6FDA Polyimide Research Articles

Insights on influence of polymer molecular weight on gas sorption, transport and plasticization in glassy 6FDA-DAM polyimide membranes

It is well-known that spinning of defect-free asymmetric polyimide hollow fibers is promoted by using high polymer molecular weight. On the other hand, effects of polymer molecular weight on microstructure and separation performance of dense film 6FDA-based polyimide membranes relevant to sheath layers of composite membranes are less well-explored. In this work, gas sorption, diffusion and permeation of CO2, N2 and CH4 for 6FDA-DAM dense films are considered for 45 kDa and 176 kDa weight average molecular weights. Such molecular weights are relevant for practical hollow fiber spinning of membranes. Earlier results for polystyrene samples show similar trends in dual mode sorption model results like those noted here for the 6FDA-DAM polyimide case; however, effects on permeation like those considered here were not addressed for the polystyrene case. Here we also address actual pure and mixed gas CO2 and CH4 permeation and show higher free volumes, higher gas permeability but lower 50:50 CO2/CH4 selectivity associated with the higher molecular weight sample. Analysis using the self-consistent dual-mode sorption and transport models help understand the observed trends. Our results suggest denser packing of polymer segments exists in the Henry's law environment of the low molecular weight sample. Moreover, the higher polymer segmental packing in the Henry's law environment suppresses CO2 plasticization with a dual-mode microstructure controlling separation performance and plasticization behavior for this important member of the 6FDA polyimide family. We show evidence of effects of charge transfer complexes as possible main features in these trends. These dense film results help guide future work on dense sheath layers on composite hollow fiber membranes.

Thermally treated ammonia functionalized graphene oxide/polyimide membranes for pervaporation dehydration of isopropanol

A novel mixed matrix membrane (MMM) consisting of a cross-linkable 6FDA polyimide matrix and ammonia functionalized graphene oxide (NHGO) particles has been molecularly designed at elevated temperatures for water/IPA separation. The reaction scheme was proposed and evidenced by FTIR and XPS analyses. The evolution of microstructure changes in each step of MMM formation was systematically studied by FESEM, XRD and PALS. The water-IPA solution-diffusion characteristics of the thermally treated MMMs at 400°C were investigated in order to evaluate their impact towards IPA/water dehydration. The MMM containing 0.5% NHGO (PI-0.5%NHGO-400) exhibited the most favorable solution-diffusion properties for water-IPA separation and displayed the best pervaporation performance. This membrane showed a water permeability of 0.198mgm−1h−1KPa−1 and a water/IPA molar selectivity of 6726 which was 35 times higher than that of the pristine co-polyimide without sacrificing the water permeability. The superior properties result from the combined contributions of (1) simultaneous amidation reactions between the polymer matrix and particles to overcome the interfacial voids in MMMs (2) thermal induced inter-chin crosslinking to enhance the MMMs solvent resistance and (3) evolution of the non-reacted functional groups of two parties during the high temperature annealing to regulate the MMMs FFV and produce a balanced membrane structure. This encouraging performance suggests a promising future of PI-0.5%NHGO-400 composite membranes for the dehydration of IPA via pervaporation process.

Pure- and mixed-gas permeation properties of highly selective and plasticization resistant hydroxyl-diamine-based 6FDA polyimides for CO2/CH4 separation

