Uncovering the Physical Mechanisms by Which Surface Bound Chains Can Increase the Local Glass Transition of Polymer Matrices

The presence of grafted or adsorbed chains to the surface of nanoparticles is central to our understanding of reinforcement of polymer nanocomposites. However, how these different types of surface bound chains alter the local material properties of the matrix is still unclear. By comparing and contrasting how and when different adsorbed and end-grafted chains alter the local glass transition temperature Tg(z=0) of matrix chains intermixed with these surface bound chains, we uncover two mechanisms by which surface bound chains can increase the local Tg of the matrix. Fluorescence is used to study the local Tg(z=0) of unattached pyrene-labeled PS chains as a function of intermixing time with surface bound PS chains in a model system with a single planar interface. Our efforts are focused on an athermal system of polystyrene (PS) matrix chains intermixed with either adsorbed or grafted PS chains on silica surfaces, making these mechanisms entirely topological in origin. Starting from known pre-adsorbed layers, we can distinguish between the impact of threading small tight loops versus large fluffy loops, and tails versus end-grafted chains.