CHARACTERIZATION OF REGENERATED CELLULOSE MEMBRANES HYDROLYZED FROM CELLULOSE ACETATE

A series of cellulose acetate membranes were prepared by using formamide as additive, and then were hydrolyzedin 4 wt% aqueous NaOH solution for 8 h to obtain regenerated cellulose membranes. The dependence of degree ofsubstitution, structure, porous properties, solubility and thermal stability on hydrolysis time was studied by chemical titration,Fourier transform infrared spectroscopy, scanning electron microscopy, wide-angle X-ray diffraction, and differentialscanning calorimetry, respectively. The results indicated that the pore size of the regenerated cellulose membranes wasslightly smaller than that of cellulose acetate membrane, while solvent-resistance, crystallinity and thermostability weresignificantly improved. This work provides a simple way to prepare the porous cellulose membranes, which not only kept thegood pore characteristics of cellulose acetate membranes, but also possessed solvent-resistance, high crystallinity andthermostability. Therefore, the application range ofcellulose acetate membranes can be expanded.     

Fabrication of honeycomb-patterned cellulose films

This paper reports for the first time on the fabrication of honeycomb-patterned cellulose films by casting water in oil emulsion of cellulose acetate onto a glass substrate and subsequent deacetylation treatment. The honeycomb pore size could be controlled from 1 to 100 microm under a saturated water vapor condition. Both cellulose and cellulose acetate films with honeycomb-pattern are expected to be a two-dimensional model of plant cell walls as well as of micro-wells for single cell cultivation. Surface topographic image of a honeycomb-patterned cellulose film (scalebar: 50 microm).     

Formation of cellulose acetate membranes using a supercritical fluid assisted process

Microporous cellulose acetate membranes have been prepared from polymer–acetone solutions using a supercritical fluid phase inversion process in which CO₂ acts as the non-solvent. Series of experiments were performed at various polymer concentrations, temperatures and pressures. The structure of the resulting membranes was analysed using scanning electron microscopy. We operated with polymer concentrations ranging between 5 and 40% (w/w) in acetone obtaining different pore dimensions and membrane structures. Increasing the percentage of polymer in the solution, the structure of the membranes changed from beads-like structure to cellular structure. Polymer concentration also influenced the mean diameter of the pores that ranged from 2 to 50 μm for polymer concentrations from 40 to 5% (w/w). We also tested membrane formation pressures between 100 and 200 bar and at temperature between 45 and 65 °C. Pressure influences the change in membrane structure from cellular to beads-like, whereas temperature has a minor influence on pore size: both the effects can be partially related to CO₂ density. Cellulose acetate membrane formation mechanisms have also been discussed.     

Construction of inorganic nanoparticles by micro-nano-porous structure of cellulose matrix

In our previous work, the CdS nanoparticles/cellulose films exhibited significantly high photocatalytic H₂ production efficiency under visible light irradiation than the ordinary CdS photocatalyst. In present paper, the CdS nanoparticles were synthesized in situ in pores of the regenerated cellulose substrate and the porous structure of cellulose, formation of the CdS nanoparticles and interactions between CdS and cellulose matrix in the composite films were investigated deeply. The experimental results indicated that the micro-nano-porous structure of the cellulose matrix could be used easily to create inorganic nanoparticles, which supplied not only cavities for the formation of nanoparticles, but also a shell (semi-stiff cellulose molecules support the pore wall) to protect their nano-structure. When the cellulose films with porous structure at wet state were immersed into inorganic ions solution, the ions interacted immediately with the –OH groups of cellulose, and then transformed into inorganic composite via another treatment, finally inorganic nanoparticles formed during the dry. The pore size of the cellulose matrix decreased from 180 nm (at wet state) to about 18 nm (at dry state), leading to the formation of nanoparticles. The results revealed that the CdS nanoparticles with a mean particle diameter about 6 nm were dispersed well, and were immobilized tightly in the cellulose matrix, resulting in a portable photocatalyst with high efficiency for photocatalytic for H₂ evolution. This is simple and “green” pathway to prepare the organic–inorganic hybrid materials.     

