In situ Evaluation of Diffusion Mechanisms in Thin Polystyrene Films Used as Integrated Optical Structures

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Abstract
The use of optical waveguide techniques allows the in situ, nondestructive examination of diffusion in thin films. Through the eigenvalue equation, the changes in thickness and refractive index, corresponding to changes in the composition of the film, can be monitored. By assuming a Case-II-like sharp diffusion front, one can model the system as a two-layer film, whose individual layer thicknesses can be determined by the analysis of the observed eigenmodes. This model is used to judge the appropriateness of the Case II description for a given set of conditions. Thin films (on the order of 1–7 μm in thickness) of polystyrene were exposed to n-hexane, methylcyclopentane, and a mixture of the two. The analysis of the observed eigenmodes indicates that n-hexane diffuses into polystyrene via a Case II mechanism, but a Fickian leading edge of the diffusion front can also be observed. In general, this approach provides a simple technique to distinguish between Fickian and Case II diffusion in planar polymer films in the micron thickness regime.