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.     

Functional and surface-active membranes from poly(vinylidene fluoride)-graft-poly(acrylic acid) prepared via RAFT-mediated graft copolymerization

Poly (vinylidene fluoride) (PVDF) with "living" poly (acrylic acid) (PAAc) side chains (PVDF-g-PAAc) was prepared by reversible addition-fragmentation chain transfer (RAFT)-mediated graft copolymerization of acrylic acid (AAc) with the ozone-pretreated PVDF. The chemical composition and structure of the copolymers were characterized by elemental analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The copolymer could be readily cast into pH-sensitive microfiltration (MF) membranes with enriched living PAAc graft chains on the surface (including the pore surfaces) by phase inversion in an aqueous medium. The surface composition of the membranes was determined by X-ray photoelectron spectroscopy. The morphology of the membranes was characterized by scanning electron microscopy. The pore size distribution of the membranes was found to be much more uniform than that of the corresponding membranes cast from PVDF-g-PAAc prepared by the "conventional" free-radical graft copolymerization process. Most important of all, the MF membranes with surface-tethered PAAc macro chain transfer agents, or the living membrane surfaces, could be further functionalized via surface-initiated block copolymerization with N-isopropylacrylamide (NIPAAM) to obtain the PVDF-g-PAAc-b-PNIPAAM MF membranes, which exhibited both pH- and temperature-dependent permeability to aqueous media.     

pH‐induced on–off switching of polycarbonate track‐etched membranes by plasma‐induced surface grafting

Surface functionalization of the plasma‐pretreated polycarbonate (PC) track‐etched membranes via plasma‐induced thermally graft copolymerization of acrylic acid (AAc) was carried out. The resulting PC membranes with grafted AAc side chains were characterized by X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric (TG) analysis. The morphology of the PC membranes was studied by scanning electron microscopy (SEM). The results showed that the grafted PAAc polymers were formed uniformly inside the pores throughout the entire membrane thickness. With increase in the pore‐filling ratio, the pore diameters of PAAc‐grafted membranes became smaller. The PC‐g‐PAAc membranes exhibit rapid and reversible response of the flux to the environmental pH as pH is switched between 3 and 9. Between pH 3.5 and 5.5, the membranes demonstrate a pH‐valve function as the carboxyl group changes from neutral to charged with a corresponding variation in chain configuration. Copyright © 2009 John Wiley & Sons, Ltd.     

Swelling of poly(methacrylic acid) gels and adsorption of L-lysine and its polymer on the gels

Poly (methacrylic acid) gels (PMAA gels) of various degrees of crosslinking were prepared and the dissociation behavior of these gels was examined; the swelling behavior was investigated as a function of the solution pH values. A reentrant phenomenon of swelling was observed and interpreted based on the Flory-Huggins equation and the Donnan equilibrium formula. Moreover, adsorption of L-lysine, oligo(L-lysine)s (Lys-n,n=3, 9, and 19) and poly(L-lysine) onto PMAA gels from aqueous solutions was investigated under different conditions of pH and concentration of adsorbate. The adsorption ratio of L-lysine onto PMAA gel is dependent on both the pH of solution and the degree of crosslinking. In a pH range between 8 and 9, the protonated form of L-lysine is strongly adsorbed on the PMAA gel by electrostatic interactions. Oligomers and polymer of L-lysine are adsorbed in a somewhat different way from the monomeric L-lysine. In addition, the desorption behavior of L-lysine from PMAA gels by a change in pH was also investigated.     

Electron-beam graft-modified membranes with externally controlled flux

Pre-irradiation grafting as a means to modify commerical poly(vinylidene fluoride) (PVDF) membranes has been studied. The membranes prepared were weak cation-exchange membranes (acrylic acid functionality), anion-exchange membranes (trimethyl ammonium functionality) and temperature-sensitive membranes (N-isopropyl amide functionality). Different graft loads were obtained by varying reaction time, radiation dose and in the case of acrylic acid the graft solution composition. The trimethyl ammonium chloride functionality was obtained by grafting vinyl benzyl chloride onto a PVDF membrane and aminating the benzyl chloride groups in a 45% trimethyl amine–water solution. For a membrane grafted with 9 wt% acrylic acid the flux increased approximately 70 times when the pH was decreased from 6 to 2. For a membrane with 5 wt% trimethyl ammonium functionality the flux increased both when pH was decreased below 3 and increased above 11. For a membrane grafted with 18 wt% N-isopropyl acrylamide a sharp increase of flux was observed when the temperature was raised above 32°C.     

