Asymmetric bipolar membrane: A tool to improve product purity

Bipolar membranes (BPMs) are catalytic membranes for electro-membrane processes splitting water into protons and hydroxyl ions. To improve selectivity and current efficiency of BPMs, we prepare new asymmetric BPMs with reduced salt leakages. The flux of salt ions across a BPM is determined by the co-ion transport across the respective layer of the membrane. BPM asymmetry can be used to decrease the co-ion fluxes through the membrane and shows that the change of the layer thickness and charge density of the corresponding ion exchange layer determines the co-ion flux. The modification of a commercial BP-1 with a thin additional cation exchange layer on the cationic side results in a 47% lower salt leakage. Thicker layers result in water diffusion limitations. In order to avoid water diffusion limitations we prepared tailor made BPMs with thin anion exchange layers, to increase the water flux into the membrane. Therefore a BPM could be prepared with a thick cation exchange layer showing a 62% decreased salt ion leakage through the cationic side of the membrane.     

Kinetics of heterogeneous free-radical polymerization of vinylidene chloride

Poly(vinylidene chloride) precipitates as a crystalline phase during the polymerization reaction. Under the conditions studied, this phase is made up of complex particles with a lamellar substructure. The detailed morphology is very sensitive to reaction medium. The morphology developed by particles formed during polymerization of vinylidene chloride in dioxane suggests a mechanism of polymerization followed by crystallization. The morphology observed in mass polymerization suggests that both processes occurs simultaneously. Kinetic data, however, suggest a solid-phase reaction mechanism for both cases. The results are analyzed by comparison with a model that takes into account the solid-phase morphology. The theoretical analysis is consistent with experimental results if it is assumed that polymerization occurs on the edges of the lamellar crystals.     

Solar driven membrane pervaporation for desalination processes

We describe details of a solar driven pervaporation process for the production of desalinated water from highly contaminated waters. The membrane material is a polyetheramide-based polymer film of 40 μm thickness. This Solar Dew^® membrane is used in a tubular configuration in a direct solar membrane pervaporation process. The feed waters used in this study are untreated seawater and waste water that is simultaneously produced with the mineral oil extraction. In all cases retention of typical ions as sodium, chloride and calcium as well as specific problematic ions (arsenic, boron and fluoride) was higher than data reported for pressure driven membrane processes like NF and RO. The condensate quality was well within WHO limits for drinking water. A reduction of almost five orders of magnitude in conductivity between brine and condensate could be realized, producing condensate with conductivities of 5 μS/cm or lower. Laboratory experiments show that the measured fluxes are independent of severe fouling and virtually independent of concentration up to 100 g/l total solids.     

Dispersion-induced insulator-to-metal transition in polyaniline

Melt dispersion processing--without solvents or “secondary dopants”--pushes polyaniline reproducibly to the metallic side of the IM transition, although the undispersed polyaniline is on the insulator side. This is the first time that a conductive polymer is found there without applying pressure. The transition to the metallic state is associated with a decrease of the C₆-N-C₆ angle from 166° to 134°. Received 11 May 1999 and Received in final form 22 November 1999     

Modeling on swelling behavior of a confined polymer network

Polymeric membranes suffer from plasticization in gas separation, extensive swollen in pervaporation, nanofiltration, and fuel cells by losing performance. Growing research has experimentally realized that the membrane performance could be stabilized by blending with inert second polymer or imbedding in a porous inert confinement. In this article, we introduce a generic model based on Flory–Rehner's swelling theory to describe various membrane processes using a measurable parameter. We assume the membrane polymer to be a network and the constraint as an expandable structure with an energy density equal to its E‐Modulus. The model reveals that the isotropic constraint is far more efficient in swelling control. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1589–1593, 2008     

Corrosion protection by the organic metal polyaniline: results of immersion, Volta potential and impedance studies

Properties of the new polyaniline containing primer CORRPASSIV sealed with two different top coats are characterised and compared with top coated samples using no or a conventional zinc-rich primer. Measurements were made using scanning Kelvin-probe (SKP), electrochemical impedance spectroscopy (EIS), voltammetry and analysis of iron and zinc content in electrolytes for testing. Results are presented and the powerful corrosion protection using the organic metal is shown.     

How to determine the correct sample volume by gravimetric sorption measurements

Determining sample volumes for gas sorption measurements by pycnometry is often considered to be trivial, as it is assumed that helium does not sorb to the sample material. The aim of this study is to show the contrary, i.e., helium may sorb to certain polymers and to introduce the proposed helium-sorption correction algorithm for compensating for such helium sorption. By using a Rubotherm magnetic suspension balance, the volume of a commercial polyether block amide (Pebax^© MH 1657) was analyzed with the new algorithm based on kinetic data. Helium was found to sorb to all tested Pebax^© MH 1657 samples. Compared to the conventional calculations, the proposed algorithm yielded more precise sample volumes. To ensure accurate sorption measurements of any gas, it is important to take into account possible helium sorption.     

Temperature‐Modulated Water Filtration Using Microgel‐Functionalized Hollow‐Fiber Membranes

In the present work, we investigate the potential of aqueous polymer microgels in membrane technology, especially for filtration applications. The poly(N‐vinylcaprolactam)‐based microgels exhibit thermoresponsive behavior and were employed to coat hollow‐fiber membranes used for micro‐ and ultrafiltration. We discuss the preparation of microgel‐modified membranes (by “inside‐out” as well as “outside‐in” filtration in dead‐end mode). The clean‐water permeability and stability of these membranes was studied not only as a function of time, but also of temperature. The microgel‐modified membranes exhibit a reversible thermoresponsive behavior whereby both the resistance and the retention increased with decreasing temperature.     

Comparing flat and micro-patterned surfaces: Gas permeation and tensile stress measurements

Micro-patterning is a suitable method to produce structured membranes that display increased flux compared to flat membranes. In this work we studied the permeation of four different gases (nitrogen, helium, oxygen and carbon dioxide) through Kraton™ polymer (SBS) membranes. It is possible to cast a micro-patterned membrane with 25 μm high and 30 μm wide lines that has a thickness of 5 μm at its thinnest point. Using this micro-pattern, the experimental diffusive gas flux was increased up to 59% compared to non-patterned membranes with the same polymer volume. Finite element simulations confirm this enhancement. Selectivities are similar for both flat and micro-patterned membranes and in accordance with literature. Tensile stress measurements confirm that the micro-patterned membranes yield only limited loss in mechanical strength. Although only one material and geometry is explored here, this principle is generally applicable to all diffusion-driven processes.