Direct observation of mobility of thin polymer layers via asymmetric interdiffusion using neutron reflectivity measurements

In this study, we investigated the diffusion dynamics at the interface between deuterated poly(methyl methacrylate) (d-PMMA) and protonated PMMA (h-PMMA) in two-layered thin films of d- and h-PMMA layers via neutron reflectivity (NR) measurements during annealing above the glass transition temperature T_g. When T_g of d-PMMA was higher than that of h-PMMA, the d-PMMA layer thickness increased with annealing time t_a, and simultaneously the h-PMMA layer thickness decreased. However, the opposite t_a dependence of the layer thicknesses was observed, if the T_g of d-PMMA was decreased. This change in the t_a dependence of the layer thickness was related to the change in the mobility of the d-PMMA layer. With the decrease in the d-PMMA layer thickness when the h-PMMA layer thickness was maintained, the t_a dependence of the layer thickness changed, and the mobility of the d-PMMA layer dramatically increased. Hence, the T_g of the d-PMMA layer decreased with the decrease in the d-PMMA layer thickness. These results suggest that the mobility and T_g of thin polymer films can be determined by interfacial dynamics via NR measurements.