Ineke G.M. Punt

CO2-induced plasticization phenomena in glassy polymers

A typical effect of plasticization of glassy polymers in gas permeation is a minimum in the relationship between the permeability and the feed pressure. The pressure corresponding to the minimum is called the plasticization pressure. Plasticization phenomena significantly effect the membrane performance in, for example, CO2/CH4 separation processes. The polymer swells upon sorption of CO2 accelerating the permeation of CH4. As a consequence, the polymer membrane loses its selectivity. Fundamental understanding of the phenomenon is necessary to develop new concepts to prevent it. In this paper, CO2-induced plasticization phenomena in 11 different glassy polymers are investigated by single gas permeation and sorption experiments. The main objective was to search for relationships between the plasticization pressure and the chemical structure or the physical properties of the polymer. No relationships were found with respect to the glass-transition temperature or fractional free volume. Furthermore, it was thought that polar groups of the polymer increase the tendency of a polymer to be plasticized because they may have dipolar interactions with the polarizable carbon dioxide molecules. But, no dependence of the plasticization pressure on the carbonyl or sulfone density of the polymers considered was observed. Instead, it was found that the polymers studied plasticized at the same critical CO2 concentration of 36±7 cm3 (STP)/cm3 polymer. Depending on the polymer, different pressures (the plasticization pressures) are required to reach the critical concentration.

Plasticization-resistant glassy polyimide membranes for CO2/CH4 separations

It is known that CO2 acts as a plasticizer in CO2/CO4 membrane separations at elevated pressures. The polymer matrix swells upon sorption of CO2, accelerating the permeation of CH4. As a consequence, the polymer membrane loses its selectivity. To overcome this effect, plasticization should be minimized. We succeeded in stabilizing the polymer membrane by a thermal treatment. For this purpose the polyimide Matrimid 5218 is used as model polymer. In single gas experiments with CO2, the untreated membrane normally shows a minimum in its pressure dependence on permeability, whereas the treated membranes do not. Membrane performances for CO2/CO4 gas mixtures showed that the plasticizing effect indeed accelerates the permeation of methane. The heat treatment clearly suppresses this undesired methane acceleration. Additionally to the pure and mixed gas permeation results, process calculations reveal valuable information as to what extent the stabilized membranes show improved membrane performance. The favourable performance of the stabilized membrane can be attributed to less methane loss and therefore a higher recovery, resulting in higher profit from gas sales.

Cation permeable membranes from blends of sulfonated poly (ether ether ketone) and poly (ether sulfone)

Sulfonated poly(aryl ether ether ketone), S-PEEK, is blended with non-sulfonated poly(ether sulfone) (PES) to adjust the properties of ion permeable and ion selective membranes. In this study, membranes are prepared from blends with (i) a S-PEEK content between 10 and 100 wt.% using one S-PEEK batch with a fixed degree of sulfonation and (ii) from batches of S-PEEK with a different degree of sulfonation, but with a fixed S-PEEK content in the blend. The transparent membranes are permeable for ions with selective transport of cations over anions. At contents of S-PEEK below 40%, phenomena related to a percolation threshold of the ion exchange functionalities are observed; the measured ion exchange capacity (IEC) indicates that not all functional groups are accessible in these blends. The transport properties of membranes with a S-PEEK content in the range of 50?80 wt.% are comparable to those known for commercial ion exchange membranes. In this range, a trade-off between resistance and selectivity with increasing IEC is observed. Both, the ion conductivity and the co-ion transport number increase with increasing IEC. This is mainly caused by the increased water content with increased IEC and the number of water molecules per fixed charge.

