The Effect of Side-Chain Length and Grafting Density on the Rheological Behavior of Branched Polymers

Abstract

Branched structures play a key role in determining the physical properties of molecular brushes, but it remains unclear how this molecular factor governs their conformational dynamics and rheological behavior. Here, we employed molecular dynamics simulations to investigate the influence of side-chain length and grafting density on the rheological behavior of branched polymers with a fixed backbone length. All systems exhibit shear-thinning behavior consistent with theoretical and experimental results. Branched polymers with high grafting density and short side chains exhibit lower viscosity at the same molecular weight. We characterized the shape and orientation of bottle-brush polymers in steady-state flow and developed a structure-performance relationship by combining Spearman correlation analysis with explainable machine learning (SHAP). Chain orientation is shown to be the dominant structural mechanism leading to viscosity reduction. The side-chain length plays a more pronounced role in dictating the trends in shear thinning; long side chains promote molecular orientation and main-chain stretching, leading to enhanced thinning effects. At high grafting density, side-chain crowding prevents alignment with the flow field, leading to a diminished thinning effect. These molecular-level insights facilitate the adjustment of the branching structure of bottlebrush polymers to regulate their rheological properties, enabling the customized design of complex branched polymers.