Sieve mechanism of microfiltration separation

A theoretical model of dead-end microfiltration (MF) of dilute suspensions is proposed. The model is based on a sieve mechanism of MF and takes into account the probability of membrane pore blocking during MF of dilute colloidal suspensions. An integro-differential equation (IDE) that includes both the membrane pore size and the particle size distributions is deduced. According to the suggested model a similarity property is applicable, which allows one to predict the flux through the membrane as a function of time for any pressure, and dilute concentration, based on one experiment at a single pressure and concentration. The suggested model includes only one fitting parameter, β>1, which takes into account the range of the hydrodynamic influence of a single pore. For a narrow pore size distribution in which one pore diameter predominates (track-etched membranes), the IDE is solved analytically and the derived equation is in good agreement with the measurements on different track-etched membranes. A simple approximate solution of the IDE is derived and that approximate solution, as well as the similarity principal of MF processes, is in good agreement with measurements using a commercial Teflon microfiltration membrane. The theory was further developed to take into account the presence of multiple pores (double, triple and so on pores) on a track-etched membrane surface.

A series of new dead-end filtration experiments are compared with the proposed initial and modified pore blocking models. The challenge suspension used was nearly monodispersed suspension of latex particles of 0.45 μm filtered on a track-etched membrane with similar sized pores 0.4 μm. The filtered suspension concentration ranged from 0.00006 to 0.01% (w/w) and the cross-membrane pressures varied from 1000 to 20,000 Pa. Three stages of microfiltration have been observed. The initial stage is well described by the proposed pore blocking model. The model required only a single parameter that was found to fit all the data under different experimental operational conditions. The second stage corresponds to the transition from the blocking mechanism to the third stage, which is cake filtration. The latter stage occurred after approximately 10–12 particle layers were deposited (mass = 0.006 g) on the surface of the microfiltration membrane.     

Pore structural studies of monodisperse porous polymer particles

Monodisperse polystyrene latex particles with molecular weight on the order of 10⁶ were used as inert diluents for the preparation of monodisperse porous styrene-divinylbenzene copolymer particles via seeded emulsion polymerization techniques. Mercury porosimetry and nitrogen adsorption-desorption isotherms were used to assess pore structure and pore size distribution. Pore size distribution was very sensitive to the molecular weight of the polystyrene latex particles used as inert diluent. Qualitative evidence from the techniques used indicated that the monodisperse porous polymer particles were macroporous (average pore diameter > 500 Å) in nature. As the molecular weight of the linear polymer decreased, the porous structure of the polymer particles ranged in complexity across the spectrum of macro/mesopore structures. Scanning electron microscope results indicated the existence of voids between the microspheres and their agglomerates within the porous polymer particle, and nitrogen adsorption isotherms confirmed that the pores were due to interstices between these crosslinked microspheres and agglomerates.     

Modeling of the permeate flux decline during MF and UF cross-flow filtration of soy sauce lees

Cross-flow ultrafiltration and microfiltration have been used to recover refined soy sauce from soy sauce lees for over 25 years. The precise mechanism which dominated the permeate flux during batch cross-flow filtration has not been clarified. In the present study, we proposed a modified analytical method incorporated with the concept of deadend filtration to determine the initial flux of cross-flow filtration and carried out the permeate recycle and batch cross-flow filtration experiments using soy sauce lees. We used UF and MF flat membrane (0.006 m² polysulfone) module under different transmembrane pressures (TMP) and cross-flow velocities. The modified analysis provided an accurate prediction of permeate flux during the filtration of soy sauce lees, because this model can consider the change in J₀ at initial stage of filtration which was caused by the pore constriction and plugging inside membrane, and these changes may not proceed when the cake was formed on the membrane surface. Mean specific resistance of the cake increased with TMP due to the compaction of the cake and decreased with cross-flow velocity due to the change of deposited particle size, but less depended on the membrane in the present study. These results indicate that the value of J₀ determined by modified method was relevant to exclude the effects of the initial membrane fouling by pore constriction due to protein adsorption and plugging with small particles. The modified analytical method for the cake filtration developed in the present study was considered to be capable of selecting an appropriate operating conditions for many cross-flow filtration systems with UF, MF membranes.     

