Combs and Bottlebrushes in a Melt

HY Liang and Z Cao and ZL Wang and SS Sheiko and AV Dobrynin, MACROMOLECULES, 50, 3430-3437 (2017).

DOI: 10.1021/acs.macromol.7b00364

We use a combination of the coarse-grained molecular dynamics simulations and scaling analysis to study conformations of bottlebrush and comb-like polymers in a melt. Our analysis shows that a crossover between comb and bottlebrush regimes is controlled by the crowding parameter, Phi, describing overlap between neighboring macromolecules. In comb-like systems characterized by a sparse grafting of side chains (Phi < 1), the side chains and backbones belonging to neighboring macromolecules interpenetrate. However, in bottlebrushes with densely grafted side chains (Phi >= 1), the interpenetration between macromolecules is suppressed by steric repulsion between side chains. In this regime, bottlebrush macromolecules can be viewed as filaments with diameter proportional to size of the side chains. For flexible side chains, the crowding parameter is given by Phi v/(lb)(3/2)(n(sc)/n(g)+ 1)/n(sc)(1/2), which depends on both the architectural parameters (degree of polymerization of the side chains, n(sc), and number of backbone bonds between side chains, n(g)) and chemical structure of monomers (bond length l, monomer excluded volume v, and Kuhn length, b). Molecular dynamics simulations corroborate this classification of graft polymers and show that the effective macromolecule Kuhn length, b(K), and the mean-square end-to-end distance of the backbone, < R-e,bb(2)>, are universal functions of the crowding parameter Phi for all studied systems.