Correlating structure-mechanics relationship of multiblock copolymers: Insights from molecular dynamics simulation

Abstract: Multiblock copolymers (MBCs) have garnered considerable interest in both industrial and fundamental polymer science. However, the phase structure and properties at a molecular level of MBCs remain largely unexplored. To address this, the effects of three structural factors (block number index $\alpha$, interaction strength between hard segments $\epsilon_{H-H}$, and hard segment stiffness $\kappa_{H}$) on the phase structure, dynamic properties and mechanical properties of MBCs have been investigated using coarse-grained molecular dynamics (CGMD) simulations. The degree of phase separation, $\phi$, has been quantified based on the number of neighboring beads designated to measure the phase separation structure. Results indicate that a lower $\alpha$(longer block chain length), stronger $\epsilon_{H-H}$, and weaker $\kappa_{H}$ significantly reduce the contacts between soft and hard segments, leading to an increase in $\phi$. An examination of higher $\phi$(greater than 0.7) reveals two $T_{g}$s that converge as $\phi$ decreases and eventually merge into one. Mechanical properties are improved with increasing $\alpha$ at low strains, whilst they change non-monotonically at high strains, likely due to differing contributions of soft and hard segments to the stress. Systems with stronger $\epsilon_{H-H}$ and $\kappa_{H}$ exhibit superior mechanical properties due to their higher non-bonded energy increments and bond orientation. In general, the mode of the variation of phase separation structure, glass transition temperature and mechanical properties of MBCs at the molecular level, has been elucidated valuable insight for the design and synthesis of high-performance MBCs materials..

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