Dried gels from linear low-density polyethylene: Morphology,thermal behavior,and mechanical properties

Dried gels of a linear low-density polyethylene cast from decalin solutions are investigated with particular attention toward structural, thermal, and mechanical properties. The number-average and weight-average molecular weights are M_n = 32,000 and M_w = 160,000. In the concentration range 1.00–0.20, the swollen gels exhibit nearly isotropic shrinkage upon drying, which is relevant to an ideal crosslinked network behavior. For the concentrations below 0.20, a strong departure from the isotropic shrinkage indicates that the chains begin to disengage from the macromolecular network owing to the dilution effect. The melting behavior of the dried gels shows that crystallization from solution improves the crystal perfection notably as concerns the more defective crystals. The concomitant decrease of the crystal thickness judged from small-angle X-ray scattering is ascribed to a reduction of the surface free energy which is consistent with the build up of regular chain-folded macroconformations. The drawability of the dried gels is considerably improved with increasing dilution as a result of the gradual disentanglement of the coils prior to the crystallization in solution. But beyond concentration 0.20, the drawability drops because of the loss of intermolecular cohesion when the chains begin to disengage from the network. The drastic change of yield behavior between the melt-crystallized and solution-crystallized samples reveals a ductile-to-brittle transition in the mechanism of failure of the crystallites at low strain. This phenomenon is related to the improvement of regular chain-folding.