Zhenghua Ping

Clustering of solvents in membranes and its influence on membrane transport properties

Abstract Solvent clustering in membranes, if it occurs, could modify the sorption and diffusion behaviors of polymer membranes and therefore their transport properties. Solvent cluster formations in hydrophilic poly(vinyl accohol) (PVA), in hydrophobic poly(dimethylsiloxane) (PDMS) and intermediate poly(vinyl acetate) (PVAc) membranes were studied by infrared spectroscopy, differential scanning calorimetry, X-ray scattering techniques, and by visual observations. A water saturated PVAc membrane contains non-freezable (bound) and liquid-like water molecules in equivalent amounts (ca. 1.6 wt%); membrane infrared studies during the sorption process indicated that water penetrates first in the membrane as monomeric molecules which aggregate later to form clusters. In PVA membranes, water molecules can exist under three states, i.e. non-freezable, freezable bound and liquid-like states, depending on the water content in the membrane. The cluster formation seems to affect the membrane selectivity towards water and the ethanol diffusivities in PVA membranes. In PDMS membranes, solvent clusters were evidenced by infrared spectroscopy and visual observations. The clustering of alcohols in PDMS makes the sorption behavior of these solvents in the membrane deviate from the Flory-type behavior observed for other solvents: the sorbed amount increases drastically with the surrounding-solvent activity in the high activity range, but the solvent diffusivity decreases.

Simple method for immobilization of bio-macromolecules onto membranes of different types

In this method, an intermediate polyelectrolyte layer is first adsorbed on an oppositely charged membrane by electrostatic interactions. This leads to a charge inversion of the original membrane. Then the bio-macromolecule is bound to the intermediate polyelectrolyte layer, always by charge interactions. The method feasibility was shown with two bio-macromolecules, glucose oxydase and heparin, which were immobilized on negatively-charged membranes via polyethyleneimine, a cationic hydrophilic polymer. The immobilization of glucose oxydase on different polymer membranes led to high-activity and stable membranes for glucose biosensor. The anti-coagulation effect of immobilized heparin was not clearly evidenced in spite of the effective bio-species immobilization. The good properties of the immobilized enzyme was explained by the hydrophilicity of the intermediate polyelectrolyte layer, its high density in sites for enzyme binding, and the mild immobilization conditions.

Pervaporation of water-ethanol mixtures through a poly(acrylic acid) grafted polyethylene membrane. Influence of temperature and nature of counter-ions

Both the permeability and the selectivity to water of poly (acrylic acid) grafted polyethylene films increase when the counter-ion is changed from Li+ to Na+ or K+. The acid membrane behaves in a singular manner: high permeability at low water contents, slight selectivity to water for ethanol-rich mixtures, reversal in selectivity at 60 wt.% of water (at 60°C) and slight selectivity to ethanol in water-rich mixtures. Both partial water and ethanol fluxes increase with pervaporation temperature, according to an Arrhenius-like relationship, but with different activation energies. The activation energy for ethanol permeation is always higher than that for water permeation, but both of them decrease with increasing water content and when the counter-ion is changed from Li+ to Na+ or K+. As a consequence, selectivity to water is strongly reduced by an increase in pervaporation temperature, especially for the K+ membrane. Most of the results can be interpreted on the basis of the extent of ionization of the carboxylate groups, and the effects of swelling on the activation energy.

Investigation of poly(dimethyl siloxane) (PDMS)–solvent interactions by DSC

Abstract Low temperature DSC was used to study the state of several solvents in poly(dimethyl siloxane) (PDMS) membrane and the influence on the sorbed solvents on PDMS crystallinity. No phase transition was detected on the DSC thermograms until −130°C, when the solvent content in polymer is low, independently of the solvent nature. This suggests a dispersion of the solvents as individual molecules in the PDMS matrix. At high solvent contents, part of the solvent crystallizes. Most of the crystallized solvent melts at a temperature close to the melting point of the bulk solvent, while the rest melts at a lower temperature. The DSC thermograms also show the change in crystallinity of swollen PDMS in different solvents. The poor solvent and the solvent of moderate power increase PDMS crystallinity due to the solvent-induced crystallization effect, while the good solvent reduces polymer crystallinity because of the solution effect.