Heterogeneous polyelectrolyte gels as stimuli-responsive membranes

Stimuli-responsive membranes may act as “on–off switches” or “permeability valves”, producing patterns of pulsatile release, where the period and rate of mass transfer can be controlled by external or environmental triggers (e.g. pH, temperature, electric field). In this work, composite-heterogeneous polyelectrolyte gel (composite-HPG) membranes consisting of polymethacrylic acid (PMAA) gel particles dispersed within a polydimethylsiloxane (PDMS) network were developed and evaluated as pH-responsive membranes.The mechanism of permeability control for caffeine and vitamin B₁₂ through composite-HPG membranes was determined to be a synergistic function of membrane hydration and the percolating volume fraction of PMAA gel. Larger changes in permeation as a function of pH were achieved when both hydration and percolation effects occurred together than when either of these effects occurred on their own. Vitamin B₁₂ permeation was observed when the hydrated gel volume fraction was above approximately 0.38, but not below. Furthermore, the percolating fraction of composite-HPG membranes containing 28% (dry basis) PMAA gel particles was manipulated via pH to fall above (pH 7) or below (pH 3) this transition in permeability, resulting in membranes that delivered solutes of high molecular weight (vitamin B₁₂) with large on/off delivery ratios (160).     

Preparation and properties of novel environment-sensitive membranes prepared by graft polymerization onto a porous membrane

The present work is concerned with the preparation and some properties of novel environment-sensitive membranes. A porous poly (vinylidene fluoride) membrane (pore size 0.22 μm) was pretreated by air plasma; subsequently, hydrophilic monomers were graft polymerized on the treated surface. Since the filtration characteristics of the obtained membranes reflect the configuration of the grafted chains, these can be changed reversibly from ultrafilter to microfilter and vice versa in response to the membrane environment such as pH, solvent composition and ionic species. Grafted chains act as a sensor and a valve to regulate filtration characteristics. The poly(acrylic acid) grafted membrane for example is very sensitive to environmental pH. In the pH region of 1 to 5, the filtration rate sharply decreased with increasing solution pH, the filtration rate at pH 1.4 being about ten times higher than at pH 5.2. Together with this decrease in filtration rate, the membrane gained the ability of ultrafiltration of macromolecular solutes such as dextran (M_w = 2,000,000) and albumin (M_w = 67,000). In the pH region of 5.2 to 7.5, filtration rate and solute rejection did not depend on pH. The pH sensitivity is reversible and reproducible. Because of characteristics such as the drastic alteration in filtration rate and solute separation properties and the quick response to solution conditions, the environment-sensitive membranes developed here may find applications in various areas of membrane technology.     

Greffage de l'acide methacrylique sur des films de poly(chlorure de vinyle) par la methode radiochimique directe

Since PVC films do not swell in pure methacrylic acid (MAA) the films were subjected to gamma-rays while dipped in various mixtures MAACHCl₂. Under such conditions, the grafting proceeds smoothly and its rate exhibits a flat maximum for the mixture containing ca. 50% (molar) monomer. The rate satisfies the relationship Rate = KI^0.6 and the over-all activation energy of the process is 4 kcal/mole. MAA grafted PVC films do not swell in solvents for PMAA (such as water or methanol) even for high grafting ratios. This result is unexpected since PTFE films grafted with either acrylic or methacrylic acid swell to a large extent in water and are excellent membranes. The swelling of the grafted PVC films was investigated in mixtures 1,2-dichloroethylene -methanol. It was found that the extent of swelling was highest in the mixture containing 35% methanol. The unusual swelling properties of these grafted films are attributed to strong polar interactions between PVC and PMAA chains.     

Protein immobilization on the surface of poly-L-lactic acid films for improvement of cellular interactions

To covalently immobilize gelatin or collagen type I on poly-L-lactic acid (PLLA) film surfaces poly(hydroxyethyl methacrylate) (PHEMA) or poly(methacrylic acid) (PMAA) was grafted via photooxidization and subsequent UV-induced polymerization [Makromol. Chem. 186 (1985) 1533.1]. For films grafted with PHEMA, methyl sulfonyl chloride was used to activate the hydroxyl groups and for films grafted with PMAA 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide was used to activate the carboxyl groups. Gelatin and collagen were finally reacted with the activated hydroxyl or carboxyl groups to obtain covalently immobilized protein layers. Grafting of PHEMA, PMAA and protein on the surfaces was confirmed using ATR-IR and XPS. Surface wettability of the modified films was improved. The protein immobilized PLLA may be widely used as a biocompatible material.     

