Relaxation behavior of polymer thin films: Effects of free surface, buried interface, and geometrical confinement

The glass transition temperature (T-g) of polymer thin films, one of the most important parameters to characterize the relaxation behavior of polymer chains in a thin film, has been extensively investigated. It was found that the relaxation behavior of polymer chains in a polymer thin film at different locations, e.g., near a free surface, near a buried interface, or inside a given position of the film bulk, can vary greatly. Such results are significant for both fundamental understanding of molecular behavior and practical use of polymer thin films in many important applications but have not been systematically summarized and reviewed. In this review, we look at two extensively studied and commonly used polymer thin films, i.e., poly(methyl methacrylate) (PMMA) and polystyrene (PS), as examples to systematically examine the effects of the free surface, buried interface (substrate), and geometrical confinement (encapsulated between two substrates) on the surface, interfacial, and bulk relaxation behavior of polymer thin films. Especially, the universal effects of "hard" and "soft" confinements and adsorbed nanolayer near a substrate on T-g are reviewed in detail. In addition, other important polymers such as poly(2-vinylpyridine) and the effects of other variables such as annealing are also discussed. This review elucidates variables which influence the relaxation behavior of polymer chains on the free surface, at buried interfaces, or in the bulk region of polymer thin films, providing in-depth understandings of such behavior. The systematic knowledge reported here will help to guide the future design of polymer thin films with desired properties used in a variety of applications, ranging from multilayer polymer packaging materials, polymer sheets, plastic microelectronics, to batteries and solar cells, and beyond. (C) 2021 Elsevier B.V. All rights reserved.