N.F.A. van der Vegt

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.

Preparation and characterization of highly selective dense and hollow fiber asymmetric membranes based on BTDA-TDI/MDI co-polyimide

In this work, the preparation, the characterization, and the permeation properties of dense flat sheet and asymmetric hollow fiber membranes, based on BTDA-TDI/MDI co-polyimide (P84), are reported. Results are shown for pure gases and for the separation of a CO2/N2 (80/20) mixture. Dope viscosity measurements were performed to locate the polymer concentrations where significant chain entanglement occurs. Asymmetric hollow fibers were spun, using the dry/wet phase inversion process. Scanning Electron Microscopy (SEM) was used to investigate the morphological characteristics and the structure of the developed fibers. The permeation rates of He, CO2, O2, and N2 were measured by the variable pressure method at different feed pressures and temperatures. P84 co-polyimide proved to be one of the most selective glassy polymers. The achieved ideal selectivity coefficients are: 285–300 for He/N2, 45–50 for CO2/N2, and 8.3–10 for O2/N2, which are in the range of the highest values reported ever for polymeric membranes. The permeability of CO2 is relatively low (1 barrer, 25 °C), however it is independent of feed pressure indicating that the P84 dense membranes are not plasticized at CO2 feed pressures up to 30 bar. To the contrary, the permeance of CO2 through the asymmetric hollow fiber membranes increases with pressure, indicating that the plasticization behavior of asymmetric membranes differs from the respective dense ones. However, no evidence of plasticization was observed when a CO2/N2 (80/20) mixture was fed to the hollow fiber membranes for pressures up to 30 bar. In all cases, CO2 permeance decreased with pressure while that of N2 remained constant.

Functionalized Carbon Molecular Sieve membranes containing Ag-nanoclusters

In Carbon Molecular Sieve (CMS) membranes, the separation of O2 and N2 is primarily based on the difference in size between the gas molecules. To enhance the separation properties of these CMS membranes it is necessary to functionalize the carbon matrix with materials that show a high affinity to one of the permeating gas species. Adding Ag-nanoclusters increases the selectivity of O2 over N2 by a factor 1.6 compared to a non-functionalized CMS membrane prepared by the same pyrolysis procedure. We have analyzed the structure of Ag-nanocluster (dcluster≈50 nm) containing membranes produced from different Ag sources, AgNO3 and AgAc, and with different Ag content (0, 6, 25, and 40 wt.%). By measuring the pure gas permeabilities of He, CO2, O2, and N2 we have determined the effect of Ag-nanoclusters in the carbon matrix, concluding that in the case of pure gases, the Ag-nanoclusters act primarily as a spacer at pyrolysis end temperatures up to 600 °C, increasing the O2 permeability by a factor of 2.4. However, they enhance the separation of O2 over N2 at higher pyrolysis end temperatures (700 and 800 °C). It was shown that the build up of an Ag layer on the surface of the membrane reduces the permeability, but does not affect the selectivity.

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.

Carbon molecular sieve membranes prepared from porous fiber precursor

Carbon molecular sieve (CMS) membranes are usually prepared from dense polymeric precursors that already show intrinsic gas separation properties. The rationale behind this approach is that the occurrence of any kind of initial porosity will deteriorate the final CMS performance. We will show that it is not necessary to produce a non-porous precursor in order to obtain a selective CMS membrane. We used tight ultra-filtration (UF) fiber membranes as a precursor. These fibers did not have any gas separation properties before the pyrolysis treatment, nor were coatings applied to these fibers before or subsequent to the pyrolysis. After a heat treatment in air followed by a pyrolysis in a nitrogen atmosphere CMS fiber membranes were obtained. The CMS fibers were analyzed using scanning electron microscopy, thermo gravimetrical analysis, and gas permeation. From the permeation rates and permselectivity values measured for He, H2, CO2, Ar, O2, N2, CH4, C2H4, C2H6, C3H6, C3H8 and SF6 the evolution of the mean pore diameter was investigated. It was found that the pore diameter increases with pyrolysis temperature up to 800 °C, but decreases as the temperature is raised to 900 °C. The overall porosity reaches its highest value at 900 °C.

