Asymmetric Polyimide Membranes Research Articles

Metal ion cross-linked nanoporous polymeric membranes with improved organic solvent resistance for molecular separation

The strategy of chemical cross-linking has been well used in the system of polyimide membranes for organic solvent nanofiltration (OSN). However, this method significantly affects the permeability and mechanical properties of polyimide membranes, as the opening of imide bond during the chemical cross-linking reaction damages the original polymer backbone structure. This work presents the formation of a new generation of OSN membranes through facile metal ion cross-linking of integrally skinned asymmetric polyimide membranes. A type of specially designed polyimide is synthesized with carboxyl functional groups introduced into the polymer backbone as organic ligands, and metal ion coordination cross-linking reaction takes place between the functionalized polyimide membranes and metal ions. Especially, the Cu2+ ion cross-linked polyimide membranes exhibit good stability in a variety of organic solvents including toluene, DMF, acetone, methanol, acetonitrile, etc., with >99% rejection to Coomassie brilliant blue (MW = 854) in acetone. Meanwhile, no significant attenuation of membrane permeability is observed after the cross-linking, as the metal ion does not occupy as much free volume as traditional chemical cross-linkers. Furthermore, the metal ion cross-linking is found to greatly improve the compaction resistance of the membrane due to the enhanced polymer-chain rigidity. This work confirms the feasibility of ion coordination based polymeric membranes for organic solvent separation applications and provides a brand-new route for the fabrication of OSN membranes with unique advantages.

Gas Permeation Properties of Asymmetric Polyimide Membranes Made by Ion–beam Irradiation

In this study, the gas permeance and selectivity of the asymmetric polyimide membrane with skin layer carbonized by He ions irradiation were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg and 35°C. Particutarly, we focused on the effects of the depth profile of the energy loss for the irradiating He ions in the skin layer on the gas permeation properties of the asymmetric membranes. Irradiation depth is gauged through the energy loss profile of the irradiating ions. The nature of the carbonised layer was investigated using AFM and ATR–FTIR. We demonstrated that the PO2 of the He+–irradiated membranes increased with the skin layer thickness and that the depth of the irradiating ions in the skin layer had a significant influence on the gas permeability and selectivity of the asymmetric membrane.

Predictive membrane transport models for Organic Solvent Nanofiltration: How complex do we need to be?

From feasibility tests at laboratory scale to large industrial scale processes, modelling can be applied to Organic Solvent Nanofiltration (OSN) for two purposes: (i) parameter estimation, when experimental data for standard solute + solvent systems are available, and it is desired to estimate relevant parameters for as yet uncharacterised systems, and (ii) prediction, when the model parameters for the solvent–solute are available, and modelling can be applied to describe the performance of a different solute + solvent system or operations at a different process scale. Both estimation and prediction require the choice of a transport model, to carry out regression and simulation, respectively. This paper reports a systematic comparison of a range of different transport models (irreversible thermodynamics, solution–diffusion, pore-flow and transient transport models, such as the solution–diffusion with imperfections model) using selected experimental data for various solutes and solvents through polymeric OSN membranes with different physico-chemical properties. Commercial integrally skinned asymmetric polyimide (PI, glassy) and thin film composite silicone-coated PI (rubbery) membranes from Evonik® MET were systematically tested in this study, with styrene oligomers and Safranin-O dye in different organic solvents under uniform operating conditions (temperature, pressure, cross-flow velocity and solute concentration). In addition, experimental data from the literature were taken for non-commercial thin film composite silicone-coated PI, glassy poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and poly[4methyl2pentyne] (PMP) membranes, to include a broad spectrum of positive and negative rejection values. The different transport models were used to perform regression of experimental data and prediction at different pressure values, based on the regressed model parameters. The models were then compared in terms of regression performance and experimental/numerical effort required to use them for parameter estimation. Negative rejection data was used to further discriminate among the models. Solution–diffusion-based models gave a better description of permeation through flexible-chain glassy membranes than pore-flow models. On the other hand, pore-flow-based models gave a better description of permeation through glassy PTMSP and PMP membranes. For integrally skinned asymmetric PI membranes, the prediction of a concentration process using the best performing regression models (i.e. the Maxwell–Stefan and the classical solution–diffusion model) was also performed. Interestingly, no significant difference was observed between the two models for both a membrane with complete solute rejection (Duramem® 200) and a membrane with partial solute rejection (Duramem® 500). Process modelling by accounting for a classical solution–diffusion-type transport mechanism is therefore sufficient to obtain a reliable transport description for this membrane family.

