Intramolecular phase separation of copolymer "bottle brushes": No sharp phase transition but a tunable length scale

TL;DR

This study uses a lattice model and Monte Carlo simulations to show that copolymer bottle brushes exhibit local phase separation without a sharp transition, allowing tunable nanoscale microphase separation through solvent quality adjustments.

Contribution

It demonstrates that intramolecular phase separation in copolymer bottle brushes is only local and lacks a sharp phase transition, contrasting with mean field predictions.

Findings

Phase separation occurs only locally, not globally.

No long-range order along the backbone.

Microphase separation range is controllable via solvent quality.

Abstract

A lattice model for a symmetrical copolymer "bottle brush" molecule, where two types (A,B) of flexible side chains are grafted with one chain end to a rigid backbone, is studied by a variant of the pruned-enriched Rosenbluth method (PERM), allowing for simultaneous growth of all side chains in the Monte Carlo sampling. Choosing repulsive binary interactions between unlike monomers and varying the solvent quality, it is found that phase separation into an $A$-rich part of the cylindrical molecule and a $B$-rich part can occur only locally. Long range order (in the direction of the backbone) does not occur, and hence the transition from the randomly mixed state of the bottle brush to the phase separated structure is strongly rounded, in contrast to corresponding mean field predictions. This lack of a phase transition can be understood from an analogy with spin models in one space dimension. We predict that the range of microphase separation along the bottle brush backbone can be controlled on the nanoscale by varying the solvent quality.