Studies on cellulose acetate and sulfonated poly(ether ether ketone) blend ultrafiltration membranes

New ultrafiltration membranes based on chemically and thermally stable arylene main-chain polymers have been prepared by blending the sulfonated poly(ether ether ketone) with cellulose acetate in various compositions in N,N^′-dimethylformamide as solvent by phase inversion technique. Prepared membranes have been subjected to ultrafiltration characterizations such as compaction, pure water flux, water content, and membrane hydraulic resistance. The pore statistics and molecular weight cut-off (MWCO) of the membranes have been estimated using proteins such as trypsin, pepsin, egg albumin and bovine serum albumin. The pore size increased with increasing concentrations of sulfonated poly(ether ether ketone) in the casting solution. Similarly, the MWCOs of the membranes ranged from 20 to 69 kDa, depending on the various polymer compositions. Surface and cross-sectional morphologies of membranes were analyzed using scanning electron microscopy. The effects of polymer compositions on the above parameters were analyzed and the results are compared and discussed with those of pure cellulose acetate membranes.     

Chiral separation of (R,S)-2-phenyl-1-propanol through cellulose acetate butyrate membranes

As the cellulose acetate butyrate possessed multichiral carbon atoms in its molecular structure unit, enantioselective membrane was prepared using cellulose acetate butyrate as membrane material. The flux and permselective properties of membrane using aqueous solutions of (R,S)-2-phenyl-1-propanol as feed solution was studied. The top surface and cross-section morphology of the resulting membranes were examined by scanning electron microscopy. When the membrane was prepared with 15 wt.% cellulose acetate butyrate and 20 wt.% DMF in the casting solution, and the operating pressure and feed concentration of racemate were 2 kgf/cm² and 5 mmol/L, respectively, over 98% of enantiomeric excess (e.e.) was obtained. This is a report, for the first time, that the cellulose acetate butyrate is used as optical resolution membrane material for isolating the optical isomers of (R,S)-2-phenyl-1-propanol.     

Structure and microporous formation of cellulose/silk fibroin blend membranes: Part II. Effect of post-treatment by alkali

Blend membranes (RCF1) were prepared from mixture solution of cellulose and silk fibroin (SF) in cuoxam by coagulating with acetone–acetic acid (4:1 by volume). The blend membranes were subjected to post-treatment with 10% NaOH aqueous solution, and their structure and properties were characterized by FT-IR, X-ray diffraction, DSC, SEM and DMTA. In previous work, cellulose/SF blend membranes (RCF2) prepared by coagulating with 10% NaOH aqueous solution formed a microporous structure, in which the SF as a pore former was almost completely removed from the membrane. However, when the blend membranes RCF1 were immersed in 10% NaOH aqueous solution for post-treatment, a strong hydrogen bonding between cellulose and SF inhibited the removal of SF. Although alkali is a good solvent for SF, the blend membranes RCF1 such obtained from cellulose and SF were alkali resistant. The crystallinity and the mean pore size of the blend membranes slightly decreased with increasing post-treatment time. This work provided a cellulose/silk blend membrane, which can be used under alkaline medium.     

Effects of polymer concentration and coagulation temperature on the properties of regenerated cellulose films prepared from LiOH/urea solution

Aqueous 5 wt% LiOH/12 wt% urea solution pre-cooled to −12 °C has a more powerful ability to dissolve cellulose compared to that of NaOH/urea and NaOH/thiourea solution system. The influences of the cellulose concentration and coagulation temperature on the structure, pore size and mechanical properties of the cellulose films prepared from LiOH/urea system were investigated. The cellulose films exhibited good mechanical properties either at wet or dry state and their pore size and water permeability at wet state can be controlled by changing the cellulose concentration or coagulation temperature. With a decrease of the coagulation temperature, the mechanical properties and optical transmittance of the cellulose films enhanced, as a result of the formation of relative smaller pore size and denser structures. This work provided a promising way to prepare cellulose films with different pore sizes at wet state and good physical properties at dry state.     