Effects of iron catalyst on gas transport properties of ion-irradiated asymmetric polyimide membranes

In this study, we focused on the effects of the carbon structures on the gas permeability and selectivity of the asymmetric polyimide membranes prepared by He +-irradiation using iron (III) acetylacetonate (AAI) as the catalyst. The irradiated asymmetric membranes consisted of a defect-free carbonized ultrathin skin layer and indicated a molecular sieving-like effect due to the surface carbonization similar to carbon molecular membranes. The Raman spectrum of the irradiated membranes included two well-known D (1360 cm −1) and G (1580 cm −1) peaks and two additional new peaks around 1150 and 1500 cm −1. Interestingly, the graphitic and amorphous structures at 1580 and 1500 cm −1 in the membranes increased with an increase in the AAI content. Especially, it was found that the catalyst had a profound effect on the graphitic and amorphous structures of the ion-irradiated membrane. Finally, we concluded that the amorphous structure in the membrane had an influence on the gas permeability and that the graphitic structure formed a barrier to the gas permeability and had an influence on the gas selectivity.

A new route of fabricating porous polyimide membranes

A novel experimental route to fabricate porous polyimide membranes (PPMs) with ideal air permeability was reported. The polymer solution layer consisting of the corresponding polyamic acid (PAA), solvent, and dibutyl phthalate (DBP) with a boiling temperature of 340°C was treated by a simple process. The polymer solution layer was first treated at a lower temperature (about 150°C), then the received solid membrane was further imidized at a higher temperature (about 270°C), and finally, DBP was removed from the membrane at a temperature above its boiling temperature. The final asymmetric polyimide membrane with a dense skin layer was obtained. To improve the air permeability of the polyimide membranes, the polymer solution layer was treated between two substrates. And the PPMs with open pores on both sides are fabricated and the air permeability of the films is improved greatly.

Structure and gas permeation properties of asymmetric polyimide membranes made by dry-wet phase inversion: Influence of the polyimide molecular weight

In this article, we report the influence of the polyimide molecular weight (1.2 × 105, 2.6 × 105, and 4.1 × 105) on the structure and the gas permeation properties of asymmetric polyimide membranes made by the dry–wet phase-inversion process. The apparent skin layer thickness of the asymmetric membrane increased with increasing molecular weight, and the thicknesses of the membranes prepared from the three polyimides with a casting polymer solution containing 8.0 wt % butanol were 132, 350, and 739 nm, respectively. That is, the gas permeance in the asymmetric membranes increased with decreasing molecular weight. In contrast, the gas selectivity of the asymmetric membranes did not depend on the skin layer thickness. The solvent evaporation in the dry phase-inversion process and the nonsolvent diffusion in the dry process were important factors that determined the formation of the asymmetric membrane. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Polyimide Ultrafiltration Membranes with High Thermal Stability and Chemical Durability

Asymmetric ultrafiltration membranes based on poly[(4,4′-oxydiphenylene)pyromelliteimide] were produced by wet technique from prepolymer casting solution, followed by solid-phase conversion of the prepolymer membranes into polyimide insoluble form at 200°C. It was demonstrated that by adding benzimidazole to the casting solution and filling of prepolymer membrane pores with inert high-boiling oil prior to thermal treatment allow us to prepare asymmetric porous polyimide membranes. The main characteristics of the membranes obtained (permeability coefficients and molecular weight cut-off) match those typical to ultrafiltration membranes. It was found that the developed asymmetric ultrafiltration polyimide membranes have excellent thermal and chemical resistance. The membranes retain rigidity above Tg (360°C) and are chemically stable at temperatures up to 400°C. The developed membranes are resistant against swelling and dissolving in aggressive and organic media including amide solvents.