Chronopotentiometry for the advanced current-voltage characterisation of bipolar membranes

Compared to steady-state current?voltage curves, chronopotentiometric measurements allow us to distinguish the contributions to the overall electric potential difference across a bipolar membrane. In this paper, the characteristic values of the electric potential difference across the bipolar membrane at different times are correlated to the corresponding concentration profiles in the bipolar membrane layers and the ion-transport processes are identified. For over-limiting current densities (i.e. current densities above the limiting current density), it is possible to distinguish the reversible and irreversible contributions to the steady-state electric potential difference. The irreversible contribution is attributed to the energy required to overcome the electric resistance whereas the reversible contribution corresponds to the electrochemical potential due to concentration gradients in the membrane layers. Further, the ohmic resistance of the membrane in equilibrium with the surrounding solution has been compared to the resistance in the transport state. For low current densities, the equilibrium resistance is lower than the transport resistance stemming from internal concentration polarisation. In contrast, the large numbers of hydroxide ions and protons produced at high current densities result in a reduced ohmic transport resistance due to their high ionic mobility. This reduced resistance is not enough to stop the increase of the irreversible contribution with higher current densities. With the possibility to split the steady-state potential into its contributions, bipolar membrane chronopotentiometry is a useful tool to identify transport limitations and to improve bipolar membranes for a reduced overall electric potential.

The sorption induced glass transition in amorphous glassy polymers

Sorption of CO2 in both the glassy and the rubbery state of an amorphous polyethylenelike polymer was investigated using molecular dynamics simulations. The temperature was chosen such that the system was in its glassy state at low solute concentrations and its rubbery state at large solute concentrations. Both the pressure and the volume isotherms changed character at the transition concentration. The physical origin of these changes was clarified by investigation of the excess thermodynamic properties of the solute both below and above the transition concentration. Dynamical changes occuring at the glass transition were studied through the self-intermediate scattering function of the polymer atoms. This function was found to excellently reveal the difference between the dynamics of the glassy and rubbery state and therefore served as an independent tool monitoring the glass transition

Novel open-cellular polysulfone morphologies produced with trace concentrations of solvents as pore opener

As a new method of membrane formation, we have investigated microcellular foaming of thin (not, vert, similar100 ?m) polysulfone films containing varying trace concentrations of tetrahydrofuran using carbon dioxide as a physical blowing agent. Membrane morphologies were obtained by first saturating the polymer with carbon dioxide at 5 MPa, and subsequently heating the sample above the glass transition temperature (Tg) of the polymer/gas mixture at atmospheric pressure. The presence of tetrahydrofuran in the polymer at concentrations above 0.04 wt.% led to a transition from a closed cellular structure into novel open-cellular morphologies. The open structure manifests itself by small spot-like openings (diameters between 10 and 100 nm) in the cell walls. The mass transport resistances of the porous films were quantified using gas permeation measurements, and a Knudsen-type separation mechanism was observed. Detailed investigation showed that the transport resistance can mainly be controlled by two variables: (1) the concentration of the residual solvent in the polymer film, and (2) the foaming temperature. At optimal foaming temperatures, thin cell walls are obtained, which break up when fluctuations in the wall thickness are amplified by plasticizing solvent molecules.

Chemically transferable coarse-grained potentials from conditional reversible work calculations

The representability and transferability of effective pair potentials used in multiscale simulations of soft matter systems is ill understood. In this paper, we study liquid state systems composed of n-alkanes, the coarse-grained (CG) potential of which may be assumed pairwise additive and has been obtained using the conditional reversible work (CRW) method. The CRW method is a free-energy-based coarse-graining procedure, which, by means of performing the coarse graining at pair level, rigorously provides a pair potential that describes the interaction free energy between two mapped atom groups (beads) embedded in their respective chemical environments. The pairwise nature of the interactions combined with their dependence on the chemically bonded environment makes CRW potentials ideally suited in studies of chemical transferability. We report CRW potentials for hexane using a mapping scheme that merges two heavy atoms in one CG bead. It is shown that the model is chemically and thermodynamically transferable to alkanes of different chain lengths in the liquid phase at temperatures between the melting and the boiling point under atmospheric (1 atm) pressure conditions. It is further shown that CRW-CG potentials may be readily obtained from a single simulation of the liquid state using the free energy perturbation method, thereby providing a fast and versatile molecular coarse graining method for aliphatic molecules.

Transport diffusion of argon in AlPO4-5 from equilibrium molecular dynamics simulations

Transport diffusion of argon in the unidirectional channels of the molecular sieve AlPO4-5 has been studied using molecular dynamics simulations. Using the Green–Kubo formalism, this nonequilibrium property is, for the first time, extracted from just one equilibrium simulation. Apart from the computational advantages above nonequilibrium simulations, the new method also provides a way to check the validity of the assumption of linear response, which is at the basis of both methods. The transport diffusion coefficient for argon at 87 K and half the maximum loading is found to be equal to Dt = (1.4±0.1)×10–5 cm2/s, of which approximately 20% can be attributed to correlated, collective motion.