Annealing effect in porous cellulose acetate membranes

On the basis of separation curves the pore size and pore dize distribution of cellulose acetate ultrafiltration membranes are determined. The change of these parameters by annealing is discussed and from the correlation between pore size and annealing temperature up to 75°C conclusions are drawn as to the structure of salt rejecting membranes annealed at 90°C.     

Annealing effect in porous cellulose acetate membranes

On the basis of separation curves the pore size and pore dize distribution of cellulose acetate ultrafiltration membranes are determined. The change of these parameters by annealing is discussed and from the correlation between pore size and annealing temperature up to 75°C conclusions are drawn as to the structure of salt rejecting membranes annealed at 90° C.     

Removal of aromatic compounds in the aqueous solution via micellar enhanced ultrafiltration: Part 1. Behavior of nonionic surfactants

The effects of nonionic surfactants having different hydrophilicity and membranes having different hydrophobicity and molecular weight cut-off on the performance of micellar-enhanced ultrafiltration (MEUF) process were examined. A homologous series of polyethyleneglycol (PEG) alkylether having different numbers of methylene groups and ethylene oxide groups was used for nonionic surfactants. Polysulfone membranes and cellulose acetate membranes having different molecular cut-off were used for hydrophobic membranes and hydrophilic membranes, respectively. The concentration of surfactant added to pure water was fixed at the value of 100 times of critical micelle concentration (CMC). The flux through polysulfone membranes decreased remarkably due to adsorption mainly caused by hydrophobic interactions between surfactant and membrane material. The decline of solution flux for cellulose acetate membranes was not as serious as that for polysulfone membranes because of hydrophilic properties of cellulose acetate membranes. The surfactant rejections for the cellulose acetate membranes increased with decreasing membrane pore size and with increasing the hydrophobicity of surfactant. On the other hand the surfactant rejections for polysulfone membranes showed totally different rejection trends with those for cellulose acetate membranes. The surfactant rejections for the polysulfone membranes depend on the strength of hydrophobic interactions between surfactant and membrane material and molecular weight of surfactants.     

Cobalt (II) chloride promoted formation of honeycomb patterned cellulose acetate films

CoCl(2) containing honeycomb patterned films were prepared from cellulose acetate (CA)/CoCl(2)/acetone solutions by the breath figure method in a wide range of humidities. Size and pore regularity depend on the CA/CoCl(2) molar ratio and humidity. When replacing CoCl(2) with Co(NO(3))(2) or CoBr(2), no formation of ordered porosity in the cellulose acetate films is observed. According to data from scanning electron microscopy (SEM), Energy Dispersive X-ray Microanalysis (EDX), X-ray Diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, the key role in the formation of honeycomb structures can be attributed to the physical and chemical properties of CoCl(2) - hygroscopicity, low interaction with CA, and extraction from CA/CoCl(2)/acetone solution by water droplets condensed on the surface of the CA/CoCl(2) solution. Obtained films are prospective for using in catalysis, hydrogen fuel cells, and optical sensing materials.     

Water in polymer membranes. Part III : Water sorption and pore volume in cellulose acetate films

Changes in waveguide properties of several cellulose acetate membranes and one polyimide membrane were measured as a function of their exposure to varying levels of relative humidity. The volume fraction of water in the films and the occupied pore volumes were determined from refractive index and thickness changes. The dependence of the refractive index on water absorption is related to a competition between two processes: one of filling pores with no film expansion and one of “free expansion” where the film expands to completely accommodate the added water volume. The term “pore” is taken to mean a volume with molecular and not macroscopic dimensions. The hydration properties of these dense cellulose acetate membranes were affected by degree of acetylation, casting temperatures and annealing treatments. Annealing CA398 membranes at 180°C decreased film water concentration by reducing the amount of free expansion. Annealed CA398 membranes that were tested in a reverse osmosis cell were found to have high salt rejection compared to unannealed films. The hydration characteristics of a polyimide membrane are compared to cellulose acetate membranes.     