Gas transport properties of asymmetric polyimide membranes prepared by plasma‐based ion implantation

AbstractIn this study, we report the gas permeance and selectivity of the asymmetric polyimide membrane prepared by plasma‐based ion implantation (PBII). The asymmetric polyimide membranes were prepared using a dry–wet phase inversion process, and the surface skin layer on the membrane was implantated by He ions at 2.5 keV. The asymmetric membranes treated by PBII were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg and 35°C. The (O2/N2) and (CO2/CH4) selectivities in the He+‐implanted asymmetric membrane at 60 sec resulted in 1.5 and 1.8 time increases, respectively, when compared to those of the asymmetric membrane before PBII. On the other hand, the O2 and CO2 permeances in the asymmetric membrane after PBII decreased with an increase in the He+ treatment time. In this paper, we addressed, for the first time, the gas permeation behavior of the asymmetric polyimide membranes prepared by PBII. Copyright © 2009 John Wiley & Sons, Ltd.

Thermal Resistance of Asymmetric Polyimide Membrane for Gas Separation

Thermal Resistance of Asymmetric Polyimide Membrane for Gas Separation

High throughput study of phase inversion parameters for polyimide-based SRNF membranes

High throughput (HT) techniques were applied for the first time for a detailed study of parameters involved in a phase inversion process. The synthesis of integrally skinned asymmetric polyimide (Matrimid ®) membranes was investigated. In spite of being one of the most important materials of reference in solvent resistant nanofiltration (SRNF), a detailed study of the phase inversion parameters for this system is still missing. Phase inversion parameters were selected both on the level of the composition of the casting solution (polymer concentration, solvent type, co-solvent/solvent weight ratio, non-solvent content) as on the level of the post-casting (evaporation time) and immersion (composition coagulation medium) conditions. The study of this extensive parameter space was conducted in a HT-fashion, in which the entire membrane preparation and testing process was miniaturized, parallellized and automated. Thanks to the availability of reliable HT techniques at all levels ( i.e. preparation of polymer solutions, membrane casting and membrane testing), 145 membranes were prepared and tested (in triplicate) in the separation of the dye rose Bengal from 2-propanol within a time frame of a few months, meaning a dramatic improvement in time- and cost-efficiency. An attempt was made to link the SRNF performances of the prepared membranes and their SEM-observed morphologies more fundamentally to the phase inversion parameters through the use of Hansen solubility parameters and viscosity measurements.

Structure and gas permeability of asymmetric polyimide membranes made by dry–wet phase inversion: Influence of alcohol as casting solution

AbstractIn this article, we have reported the influence of alcohol as a casting solution on the structure and the gas permeability of asymmetric polyimide membranes made by dry–wet phase inversion. The apparent skin layer thickness of the asymmetric membrane decreased with an increase in molecular weight of the alcohol, and the thicknesses of the membranes made from methanol, ethanol, propanol, and butanol were 250, 120, 61, and 31 nm, respectively. We found that χ12 as an interaction parameter of solvent–nonsolvent had a significant influence on the phase inversion occurring in the coagulant medium. On the other hand, the gas permeance and the gas selectivity in the asymmetric membranes increased with the increasing molecular weight of the alcohol. We believe that a more packed structure formed in the asymmetric polyimide membrane with a thinner surface skin layer is also responsible for the thickness‐dependence of the gas selectivity obtained in this study. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2739–2746, 2007

Gas transport properties of asymmetric polyimide membranes prepared by ion‐beam irradiation

AbstractIon beam irradiation has been widely used to modify the structure and properties of membrane surface layers. In this study, the gas permeability and selectivity of an asymmetric polyimide membrane modified by He ion irradiation were investigated using a high vacuum apparatus equipped with a Baratron absolute pressure gauge at 76 cmHg and 35 °C. Specifically, we estimated the effects of the gas diffusion and solubility on the gas permeation properties of the asymmetric membranes with the carbonized skin layer prepared by ion irradiation. The asymmetric polyimide membranes were prepared by a dry–wet phase inversion process, and the surface skin layer on the membrane was irradiated by He ions at fluences of 1 × 1015 to 5 × 1015 ions/cm2 at 50 keV. The increase in the gas permeability of the He+‐irradiated asymmetric polyimide membrane is entirely due to an increase in the gas diffusion, and the gas selectivity increases of the membranes were responsible for the high gas diffusion selectivities. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 262–269, 2007.

