Nonuniformity in Thin Polymer Films

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Abstract
Thin polymer films solidified from solutions can display a broad range of barrier and mechanical properties, depending on the quality of the solvent and the initial polymer concentration. These findings apply specifically to polymers containing polar groups, exemplified in this work by polymethyl methacrylate and a styrene-butyl acrylate copolymer, but not to polymers like polystyrene. In the former two cases it has also been shown that very different surface tensions exist at the air and the substrate interface, provided the films were solidified against high surface energy solids. These observations have been attributed to the ability of polar-group-containing polymers to adopt various chain conformations at and near interfaces, these conformations reflecting interactions between polymer and solvents and between polymer and substrate. An additional contribution is attributed to the degree of chain entanglement persisting in solutions. The measured properties often pertain to metastable states of the films, and the findings thus indicate the existence of a thermodynamic driving force for the time-dependent change (aging) in properties of protective films.

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