Solvent Permeation, Swelling Profiles and Mechanical Properties of Thin Polymer Films

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This research will study thin siloxane and epoxy polymeric films which will be spin cast as linear chains onto a variety of substrates (sapphire, BK7 glass, and silica) to thicknesses of 50 nm to several microns, and then crosslinked to form a rigid structure. The study focuses specifically on the mechanisms by which these films uptake solvent, and how solvent permeation and swelling affects the adhesion of the film to the substrate, and the membrane mechanics. Solvent permeation will be studied using total internal reflectance fluorescence (TIRF), a technique in which the substrate onto which the film is adhered is placed on an optical prism, and a laser beam impinges at a grazing angle to the substrate /polymer film surface producing an exponentially decaying evanescent wave which penetrates through the film. By varying the angle of incidence, different penetration depths can be achieved. Using TIRF, two types of experiments will be undertaken. In the first, diffusion of a free dye into an equilibrium swollen film or a partially swollen film will be followed by placing a chromophore on the dye. The radiation due to the evanescent wave causes the chromophore to fluoresce, and by detection of the total fluorescence of the film as a function of the penetration depth (by changing the grazing angle), the dye concentration can be computed by deconvolution (cf. eq. 3). By repeating the TIRF angling as a function of time, the time dependent concentration profile of the dye can be constructed. In the second type of experiment, the polymer network of an unswollen film will be tagged. The face of the film will then be exposed to a permeating solvent, and as the film swells due to the solvent uptake, the polymer chains will deform and the concentration of the tag will vary. Again, by using time resolved TIRF angling, the time dependent concentration profile of the labels can be computed, and from these, insight into the spatial distribution of the labels during swelling can be obtained. The dynamical mechanical response of the membrane will be measured, and a microhardness tester will be used to determine the bond between the polymer film and the substrate.