Simulations of sphere-forming diblock copolymer melts

Abstract

Molecular dynamics simulations are used to study melts of asymmetric sphere-forming diblock copolymers with two significantly different values of the invariant degree of polymerization, $\overline{N}=$ 3820 and 960. In both systems, changes in parameters that correspond to decreasing temperature lead to the appearance of micelles at a critical micelle temperature (CMT) and crystallization at a lower order-disorder transition temperature (ODT). The CMT is identifiable in simulations by the appearance of large clusters with a strongly segregated core region, but has no equally clear signature in scattering experiments on systems of modest $\overline{N}$. The value of the product $\ensuremath{\chi}N$ at the CMT (where $\ensuremath{\chi}$ is the Flory-Huggins parameter and $N$ is degree of polymerization) is close to that predicted by SCFT for the ODT, while the value at the actual ODT is larger and increases with decreasing $\overline{N}$. Micelles exhibit significant and comparable dispersity in aggregation number in the crystalline and liquid phases near the ODT. Both the liquid and crystal phases exhibit transient dimers consisting of pairs of neighboring spherical micelles with cores connected by a bridge of core-block material.