Computer simulation of bottle brush polymers with flexible backbone: Good solvent versus Theta solvent conditions

TL;DR

This study uses molecular dynamics simulations to analyze the structural properties of bottle brush polymers with flexible backbones under good and Theta solvent conditions, revealing how solvent quality influences chain stretching and persistence lengths.

Contribution

It provides a comparative analysis of bottle brush polymers under different solvent conditions using coarse-grained simulations and explores the dependence of structural parameters on solvent quality and chain length.

Findings

Side chains are significantly stretched even at the Theta point.

Different measures of persistence length yield inconsistent results.

Solvent quality weakly affects the linear dimensions of side chains.

Abstract

By Molecular Dynamics simulation of a coarse-grained bead-spring type model for a cylindrical molecular brush with a backbone chain of $N_b$ effective monomers to which with grafting density $\sigma$ side chains with $N$ effective monomers are tethered, several characteristic length scales are studied for variable solvent quality. Side chain lengths are in the range $5 \le N \le 40$, backbone chain lengths are in the range $50 \le N_b \le 200$, and we perform a comparison to results for the bond fluctuation model on the simple cubic lattice (for which much longer chains are accessible, $N_b \le 1027$, and which corresponds to an athermal, very good, solvent). We obtain linear dimensions of side chains and the backbone chain and discuss their $N$-dependence in terms of power laws and the associated effective exponents. We show that even at the Theta point the side chains are considerably stretched, their linear dimension depending on the solvent quality only weakly. Effective persistence lengths are extracted both from the orientational correlations and from the backbone end-to-end distance; it is shown that different measures of the persistence length (which would all agree for Gaussian chains) are not mutually consistent with each other, and depend distinctly both on $N_b$ and the solvent quality. A brief discussion of pertinent experiments is given.