Surface Capillary Waves on Polymer Films Studied by Atomic Force Microscopy

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TECHNICAL SUMMARY: Understanding the dynamics of polymers confined in thin films with thicknesses comparable to the radius of gyration of the polymer chains has been one of the most challenging problems of polymer physics in the recent decade. In particular, there have been numerous experimental and theoretical attempts to understand the cause for the observed precipitously reduction in the glass transition temperature of some polymer films by ~ 60 K when the film thickness is decreased to the nanometer range. One difficulty stems from the fact that the glass transition only represents one parameter of the dynamic relaxation curve characterizing the full dynamic behavior of a polymer film; the Vogel-Tamman-Fucher scaling generally exhibited by the dynamic curves of polymers requires three parameters. However, dynamic measurements of polymer films in the 10 nm thickness range are scarce, attributable to the small amount of polymer material contained in these films. It is noticed that many of these films would rupture when subjected to prolonged heating. In this program, a new method based on atomic force microscopy, and complemented with grazing incidence small-angle x-ray scattering, will be developed to probe the surface structure of the films and their dynamics in the 1 ~ 100 s time range, directly relevant to the glass transition, for polymer films with thicknesses from 2 to 10 nm - a regime that has been seldom explored for the dynamics in the 1 ~ 100 s range. The method is benign and involves monitoring of the time evolution of the surface structure of (or capillary waves on) a polymer film due to the innate molecular forces (such as van der Waals interaction forces and surface tension) and the stochastic thermal forces from which the viscosity of the polymer film can be determined. NON-TECHNICAL SUMMARY: Applications of polymer films are ubiquitous including protective coatings, adhesives, electronic packaging, and photoresist films for microelectronics and micro-electro-mechanical systems (MEMS) fabrication, etc. Results of this program will lead to better understanding of the materials properties of polymers in thin films, which have frequently been found to differ noticeably from those of the bulk. A good understanding on the cause of these findings would allow us to accurately premeditate which polymer materials to use for a given thin film application. This program provides an excellent training ground for graduate students to be involved in the academically challenging and technologically important problem of polymer thin films, and at the same time, novel applications of atomic force microscopy, which is a rapidly developing technology in materials research. This program will actively recruit undergraduates - especially the underrepresented female students - to the program through Boston University's Undergraduate Research Opportunities Program (UROP). The PI has a strong record of getting undergraduates and female students participating in her research.