Diamine modification of P84 polyimide membranes for pervaporation dehydration of isopropanol

AbstractThe effectiveness of chemical crosslinking modification of P84 copolyimide membranes using diamine compounds for pervaporation dehydration has been investigated, and the scheme to enhance separation performance of asymmetric polyimide membranes has been developed. Two diamine crosslinking agents, p‐xylenediamine and ethylenediamine (EDA), were used in this study for both dense and asymmetric P84 membranes. Experimental results suggest that the crosslinking reaction induced by EDA is much faster than that by p‐xylenediamine because the former has a smaller and linear structure than that of the latter. However, membranes crosslinked by p‐xylenediamine are thermally more stable than those by EDA. Membranes modified by p‐xylenediamine or EDA have increased hydrophilicity. An increase in the degree of crosslinking reaction initially results in an increase in separation factor with the compensation of lower flux for pervaporation dehydration of isopropanol (IPA). However, a further increase in the degree of crosslinking reaction may swell up the polymeric chains because of the hydrophilic nature of these diamine compounds, thus resulting in low separation performance. It is found that post treatment after crosslinking reaction can significantly enhance as well as tailor membrane performance because of the formation of charge transfer complexes (CTCs) and the enhanced degree of crosslinking reaction. A low‐temperature heat treatment may develop pervaporation membranes with high flux and medium separation factor, whereas a high‐temperature heat treatment may produce membranes with high separation factor with medium flux. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Gas diffusion and solubility in He +-irradiated asymmetric polyimide membranes

In this study, the gas diffusion and solubility of the asymmetric polyimide membrane irradiated by He ions were investigated using a high vacuum apparatus equipped with a Baratron absolute pressure gauge at 76 cmHg and 35 °C. Specifically, we estimated their effects on the gas permeation properties of the asymmetric membranes. The asymmetric polyimide membranes were prepared by a dry–wet phase inversion process, and the surface skin layer on the membrane was irradiated by He ions at fluences of 1 × 10 15 or 3 × 10 15 ions/cm 2 at 150 keV. We demonstrated that the gas diffusion had a significant influence on the gas permeability and selectivity of the He +-irradiated asymmetric membrane.

Preparation of novel organic‐inorganic nanoporous membranes

AbstractIn this study, the water permeability, the rejection property of sucrose and glucose, the fouling property of humic acid as the foulant for a novel porous fluorinated polyimide membrane made by combining the ion irradiation and plasma treatment have been reported. First, an asymmetric polyimide membrane with a defect‐free and thin skin layer was prepared, then ions on the skin layer were irradiated and the ion‐irradiated layer was treated by plasma to form nanopores in the layer. The asymmetric polyimide membranes with a defect‐free skin layer were irradiated with 50 keV He+ at 1 × 1015 ions/cm2, and the irradiated polyimide surfaces were treated by Ar glow discharge. The porous polyimide membrane showed a high water flux and excellent rejection properties and fouling resistance when compared with NTR‐7250, which is commercially available. These findings indicated that the pore size formed on the porous polyimide membrane was effectively controlled by the plasma treatment time and the skin layer thickness. Copyright © 2005 John Wiley & Sons, Ltd.

Gas Permeation Properties of Asymmetric Polyimide Membranes with Partially Carbonized Skin Layer

In this study, the gas permeance and selectivity of the asymmetric polyimide membrane irradiated by He ions were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg and 35 °C. The asymmetric polyimide membranes were prepared from a dry−wet phase inversion process, and the surface skin layer on the membrane was irradiated by He ions in the fluence range from 1 × 1012 to 3 × 1015 (He+/cm2) at 50 keV. We demonstrated that the gas selectivity of the He+-irradiated membrane increased with an increase in the ion fluence and that the depth profile of the energy loss for the irradiating ions in the skin layer had a significant influence on the gas permeability and selectivity of the asymmetric membrane.

