The dynamic mechanical relaxation behaviour of polyethylene copolymers cast from solution

Three polyethylene copolymers, which differ widely with respect to their short chain branch content, have been cast from solution and subject to extensive structural and Dynamic Mechanic Thermal Analysis (DMTA) to establish the effect of the morphology on the alpha-, beta- and gamma -relaxations. The results have shown that the alpha -relaxation temperature, T-alpha, increases with the lamellar thickness, irrespective of grade, initial solvent concentration or crystallinity and is associated with c-shear within the crystalline lamellae. The beta -relaxation temperature, T-beta, shows a more complex morphological dependence depending on the grade. The results indicate that this relaxation is associated with interlamellar shear and is determined, to some extent, by fold surface morphology. The gamma -relaxation temperature, T-gamma, is shown to be dependent on the initial solution concentration only. It is concluded that this relaxation is associated with short-range motions in the amorphous region, with residual solvent acting to plasticise the amorphous regions. (C) 2001 Elsevier Science Ltd. All rights reserved.