Heyi Liang, Zilu Wang, Sergei S. Sheiko, and Andrey V. Dobrynin
We have studied properties of comb-like and bottlebrush-like graft copolymers made of chemically different backbones and side chains in a melt. A diagram of states for this class of copolymers was calculated in terms of architectural parameters (degree of polymerization of the side chains, nsc, and the number of backbone bonds between grafting points of side chains, ng) and structural parameters (backbone and side-chain monomer projection lengths, excluded volumes, and Kuhn lengths). We apply the concept of the crowding parameter, Φ, describing overlap between neighboring macromolecules for classification of graft copolymers into combs and bottlebrushes. In this classification, the sparsely grafted side chains of comb-like macromolecules allow interpenetration of both side chains and backbones belonging to neighboring macromolecules (Φ < 1). However, in the case of bottlebrush-like macromolecules, the densely grafted side chains preclude molecular interpenetration because of steric repulsion, resulting in Φ ≥ 1. Coarse-grained molecular dynamics simulations corroborate this classification of graft copolymers and show that the effective macromolecule Kuhn length, bK, is a function of the crowding parameter, Φ. An increase in the backbone Kuhn length or monomer size results in a shift of the boundaries between different regimes in the diagram of states in comparison with those obtained for graft homopolymers with chemically identical backbones and side chains. Graft copolymers with backbones stiffer than side chains are comb-like in a broader range of parameter space. However, by increasing the excluded volume of the backbone monomers, one expands the parameter space where macromolecules demonstrate bottlebrush-like behavior.