Elaboration of hydrophilic aminodextran containing submicron magnetic latex particles

Aminodextran containing submicron magnetic latex particles were prepared in two steps: (a) transformation of oil-in-water magnetic emulsion into structured magnetic latex particles via combination of seed and miniemulsion-like polymerization process and (b) immobilization (adsorption and chemical grafting) of prepared aminodextran onto negatively charged seed magnetic latex particles. The elaborated magnetic latex particles were characterized in terms of particle size, size distribution, morphology, surface charge density, chemical composition, magnetic properties, and also colloidal stability. The results showed that the morphology of the prepared seed magnetic latex is core–shell like and the cationic latex particles are hydrophilic and of high colloidal stability, irrespective of the aminodextran immobilization process.     

Effects of the surface pressure on the formation of Langmuir-Blodgett monolayer of nanoparticles

The effects of the surface pressure on the particle arrangement of Langmuir-Blodgett (LB) monolayers of alkanethiol-capped gold nanoparticles were studied. The LB monolayers were prepared from a highly concentrated particle solution, which increases film fabrication efficiency but readily causes small particle voids in the particle array. Overcompressing the LB monolayer to a high surface pressure restructured the particles and eliminated the voids. When the gold particles capped by dodecanethiol were 8.5 nm in diameter, the particle arrangement was vastly improved and a wafer-scale LB monolayer was transferred onto a substrate at the surface pressure of 20 mN/m.     

交联剂对无皂P(MMA-EA-MAA)种子聚合乳胶粒成孔的影响

采用完全无皂种子乳液聚合技术合成了粒径窄分布的P(MMA-EA-MAA)乳胶粒,通过对上述胶乳进行碱处理,制备出了具有空腔结构和多孔结构的聚合物乳胶粒,研究了交联剂的种类和用量对聚合过程、胶粒特性及胶粒结构形态的影响.结果表明,体系中加入交联剂后,单体转化率都有不同程度的提高;随交联剂用量的增加,乳胶粒粒径略有减小,交联剂用量较高时,乳胶粒粒径分布加宽;二乙烯基苯(DVB)的交联效率稍高于双甲基丙烯酸乙二醇酯(EGDMA);不加入交联剂及EGDMA用量低于0.5%时,处理后乳胶粒呈空腔结构,加入DVB及EGDMA用量高于1.0%时,处理后乳胶粒呈多孔结构,并且乳胶粒体积增量随交联剂用量的增加而减小.     

Poly(N-isopropylacrylamide) grafted into nanopores: Thermo-responsive behaviour in the presence of different salts

Surface initiated polymerization of N(isopropylacrylamide) (NIPAM) was performed by controlled radical polymerization on PET track-etched membranes presenting two different pore diameters (narrow pores: ∼80 nm and large pores: ∼330 nm). The opening and closing characteristics of the resulting PNIPAM-g-PET membranes were investigated by conductometric measurements carried out at different temperatures below and above the LCST of PNIPAM and in the presence of different salts. Depending on the membrane pore size, two types of permeation control mechanisms are observed. In large pore membranes, expanded PNIPAM chains conformations result in reduced effective pore size and therefore lower permeabilities relative to collapsed macromolecules chain conformations. In contrast, in narrow pore membranes, the expanded PNIPAM brush presents greater degrees of hydration in the surface layer and therefore gives rise to higher permeabilities than the collapsed conformation. In this situation, the overall permeability is thus comparable to that of a hydrogel membrane.     

