Block copolymers confined in a nanopore: Pathfinding in a curving and frustrating flatland

TL;DR

This study uses dynamic density functional theory to explore the diverse and complex structures formed by block copolymers confined in cylindrical nanopores, revealing metastable states, transition pathways, and the influence of pore roughness on structure formation.

Contribution

It introduces a classification scheme for structures and demonstrates how pore roughness and kinetics influence the formation of exotic block copolymer structures in nanopores.

Findings

Multiple exotic structures, including toroids, helices, and catenoids, are metastable.

Pore roughness significantly affects the stability and formation pathways.

Matching model parameters to experiments reproduces observed toroidal structures.

Abstract

We have studied structure formation in a confined block copolymer melt by means of dynamic density functional theory (DDFT). The confinement is two-dimensional, and the confined geometry is that of a cylindrical nanopore. Although the results of this study are general, our coarse-grained molecular model is inspired by an experimental lamellae-forming PS-PBD diblock copolymer system (Shin et al, Science, 306, 76 (2004)), in which an exotic toroidal structure was observed upon confinement in alumina nanopores. Our computational study shows that a zoo of exotic structures can be formed, although the majority, including the catenoid, helix and double helix that were also found in Monte Carlo (MC) nanopore studies, are metastable states. We introduce a general classification scheme and consider the role of kinetics and elongational pressure on stability and formation pathway of both equilibrium and metastable structures in detail. We find that helicity and three-fold connections mediate structural transitions on a larger scale. Moreover, by matching the remaining parameter in our mesoscopic method, the Flory-Huggins parameter, to the experimental system, we obtain a structure that resembles the experimental toroidal structure in great detail. Here, the most important factor seems to be the roughness of the pore, i.e. small variations of the pore radius on a scale that is larger than the characteristic size in the system.