Effect of grafting density and side chain length on the mechanical properties of comb polymers under shear flow: Insights from molecular dynamic simulations

In this study, we explore the influence of grafting density and side chain length on the mechanical characteristics of comb polymers subjected to shear flow through molecular dynamics simulations. Our simulation results indicate that higher grafting density leads to slow down the relaxation dynamics of the grafted polymer. Moreover, the increase in grafting density induces a higher shear stress overshoot in the corresponding polymer melt. Subsequent analysis uncovers a direct correlation between the overshoot peak stress and peak strain concerning grafting density. Through an examination of the stretch ratio and structural evolution of molecular chains, it can be inferred that during the initial stage of shear strain, the predominant molecular chain behavior manifests as rotational orientation. As strain grows larger, systems with higher grafting density exhibit heightened dihedral rotational energy and reduced inter-chain interaction energy. In instances of high-density grafting, the chains encounter increased dihedral rotational energy that must be overcome. In contrast to the impact of grafting density, with increase in side chain length does not result in a linear increase in overshoot peak stress under high shear flow. Instead, a critical value for side chain length exists, beyond which the overshoot peak stress decreases as side chain length increases. Further analysis indicates that longer side chains infiltrate the backbone tube under shear flow, thereby weakening inter-chain interactions and expediting the disentanglement of backbones. Consequently, both the overshoot peak stress and peak strain diminish. This study establishes a theoretical foundation for advancing polymer processing techniques for grafted polymers.