The effect of heat sealing variables (platen temperature and dwell time) on seal strength of a linear low-density polyethylene (LLDPE) was examined. In order to characterize the development of interfacial strength, blown films were heat-sealed for times from 1 to 100,000 s, much longer than the typical sealing times of less than 1 s. The seal temperature ranged from 100 to 130 degrees C. From the differential scanning calorimetry thermogram, the LLDPE was determined to be completely melted at 130 degrees C. Therefore, the films ranged from partially to fully melted when they were heat-sealed. The seal strength was measured in the T-peel configuration, and the peel fracture surfaces were examined in the scanning electron microscope. A temperature of 115 degrees C or higher was required to form a good seal. The strong effect of seal temperature was related to the heterogeneous composition of the LLDPE studied. At 115 degrees C, the lower-molecular-weight, more highly branched chains easily diffused across the interface. Crystallization upon cooling produced connections across the interface. However, because these chains represented a small fraction of the crystallinity and the molecular weight was low, they contributed much less than the full peel strength. Conversely, chains with less branching represented the main fraction of crystallinity (anchors for tie chains) and the highest molecular weights (more entanglements). Only at temperatures at which the higher-molecular-weight, less branched chains began to melt and diffuse across the interface could high peel strengths be achieved.