Polyacrylonitrile enzyme ultrafiltration and polyamide enzyme microfiltration membranes prepared by diffusion and convection

Glucose oxidase (GOD) and catalase (CAT) were covalently immobilized onto three types of polyacrylonitrile (PAN 1, PAN 2, and PAN 3) ultrafiltration (UF) membranes with different pore sizes and one type of polyamide (PA) microfiltration (MF) membrane by the bifunctional reagent, glutaraldehyde. The initial membranes were pre-modified to generate active amide groups in the PAN membranes and active amino groups in the PA membranes. The PAN 3 membrane contained the highest amount of active groups, and the membrane PA the lowest. The modified membranes were enzyme-loaded by diffusion and convection (UF). The effect of membrane pore size and immobilization methods on enzymatic activity and bound protein were studied. The most effective immobilized system was prepared by diffusion using a PAN 3 membrane as a carrier (bound protein: 0.055 mg/cm(2), relative activity: 87.6%). This membrane had the highest pore size of all the PAN membranes. Despite the highest pore size of PA membrane, the enzyme PA membranes prepared by diffusion showed the lowest amount of bound protein (0.03 mg/cm(2)) and the lowest relative activity (35.38%). This correlates with the lowest amount of active groups found in these membranes. The relative activity was higher for all the enzyme systems loaded by diffusion. The systems prepared by convection of the enzyme solution contained higher amounts of enzymes (0.035-0.13 mg/cm(2) protein), which led to internal substrate diffusion resistance and a decrease in the GOD relative activity (21.55-68.5%) in these systems. The kinetic parameters (V(max) and K(m)) and the glucose conversion of the immobilized systems prepared by diffusion were also studied. [diagram in text].     

The preparation of composite microsphere with hollow core/porous shell structure by self-assembling of latex particles at emulsion droplet interface

A submicrometer-scaled polystyrene/melamine-formaldehyde hollow microsphere composite was prepared by self-assembling of sulfonated polystyrene (SPS) latex particles at the interface of emulsion droplets and then being fixed in place using a hard melamine-formaldehyde (MF) composite layer. For control-released purposes, the influential factors that control the size and uniformity of the packed-droplets and the permeability of the composite shell, including the initial particle location, the hydrophilicity and the size of colloidal templates, the oil phase solvent and reserving time of emulsions after the addition of MF prepolymer, were further studied. Relatively uniform sized particle packed-droplets with an average diameter of 10 microm were obtained. The assembled SPS particles kept ordering and minimal conglutination after the preparation of composite microspheres, which allows of controlling the permeability from the interstices between the particles. Porous-mesh-structured MF composite layer was formed to further control the permeability. The morphology of emulsions and composite microspheres were characterized by optical microscopy, scanning and transmission electron microscopy.     

Some aspects of polymer colloids I. Preparation and properties of different types of latex particles

The boundary region separating a latex particle from the surrounding medium has a great influence on the properties of latex dispersions. Four types of polystyrene and polystyrene/comonomer latices differing greatly in the structure of the boundary region were prepared. The first part of a series of papers reports on the preparation of the various latex dispersions. Mean particle sizes were obtained from simple turbidity measurements, quasi-elastic light scattering, and electron micrographs. The behavior of the particles in the centrifugal force field is a simple tool for detecting aggregation tendencies that are not directly related to salt stability. The BET-surface area agrees with the area calculated from the mean particle size when a sharp boundary and smooth surface is developed between the particle and the surrounding medium. In the case of particles with extended boundary regions (core/shell particles or particles with hairy envelopes), film formation reduces the specific surface area. Removal of soluble oligomers and polymers from the boundary region during subsequent treatments (purification and centrifugation before freeze-drying) can increase the surface area considerably.     

Improving Specific Activity of Aspergillus carbonarius Polygalacturonase Using Polymeric Membranes

Microfiltration (MF) and ultrafiltration (UF) membranes were screened for improving the specific activity of polygalacturonases (PG) in the culture broth of Aspergillus carbonarius obtained after submerged fermentation. While 200 and 450 nm MF membranes eliminated some of the larger non-enzymatic proteins, 50 kDa UF membrane exhibited a marginal selectivity between the enzyme and other smaller proteins. The 450 nm MF and 50 kDa UF membranes selected were further evaluated under different process conditions for an integrated membrane process. The process efficacy of three different schemes was also studied for enzyme purification. A two-stage membrane process employing MF followed by UF improved the enzyme-specific activity (5,590 U/mg) by 4.69-fold eliminating the larger and smaller non-enzymatic proteins as well as non-protein impurities with a recovery of 76% enzymes, besides resulting in higher productivity. Thus, adoption of integrated membrane process with appropriate selection of membranes could result in high recovery of enzymes with improved specific activity.     

