Melt memory in random ethylene-1-alkene copolymers

Chain flexibility or stiffness based polymer conformation plays a crucial role in affecting the dynamics and kinetics of polymers, which is related to the hierarchical architecture of chains. A series of random copolymers of ethylene and 1-alkenes including 1-hexene, 1-octene, and 1-dodecene were synthesized with metallocene catalysts. The crystallization behavior and memory effect in random ethylene-1-alkene copolymers with different side groups were investigated via differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). Rheological tests were performed for understanding their dynamical behavior. The results show that the melting peak and the viscosity decrease but the orthorhombic crystal dimensions increase with co-unit contents increasing in the copolymers. It was found that the scaling relationship between the zero shear viscosity (η₀) and molecular weight (M_n) of the copolymers containing ethylene-1-hexene and ethylene-1-octene is 3.6, which is higher than the classical scaling value of 3.4. The memory of crystals in the melt is enhanced with the increase of 1-alkene contents but is independent of the types of 1-alkenes. The enhanced melt memory effect in the copolymers was proposed due to the effect of the 1-alkene based side groups on the dynamics of polymer chains. The present work would be helpful to understand the chain stiffness based polymer dynamics and processing of polyolefins and copolymers prepared with the metallocene catalyst.