Effect of Crystal Morphology and Crystallinity on the Mechanical α- and β-Relaxation Processes of Short-Chain Branched Polyethylene

The temperature and the Gibbs free energy of activation of the α-relaxation in ethylene-1-octene and ethylene-1-hexene copolymers drop with decreasing crystal size which is triggered by increasing concentration of short-chain branches and/or increasing cooling rate during melt crystallization, respectively. The activation enthalpy and entropy, unexpectedly, increase with decreasing crystal size, obtained by increasing the cooling rate at constant branch concentration. Both effects can be explained by hindrance of the intracrystalline translational mobility and the restricted segment exchange between the crystalline and amorphous phases, caused by branch points accumulated at the crystal surface. The significant drop of the activation enthalpy and the temperature of β-relaxation with increasing branch-concentration point to decreasing restriction of cooperative segmental mobility by crystals. The restriction is reduced as result of decreasing crystallinity or increased crystal perfection, respectively. Thus, the distribution of the local branch concentration controls the molecular mobility of short-chain branched polyethylene expressed both in the α- and β-relaxation.