(742h) Residual Solvent Effect On CO2-Induced Plasticization of Polyimide Membranes

Previous
experimental studies on gas permeabilities of polyimide membranes showed that
the presence of the residual solvent in the membrane has a significant effect
on the free volume size distribution, and it alters the gas sorption behavior, hence
the gas permeability. While small amounts of solvents in membrane act as an anti-plasticizing
agent, larger amounts exhibit an opposite effect. Molecular simulation methods
were previously used to investigate the effect of the residual solvents on free
volume changes of the polyimides.¹

In
this study, we investigated the change in the CO₂ sorption capacity
of 6FDA-DAM, 6FDA-ODA, 6FDA-DABA, 6FDA-25DPX and BTDA-DABA as a function of
residual solvent type and amount. A low-boiling-point solvent, tetrahydrofurane
(THF) (BP: 66ºC), and a high-boiling-point solvent, n-methyl-2-pyrrolidone
(NMP) (BP: 197ºC), were selected for this purpose.  

First solvent-free polyimides
are constructed and their sorption capacities and plasticization behavior are
investigated. To reproduce the CO₂-induced
plasticization effect, sorption-relaxation cycles were applied until the CO₂
concentration converges: At each cycle, the polymer matrix was loaded with CO₂
through Grand Canonical
Monte Carlo simulations at the considered pressure, and then molecular dynamics
simulation runs in the NPT ensemble
were applied to obtain an equilibrated matrix. The cycle was repeated until CO₂
concentration converges. Simulation results showed that the increase in the
fractional free volume and the flexibility of the diamine-dianhydride linkage are
correlated. Next, the procedure was repeated for model polyimides containing different
amounts of NMP or THF up to 4% by weight. The
change in the linkage flexibility due to solvent presence and its effect on CO₂
sorption capacity and plasticization are analyzed.

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; 25DPX: 2,5-dimethyl-p-phenylenediamine.

¹Chang, K. S.; Tung, C.
C.; Lin, C. C.; Tung, K. L. Residual Solvent Effects on Free Volume and
Performance of Fluorinated Polyimide Membranes: A Molecular Simulation Study J.
Phys. Chem. B. 2009, 113, 10159.