The effect of hydroxyl functionalization on the m-phenylene diamine moiety of 6FDA dianhydride-based polyimides was investigated for gas separation applications. Pure-gas permeability coefficients of He, H2, N2, O2, CH4, and CO2 were measured at 35°C and 2atm. The introduction of hydroxyl groups in the diamine moiety of 6FDA-diaminophenol (DAP) and 6FDA-diamino resorcinol (DAR) polyimides tightened the overall polymer structure due to increased charge transfer complex formation compared to unfunctionalized 6FDA-m-phenylene diamine (mPDA). The BET surface areas based on nitrogen adsorption of 6FDA-DAP (54m2g−1) and of 6FDA-DAR (45m2g−1) were ~18% and 32% lower than that of 6FDA-mPDA (66m2g−1). 6FDA-mPDA had a pure-gas CO2 permeability of 14Barrer and CO2/CH4 selectivity of 70. The hydroxyl-functionalized polyimides 6FDA-DAP and 6FDA-DAR exhibited very high pure-gas CO2/CH4 selectivities of 92 and 94 with moderate CO2 permeability of 11 and 8Barrer, respectively. It was demonstrated that hydroxyl-containing polyimide membranes maintained very high CO2/CH4 selectivity (~75 at CO2 partial pressure of 10atm) due to CO2 plasticization resistance when tested under high-pressure mixed-gas conditions. Functionalization with hydroxyl groups may thus be a promising strategy towards attaining highly selective polyimides for economical membrane-based natural gas sweetening.

Mixed matrix membranes based on 6FDA polyimide with silica and zeolite microsphere dispersed phases

Mixed matrix membranes (MMMs) prepared with 6FDA‐DAM polymer using ordered mesoporous silica MCM‐41 spheres (MSSs), Grignard surface functionalized MSSs (Mg‐MSSs) and hollow zeolite spheres are studied to evaluate the effects of surface modification on performance. Performance near or above the so‐called permeability‐selectivity trade‐off curve was achieved for the H2/CH4, CO2/N2, CO2/CH4, and O2/N2 systems. Two loadings (8 wt % and 16 wt %) of MSSs were tested using both constant volume and Wicke–Kallenbach sweep gas permeation systems. Besides single gas H2, CO2, O2, N2, and CH4 tests, mixed gas (50/50 vol %) selectivities were obtained for H2/CH4, CO2/N2, CO2/CH4, and O2/N2 and found to show enhancements vs. single gases for CO2 including cases. Mg‐MSS/6FDA‐DAM was the best performing MMM with H2/CH4, CO2/N2, CO2/CH4, and O2/N2 separation selectivities of 21.8 (794 Barrer of H2), 24.4 (1214 Barrer of CO2), 31.5 (1245 Barrer of CO2), and 4.3 (178 Barrer of O2), respectively. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4481–4490, 2015

Effect of polymer structure on gas transport properties of selected aromatic polyimides, polyamides and TR polymers

Thermally rearranged (TR) polymers are formed through a thermally induced solid-state reaction of polyimides or polyamides that contain nucleophilic reactive groups ortho-positioned to their diamine. Naturally, the transport properties of TR polymers are intimately related to the chemical structure and reactivity of their precursors. Herein, we report characterization and transport properties for three poly(hydroxyimide) precursors prepared via thermal imidization in solution and for their corresponding TR polymers. Structural modifications to the polymer backbone can be used to control thermal rearrangement reaction kinetics. In regards to TR polymer formation, samples prepared from diamines with biphenyl functionality reacted more efficiently than those prepared from diamines with hexafluoroisopropylidene-linked aromatic units. However, hexafluoroisopropylidene functional units provided the highest combinations of permeability and selectivity for separations involving H2, N2, O2, CH4, and CO2. Differences in permeability between samples correlated well with changes in free volume, and 3 poly(hydroxyimide)s showed unusually high selectivities for their given free volume. The effect of synthesis route was also investigated for a specific TR polymer derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). Poly(hydroxyimide) precursors prepared via thermal imidization in solution and thermal imidization in the solid-state showed nearly identical permeabilities and selectivities regardless of synthesis route. However, after thermal rearrangement, the TR polymers prepared from polyimides synthesized via solid-state imidization have higher gas permeabilities than their solution-imidized analogs. In addition to light gas permeabilities, plasticization effects were investigated with CO2 hysteresis loops for all samples, and pure-gas olefin/paraffin permeabilities were determined for a TR polymer derived from 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF) and 6FDA. With the exception of HAB–6FDA polyimides, pure-gas CO2 feed pressures up to approximately 50bar do not reveal a plasticization pressure point, but conditioning effects are observed for most samples. APAF–6FDA TR polymers have pure-gas permeabilities and selectivities beyond the propylene/propane upper bound.

