Simulation\nStudy on the Mechanical Properties of Lamella-Forming\nABABA Linear Multiblock Copolymers

The\nA′BA″BA′ multiblock copolymers usually\nexhibit superior mechanical performance due to the increased bridging\nconformations that could bridge multiple domains compared to their\nABA triblock counterparts. However, the underlying molecular mechanisms\nare not completely understood due to the difficulty in experimental\ncharacterization. Moreover, for the A′BA″BA′\npentablock copolymers, the molecular conformations and the segregation\nbetween the blocks are significantly influenced by the length ratio\nτ of A″-block relative to total A-blocks, and therefore\nthere should be an optimal value of τ for the best mechanical\nperformance. To elucidate the effect of the middle A″-blocks\non the mechanical properties and optimize the molecular architecture\nfor A′BA″BA′ pentablock copolymers, we investigated\nthe mechanical properties of lamella-forming A′BA″BA′\nmultiblock samples using self-consistent field theory (SCFT) coupled\nwith coarse-grained molecular dynamics (CGMD) simulations. It is revealed\nthat the bridging A″-blocks significantly enhance the mechanical\nperformance of A′BA″BA′ pentablock samples by\nbearing and transmitting stress during elongation. Furthermore, the\nmechanical properties of A′BA″BA′ samples are\ngreatly affected by the length ratio τ of the A″-block\nto the total A-blocks. Specifically, the maximal stress σ<sub><i>m</i></sub> is mainly influenced by the bridging ratio\ntogether with the contact degree of A-blocks extending from adjacent\nB-domains, becoming the largest at τ ≈ 0.4 before the\nbridging ratio reaches the highest value. The elongation ratio ε<sub><i>m</i></sub> reaches its optimum value at a larger τ\naround 0.7. Combining the maximal stress and elongation ratio, the\npentablock samples exhibit optimal toughness when τ is close\nto 0.6, which is estimated to be 50% larger than that of the A′BA″BA′\nsample with simply equal A-blocks.