Optical vs. direct sorption and swelling measurements for the study of stiff-chain polymer-penetrant interactions

This work involves interferometric ‘optical thickness’ and refractive index measurements performed in an optical thickness meter (OTM), on supported cellulose acetate (CA) films equilibrated with various activities of methylene chloride (MC) vapor. The relevant equilibrium sorption and volume swelling isotherms were determined by application of the Claussius-Mossotti equation on the assumption that these films swell unidimensionally along the thickness direction, and were compared with corresponding direct equilibrium sorption (weight gain), elongation and thickness dilation measurements on similar free films performed in a vacuum sorption/swelling apparatus (VSA) and complemented with refractive index data. Combined elongation and thickness dilation data from the VSA showed that free glass-cast CA films exhibit pronounced swelling anisotropy. The said anisotropy, although it cannot be completely eliminated, by conditioning at high degrees of swelling, does not appear to affect the extent of volume swelling significantly, thus permitting quantitative comparison of sorption and swelling isotherms determined by the VSA and the OTM. Such comparison showed satisfactory agreement between these two sets of results up to an MC uptake of ca. 0.4 gMC/cm³ of dry CA corresponding to a degree of swelling of ca. 0.2. Increasing discrepancies are observed at higher MC concentrations, which are attributable to breakdown of the assumption used that the supported films swell unidimensionally along the thickness direction. The present CA-MC volume swelling data exhibit the negative deviation from volume additivity on mixing typical glassy polymers.     

A High Cell‐Bearing Capacity Multibore Hollow Fiber Device for Macroencapsulation of Islets of Langerhans

Macroencapsulation of islets of Langerhans is a promising strategy for transplantation of insulin‐producing cells in the absence of immunosuppression to treat type 1 diabetes. Hollow fiber membranes are of interest there because they offer a large surface‐to‐volume ratio and can potentially be retrieved or refilled. However, current available fibers have limitations in exchange of nutrients, oxygen, and delivery of insulin potentially impacting graft survival. Here, multibore hollow fibers for islets encapsulation are designed and tested. They consist of seven bores and are prepared using nondegradable polymers with high mechanical stability and low cell adhesion properties. Human islets encapsulated there have a glucose induced insulin response (GIIS) similar to nonencapsulated islets. During 7 d of cell culture in vitro, the GIIS increases with graded doses of islets demonstrating the suitability of the microenvironment for islet survival. Moreover, first implantation studies in mice demonstrate device material biocompatibility with minimal tissue responses. Besides, formation of new blood vessels close to the implanted device is observed, an important requirement for maintaining islet viability and fast exchange of glucose and insulin. The results indicate that the developed fibers have high islet bearing capacity and can potentially be applied for a clinically applicable bioartificial pancreas.     

Phenomenology and mechanisms of unidimensional stress-dependent micromolecular transport in a stiff-chain polymer. III. Effect of modification of the polymer

A detailed study of the kinetics and mechanism of micromolecular transport in cellulose acetate films containing 2.0 acetate groups per glucose unit (CA-2.0) is reported. The polymer was prepared by controlled hydrolysis of CA-2.45 films studied in preceding articles. The same series of simple liquid penetrants varying from weak swelling agent to good solvent of the polymer was used. As before, measurement of rates of penetration along the polymer film confined between glass plates was supplemented with information on penetrant distribution profiles in the polymer film and on the corresponding deformation and structural relaxation of the swelling polymer, deduced from refractive index and birefringence profiles, respectively. Transport was studied in (a) unoriented CA-2.0 films and (b) uniaxially oriented films with penetration normal and parallel to the orientation axis. This was equivalent to varying the viscoelastic polymer properties affecting transport, under otherwise identical experimental conditions. The results complemented and extended those previously obtained with CA-2.45 in interesting ways and were successfully interpreted on the basis of a previously developed theoretical model designed to represent the influence of (a) the stress generated by the constraints imposed on the swelling polymer, and (b) the viscoelastic response of the latter thereto, on the transport mechanism. It was shown that the observed differences in transport mechanism in CA-2.45 and CA-2.0 are primarily related to the corresponding changes in the sorptive capacity of the polymer for the relevant penetrant rather than the chemical constitution of the latter. The most striking result in this respect was that the remarkable kinetic pattern (which involved a drastic change from Case I kinetics for penetration across, to Case II kinetics for penetration along, the axis of orientation) exhibited by oriented CA-2.45 film penetrated by the strong swelling agent of the series of penetrants used, namely methylene chloride, was reproduced here for the penetration of acetone, which occupies the slot of strong swelling agent in the case of CA-2.0. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2593–2607, 1997     

Porous membrane structures as stationary phase for capillary electrochromatography

This work presents the application of membrane technology for the fabrication of stationary phase for CEC columns using the technique based on phase inversion of polymer solution. A blend of polyimide P84 and sulphonated poly(ether ether ketone was processed via immersion precipitation dry‐wet spinning into small‐bore porous fiber. The morphology, zeta potential, and performance of the porous structure in the CEC separation were investigated. Noncharged molecules (as markers of the electroosmotic flow) and small organic compounds were injected into the column, driven under the application of voltage, and detected on the electropherogram. The proof of concept of applying porous membrane structure as stationary phase for CEC was shown and possible optimization to improve efficiency and selectivity was suggested.     

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.     

Free volume in C60 modified PPO polymer membranes by positron annihilation lifetime spectroscopy

PPO (poly(2,6-dimethyl-1,4-phenylene oxide)) is a well-known membrane material showing good gas separation properties. The incorporation of nanoparticles can enhance or deteriorate the performance of composite membranes, sometimes depending only on the way of the composite preparation. We have modified the PPO polymer with C60 fullerenes up to a content of 2 wt %. Previous investigations showed a strong dependence of permeability on whether the C60 is simply dispersed in the polymer or chemically bonded to the polymer chains. Free volume effects were suggested as an explanation but not experimentally confirmed. Here, we present free volume studies by positron annihilation lifetime spectroscopy. An additional long positron lifetime shows the increased free volume of composite samples, while the high electron affinity of C60 helps to indicate the homogeneity of the samples. Combining the presented results with permeability measurements refines the understanding of this promising membrane material.     

Novel Gas Separation Membranes Containing Covalently Bonded Fullerenes

Summary: In this work, we report superior mass transport properties of polymers prepared by the covalent coupling of supermolecular carbon cages (e.g., fullerenes, bucky balls) to a poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) polymer. Dispersing the bucky balls into the polymer reduces gas permeability, whereas covalent bonding enhances permeability up to 80% in comparison to the pure PPO. Gas pair selectivity, however, is not compromised and stays constant.

Schematic representation of the PPO polymer membrane and the PPO‐covalently bonded C₆₀ polymer membrane.