In situ ellipsometry studies on swelling of thin polymer films: A review

The properties of a thin polymer film can be significantly affected by the presence of a penetrant. This can have potential implications for many technological applications, suchas protective and functional coatings, sensors, microelectronics, surface modification andmembrane separations. In situ ellipsometry is a powerful technique for the characteriza-tion of a film in contact with a penetrant. The main advantages of ellipsometry includethe very high precision and accuracy of this technique, combined with the fact that it isnon-intrusive. Recent advances in the speed and automation of the technique have furtherexpanded its application.This article provides an overview of the research that has been done with in situ UV–visellipsometry on penetrant-exposed polymeric films, in the last 15–20 years. The focus ispredominantly on films that are not attached covalently to a substrate. Polymer brushesand grafts are therefore excluded. This review addresses a variety of topics, covering instru-mental aspects of in situ studies, approaches to data analysis and optical models, reportedprecision and repeatability, the polymer-penetrant systems that have been studied, thekind of information that has been extracted, and other in situ techniques that have beencombined with ellipsometry. Various examples are presented to illustrate different practi-cal approaches, the consequences of the optical properties of the ambient, and the variousways that have been employed to bring polymer films in contact with a penetrant, rangingfrom simple ex situ-like configurations (i.e., drying studies) to complex high pressure cells.The versatility of in situ ellipsometry is demonstrated by examples of the distinctive phe-nomena studied, such as film dilation, penetrant diffusion mechanisms, film degradation,electrochemical processes, and the broad variety of polymer-penetrant systems studied(glassy and rubbery polymers, multilayer stacks, etc.). An outlook is given on possible future trends