Molecular Simulation of Permeation of Small Penetrants through Membranes.
1. Diffusion coefficients.
Y. Tamai, H. Tanaka, K. Nakanishi
Macromolecules, 27, 4498-4508 (1994)
Abstract
Molecular dynamics simulations have been carried out
in order to examine the mechanism of diffusion of small penetrants
in amorphous polymer membranes.
Diffusion processes of methane, water, and ethanol
in poly(dimethylsiloxane) (PDMS) and in polyethylene (PE) were investigated.
Pure liquid water and ethanol were also simulated.
The insertion probabilities P(R) of hard sphere atoms
of radius R
into the polymers and the liquids were calculated.
The free volume fraction, P(0), of PDMS is large
and the insertion probability of a finite size atom into PDMS is
widely distributed compared with the other polymers and liquids.
Simulations of 5 ns were performed
for PDMS and in PE with a penetrant species, methane.
The diffusion of methane in the polymer matrix
exhibits anomalous (non-Einstein) behavior for time scales of
1 and 0.3 ns in PDMS and PE, respectively.
Aggregates of water and ethanol are found to be formed in PDMS.
Diffusion coefficients of water and ethanol in PDMS
are reduced by more than 1 order of magnitude due to the aggregation.
The calculated diffusion coefficients
of the nonaggregated penetrants in PDMS and in the pure liquids
agree well with the experimental values.
Copyright (c) 1997 Yoshinori Tamai