Coarse-grained molecular dynamics model of conformationally asymmetric AB diblock copolymers forming lamellar structures

K Hagita and T Murashima, COMPUTATIONAL MATERIALS SCIENCE, 243, 113105 (2024).

DOI: 10.1016/j.commatsci.2024.113105

We performed coarse-grained molecular dynamics simulations to investigate the lamellar domain spacing D of a structurally asymmetric AB block copolymer (BCP) with equal number and size of beads in the A- and B-segments. The semiflexible Kremer-Grest model with the bending- potential coefficients (kappa(A), kappa(B)) of the A- and B-segments was used because it was considered that the conformational asymmetry parameter epsilon(asym) = (rho(bulk)(B)b(B)(2))/ (rho(bulk)(A)b(A)(2)) characterized the asymmetry of the AB BCP. Here, the bulk densities (rho(bulk)(A), rho(bulk)(B)) were changed by the Lennard-Jones (LJ) parameter (epsilon((B,B))) of the B-segments under epsilon((A,A)) = 1 and the statistical segment length (b(A), b(B)) by (kappa(A), kappa(B)). First, we investigated the partial domain spacings (D-A, D-B) and confirmed that D-A < D-B due to rho(bulk)(A) > rho(bulk)(B) for epsilon((A,A)) > epsilon((B,B)) when kappa(A) = kappa(B) = 0. For different (kappa(A), kappa(B)) cases with epsilon((B,B)) = 1 (epsilon((A,A)) = epsilon((B,B))), we found that D depended on kappa(A) + kappa(B) and D-A = D-B was maintained due to rho(bulk)(A) = rho(bulk)(B) even for kappa(A) not equal kappa(B). Second, we confirmed that all scaling relations of D similar to Delta epsilon (1/6), D/N-0.7 similar to Delta epsilon, and D/N-1/2 similar to Delta epsilon N with Delta epsilon = ((1 + epsilon((B,B)))/2 - epsilon((A,B))) were maintained. We concluded that the asymmetry of the LJ parameters has a significant impact on the behaviors of D and our detailed investigations in this direction lead to further research and development of BCPs.

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