Structure of Irreversibly Adsorbed Star Polymer Layers*

Viscosity, glass-transition temperature, and diffusivity of polymer chains in thin films deviate from bulk values due to the presence of adsorbed polymer chains on the substrate. The structure of irreversibly adsorbed layers (IALs) formed by linear chains has been extensively investigated and found to be composed of two layers: high density inner layer in which chains are flattened and bulk-like density outer layer in which chains adopted more tail and loop conformations. However, the role of chain branching on the structure of adsorbed layer is not clear.
Well-defined linear polystyrene (PS), 4-arm and 8-arm star PS chains with the same total molecular weight, and 8-arm star PS with different molecular weights are used to elucidate the effect of polymer architecture on the structure of IAL by ellipsometry, X-ray reflectivity (XR) and atomic force microscopy measurements. Our results indicated that the normalized layer thickness increases as the number of arms increases and arm length decreases on both SiO_x and Si-H surfaces. Adsorbed star polymers formed thicker layers on Si-H surfaces compared to SiO_x surfaces. Adsorbed layer thickness is found to be a function of initial film thickness up to 100 nm.