Studies on syntheses and permeabilities of special polymer membranes. VI. Permeation characteristics of cellulose nitrate membranes and cellulose membranes

The permeation characteristics of cellulose nitrate membranes and cellulose membranes were investigated using aqueous solutions of poly(ethylene glycol) as feed. To gain cellulose membranes the nitro groups in cellulose nitrate were converted to hydroxyl groups. It was found that cellulose nitrate membranes separate poly(ethylene glycol) 6000 but not any cellulose membrane did separate poly(ethylene glycol) 20 000.     

Permeation of propane and propylene thrugh cellulosic polymer membranes

Cellulosic polymers were tested by measuring their permeabilities of propane and propylene. High permeabilities and selectivities were obtained with ethyl cellulose membranes. Hollow fibers coated with ethyl cellulose were made through a solution coating method. Microporous polypropylene hollow fibers were used as base material. A dense layer of ethyl cellulose about 6–10 μm in thickness was formed on the microporous hollow fiber. The performance of a coated membrane column was tested for the separation of propane and propylene mixture gas. It was concluded that ethyl cellulose membranes have a good potential for the separation of olefins form saturated hydrocarbons.     

Studies on syntheses and permeabilities of special polymer membranes

Summary The permeabilities of various cellulose ester membranes and cellulose membranes were investigated using pure water and aqueous solution of glycerol as feed. In all membranes, the permeation rate for aqueous solution of glycerol was greater than for pure water. The permeation rates of cellulose ester membranes increased with an increase in number of carbon atoms of the substituent group in cellulose ester.     

Development and characterization of cellulose acetate benzoate flat osmotic membranes

Different parameters of casting solutions and casting conditions were studied for the development of cellulose acetate benzoate flat osmotic membranes. Casting solutions were prepared with different concentrations of the polymer, the additive, and the solvent; viscosity of the casting solution; and the thickness of the membrane developed. The membranes were given different evaporation periods and annealing temperatures under different RH. Different annealing baths were also used. Based on these, conditions were optimized for the development of cellulose acetate benzoate flat osmotic membranes. These membranes were characterized with respect to bound water content, specific water content, transport properties by direct osmosis, salt intake by direct immersion, water permeability coefficient of the dense membrane, diffusion coefficient, salt permeability, and salt distribution by electrical conductivity. Also, cellulose acetate benzoate membranes were compared with conventionally used cellulose acetate membranes. © 1994 John Wiley & Sons, Inc.     

Cellulose nanopapers as tight aqueous ultra-filtration membranes

Recently, we have demonstrated the use of wood-derived nanocellulose papers, herein termed nanopapers, for organic solvent nanofiltration applications. In this study, we extend the use of these nanopapers to tight ultrafiltration (UF) membranes. The feasibility of such nanopaper-based UF membranes intended for use in water purification is shown. Four types of nanocelluloses, namely bacterial cellulose, wood-derived nanocellulose, TEMPO-oxidized cellulose nanofibrils and cellulose nanocrystals, were used as raw materials for the production of these nanopaper-based membranes. The resulting nanopapers exhibit a transmembrane permeance in the range of commercially available tight UF membranes with molecular weight cut-offs ranging from 6 to 25 kDa, which depends on the type of nanocellulose used. These molecular weight cut-offs correspond to average pore sizes of a few nanometres. The rejection performance of the nanopapers is on the border of nanofiltration and UF. We demonstrate that the pore size of the nanopapers can be controlled by using different types of nanocellulose fibrils.     

Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using the electron spin resonance (ESR) method

The electron spin resonance technique (ESR) was used to study the structure and transport of asymmetric aromatic polyamide membranes. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy free radical) was used as a spin probe that was brought into the membrane either by (a) immersion ofthe membranes in aqueous TEMPO solutions, (b) reverse osmosis (RO) experiments with feed solutions involving TEMPO or (c) blending TEMPO in casting solutions. The membranes were further tested for the separation of sodium chloride and TEMPO from water by RO. It was concluded that aromatic polyamide membranes contain water channels in the polymer matrix like cellulose acetate membranes. The presence of such water channels allows aromatic polyamide membranes to be used as RO membranes. The diffusion of organic solutes through the water channels seems much slower in aromatic polyamide membranes than in cellular acetate membranes, which probably causes a higher separation of organic solutes by aromatic polyamide membranes than cellulose acetate membranes. A comparison was made with other RO membranes (cellulose acetate, CA) and ultrafiltration membranes (polyethersulphone, PES). It was observed that the ESR technique can be used to study the structure of OF and RO membranes. The presence of water channels in the polymer matrix seems indispensable for the RO membrane.     

