Investigation of
Concentration-Dependent Moisture Diffusion in Polymers

A. Cetegen1,2, Hom N. Sharma1, Yunwei Sun1, and
Elizabeth A. Glascoe1

Livermore National Laboratory, Livermore, CA

of Connecticut, Storrs, CT

The concentration
dependency of diffusion coefficients has long been studied. However, due to the
extreme complexity of many physical processes, the diffusion coefficient is
often assumed to stay constant throughout the process. While this
simplification may be an appropriate representation of some systems, the
constant diffusion coefficient assumption should be validated for the specific
process being investigated prior to its implementation. In this work, we
present a simple, easy-to-implement approach to determining the
concentration-dependent moisture diffusion in polymers. This method can be
easily adapted to gain useful insight into any process involving liquid or
vapor diffusion into a solid material.

In this study, full
profiles of sorption and desorption data were collected for Kapton H, Kapton
HN, and Pyralux polymers. A Fickian diffusion model was then implemented to
determine the best fitting diffusion coefficient for each sorption and
desorption step using a least squares optimization approach. This approach yields
the information necessary to map the concentration and temperature dependencies
of the diffusion coefficient, track the transient behavior of the water vapor
concentration throughout the sample thickness, and determine the activation
energy of the diffusion process for these materials. It was observed that both
Kapton materials showed a unique concentration dependency during the desorption
process and all diffusion coefficients resulting from implementation of the
Fickian model were in good agreement with literature-reported values.

This work was performed under the
auspices of the U.S. Department of Energy by Lawrence Livermore National
Laboratory under the contract DE-AC52-07NA27344. LLNL-ABS-758269-DRAFT.