Sieving variations due to the choice in pore size distribution model

The effect of the choice of the standard probabilistic model to describe the pore size distribution was theoretically studied on predicting membrane performance parameters, area average water flux and area average membrane sieving coefficient. Preliminary discrete pore size distributions were generated from rejection profiles of dextran and PEG for 10,000, 30,000 and 100,000 molecular weight cutoff (MWCO) polysulfone and cellulose acetate membranes. The standard probability distribution functions (PDF), gamma, lognormal, normal, Weibel and Rayleigh were used to fit the resulting pore size distribution data. It was observed that the area averaged sieving coefficients are sensitive to the choice of the PDF. These results implied that an uncertainty in the choice of distribution in describing the membrane morphology could lead to a propagated uncertainty in predicting overall membrane performance.     

Breath figure patterns prepared by spin coating in a dry environment

We introduce a novel method for fabricating breath figure patterns on a homopolymer film by spin coating of polymer solutions with various solvents. The homopolymers employed in this study were cellulose acetate butyrate, monocarboxylated end-functional polystyrene, and poly(methyl methacrylate). Breath figure patterns were generated even when a water-miscible solvent such as tetrahydrofuran (THF) was used as a solvent. We even succeeded in generating breath figure patterns by spin coating even under a dry environment (relative humidity less than 30%), when water was directly added into THF solution. With the combination of the spin coating method, pores with a few hundred nanometers to several micrometers have been generated. We found that the pore size becomes larger with increasing water content in THF solution and decreasing rotating speed. This is equivalent to increasing humidity and decreasing evaporation speed, respectively, in the conventional method, which is direct solvent evaporation under a humid environment. Thus, compared with the conventional method for making breath figure patterns, this method would be very convenient for fabricating large-scale films with various pore sizes.     

Facile preparation of regenerated cellulose film from cotton linter using organic electrolyte solution (OES)

Regenerated cellulose (RC) film is an important cellulose-based product with a wide range of applications in the packaging industry and drug delivery. In this study, RC film was prepared using an organic electrolyte solution (OES)/ionic liquid (IL) system consisting of 1-ethyl-3-methylimidazolium acetate (EmimAc) and dimethyl sulfoxide (DMSO) to dissolve cotton linter at room temperature. Results showed that OES with EmimAc molar fraction in the range from 0.2 to 0.4 could rapidly dissolve cotton linter at room temperature, in contrast to 80 °C for EmimAc solution (control). The cellulose degradation was therefore much less than for the control, resulting in significant improvement in the mechanical properties and thermal stability of the RC film. In addition, the presence of DMSO significantly decreased the viscosity of the cellulose/OES system, being advantageous for casting of the RC film.     

Selective behaviour of hyperfiltration cellulose acetate membranes: Part II. Streaming potential

Streaming potentials have been measured by the fast pulse method for cellulose acetate membranes cured at 85 and 95°C. Bounding, solutions of Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Sr²⁺, Ba²⁺ and La³⁺ chlorides were used in the concentration of 10^?4-10^?1M. The streaming potenial was shown to depend on the nature of the cation and to increase with decreasing solution concentration.Rather good consistency was obtained in dilute solutions between the electroosmotic cross coefficients calculated from the streaming potentials and from the electroosmosis measurements for Part I (M.G.A. Khedr, S.M. Abd el Haleem and A. Baraka, J. Electronal. Chem. 182 (1985) 157), according to the Onsager reciprocal relations. The deviation observed at higher concentrations, independent of the cation nature, is attributed to concentration polarization and/or excessive water transport encountered in electroosmosis measurements.Determination of additional phenomenological coefficients of the same system, the hydraulic permeability and the electric resistance enabled the estimation of the effective exchange capacity of modified cellulose acetate membranes. The resulting value of 2.1×10^?3 mol of unit charge/1 of pore liquid is compared to the corresponding values previously reported on a different basis.     

