(733g) Assessment of Structure-Solubility Relationship for Copolyimide Gas Separation Membranes Via Molecular Simulation

Aromatic polyimides are a promising group of polymers, due to
their outstanding thermal and chemical stability, to be used as polymeric gas
separation membranes.  Polyimides are synthesized by the reaction of a
dianhydride and a diamine in a solvent and by dehydration of the resultant
polyamic acid by imidization. Copolyimides are obtained when more than two
monomers are used. Copolyimides allow us to optimize the separation properties
by using different dianhydride and diamine combinations.  Hundreds of different
copolyimide structures can be proposed, however, the experimental analysis of
the separation properties of all these structures is a tedious and
time-consuming task. 

In this study, a combination of molecular dynamics (MD) and
Monte Carlo (MC) simulation methods was applied to estimate the structural
properties (d-spacing, fractional free volume, radius of gyration and glass
transition temperature), solubility coefficients, and solubility selectivities
of small gases (CO₂, CH₄, O_2, N₂)
in different copolyimide membranes, for which no experimental data are
available: 6FDA/BTDA-DABA, 6FDA/BTDA-pBAPS, 6FDA-pBAPS/DAM, 6FDA-pBAPS/DABA,
and 6FDA-pBAPS/mPDA.  The selection of these copolyimide structures was
based on our previous study [1] in which the permeability coefficients of H₂,
O₂, He, CO₂, N₂, and CH₄ gases for
more than 2200 possible copolyimide structures were estimated by the group
contribution method of Alentiev et al. [2].  The predicted
permselectivities of the above mentioned copolyimides were located above the
polymer upper bound of Robeson [3] for CO₂/CH₄ and O₂/N₂
separation applications.

This methodology was first validated by predicting the
membrane properties for two polyimides (6FDA-DAM and 6FDA-ODA) for which
experimental data were available in the literature. The estimated properties
for 6FDA-DAM and 6FDA-ODA were in excellent agreement with the experimental
data, with a maximum deviation of ~5%.  Among the five copolyimides,
6FDA/BTDA-DABA exhibited two distinct d-spacing values in the simulated X-ray
patterns, provided the highest CO₂, CH₄, O₂ and
N₂ solubility coefficients and lowest CO₂/CH₄
and O₂/N₂ solubility selectivities indicating that
fractional free volume of 6FDA/BTDA-DABA is highest. This may be attributed to
the BTDA-DABA linkage of this copolyimide. To better understand this finding,
BTDA-DABA and 6FDA-DABA polyimides were constructed and their structural
properties were estimated. Results indicated that the fractional free volume of
BTDA-DABA polyimide is much larger than those of 6FDA-DABA.

Acknowledgment

This work was partially supported by Turkish Scientific and
Technological Research Council (Grant No. 106M339) and National Center for High
Performance Computing (Grant No. 10932010).

Abbreviations

6FDA: 4,4-hexafluoro isopropylidene
diphthalicanhydride; ODA: 4,4-oxydianiline; DABA: 3,5-diamino benzoic acid;
BTDA: 3,3-4,4-benzophenone tetracarboxyclic dianhydride; DAM:
2,4,6-trimethyl-m-phenylene diamine.

References

[1]   S. Halitoglu, S.B. Tantekin Ersolmaz, Prediction of
gas permeability coefficients of copolyimides by group contribution methods,
presented at the NAMS meeting, May 12-16, 2007, Orlando, Florida.

[2]   A. Y. Alentiev, K.A. Loza, Y. Yampolski, J. Memb.
Sci. 167, 91-106  (2000).

[3]   L. M. Robeson, J. Memb. Sci., 62, 165-185
(1991).