Mobility of Star Polymers under Confinement

Thin polymer films made of star polymers have applications in biomedical coatings and microelectronics. As the thickness of the films decreases, the surface and interfacial behavior of the polymer chains play a major role in determining the physical properties of the materials. Interdiffusion measurements can reveal the mobility of polymer chains at the interfaces. Here we have determined the thickness dependence of diffusion coefficients of the star polystyrene (PS) chains in thin films as a function of the number of polymer arms and the molecular weight per arm using neutron reflectivity (NR) and compare our results with linear chains of identical total molecular weight. Bilayer samples of 4-arm and 8-arm protonated PS (hPS) and deuterated PS (dPS) were used to elucidate the effect of polymer chain architecture on interdiffusion. NR measurements indicate that the mobility of polymer chains in thin films get faster as the number of polymer arms increases and the arm molecular weight decreases. The apparent diffusion coefficient of star PS chains has a weak dependence on the thickness of the bottom layer most likely due to the adsorption of chains on the substrate.