Small Volume Fraction Limit of the Diblock Copolymer Problem: II. Diffuse-Interface Functional

TL;DR

This paper analyzes the small volume fraction limit of a diffuse-interface functional modeling diblock copolymer microphase separation, deriving effective energies that describe particle concentration and interactions.

Contribution

It extends previous sharp-interface results to the diffuse-interface case, deriving first- and second-order Gamma-limit energies that reveal particle sizes and Coulomb-like interactions.

Findings

Effective energies are finite on sums of delta functions.

Particles exhibit local size information and Coulomb-like interactions.

Results are presented in three dimensions with comments on two-dimensional cases.

Abstract

We present the second of two articles on the small volume fraction limit of a nonlocal Cahn-Hilliard functional introduced to model microphase separation of diblock copolymers. After having established the results for the sharp-interface version of the functional (arXiv:0907.2224), we consider here the full diffuse-interface functional and address the limit in which epsilon and the volume fraction tend to zero but the number of minority phases (called particles) remains O(1). Using the language of Gamma-convergence, we focus on two levels of this convergence, and derive first- and second-order effective energies, whose energy landscapes are simpler and more transparent. These limiting energies are only finite on weighted sums of delta functions, corresponding to the concentration of mass into `point particles'. At the highest level, the effective energy is entirely local and contains information about the size of each particle but no information about their spatial distribution. At the next level we encounter a Coulomb-like interaction between the particles, which is responsible for the pattern formation. We present the results in three dimensions and comment on their two-dimensional analogues.