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.