Transport through zeolite filled polymeric membranes

In this paper the effect of zeolite particles incorporated in rubbery polymers on the pervaporation properties of membranes made from these polymers is discussed. Pervaporation of methanol/toluene mixtures was carried out with membranes prepared from the toluene selective polymer EPDM and the methanol selective polymers Viton and Estane 5707. From the results of the pervaporation experiments it could be concluded that the addition of the hydrophilic zeolite NaX as well as the hydrophobic zeolite silicalite-1 leads to an increase in methanol flux and a decrease in toluene flux through the membranes. Pervaporation experiments with bi-layer membranes consisting of an unfilled polymer layer filled with zeolite particles demonstrated that the effect of addition of particles depends on their position in the membrane. Furthermore, the component flux through the membranes as a function of the volume fraction of zeolite is modelled with existing theories describing the permeability of heterogeneous materials. The results show that the apparent permeability of the dispersed phase is lower than the intrinsic permeability of the dispersed phase when the flux through the particle is restricted by the polymer phase. This phenomenon was confirmed by numerical simulation of the transport in the membrane through a plane parallel to the transport direction. The simulations are carried out for an unfilled membrane, a membrane filled with an impermeable particle, a rubber particle and with a particle which shows Langmuir sorption behaviour. The reason for the discrepancy between the apparent permeability and the intrinsic permeability is that the apparent permeability of the zeolite phase is calculated by dividing the flux with the driving force over the entire membrane which is larger than that over the particle. In case of numerical simulation the concentration in every position in the plane is known and therefore the intrinsic permeability of the filler can be calculated on basis of the actual driving force. This treatment results in a permeability which is correct over several orders of magnitude.

Suppression of CO2-plasticization by semiinterpenetrating polymer network formation

CO2-induced plasticization may significantly spoil the membrane performance in high-pressure CO2/CH4 separations. The polymer matrix swells upon sorption of CO2, which accelerates the permeation of CH4. The polymer membrane looses its selectivity. To make membranes attractive for, for example, natural gas upgrading, plasticization should be minimized. In this article we study a polymer membrane stabilization by a semiinterpenetrating polymer network (s-ipn) formation. For this purpose, the polyimide Matrimid 5218 is blended with the oligomer Thermid FA-700 and subsequently heat treated at 265°C. Homogeneous films are prepared with different Matrimid/Thermid ratios and different curing times. The stability of the modified membrane is tested with permeation experiments with pure CO2 as well as CO2/CH4 gas mixtures. The original membrane shows a minimum in its permeability vs. pressure curves, but the modified membranes do not indicating suppressed plasticization. Membrane performances for CO2/CH4 gas mixtures showed that the plasticizing effect indeed accelerates the permeation of methane. The modified membrane clearly shows suppression of the undesired methane acceleration. It was also found that just blending Matrimid and Thermid was not sufficient to suppress plasticization. The subsequent heat treatment that results in the s-ipn was necessary to obtain a stabilized permeability.

Optimisation strategies for the preparation of bipolar membranes with reduced salt ion leakage in acid–base electrodialysis

The salt ion fluxes across commercial bipolar membranes (BPMs) result in the salt contamination of the produced acids or bases especially at increased product concentrations. Often, bipolar membrane electrodialysis can only be applied when these fluxes are reduced. Here, a model is presented to predict the salt impurities using the limiting current density measured for a single bipolar membrane. The model is extended to relate the limiting current density to the experimentally determined properties of the separate membrane layers. A direct dependence has been found for the salt ion fluxes across the bipolar membrane on the square of the solution concentration and the effective salt diffusion coefficient. Further, the salt ion transport is inversely dependent on the fixed charge density and the thickness of the layers. The latter is not trivial ? the thickness in general does not play a role in the selectivity of separate anion or cation exchange membranes. The dependence of the salt ion transport on the membrane layer properties has been verified experimentally by characterising membranes prepared from commercially available anion exchange membranes and tailor-made cation-permeable layers. The presented model has proven to be both, simple and accurate enough to guide bipolar membrane development towards increased selectivity.

Micropatterned Polymer Films by Vapor-Induced Phase Separation Using Permeable Molds

Microstructured polymeric films are fabricated by a novel replication method. A polymer solution is applied and contained between two substrates, of which at least one is a patterned PDMS mold. The ensemble is then put in an atmosphere containing water vapor, which diffuses through the PDMS. The absorption of water into the polymer solution causes the precipitation (phase separation) of the polymer while in contact with the microstructured molds. The thickness of the PDMS slab can be exploited to tune the water vapor transport and hence the phase separation kinetics and resulting polymer morphology. Removal of excess polymer solution from between two PDMS slabs, followed by vapor induced phase separation, can also result in microperforated polymer films with great control over the dimensions.