Nanofiber cellulose di- and tri-acetate using ferric chloride as a catalyst promoting highly efficient synthesis under microwave irradiation

An efficient method for the generation of cellulose di- and tri-acetate nano-structures is obtained through testing ferric chloride hydrate (FeCl₃·6H₂O) as a valuable Lewis acid catalyst with acetic anhydride under microwave irradiation. Our target was to evaluate the effects of the reaction conditions on the products' properties such as surface area and particle size distribution. It was found that changes in the degree of substitution (DS), the surface area, the degree of polymerization and the particle size distribution of the products correlated with reaction conditions. Cellulose tri-acetate nanofibers with DS of 2.94 with 98.03% yield was prepared using 200 mg of FeCl₃·6H₂O, 25 ml of Ac₂O and 4 minutes of microwave irradiation. Also, cellulose di-acetate nanofibers were prepared with DS values ranged between 2.37 and 2.72 with yield ranged between 78.92 and 90.58%. The percentage of acetyl groups (Ac%) as well as the BET specific surface area, total pore volume, mean pore diameter, mono layer volume and the mean particle size of the products were determined. The maximum specific surface area obtained for the acetylated cellulose was about ten times larger than that measured for the commercial cotton cellulose and about six times larger than that of the commercial cellulose acetate. The lowest mean particle size (34.90 nm) was about eleven times smaller than the mean particle size of the commercial cellulose acetate (394 nm). The present work has proved that FeCl₃·6H₂O was a highly active catalyst for the esterification of cellulose with unexpected yields and for the formation of nanofibers with low molecular weight.     

Effect of Polyvinylpyrrolidone on Separation Performance of Cellulose Acetate-Polysulfone Blend Membranes

Blend membranes comprising cellulose acetate and polysulfone (CA/PSf) were prepared through a solution casting method using a different concentration of polyvinylpyrrolidone (PVP) as the pore former. Fourier transform infrared spectroscopy (ATR-FTIR) was used to investigate structural properties of membranes. Membranes morphology and its thermal properties were characterized by scanning electron microscope (SEM) and thermogravimetric analysis (TGA). The strength of membranes was studied by mechanical stability. The effect of PVP concentration on separation performance of the prepared membranes was studied. The separation performance of prepared membranes was tested by using an aqueous solution of cadmium metal complexed with humic acid. The results showed that an increase in the PVP concentration in the cast film from 0 to 3 wt% increased the thermal stability, water content (%), pure water flux, and solute rejection. SEM results showed that the pore size decreased but the number of pores increased on an increase in the PVP concentration.     

Some aspects of acylation of cellulose under homogeneous solution conditions

Commercially available cellulose (Avicell PH101) was successfully acylated under homogeneous solution conditions by the following procedure: 2.0 g of cellulose were stirred with 75 mL of N,N‐dimethylacetamide for 1 h at 150°C, 3.5 g of LiCl were added, the temperature was raised to 170°C, ca. 18.5 mL of the solvent were distilled and the suspension was cooled to room temperature and stirred overnight. The temperature of the clear cellulose solution was raised to 110°C, kept at that temperature for 1 h, an acid anhydride was added and the solution stirred at 110°C for additional 4 h. Acetates, propionates, butyrates, and acetate/propionate mixed ester were prepared with excellent control of the degree of substitution, DS, 1 to 3 for acetates, 2 and 3 for propionates and butyrates, and 3 for acetate/propionate. The degree of polymerization of cellulose is negligibly affected under these reaction conditions. The distribution of the acetyl moiety among the three OH groups of the anhydroglucose unit shows a preference for the C₆ position. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1357–1363, 1999