Crystallization in thin films of polymer glasses: The role of free surfaces, solid interfaces and their competition

Polymer coatings of nanometric thickness are about to enter in everyday life as part of a wide range of applications such as protective layers, stimuli-responsive membranes or as components of flexible electronics devices. In the past 30 years, these polymer nanomaterial systems have been at the center of research interest due to the opportunities to control relevant material properties like the phase transition temperature, viscosity, permeability, or thermal expansion by variation of the film thickness. One of the key factors responsible for the deviation from bulk behavior is known as 1D confinement that describes the increasing impact of interfacial layers when reducing film thickness. This review provides a comprehensive discussion of the role of the free surface at the boundary with air and the interfacial layer in proximity of a supporting substrate on the crystallization of thin polymer films. First, the dynamics of polymers near the free surface and its impact on the crystallization of films is discussed. Subsequently, the effect of solid substrates on crystallization in thin films is elaborated, including the formation of irreversible adsorption layers, alteration of crystalline structure and the kinetics of crystallization. Subsequently, the competition between surface and interface effects on the formation of ordered structures in thin polymer films is discussed. A perspective on challenges and opportunities in the field of thin film crystallization is provided to inspire future research and development in the field. This review thus provides an up-to-date analysis of current understanding of crystallization of polymer glasses under 1D confinement, aimed at supporting the manipulation and control of the properties of polymer-based nanodevices.