Sorption and transport behavior of water vapor in dense and asymmetric polyimide membranes

Sorption and transport behavior of water vapor in two copolyimide dense films and their asymmetric capillary membranes were studied at 30 °C. Water vapor permeation and separation behavior in the copolyimide dense and asymmetric membranes were also investigated in vapor feeds of pure water, 1-propanol/water, and acetic acid/water with concentration of 10 wt.% of organic compounds at 85–135 °C. Experimental results showed that a solvent-resistant, defect-free asymmetric polyimide membrane could be prepared from its precursor poly(amic acid) (PAA) by a dry/wet phase inversion process to obtain a PAA asymmetric membrane first that was then thermally imidized. The skin structure of the asymmetric polyimide membrane differed somewhat from that of polyimide dense films. Water sorption behavior in the polyimide dense and asymmetric membranes could be successfully interpreted by the Guggenheim–Anderson–de Boer model. The sorption and transport behavior of water vapor in the skin layer of an asymmetric membrane could be approximately expressed by the effective thickness of that of a homogeneous dense membrane, which determines the membrane permeation and separation properties.

Preparation of polyimide composite membranes grafted by electron beam irradiation

In this study, the preparation of polyimide composite membranes was investigated by the electron beam (EB) irradiation method. The asymmetric polyimide membranes as substrates were prepared on the non-woven fabric by the phase inversion process. Electron beam irradiation was done on the polyimide membrane using the EB irradiation equipment with low acceleration voltage (50 keV) in the nitrogen atmosphere. Graft polymerization took place by soaking membranes after the EB irradiation in aqueous ethanol solution with various vinyl monomers. The performance of prepared membranes was evaluated by nitrogen permeability and by flux and separation factor for a benzene/cyclohexane (50/50, w/w) azeotropic mixture at 30 °C on pervaporation.

Effect of molecular weight on gas selectivity of oriented thin polyimide membrane

AbstractIn this study, we focused on effect of the molecular weight of polyimide on the gas selectivity of the asymmetric membrane with an oriented surface skin layer prepared at different shear stresses. Asymmetric polyimide membranes, which have a defect‐free surface skin layer supported by a porous substructure, were prepared by a dry/wet phase inversion process. The structures of the asymmetric polyimides consisted of a thin skin layer and a porous substructure characterized by the presence of finger‐voids. The gas selectivities of the asymmetric polyimide membranes increased with an increase in the shear rate or a decrease in the molecular weight, indicating that the oriented polyimide structure in the surface skin layer provided a high size and shape discrimination between the gas molecules. The selectivity values of (O2/N2) and (CO2/CH4) in the asymmetric polyimide membrane prepared from the 7.2 × 104 molecular weight material at 1000 sec−1 shear rate were 12 and 143, respectively. Copyright © 2003 John Wiley & Sons, Ltd.

Gas permeation stability of asymmetric polyimide membrane with thin skin layer: effect of polyimide structure

The gas permeation stability of an asymmetric polyimide membrane with a thin and defect-free skin layer has been investigated. The polyimide used in this study was synthesized from the fluorinated dianhydride (6FDA) and the aromatic diamine not containing a CF 3 group. The asymmetric polyimide membranes were prepared by a dry/wet phase inversion, and the apparent skin layer thickness of the asymmetric membrane was 55 nm. The gas permeances of O 2, N 2, CH 4, and CO 2 through the asymmetric membranes at 35 °C and at pressures up to 760 cmHg have been determined using a high vacuum apparatus. We specifically focused on the CO 2 permeation stability of the asymmetric polyimide membrane with a 55 nm skin layer exposured to a CO 2 pressure of 760 cmHg. The CO 2 permeances of the asymmetric membrane showed almost constant values during the repeated permeation experiments. Additionally, the 13 C T 1 values of the asymmetric membrane repeatedly measured at 760 cmHg were quite similar to those for the original asymmetric membrane before the gas permeation experiment. These results indicate that the plasticization of the asymmetric polyimide membrane was not caused by CO 2. We postulated that the absence of bulky CF 3 groups on the aromatic diamine inhibited plasticization of the polyimide by CO 2.