Reorganization of the Near-Surface Composition in Pressure Sensitive Adhesive Films Stored in Nitrogen Atmosphere

The reorganization of the near-surface composition of a pressure sensitive adhesive (PSA) film upon exposure to nitrogen atmosphere is investigated. PSA films are prepared via solution casting of the statistical copolymer P(EHA-stat-20MMA) which consists of 80% ethylhexylacrylate as the sticky majority component and 20% methylmethacrylate as the glassy minority component. The resulting films are exposed to a nitrogen atmosphere representing an apolar and inert gas. This treatment causes a reorganization of the inner morphology in the PSA film. Using X-ray reflectivity measurements it is shown that the freshly prepared films show a solubility-driven enrichment of the PMMA component at the very surface. Due to the interaction with the apolar nitrogen atmosphere the less polar component of the copolymer, PEHA, enriches on top with time. The timescale of this internal rearrangement lies in the order of days; a complete inversion is found to take 12 days. An investigation of the local homogeneity of the sample surface on a nanoscale with atomic force microscopy proves a smooth and featureless surface right after casting.