Effect of Chain Architecture on the Structure, Diffusion, and Swelling in Thin Polymer Films*

Polymers with special architectures are utilized in wide variety of technologies such as in pressure sensitive adhesives, biomedical coatings, photoresists, targeting multi-drug resistant gram-negative bacteria, and electrochromic displays. Being able to control the chain architecture opens new pathways to tune physical properties of the polymer thin films through alterations in the entropic contribution. In this work, we have investigated the role of chain architecture on the adsorption, diffusion and swelling in supercritical carbon dioxide (ScCO₂) using well-defined linear, star, centipede and comb polystyrene (PS). Structure of irreversibly adsorbed layers are determined using ellipsometry and X-ray reflectivity. Vertical diffusion in bilayer films and ScCO₂ swelling in single layer films were determined using in-situ neutron reflectivity measurements. Our results indicated that the normalized equilibrium thickness of the adsorbed layer increases as the number of branches increases and the length of the branches decreases. Independent of the architecture the films are consistently fit using a single layer of uniform density. For the same total molecular weight, thin films composed of more branched polymers have larger diffusion constants than that are made of linear chains. Increase in the fraction of adsorbed chains in a thin film causes a decrease in the vertical diffusion and ScCO₂ swelling.