Morphology and extensibility of dried gels from an ethylene copolymer in relation to initial concentration

Abstract

Dried gels of a linear low-density ethylene copolymer cast from decalin solutions are investigated from the standpoint of structural, thermal, and mechanical properties. In the concentration range 1.00–0.20 the swollen gels exhibit a nearly isotropic shrinkage upon drying which is the expected behavior of an ideal crosslinked network. For concentrations below the critical value C^†=0.20, non-isotropic shrinkage is obtained, indicating that the chains begin to disengage from the macromolecular network due to the dilution. The melting behavior of the dried gels shows that crystallization from solution notably improves the perfection of 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 extensibility of the dried gels is considerably improved with increasing dilution as a result of the gradual disentanglement of the coilsd prior to the crystallization in solution. But at concentrations below C^†=0.20, the extensibility drops because of the loss of intermolecular cohesion when the chains begin to disengage from the network. The fact that C^† is much greater than the critical concentration for gelation shows that a large molecular overlap is necessary for chain intertwining.