Solid-state covalent cross-linking of polyimide membranes for carbon dioxide plasticization reduction

Solid-state covalent cross-linking of 6FDA-based polyimides with esterification reactions is shown to be effective in stabilizing membranes against CO2 plasticization up to 40 atm feed pressure. The selection of cross-linking agent has a major impact on the cross-linking degree and the gas transport properties. A generalized cross-linking strategy is presented that enables analysis of the effects of the cross-linking agent structure and thermal treatment on pure gas permeation and sorption for CO2 and CH4 at 35 degreesC. The polyimide 6FDA-DAM:DABA 2:1 was cross-linked with ethylene glycol, 1,4-butylene glycol, 1,4-cyclohexanedimethanol, and 1,4-benzenedimethanol to illustrate these effects. The cross-linking degree is evaluated by a combination of solution H-1 NMR and solid-state IR spectroscopy. The annealing temperature is a significant factor in determining the membrane transport properties, since it affects the polymer chain rigidity and free volume distribution. It is believed that this cross-linking approach can be implemented in an industrial hollow fiber formation process since the reaction is shown to occur in the solid state, well below the glass transition temperature.