Catalysis of a β-elimination applying membranes with incorporated molecularly imprinted polymer particles

Summary Molecularly imprinted membranes are known to be applicable as recognition elements in separation, extraction and sensors, however, not as catalysts. In this paper, two different formats of molecularly imprinted membranes were generated and used as catalysts in a dehydrofluorination reaction. The first format was based on cellulose filter membranes which were coated with a polymer imprinted with a transition-state analogue of the chosen reaction. Alternatively, ground bulk polymers have been incorporated in polyvinyl alcohol (PVA) matrices on cellulose membranes. Both types of membranes were evaluated in membrane reactors. Either the substrate solution was pumped once only through the membrane, or the substrate solution was recirculated through the membrane. The coated membranes did not show any specific catalytic effect when comparing the molecularly imprinted with a non-imprinted control polymer membrane. Nevertheless, the PVA membranes containing the imprinted polymer particles showed specific catalytic effects, compared to PVA membranes with incorporated non-imprinted control polymer particles.     

Permeability through cellulose membranes grafted with vinyl monomers in a homogeneous system. VIII. N-vinylpyrrolidone-grafted cellulose membranes

The homogeneous grafting of N-vinylpyrrolidone (NVP) onto cellulose was carried out in a dimethyl sulfoxide/paraformaldehyde (DMSO/PF) solvent system. The diffusive permeabilities of solutes through the NVP-grafted cellulose membranes, apparent activation energy for solute permeation through them, states of water in them, and their microphaseseparated structures were investigated. The solute permeability through the grafted membranes was superior to that through the cellulose membrane cast from the DMSO/PF solution of cellulose. The total water and nonfreezing water contents of the grafted membranes were larger than those of the cellulose membrane. The difference in permeability through the membranes was not correlated quantitatively with the amount of each state of water in them. Activation energies for permeation of solutes through the grafted membranes were similar to those through the cellulose membrane. © 1994 John Wiley & Sons, Inc.     

Influence of coagulation bath on morphology of cellulose membranes prepared by NMMO method

To control the morphology of cellulose membranes used for separation, they were prepared by the NMMO method using water, methanol, ethanol and their binary solution as coagulation baths. Morphologies of the surface and cross section of dry membranes were observed. The pore structure parameters of wet membranes were determined. By comparison, the process and mechanism of pore formation in dry membranes were suggested, and the relativity of cellulose crystal size to average pore diameter in wet membranes and their influences were discussed. The results show that the morphology of dry membranes is clearly varied with coagulation baths, while the porosity of wet membranes is almost constant. Porous structures can appear in the compact region of dry membranes due to swelling from water. These pores have a virtual effect on the average pore diameter of wet membranes. By changing the composition of coagulation baths, the microstructure of cellulose membranes in a dry or wet environment can be adjusted separately.     

Influence of electrolyte on electrical properties of thin cellulose acetate membranes

The electrical behavior of thin cellulose-2.5-acetate membranes for chemical semiconductor sensor applications was investigated. The influence of electrolyte on the electrical membrane properties was studied by means of electrochemical impedance spectroscopy. The polymeric membranes can be described using the electrical model of Cole and Cole. The membrane electrolyte distribution equilibria were investigated by means of an absorption–desorption method using conductivity measurements. The desorption kinetics for thin cellulose acetate membranes differ significantly from those of thicker cellulose acetate reverse osmosis membranes because here the rate of the ionic exchange at the membrane interface cannot be neglected. The experimentally determined distribution coefficients were used for the discussion of the electrolyte influence on the electrical membrane parameters. A functional dependence was found between the specific membrane resistance of cellulose acetate membranes and the ionic hydration enthalpy. The effect of electrolyte on the relative permittivity can be explained by a theoretical model. © 1994 John Wiley & Sons, Inc.     

