The Role of Architecture on the Structure of Irreversibly Adsorbed Polymer Layers

Existence of irreversibly adsorbed polymer chains in thin polymer films has a huge impact on the wetting, glass transition, aging and polymer chain mobility. Adsorption of polymer chains on the solid substrates is possible due to a delicate balance between enthalpic gain through segment-substrate interactions and reduction in conformational entropy. In this work we have used 4-arm star, 8-arm star, centipede and comb polystyrene (PS) architectures to understand the role of entropy on the structure of irreversibly adsorbed polymer layers using ellipsometry, X-ray reflectivity and atomic force microscopy. Normalized equilibrium adsorbed layer thickness for all architectures is larger than that of the linear PS and for 8-arm star PS it is above 1Rg. Substrate surface energy affects both the adsorbed layer thickness and the density of the layers. Adsorbed star polymers formed thicker and denser layers on HF etched substrates compared to hydrophilic substrates. It is also interesting to note that the adsorbed layer thickness increases with as-cast film thickness initially and reaches a plateu for thicknesses above 200 nm.