Characterization of Diffusion Processes in Thin Polymer Films via Optical Waveguide Techniques
Fell, Nicholas Francis, Jr.
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https://hdl.handle.net/2142/72289
Description
Title
Characterization of Diffusion Processes in Thin Polymer Films via Optical Waveguide Techniques
Author(s)
Fell, Nicholas Francis, Jr.
Issue Date
1993
Doctoral Committee Chair(s)
Bohn, Paul W.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Chemistry, Physical
Chemistry, Polymer
Abstract
The goal of the work presented here is the development of optical waveguide techniques for the characterization of diffusion mechanisms and mass transport processes in uncoated polymer films and Langmuir-Blodgett mono- and multilayer coated polymer films. Optical waveguide techniques provide a unique ability to monitor the temporal changes in the physical properties of the film, the film thickness and refractive index, and the spectroscopic response from the film components through the collection of Raman scattering or fluorescence from the film components. These techniques are non-destructive and can be applied in situ, as long as the refractive indices of the material surrounding the film are lower than the film refractive index.
The mechanism of solvent diffusion in polymer films are classified by two phenomenological extremes: Fickian diffusion and Case II diffusion. They differ in the temporal dependence of the diffusion front propagation. By modeling the polymer film as a two layer system, where the interface between the two layers represents the diffusion front, the temporal dependence of the front propagation can be determined and the diffusion mechanism for the solvent/polymer system identified. Changes in the eigenmode distributions permit the determination of the individual layer thicknesses and hence, the position of the interface as a function of time. The polystyrene/hexane isomers system is examined in this work. The diffusion mechanism is identified as Case II for n-hexane and the details of the determination are described.
The mass transport properties of analyte diffusion from bulk solution into polymer films can also be examined with optical waveguide techniques. The transport of fluorescein from aqueous solutions into 2-5 $\mu$m thick poly(2-vinylpyridine) films is detailed. The collection of the fluorescence from the film as a function of time permits the determination of the mass transport or diffusion coefficient. The mass transport coefficients for both uncoated and Langmuir-Blodgett coated films are reported. These results indicate that there is no concentration dependence in the mass transport coefficient from 1 nM to 10 $\mu$M and only multilayer LB films will hinder transport of fluorescein into the poly(2-vinylpyridine) films.
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