Experimental study of substrate roughness on the local glass transition of polystyrene

Numerous computer simulations have shown that local dynamics associated with the glass transition can be slower next to rough interfaces compared with smooth interfaces. Even though the impact of surface roughness has been frequently considered computationally and theoretically, almost no experimental studies exist investigating these effects. Using a hydrogen fluoride vapor treatment, we created silica substrates with an increase in roughness that left the surface chemistry unchanged. The local glass transition temperature T_g near silica substrates with an increase in roughness was measured using fluorescence, finding an increase in local T_g of 10 K with an increase in the root-mean-square roughness R_rms from 0.5 nm to 11 nm. Characterization of the substrate roughness needed to create an experimental change in local T_g was found to be quite large, leaving the mechanism for this observed behavior uncertain. We discuss possible causes associated with polymer chains being more readily able to make surface contacts and adsorb to roughened interfaces.