Characterization of latex particle arrays by gas adsorption

Assemblies of colloidal particles are frequently used in novel applications, and this requires nondestructive methods allowing overall characterization of the sample and collection of information about the quality of the arrays. From suspensions of polystyrene, poly[styrene-co-(2-hydroxyethylmethacrylate)], poly[styrene-co-acrylic acid], and poly[styrene-co-methacrylic acid], assemblies of spherical particles were obtained by elimination of the solvent in different ways-evaporation, gravity deposition, and filtration. These latex particle packings were characterized by scanning and transmission electron microscopy and by gas adsorption to determine the efficiency of packing. The surface area, total pore volume, and pore size distributions obtained from the adsorption and desorption data were related to characteristic parameters calculated for cubic close-packed spherical particles.     

Emulsion Polymerization of Acrylic Monomers Stabilized by Poly(Ethylene Oxide)

Abstract

The stability of acrylic latices stabilized by poly(ethylene oxide) (PEO) is governed by the bridging flocculation process during polymerization. The final latex particle size increases with increasing concentration of initiator, PEO, or NaCl. The total scrap formed during the reaction increases rapidly with increasing NaCl concentration due to the ionic strength effect. It is shown that the final latex particle size decreases rapidly with an increase in the agitation speed. The amount of total scrap formed during polymerization is generally greater at a higher agitation speed. These results suggest that the fraction of the particle surface covered by PEO and the ratio of the thickness of the PEO adsorption layer to that of the electric double layer of the latex particles should play an important role in determining the final latex particle size and colloidal stability.     

Microfiltration membranes prepared from polyethersulfone powder grafted with acrylic acid by simultaneous irradiation and their pH dependence

Polyethersulfone (PES) powder was grafted with acrylic acid (AAc) by simultaneous γ-ray irradiation. The kinetics of the radiation induced graft polymerization was studied and the grafted PES powder was characterized. Then, microfiltration (MF) membranes were prepared from PES-g-PAAc powder with different degrees of grafting (DG) under phase inversion method. The swelling behavior and the mean pore size of MF membranes were measured, and the filtration property was tested. The results showed that the pore size and the flux of MF membranes increased with the increase in DG. And, MF membranes’ properties were dependent on the pH value.     

Preaparation of cationic latexes of poly(styrene-CO-butyl acrylate) and their properties evolution in latex dilution

Cationic latexes were prepared through emulsion copolymerization of styrene(St) and butyl acrylate(BA) with a cationic surfactant,cetyl trimethyl ammonium bromide(CTAB).Latex properties,including particle size,size distribution,ζpotential,surface tension and monomer conversion,were determined for latexes prepared with different CTAB amounts. Evolution of these properties during emulsion polymerization was followed in order to understand the mechanism of the particles formation.Results showed that both particle size andζpotential were function of polymerization time and latex solids.Parallel emulsion polymerizations with cationic,anionic charged initiator and charge-free initiators were also carried out,the latex properties were determined at different polymerization time.All these results were attentively interpreted based on the mechanisms of emulsion polymerization,surfactant adsorption and latex particle stabilization.     

