Preparation and characterization of Para-Phenylenediamine cross-linked polyimide membranes for gas separation

Abstract

The chemical cross-linking modifications of asymmetric 3,3',4, 4'-benzophenone tetracarboxylic dianhydride and diamino-phenylindane (BTDA-DAPI) polyimide membranes with para-phenylenediamine (pPD) have been investigated. The modification was performed by immersing BTDA-DAPI polyimide membranes in a pPD/methanol solution at ambient temperature for certain periods of time. The effects of immersion time and cross-linker concentrations on degree of cross-linking, physicochemical properties and gas transport properties of the membranes have been studied. The cross-linked BTDA-DAPI polyimide membranes were characterized using gel test, fourier transform infrared (FTIR), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA) and x-ray diffractometer (XRD). The FTIR spectra confirmed that the cross-linking process had taken place, with the imide groups turning into amide groups during the modification process. The degree of cross-linking was increased with the increase in the immersion time and the cross-linker concentrations. It is also showed that there are small amount of insoluble materials remaining from the gel test. The XRD results revealed that the d-space of the polymeric chains changed after the modification due to the methanol swelling and structure tightening effects induced by cross-linking. The TGA and DSC results indicated that the degradation and glass transition temperature were sufficient for gas separation applications. The gas transport properties of the pure and cross-linked BTDA-DAPI polyimide membranes were investigated using gas permeation tests. The results showed that the cross-linked BTDADAPI polyimide membranes decreased the oxygen (O2), nitrogen (N2), carbon dioxide (CO2) and methane (CH4) permeabilities compared to the unmodified membranes. Interestingly, 10% pPD-120 min samples showed an impressive enhancement of selectivity for O2/N2 compared to the unmodified membrane from 6.19 to 10.01 with reasonable permeability. However, less favorable CO2/CH4 and CO2/N2 selectivity have been observed after diamine cross-linking for all samples. Thus, it appears that the pPD is a good alternative cross-linker agent for BTDA-DAPI polyimide membrane mainly for the separation of O2/N2 with 10% pPD and 120 minutes as the optimum cross-linker concentrations and cross-linking time, respectively.