Study on the Complex Structure–Property Relationships of Styrenic Thermoplastic Elastomers and Their Derivative Based on Molecular Dynamic Simulations

Abstract

Transformations in the chemical bonds of polymers profoundly influence their performance and functional characteristics, offering critical insights into their structure–property relationships. Styrenic thermoplastic elastomers, represented by styrene–butadiene-styrene (SBS), styrene-ethylene-butylene-styrene (SEBS), and their hydrogenated derivative vinylcyclohexane-b-(ethylene-co-butene)-b-vinylcyclohexane (CBC) triblock copolymers, exhibit distinct thermophysical properties, such as density and glass transition temperature (T_g), despite sharing similar molecular chain structures. In this study, fully atomistic models of these systems were constructed, with their densities and T_g values predicted via molecular dynamics (MD) simulation demonstrating less than 5% relative error. Evolution of the microphase separation degree, after hydrogenation of unsaturated bonds in the macromolecular backbone and side phenyl rings, respectively, was analyzed through characteristic peak shifts in the radial distribution function. Glass transition mechanisms were elucidated by monitoring segmental dynamics via mean square displacement analysis and evaluating intermolecular interactions through cohesive energy density calculations, revealing that fully hydrogenated segments exhibit enhanced rigidity accompanied by a reduction in intermolecular forces. Especially, the optimal block sequence length for the simulation scale was determined by examining the solubility parameters across varying degrees of polymerization in CBC systems. And AB-type diblock and ABA-type triblock copolymers with extended sequence lengths display higher T_g values, while (AB)_n-type multiblock copolymers show T_g values comparable to or marginally lower than random copolymers upon sequence length reduction, providing a feasible approach for the intelligent design of chain segment structures based on the required thermophysical properties of thermoplastic elastomer.