Gas Separation of Asymmetric 6FDA Polyimide Membrane with Oriented Surface Skin Layer

A novel asymmetric polyimide membrane with an oriented surface skin layer was prepared by a dry−wet phase inversion process at different shear stresses. The surface morphology and the structure of ...

Fabrication of hollow fiber membrane of 6FDA polyimide and design of novel membrane oxygenator

Fabrication of hollow fiber membrane of 6FDA polyimide and design of novel membrane oxygenator

Packing density and gas permeability of copolyimides containing bicyclo[2,2,2]oct‐7‐ene‐2,3,5,6‐tetracarboxylic dianhydride constituent

AbstractPacking density and gas permeability of two copolyimide series containing an alicyclic monomer, bicyclo[2,2,2]oct‐7‐ene‐2,3,5,6‐tetracarboxylic dianhydride (BCDA), and a fluorinated monomer, hexafluoroisopropylidene,2,2‐bis(phthalic acid anhydride) (6FDA), were measured. Incorporation of a rigid BCDA segment in 6FDA polyimides either increases gas permeability or improves gas selectivity, depending on the base 6FDA polyimide. In the case of the highly gas permeable 6FDA polyimide, introduction of BCDA further increases gas permeability despite a loss in the amount of free volume. In the moderately gas permeable 6FDA polyimide, on the other hand, the introduction improves gas selectivity. © 1995 John Wiley & Sons, Inc.

Structure/permeability relationships of polyimides with branched or extended diamine moieties

AbstractMean permeability coefficients for CO2, O2, N2, and CH4 in seven types of 6FDA polyimides with branched or extended diamine moieties were determined at 35.0°C (95.0°F) and at pressures up to 10.5 atm (155 psia). In addition, solubility coefficients for CO2, O2, N2, and CH4 in six of these polyimides were determined at 35.0°C and at 6.8 atm (100 psia). Mean diffusion coefficients for the six gas/polyimide systems were calculated from the permeability and solubility data. The relationships between the chemical structure of the polyimides, some of their physical properties (glass transition temperature, mean interchain spacing, specific free volume), and their gas permeability, diffusivity, and solubility behavior are discussed. The 6FDA polyimides studied here exhibit a considerably lower selectivity for the CO2/CH4 and O2/N2 gas pairs than 6FDA polyimides with short and stiff aromatic diamines with comparable CO2 and O2 permeabilities. © 1993 John Wiley & Sons, Inc.

Gas permeation of aromatic polyimides. II. Influence of chemical structure

The permeability of CO 2 and CH 4 in 17 different types of polyimides membranes has been determined at 25°C and at an applied pressure of 4.87 atm. The relationship between structures of polyimides and their gas permeability was studied. Polyimides containing an O group in the dianhydride part of the polymer chain showed low permselectivity and low permeability. Polyimides with an SO 2 group in the dianhydride exhibited high permselectivity but low permeability. On the other hand, a C(CF 3) 2 group in the dianhydride showed both high permselectivity and high permeability. The effects of the diamine part of 6FDA polyimides on gas permeability were studied. Aromatic diamines with meta-connections in the polymer chain showed a higher permselectivity and lower permeability compared with para-connected polyimides. Polyimides with longer diamine chains with O linkages show decreased permselectivity and permeability. Polyimides having C(CF 3) 2 groups both in the dianhydride part and the diamine part showed both high permeability and high permselectivity.

Gas permeation of aromatic polyimides. I. Relationship between gas permeabilities and dielectric constants

The permeability of 17 different 6FDA polyimides membranes to CO 2 and CH 4 has been determined at 25°C and at an applied pressue of 4.87 atm. Dielectric constants g of polyimides were measured with 1 MHz frequency at 23°C and at 55% rh (relative humidity). Fractional free volumes and dielectric constants were also obtained by theoretical calculation for comparison. Good correlation between dielectric constant and gas permeability was observed. Since the dielectric constant is a function of molar polarization and molar volume, polymer structure can be much better characterized by using ϵ than by using the fractional free volume. Good relationships between gas permeability and the solubility parameter and the cohesive energy density were also found. It was found that the gas permeability of the polymer can be estimated by measuring the dielectric constant, due to the correlation between ϵ and the gas permeability in common commercially available polymers. The dielectric constant can be applied to identify the structural factors that will lead to the preparation of polymer membranes that exhibit both a high permeability and permselectivity toward specific gases.