Desorption and pervaporation properties of zeolite-filled Poly(dimethylsiloxane) membranes

Abstract The role played by zeolite fillers in PDMS membranes was studied by desorption and pervaporation. The data on the desorption of different solvents from the membranes made of poly(dimethylsiloxane) (PDMS) filled with silica and different zeolites show that the solvent molecules are extracted in two stages. The first stage, which occurs at a low temperature (ca.50 °C), would correspond to the extraction of the solvent molecules in the ’bulk-like state’ in PDMS, while the last stage, at high temperature, would correspond to the extraction of the solvent molecules in the ’bound state’. There is a clear contribution of the zeolites to the membrane performances in pervaporation, even at a low zeolite content in PDMS (ca. 20 wt. %). The hydrophobic zeolites enhance significantly the organic flux, leading to an improvement of both the flux and the selectivity of PDMS. The ZSM-5 zeolite imparts to PDMS a higher selectivity but lower flux compared with SY-2 zeolite. The silica filler exhibits a crosslinking effect on PDMS, i.e. a slightly higher selectivity and lower flux than the pure PDMS membrane.

Characterization of solvent-filler-poly(dimethylsiloxane) membranes by Differential Scanning Calorimetry

Abstract The properties of three polydimethylsiloxane (PDMS) membranes filled with crystalline zeolites and with amorphous SiO2, respectively, before and after sorption of organic solvents were studied by DSC. The PDMS crystallinity of the membranes with sorbed solvents is affected by both the solvent and the filler natures. For SiO2 -filled membrane, the PDMS crystallinity increases first due to the weakening of the physical crosslinks by the solvent. In the case of zeolite fillers, the PDMS crystallinity decreases first with the increase in the solvent content due to a decrease in the zeolite-induced crystallization effect by the solvent adsorption on zeolites. At high solvent contents, the solvent influence on the PDMS crystallite prevails. A medium solvent (ethyl acetate) induces the polymer crystallization, while a good solvent (cyclohexane) disrupts PDMS crystallites.

MFI-type zeolite filled silicone rubber membranes: preparation, composition, and performance

Abstract The pervaporation properties of silicone rubber membranes filled with ZSM-5 and silicalite-1 for organic solvent/water systems are relative to the composition, structure, adsorption and desorption behavior of the membranes. The key factors are the structural properties of filled zeolite, including silica- alumina ratio, cation, Si-OH defect and crystallinity. The silicalite- 1 with perfect framework shows excellent hydrophobicity, high selectivity for adsorbing organic solvents, and low desortion temperature. the pervaporetion selectivity (α) for 4.3 wt% EtOH solution through the filled membrane prepared thereof can reach 30. The separation effect can be improved by using the silicone rubber with high selectivity as raw material. As difference in molecule dimension and polarity, the permeation effects for 1-propanol/water, isopropanol/water and acetone/water system are different evidently. The influence of filling zeolite on the mechanical properties of the membrane are also studied in this article.

Preparation and properties of ZSM-5 zeolite membrane obtained by low-temperature chemical vapor deposition

Abstract A zeolite membrane was obtained by in situ crystallization of silicalite-1 from a layer of silica species prepared by the novel method of low-temperature chemical vapor deposition (LTCVD) on a porous cordierite support. The XRD and SEM analyses showed a three-layer membrane: a 3–5 μm-thick compact layer, a 45 μm-thick zeolite-in-pore layer and the support. The pure gas permeation data indicate that the synthesized zeolite layer is dense and pinhole-free. In water-alcohol mixture pervaporation, the membrane exhibited a behavior of hydrophilic materials, with a high selectivity to water and a fair flux. This behavior can be explained by the presence of Al in the framework due to Al migration from the support. The membrane flux and selectivity decreased fairly slowly with time in the transient regime of water—ethanol pervaporation with the dry membrane, suggesting slow sorption equilibria of water and ethanol with the dry zeolite.

Structure–property relationships in sorption of vapours with different polarities in zeolites and zeolite-filled poly(dimethylsiloxane)

Three faujasite (FAU) zeolite samples with different Si:Al ratios and OH contents were prepared from a NaY-type zeolite and their structure was studied by XRD, 29Si NMR, 27Al NMR and IR. These studies showed that the SiCl4 treatment mainly dealuminated the starting NaY zeolite, while the additional hydrothermal treatment reduced the OH groups on the zeolite surface, and the final acid treatment removed further the residual Al traces in the framework. The sorption measurement shows that the zeolite affinity to a non-polar organic solvent increases with increasing Si:Al ratio and decreasing surface OH groups. When zeolites were used to fill a poly(dimethylsiloxane) membrane, the affinity of the sorption sites and their capacity decreased, leading to a low contribution of the zeolites to the sorption capacity of the composite membranes. The sorption selectivity of zeolite-filled membranes was consistent with that of pure zeolites at low solvent activities, but at high activities, the sorption property of the PDMS matrix prevailed.