HDPE and LLDPE geomembranes are typically used as liners in various engineering appli-cations including municipal solid waste landfills, brine ponds, and mining facilities. The liner temperature differs from one application to another and for some applications such as landfills containing aluminum production waste the temperature can exceed 85°C and in some cases, it can reach up to 143°C. It has been reported that the leachate temperature above the liner can exceed 80°C and in some cases in can ex-ceed 100°C. This study focuses on the investigation of morphological changes associated with the subjec-tion of five different 1.5 mm-thick geomembranes (two HDPE, two LLDPE and one blended polyolefin) incubated in air at 105°C in a forced-air oven for one month. The morphological changes are assessed via differential scanning calorimetry, the peak melting temperatures associated with lamellae thickening as well as crystallinity increased with testing time for all tested geomembranes. All geomembranes exhibited changes in melting endotherms from the unaged state upon annealing, exhibiting different features which are associated with crystalline regions forming during annealing and upon cooling from annealing temper-ature. The tie-molecule density was reduced upon annealing due to both lamellae thickening and the for-mation of crystalline regions just above the annealing temperature which accelerates the stress-crack growth. The reduction in tie-molecules density is counteracted by the tautness of remaining tie-molecules upon lamellae thickening. With further annealing, it is expected that the taut tie-molecules will slip and get incorporated in one lamellae hence accelerating stress-crack growth.
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