SEM study of the morphology of asymmetric cellulose acetate membranes produced from recycled agro-industrial residues: sugarcane bagasse and mango seeds

Cellulose, obtained both from sugarcane bagasse and mango seeds, was used for synthesizing cellulose acetate in order to produce asymmetric membranes. These were compared to membranes of commercial cellulose acetate (Rhodia). All produced membranes were asymmetric, characterized by the presence of a dense skin and a porous support. Differences regarding the morphology of the surfaces as well as of the porous support can be noticed. Scanning Electron Microscopy (SEM) showed that the morphology of the superficial layer, responsible for transport, depends on the different lignin content of the starting material and also on the viscosity average molecular weight of the cellulose acetates produced from sugarcane bagasse, mango seed, and Rhodia’s commercial cellulose acetate.     

Development of new membranes

Asymmetric membranes made from various polymers, such as cellulose acetate, polysulfone, polyamide, etc. by a phase inversion process are widely used today in ultra- and hyperfiltration. In spite of their good filtration properties, these membranes have certain short-comings as far as their selectivity and chemical stability are concerned. Considerable effort has therefore been spent in recent years to develop new membranes with better separation characteristics and better overall properties tailor-made for special applications. For desalination of sea- and brackish waters, e.g., composite membranes with better chemical stability and higher fluxes and salt rejection have been developed. For the treatment of certain industrial effluents, liquid membranes with carrier facilitated transport properties for certain metal ions are available today. Membrane preparation techniques are no longer limited to the phase inversion process; track etching procedures, plasma polymerization and radiation induced grafting are used today to produce membranes for various applications. The paper gives a review of recent developments in the preparation techniques of membranes and membrane materials.     

Studies on syntheses and permeabilities of special polymer membranes. IV. Permeation characteristics of improved cellulose nitrate membranes

For the purpose of the application of cellulose nitrate membranes for artificial kidneys, the permeation characteristics and the bursting strength of three types of membranes, which were prepared from a system of cellulose nitrate/methyl alcohol/1,4-dioxane/barium perchlorate (CN-1), from a system of cellulose nitrate/methyl alcohol/formamide (CN-2), and a poly(ethylene terephthalate) cloth coated with the casting solution of CN-2 (CN-3), were investigated under various conditions. It was found that permeabilities of these membranes were influenced significantly by the preparation methods of the membranes and the permeation conditions, and they depended upon the structure of the resulting membranes and the interaction between the polymer in the membrane and the feed. In blood filtrations, the permeation rates of our three types of membranes had about four-fold values than a cuprophane membrane, and moreover no protein was found in the blood filtrate. The bursting strength of the membranes swollen with water was in the order CN-3 > cuprophane > CN-2 > CN-1, and this ratio was calculated to be about 5.9:1.0:0.5:0.3     

Water flux,DSC, and cytotoxicity characterization of membranes of cellulose acetate produced from sugar cane bagasse,using PEG 600

Summary In this article, cellulose acetate produced through the homogeneous acetylation of sugar cane bagasse cellulose was used to produce membranes, using poly(ethyleneglycol) 600 (PEG 600) as an admixture. The membranes were characterized using water flux measurements (Payne’s cup), differential scanning calorimetry (DSC) and neutral red uptake (cytotoxicity). The results showed that PEG 600 acts as a crystallinity inductor and/or pore former in the cellulose acetate matrix. The induction of crystallinity is important for this system since it had not been reported on the literature yet. The results also demonstrated that the studied membranes present a nontoxic behavior.     

功能性再生纤维素复合膜的制备及性能研究进展

纤维素是自然界中储量最大的天然高分子化合物,被认为是未来能源和化工的主要原料。然而,天然纤维素聚合度高、结晶度高的特性,使其难以溶于常规溶剂,极大限制了纤维素的应用。近年来,人们发现了多种新型纤维素溶剂体系,本文简要介绍了基于新型纤维素溶剂体系制备而来的再生纤维素膜以及一系列功能性再生纤维素基有机/无机复合膜材料。通过新型纤维素溶剂体系溶解再生得到的再生纤维素基复合膜在多孔性、热稳定性、强度等性能方面得到一定程度的改善,有望应用于包装、污水处理、传感器、生物医学等领域。本文基于再生纤维素膜及其复合膜材料的最新研究进展,对今后发展的热点方向进行了展望,旨在为纤维素溶解和功能性再生纤维素新材料的开发提供参考。     

