NAMS 2002 Workshop - ICOM 2008

Membrane Modeling II - Gas Separation – 3
Tuesday July 15, 3:30 PM-4:00 PM, O’ahu/Waialua
Development of a Microscopic Free Volume Theory for Molecular Self-
Diffusivity Prediction in Polymeric Systems
H. Ohashi (Speaker), The University of Tokyo, Tokyo, Japan - yamag@res.titech.ac.jp
T. Ito, Tokyo Institute of Technology, Tokyo, Japan
T. Yamaguchi, Tokyo Institute of Technology, Tokyo, Japan
Molecular diffusivity in polymer matrices is an important dynamic physical
property for membrane transports. Prediction of the diffusivity using some
theoretical models is favorable, and thus, several diffusion models for polymeric
systems have been proposed up to now. However, diffusivity prediction model
without adjustable parameter has not been proposed yet because microscopic
phenomena are not taken into account in the previous models.
Microscopically, molecular self-diffusion originates in the common mechanisms
of molecular collisions and random walk motion in polymer systems as well as in
simple liquids. Therefore, we developed a novel model for molecular diffusion in
polymer by incorporating these two notions into the free volume theory. The free
volume theory for polymeric systems contains two unknown parameters, so we
introduced a newly developed concept, “shell-like free volume” around a
molecule, and “random walk movement into neighbor free volume hole” into both
of the unknown parameters. Incorporation of these microscopic concepts
provides a predictive model, which can calculate self-diffusivity of mixing property
using only pure component properties derived from experimental viscoelasticity
and quantum chemical calculation.
Using this model, we can predict self-diffusivity of various molecules in polymer
matrices without using any adjustable parameter. Our model can be applied to
molecules having various shapes and molecular types of gas, solvent, and solute
in several polymeric systems. The predictive ability of our model was found to be
fairly acceptable in every case and thus, the model can be a useful tool for
polymeric membrane designs.