characterize the micrographs of blown films made from resins of low E-R value. A model is proposed to explain the relationship between orientation function and melt relaxation times. (C) 2001 Elsevier Science Ltd. All rights reserved.