Tuning Adsorption using Entropy to Alter Structure, Diffusion, and Swelling in Thin Polymer Films*

Various polymeric architectures are gaining importance in variety of applications ranging from biomedical coatings to electrochromic displays. Creating well-defined star, comb, and/or branched polymer chains through controlled polymerization techniques paving the way to elucidate the role of entropic effects in adsorption, diffusion and swelling in thin films. We have determined the effects of number of arms, arm molecular weight as well as film thickness and molecular weight on the adsorption using well-defined linear and star polystyrene (PS) by ellipsometry and X-ray reflectivity. Interdiffusion in bilayer films and in-situ swelling of single layers in supercritical carbon dioxide (ScCO₂) were determined using neutron reflectivity. When the total molecular weight is constant, the normalized equilibrium thickness of the adsorbed polymer layer increases as the number of arms increases or arm molecular weight decreases. In contrast to the linear adsorbed layers, structure of star, comb or centipede adsorbed layers is well-described by a single layer of uniform density. Star polymers have larger apparent diffusion coefficients than their linear counterparts of identical total molecular weight. The increase in the adsorbed layer thickness as a function of polymer architecture results in a decrease in the swelling and diffusion coefficient as the films become thinner.