NAMS 2002 Workshop - ICOM 2008

Membrane Modeling II - Gas Separation – 6
Tuesday July 15, 5:00 PM-5:30 PM, O’ahu/Waialua
Free-Volume Holes in Amorphous Polymers for Solvent Diffusion:
Reconsideration of the Free-Volume Theory By Equation-of-State, Group
Contribution Method, PALS Measurement and Molecular Simulation
H. Lv (Speaker), Tsinghua University, China
B. Wang, Tsinghua University, China - bgwang@tsinghua.edu.cn
J. Yang, Tsinghua University, China
In many processes such as gas separation, pervaporation and vapor permeation
with a polymeric dense membrane, solvent diffusion behaviors in polymer matrix
have attracted much attention, since the diffusivity is normally the rate- limiting
step. Prediction of solvent diffusivity is of fundamental importance in the
development of polymeric membrane design methodology for organic mixture
separation [1-3]. In the past decades, the free-volume theory, which emphasizes
the amount of free-volume vacancies as the dominant factor for diffusion, has
served as the main basis for the correlation of diffusion behaviors in polymersolvent
systems. The model proposed by Vrentas and Duda is the representative
of free-volume theory, in which most parameters can be obtained from pure
component properties and no adjustable parameters are used [4-8]. However,
free-volume parameters of polymer are usually determined by fitting the results
from measurement of polymer viscoelasticity, meaning a great deal of time and
cost consumption [9-11]. Moreover, the relationship between detailed information
about the atomic-scale holes, which collectively constitute the free volume in
polymers, and solvent transport properties still remains uncertainty. In order to
remove these shortcomings, this study proposes four approaches to estimate
polymer free volume and compare with the original model both theoretically and
experimentally.
The first two approaches are equation-of-state (EOS) and group contribution
method, both of which are based on macroscopic viewpoint of the free volume.
For the former, the Simha-Somcynsky hole theory EOS is introduced into the
free-volume theory; for the latter, the universal constant of the van der Waals
volume of functional groups in polymer repeating units is introduced. Both of the
modified models provide agreeable prediction of infinite dilution diffusion
coefficients and solvent self-diffusion coefficients in several polymer- solvent
systems without measuring polymer viscoelasticity. Furthermore, the individual
predominance of these two approaches is discussed. In the EOS-modified
model, the influence of pressure on solvent diffusivity in dilute polymer solutions
can be included. In the group contribution-modified model, since all the
parameters related to polymer can be determined only based on the knowledge
of polymer structural units, a real process of membrane design with polymer