Structural changes in block copolymer solution under shear flow as determined by nonequilibrium molecular dynamics

TL;DR

This study uses nonequilibrium molecular dynamics to explore how block copolymer solutions undergo phase transitions and structural changes under shear flow, revealing critical shear rates that influence morphology.

Contribution

It provides detailed insights into shear-induced structural transitions in block copolymer solutions, including the identification of critical shear rates and the formation of various morphologies.

Findings

Phase transitions from BCC micelles to lamellae with increasing concentration.

Shear flow induces domain rearrangements into layered structures.

Identification of critical shear rates affecting morphology.

Abstract

A nonequilibrium molecular dynamics computer simulation on microsegregated solutions of symmetrical diblock copolymers is reported. As the polymer concentration increases, the system undergoes phase transitions in the following order: body centered cubic (BCC) micelles, hexagonal (HEX) cylinders, gyroid (GYR) bicontinuous networks, and lamellae (L), which are the same morphologies that have been reported for block copolymer melts. Structural classification is based on the patterns of the anisotropic static structure factor and characteristic 3-dimensional images. The systems in the BCC micellar ($\rho\sigma^{3}=0.3$) and HEX cylindrical ($\rho\sigma^{3}=0.4$) phases were then subjected to a steady planar shear flow. In weak shear flow, the segregated domains in both systems tend to rearrange into sliding parallel close-packed layers with their normal in the direction of the shear gradient. At higher shear rates both systems adopt a perpendicular lamellar structure with the normal along the neutral direction. A further increase in the shear rate results in a decrease in lamellar spacing without any further structural transitions. Two critical shear rate values that correspond to the demarcation of different structural behaviors were found.