Molecular dynamics simulation of crystallization and stretching mechanisms in ethylene/1-octene block copolymers: insights into the role of the block structure

Abstract

Ethylene/1-octene block copolymers (OBCs) combine the excellent elasticity of crosslinked rubbers with the high toughness of thermoplastic polymers, with their mechanical properties strongly influenced by the block structure. In this study, we developed OBC models with varying soft block content, 1-octene insertion rates, and block numbers and distributions, based on commercially available OBCs synthesized by Dow Chemical. Using molecular dynamics simulations, we investigated the effects of the block structure on crystallization behavior and mechanical properties. The results show that increasing soft block content and 1-octene insertion rate enhances microphase separation, limits hard block crystallization, reduces lamellar size, and lowers crystallinity. The dumbbell structure forms more compact and cohesive crystalline regions, while the random copolymer exhibits the weakest crystallization capability. The dual network structure formed at moderate soft block content improves toughness, while higher soft block content results in rubber-like elasticity. Different 1-octene insertion rates lead to distinct tensile deformation mechanisms. OBCs with varying block numbers and distributions exhibit similar stress–strain behavior. The 20dumbbell model forms more stable crystalline regions and shows higher modulus, while the 20random model has the loosest crystalline regions and the lowest modulus. Energy analysis reveals that non-bonded interactions are the primary driving force behind crystalline conformational changes, with the rate of variation increasing with soft block content and 1-octene insertion, while the number of blocks has minimal impact on internal energy changes.