Tailoring surface properties of cellulose acetate membranes by low‐pressure plasma processing

The aim of this study was to tailor the surface properties of cellulose acetate membranes using low‐pressure plasma processing. Argon (Ar) plasma and Difluoromethane (CH₂F₂) plasma were used to control the surface wettabilities of cellulose acetate membranes. Optical emission spectroscopy was used to examine the various chemical species of low‐pressure plasma processing. In this investigation, the plasma‐treated surfaces were analyzed by X‐ray photoelectron spectroscopy, while changes in morphology and surface roughness were determined with confocal laser scanning microscopy. Ar plasma activation resulted in hydrophilic surface. CH₂F₂ plasma deposited hydrophobic layer onto the cellulose acetate membrane because of strong fluorination of the top layer. The results reveal low‐pressure plasma processing is an effective method to control the surface properties of cellulose acetate membranes. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Cellulose membranes for reverse osmosis Part I. RO cellulose acetate membranes including a composite with polypropylene

With the aim of obtaining RO membranes for brackish water desalination from purified celluloses (cotton linters and bleached bagasse pulp), two reactions (heterogeneous and homogeneous) were applied for the synthesis of cellulose acetate (CA). The efficiency of the membranes was measured and compared with those prepared from purchased CA and prepared CA by acetylation of imported high-grade viscose wood pulp. The effect of blending CA with polypropylene (PP), on the efficiency of the prepared RO membranes was also studied. Results showed that the method of preparation of CA plays a profound effect on the salt rejection and water flux of the RO membranes. The efficiencies of RO membranes formed from heterogeneously acetylated celluloses are higher than those prepared from homogeneous ones. Blending the acetylated cellulose with 9% PP wastes improves the efficiency of membranes prepared from the homogeneously acetylated celluloses.     

Studies on syntheses and permeabilities of special polymer membranes: 41. Finger-like cavities in cellulose nitrate membranes from binary organic solvents

The formation of finger-like cavities in asymmetric cellulose nitrate membranes was investigated by changing the composition of binary mixed casting solvent and the evaporation period during the membrane formation process. The nature of the casting mixture, during the solvent evaporation process, was of coacervated droplets with absorbed water molecules from the atmosphere. Changes in the concentration of cellulose nitrate and the dissolution state of the cellulose nitrate molecules in the evaporation loss of the casting solvent, were clearly related to the formation of the finger-like cavity in the back sponge layer in asymmetric cellulose nitrate membranes.     

Studies on syntheses and permeabilities of special polymer membranes. XVI. Permeation and separation characteristics of aqueous polymer solutions for acrylonitrile-styrene copolymer membranes

The permeation and separation characteristics of aqueous polymer solutions using acrylonitrile-styrene copolymer membranes were investigated under various conditions. The membranes obtained from dimethyl sulfoxide solution of acrylonitrile-styrene copolymer have not a sufficient reproducibility and stability of permeation. These lacks were improved by adding ethylene glycol or glycerol to the casting solution and treating the membranes with pressure. The permeation and separation characteristics were influenced significantly by the additional amount of above additives, the heat treatment temperature, and the operating pressure. It was found that the concentration polarization of poly(vinyl alcohol) molecules onto the surface of the acrylonitrile-styrene copolymer membranes is smaller than that onto hydrophilic polymer membranes such as membranes of cellulose acetate, cellulose nitrate, and nylon 12, etc. Moreover, the acrylonitrile-styrene copolymer membranes show better performance for separation and concentrating of aqueous polymer solutions than hydrophilic membranes.     

Permeability of alkali chlorides through charged cellulosic membranes

The permeation of alkali chlorides through charged cellulosic membranes was investigated. The membranes used were partially carboxymethylated and carboxyethylated cellulose with various degrees of substitution. The permeability coefficients were found to increase in a sequence of LiCl < NaCl < CsCl < KCl. This sequence is explained by considering the partition and the hydration of the ions in these hydrophilic membranes. The dependence of the permeability on the salt concentration was interpreted by means of Teorell-Meyer-Sievers (TMS) theory. The effectiveness of the charged groups in the membranes is explained by the counterion binding mechanism.