General weak segregation theory with an application to monodisperse semi-flexible diblock copolymers

TL;DR

This paper develops a comprehensive weak segregation Landau theory for polydisperse semi-flexible multi-block copolymer melts, predicting various phase structures influenced by composition, interactions, and chain flexibility.

Contribution

It introduces a general Landau free energy framework incorporating density and orientation order-parameters for semi-flexible copolymers, extending understanding of phase behavior.

Findings

Predicts phase diagrams with bcc, hexagonal, smectic-A, smectic-C, and nematic phases.

Analyzes the impact of Maier-Saupe interactions on microphase structures.

Provides a theoretical basis for designing copolymer materials with desired phases.

Abstract

A general theory has been developed for a polydisperse semi-flexible multi-block copolymer melt. Using the Bawendi-Freed approach to model semi-flexible chains an expression for the Landau free energy is derived in the weak segregation regime which includes the density and orientation order-parameters. The orientation order-parameter is described in the smectic phase and in more complicated structures such as the hexagonal phase. The Landau free energy contains contributions of two kinds of interactions. The first kind is the Flory- Huggins interaction which describes the incompatibility of chemically different blocks and may induce microphase separation. The second kind is the Maier- Saupe interaction which may induce nematic ordening. In the framework of the weak segragation limit the Landau theory allows to predict phase structures in the melt as a function of the composition, persistence length and the strength of the Flory-Huggins and Maier-Saupe interaction. The general theory is applied to a simple system of monodisperse semi-flexible diblock copolymers. In several phase diagrams a number of possible phase structures is predicted such as the bcc, hexagonal, smectic-A, smectic-C and nematic phase. The infuence of the Maier-Saupe interaction on the microphase structure is thoroughly discussed.