How Do End-Tethered Chains Alter the Local Glass Transition Temperature Next to Solid Substrates?

Grafted polymer chains are widely used for modifying interfacial interactions in polymer materials and nanocomposites, but the mechanisms by which the changes to material properties are conferred to the system are not well understood.  Recently, our group demonstrated that end-tethered polystyrene (PS) chains with a molecular weight M_w = 100 kg/mol increased the local glass transition temperature T_g(z) of PS matrices by several decades and persisted out to distances of z = 100-125 nm away from the substrate interface [Huang, Roth, ACS Macro Lett. 2018, 7, 269].  Surprisingly, the largest local T_g(z=0) increase next to the substrate interface of +50 K above the bulk T_g of PS occurred for a very low grafting density of 0.011 chains/nm², within the mushroom-to-brush transition regime.  These results are puzzling because typically the glass transition in polymers is not associated with chain connectivity.  We investigate this phenomenon further by studying the local glass transition temperature T_g(z=0) next to the substrate interface as a function of tethered chain length, end functional group, and grafting density to identify the underlying factors controlling this behavior.  Good interpenetration between the grafted and matrix chains are central to creating an elevated T_g.