Shear Thinning from Bond Orientation in Model Unentangled Bottlebrush Polymer Melts

The rheology of molecular brushes remains challengingto controldue to the multiple length scales and relaxation processes involvedand the lack of direct observation of molecular conformation duringflow. We use molecular dynamics simulations to determine the shearthinning of unentangled bottlebrush polymers with varying architecture,from linear chains to combs, to densely grafted bottlebrushes, tostar-like and star polymers. We find shear thinning exponents in linewith theoretical and experimental results and characterize the shapeand orientation of bottlebrushes in steady-state flow. Many shapeparameters derived from the gyration tensor show molecular alignmentwith the flow for all systems. Yet, the orientation of individualbonds is what most strongly correlates with the architecture-dependentshear-thinning exponents. In densely grafted bottlebrushes, the packingof side chains prevents alignment with the flow, causing a reductionin shear thinning. The molecular insight from our simulations is usefulto tune the architecture of bottlebrushes to control their rheology.