Special issue on properties of thin polymer films.

Abstract

The behavior of polymers in thin films or close to interfaces is far from being understood. Many observations, encompassing both structural and dynamical behavior, indicate that the properties of polymers in thin films deviate from what we know from the bulk. Questions about the density (irreversible and reversible changes after annealing even above the bulk glass transition temperature), film stability and dewetting, glass transition temperature, diffusion coefficient and chain conformation and relaxation are intensively debated. In particular, it is not yet clear how the size of such chain-like molecules (their molecular weight) comes into play, especially if the thickness of the film is less than the radius of gyration of these macromolecules. In addition, due to the high surface-to-volume ratio the influence of surface and interfacial properties becomes important if not dominant. This interfacial sensitivity highlights the importance of the properties of the near surface region in polymer films; a topic whose importance is beginning to be recognized. This special issue presents experimental and theoretical works on a variety of questions related to polymers at interfaces and in thin films, ranging from space-averaged properties like adhesion to surface ordering or dynamic molecular (segmental) motion in confining geometries. Since the first reports of anomalous dynamics in thin polymer films almost a decade ago, the subject of dynamics in thin films has gained considerable momentum. Until very recently, this body of work focused almost solely on measurements of the thermodynamic signature of the kinetic glass transition. Such measurements are, at best, a very indirect probe of the microscopic dynamics and convolute the temperature dependence, time dependence and sometimes even the thermal history into a single measured value. The articles focusing on dynamics in thin films published in this special issue illustrate an important shift in this rapidly evolving field. There is now a strong focus on many different fronts. Measurements of dynamics are more varied, ranging from indirect studies such as adhesion to direct measurement of the segmental relaxation using dielectric spectroscopy. There is a concerted effort now to draw analogies to bulk systems in order to learn about what effects may contribute to the observation made in thin films. There is also a strong effort using numerical simulations to make quite direct comparisons to measured values in thin films. Finally, in a way that signals a new maturity to this field, a significant fraction of literature being currently published concerns ideas as to why the dynamics in thin films behave the way they do. In this special issue we have aimed at capturing a cross-section of problems of high current interest. While all contributions definitely provide highly valuable insight in the behavior of polymers in thin films, many questions are still unanswered and await further in-depth-going studies. We just want to note a few of these questions, also highlighted in several commentaries published in this issue: Can polymers in experimental studies on thin films ever be fully equilibrated? What is the relaxation behavior of macromolecules in confining geometries with interacting boundaries? How does the relaxation behavior depend on the length scale over which it is measured? What are the mechanical properties of thin polymer films? We tried to assemble different approaches and opinions from various viewpoints. We hope that such a complementary presentation is helpful and stimulates further discussions in order to dissolve some of the confusion in this area, leading eventually to a clear understanding of thin-film properties of polymers. The European Physical Journal E - Soft Matter will continue to provide a forum for the discussion of such questions and a place for the publication of future work on properties of polymers in thin films for all colleagues interested in these questions. Gunter Reiter (Editor) James Forrest (Guest Editor)