Synthesis, preparation and characterization of novel hyperbranched 6FDA-TTM based polyimide membranes for effective CO2 separation: Effect of embedded mesoporous silica particles and siloxane linkages

Abstract

We report on synthesis, preparation and characterization of newly prepared hyperbranched polyimide (HBPI) based on 4,4´-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 4,4′,´´-triaminotriphenylmethane (TTM). HBPI material was synthesized via the amine-terminated hyperbranched polyamic acid (HBPAA) precursor and the absence of corresponding HBPAA amide band at 1670 cm−1 in IR spectra indicated complete imidization. TGA analysis of 6FDA-TTM proved the excellent thermal stability up to ca. 500 °C. Gas permeation measurements (H2, CH4, and CO2) revealed the excellent separation properties for the CO2/CH4 gas pair, with the best-reached selectivity value of 137 and 27.4 Barrer of CO2 permeability that exceeds the Robeson 2008 upper bound. The 6FDA-TTM based-analogs containing 5 and 20 mol% of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (DMQ) were prepared for comparison to investigate the effects of reduced HBPI branching on material properties. 6FDA-TTM-DMQ displayed lower CO2 sorption in the whole pressure interval (1–15 bar) while for CH4 only at higher pressures, presumably due to reduced chains mobility to accommodate excessive amounts of CO2. Similarly, 6FDA-TTM-DMQ with increasing of DMQ content exhibited lower H2 and CO2 permeabilities and lower corresponding CO2/CH4 selectivities. However, in the case of H2/CO2 were selectivities higher (2.3 and 2.1, respectively) compared to a neat polymer (1.5). Simultaneously, the effect of mesoporous hollow and solid silica spheres with diameters 600 and 400 nm, respectively, in amounts of 1, 5 and 10 wt % embedded in 6FDA-TTM and 6FDA-TTM-DMQ membranes was studied too. Prepared mixed matrix membranes exhibited rather enhanced permeability due to the introduction of the additional free volume, but without clear trend with respect to the silica morphology and loading, and, in most cases, it reduced corresponding selectivities compared to a neat polymer. Consistent with that, experimental permeabilities were comparable or higher than those obtained by Maxwell model due to the mesoporosity of the silica particles and, to some extent, due to probably the non-homogeneous distribution of filler within the matrix. However, our work proves the concept of 6FDA-TTM materials for effective gas separation and outlined the possibilities for further tailoring the material properties.