jp6b09690_si_001.pdf (16.5 MB)
Molecular Simulation of Gas Solubility in Nitrile Butadiene Rubber
journal contribution
posted on 2016-12-06, 00:00 authored by M. Khawaja, A. P. Sutton, A. A. MostofiMolecular simulation
is used to compute the solubility of small gases in nitrile butadiene
rubber (NBR) with a Widom particle-insertion technique biased by local
free volume. The convergence of the method is examined as a function
of the number of snapshots upon which the insertions are performed
and the number of insertions per snapshot and is compared to the convergence
of the unbiased Widom insertion technique. The effect of varying the
definition of local free volume is also investigated. The acrylonitrile
content of the polymer is altered to examine its influence on the
solubility of helium, CO2, and H2O, and the
solubilities of polar gases are found to be enhanced relative to those
of nonpolar gases, in qualitative agreement with experiment. To probe
this phenomenon further, the solubilities are decomposed into contributions
from the neighborhoods of different atoms, using a Voronoi cell construction,
and a strong bias is found for CO2 and H2O in
particular to be situated near nitrogen sites in the elastomer. Temperature
is shown to suppress the solubility of CO2 and H2O but to increase that of helium. Increasing pressure is found to
suppress the solubility of all gases but at different rates, according
to a balance between their molecular sizes and electrostatic interactions
with the polymer. These results are relevant to the use of NBR seals
at elevated temperatures and pressures, such as in oil and gas wells.