Gas permeation properties of hexafluoro aromatic polyimides

AbstractTwo hexafluoro‐substituted aromatic polyimides, 6FDA‐p‐PDA and 6FDA‐4,4′‐ODA, were synthesized. The influence of some synthetic conditions on physical properties and gas permeabilities of the 6FDA polyimide membranes were studied. Gas permeabilities and permselectivities of six different gases for the homogeneous films have been determined at 15–35°C and at pressures up to 10 atm. 6FDA polyimide composite membranes with asymmetric structure were prepared by the solution‐casting method. The membranes showed exceptionally high permeation fluxes and permselectivities. No capability deterioration of asymmetric membrane was observed during 19 days operation with pure gases. © 1993 John Wiley & Sons, Inc.

Molecular Simulation of Polyimide Structure and Gas Permeation Properties.

Polyimide membranes of 6 FDA- p- PDA, 6 FDA-4, 4'-ODA and BTC-4, 4'-ODA were prepared. Gas permeabilities and selectivities of carbon dioxide and methane for the films were measured. 6 FDA polyimides exhibit both high selectivities and high permeabilities in CO2/CH4 separation. Molecular structure of polyimides were investigated by using molecular simulation technique. 6FDA polyimides show helix configuration due to bending and twisting of polymer chain at carbon atom having-C (CF3) 2- substituent groups. On the other hand normal polyimides without-C (CF3) 2- give straight configuration. Helix configuration can be one of the important reason for high selectivity and high permeability of hexafluoro-substituted polyimide.

The solubility of Carbon Dioxide and Methane in polyimides at elevated pressures

AbstractThe solubility of CO2 and CH4 in five polyimides was measured at 35.0°C and at pressures up to 10 atm (147 psia). The concentration of the penetrant gases dissolved in the polymers can be represented satisfactorily as a function of penetrant pressure by the “dual‐mode sorption” model. The solubility coefficients for CO2 and CH4, S(CO2) and S(CH4), increase in the polyimide order: The magnitude of the solubility coefficients appears to depend primarily on the intermolecular forces between the penetrant gases and the polymers. The values of these coefficients are greater for the polyimides with larger mean interchain spacings, but no one‐to‐one correspondence appears to exist in this respect. The lower solubility of CO2 in PMDA‐4,4'‐m‐APPS compared with that in the 6FDA polyimides may be due to the lower “excess” free volume of the former polymer. The ratio S (CO2)/S (CH4) varies relatively little for a variety of PMDA and 6FDA polyimides.

Structure/permeability relationships of polyimide membranes. Applications to the separation of gas mixtures

AbstractThe permeability of nine different polyimide membranes to H2, N2, O2, CH4, and CO2 has been determined at 35°C and at applied pressures of up to 9 atm. The dianhydride monomers used for the synthesis of the polymides were PMDA and 6FDA, whereas the diamine monomers were ODA, BDAF, and p‐PDA. The selectivities of the 6FDA polymides toward CO2 relative to CH4 are higher than those of the PMDA polyimides at comparable CO2 permeabilities. Both types of polyimides exhibit significantly higher CO2/CH4 selectivities than more common glassy polymers, such as cellulose acetate, polysulfone, and polycarbonate. The selectivities of the PMDA and 6FDA polyimides to O2 relative to N2 are of the same magnitude and generally higher than those of common glassy polymers with similar O2 permeabilities. The polymides are more permeable to N2 than to CH4, whereas the opposite is true for many other glassy polymers. Possible factors responsible for the above behavior, such as segmental mobility, mean interchain distance, and formation of charge transfer complexes, are examined. The relevance of the study to the development of more highly gas‐selective and permeable membranes for the separation of gas mixtures is also discussed.