The relationship between membrane surface pore characteristics and flux for ultrafiltration membranes

Surface porosities of Amicon XM100A and XM300 membranes have been measured by electron microscopy and found to be less than 1 per cent. From the measured pore size distributions it is deduced that 50 per cent of the solvent flow is through 20 to 25 per cent of the pores.The conventional model for concentration polarisation in ultrafiltration (UF), which assumes a homogeneously permeable membrane surface, has been modified to account for regions of differing permeability. An effective free area correction factor (≤ 1.0) has been introduced to allow for the effect of membrane surface properties on gel-polarised UF flux.Ultrafiltration experiments with protein solutions and membranes with a range of water fluxes confirm that gel-polarised UF flux is dependent on membrane permeability and surface properties. Effective free area correction factors vary from about 0.4 to 1.0 with values < 1.0 obtained for membranes with water fluxes typically < 150 1/m² hr at 100 kPaSupport for the effective free area concept in UF is provided by an analogy between a gel-polarised UF membrane and a composite reverse osmosis membrane. In both cases the magnitude of the upper ‘controlling’ resistance may be influenced by the pore size and spacing of the lower supporting structure.     

聚硅氧烷/丙烯酸酯核/壳复合胶乳的粒径分布与成核机理

通过种子乳液法合成出具有高有机硅含量核 壳结构的聚硅氧烷 丙烯酸酯复合粒子 .研究了聚合方法、乳化剂浓度、引发剂浓度、单体滴加速度等工艺条件对复合乳液粒径尺寸、分布与形态的影响 ,并对复合乳液的成核机理进行了探讨 .研究表明 ,乳化剂浓度对乳液粒子的粒径分布和形态、结构有显著影响 ,引发剂浓度增加将使粒子粒径减小 ;相对一次投料法 ,种子乳液法生成的粒子分布窄 ,具有明显核壳结构 ,通过壳层单体滴加速度可以控制粒子的粒径尺寸和分布 ;而壳层丙烯酸酯聚合物主要是在聚硅氧烷种子表面的“过渡层”聚合、富集而成 .     

The influence of phase inversion process modified by chemical reaction on membrane properties and morphology

In this study, the chemical reaction between acetic acid (CH₃COOH) used as non-solvent additive of casting solution and sodium carbonate (Na₂CO₃) dissolved in water as coagulant was employed to modify the classical phase inversion process. By means of this method, the polyethersulphone (PES) ultrafiltration (UF) membranes were prepared. The influence of acetic acid on the properties of the polymer solution was examined by viscometry and related to the morphology of the membrane prepared from the casting solution. The membranes were characterized in terms of the pure water flux, solute transport and field emission scanning electron microscope (FESEM) observation. It was found that chemical reaction between the additive and coagulant increases membrane permeability and mean pore size while maintaining the relatively narrow pore size distribution. FESEM images also confirmed that the chemical reaction contributes to suppress the formation of macrovoid and enhance the interconnectivity of pore. Furthermore, the potential mechanism of membrane formation influenced by chemical reaction was explored tentatively.     

Cryo-SEM studies of latex/ceramic nanoparticle coating microstructure development

Cryogenic scanning electron microscopy (cryo-SEM) was used to investigate microstructure development of composite coatings prepared from dispersions of antimony-doped tin oxide (ATO) nanoparticles (approximately 30 nm) or indium tin oxide (ITO) nanoparticles (approximately 40 nm) and latex particles (polydisperse, D(v): approximately 300 nm). Cryo-SEM images of ATO/latex dispersions as-frozen show small clusters of ATO and individual latex particles homogeneously distribute in a frozen water matrix. In contrast, cryo-SEM images of ITO/latex dispersions as-frozen show ITO particles adsorb onto latex particle surfaces. Electrostatic repulsion between negatively charged ATO and negatively charged latex particles stabilizes the ATO/latex dispersion, whereas in ITO/latex dispersion, positively charged ITO particles are attracted onto surfaces of negatively charged latex particles. These results are consistent with calculations of interaction potentials from past research. Cryo-SEM images of frozen and fractured coatings reveal that both ceramic nanoparticles and latex become more concentrated as drying proceeds; larger latex particles consolidate with ceramic nanoparticles in the interstitial spaces. With more drying, compaction flattens the latex-latex particle contacts and shrinks the voids between them. Thus, ceramic nanoparticles are forced to pack closely in the interstitial spaces, forming an interconnected network. Finally, latex particles partially coalesce at their flattened contacts, thereby yielding a coherent coating. The research reveals how nanoparticles segregate and interconnect among latex particles during drying.