Reversible pH-controlled switching of poly(methacrylic acid) grafts for functional biointerfaces

Responsive polymeric brushes of poly(methacrylic acid) (PMAA) were grafted from silicon surfaces using controlled surface-initiated atom-transfer radical polymerization (SI-ATRP). The growth kinetics of PMAA was investigated with respect to the composition of the ATRP medium by grafting the polymer in mixtures of water and methanol with different ratios. The dissociation behavior of the polymer layers was characterized by FTIR titration after incubating the polymer-grafted substrates in PBS buffer solutions with different pH values. PMAA layers show a strong pH-dependent behavior with an effective pK(a) of the bulk polymer brush of 6.5 ± 0.2, which is independent of the polymer brush thickness and methanol content of the ATRP grafting medium. The pH-induced swelling and collapse of the grafted polymer layers were quantified in real time by in situ ellipsometry in liquid environment. Switching between polymer conformations at pH values of 4 and 8 is rapid and reversible, and it is characterized by swelling factors (maximum thickness/minimum thickness) that increase with decreasing the methanol content of the SI-ATRP medium.     

Aggregation properties of oligo(methacrylic acid)-shelled dendrimer and its microenvironment in aqueous solutions

A third-generation poly(amido amine) dendrimer having poly(methacrylic acid) segments on the periphery (G3-PMAA) was newly synthesized. A xanthate-modified dendrimer (G3-X) was first prepared by condensation of the terminal amino groups of the poly(amido amine) dendrimer with an activated-ester xanthate. G3-PMAA was then synthesized by polymerization of methacrylic acid initiated with G3-X. The number-average degree of polymerization of PMAA segment was estimated to be 10. The pK_a value for G3-PMAA was evaluated to be 7.3 that is somewhat higher than that (6.8) of the corresponding linear PMAA with the same segment length, indicating the interaction of PMAA segments caused by assembling them at the dendrimer surface. When the diameter of G3-PMAA in aqueous solution at various pHs was measured by a dynamic light scattering, G3-PMAA was found to self-aggregate in a pH region, where the PMAA segment took a hydrophobic, compact coil conformation. Subsequently, the interaction of a fluorescent probe (1-anilino-8-naphthalenesulfonic acid ammonium salt (ANS)) with G3-PMAA was examined by means of fluorescence spectroscopy. As a result, ANS was found to bind to the hydrophobic site of G3-PMAA aggregates at lower pH, and to be released into water phase at higher pH.     

Investigation and characterization of PVA-g-AAc/Zol membranes for possible practical use in separation processes

Poly(vinyl alcohol) films were grafted with two monomers (acrylic acid and N-vinyl imidazole) using the gamma irradiation technique. The melting temperature (T_m) and glass transition temperature (T_g) of the grafted membranes were determined with respect to the grafting yield. The ability of these membranes to separate cobalt from nickel has been investigated. The diffusion of cobalt and nickel ions from the feed compartment to the receiver compartment depends on the grafting yield and the pH of the feed solution. Cobalt ions do not diffuse through the membrane when the pH of the feed solution is >4.5. Thus, the prepared membranes could be considered for the separation of cobalt ions from nickel ions. The temperature of thermal decomposition of pure PVA-g-AAc/Zol membrane, PVA-g-AAc/Zol membranes containing cobalt ions, and PVA-g-AAc/Zol membranes containing nickel ions are determined using TGA analyzer; it was shown that the presence of cobalt and nickel increases the decomposition temperature. Also the membranes bonded with cobalt ions are more stable than the membranes containing nickel ions.     

Titania membrane preparation with chemical stability for very hash environments applications

The high-quality tubular titania MF membranes are successfully prepared by dip-coating techniques and systematically investigated with regard to their corrosive resistances. The experiments show that dispersants PAA and anatase powder were preferably employed to prepare desired suspensions with solid loading 10–15 wt.% and that suspensions properties significantly affect the final membranes quality. The titania MF membranes with pure water permeability 742.42 l m⁻² h⁻¹ bar⁻¹ and 0.1 μm pore diameter have been obtained using the prepared suspensions. The corrosion resistance of titania membranes exhibits that the anatase layers are more stable than the alumina supports in boiling corrosive medium and that the poor quality titania MF membranes become more deteriorated due to its top layer pore blockages and fouling layer formation, which provides a wide range of practical application fields in very hash environments with reliable data supports.     

Permeability and electrokinetic characterization of poly(ethylene terephthalate) capillary pore membranes with grafted temperature-responsive polymers

Poly(ethylene terephthalate) (PET) track-etched membranes with average pore diameters of 692 and 1629 nm were functionalized using the monomer N-isopropylacrylamide (NIPAAm) and a photoinitiated "grafting-from" approach in which a surface-selective reaction has been most efficiently achieved by combinations of the unmodified PET surface with benzophenone and, alternatively, of an aminated PET surface with benzophenone carboxylic acid. Consistent estimations of the pore diameters of the base PET membranes and of the effective grafted polyNIPAAm layer thicknesses on the PET pore walls were possible only on the basis of the permeabilities measured with aqueous solutions of higher ionic strength (e.g., 0.1 M NaCl). However, the permeabilities measured with ultrapure water indicated that the "electroviscous effect" was significant for both base membranes. The influences of membrane pore diameter, surface charge, and solution ionic strength could be interpreted in the framework of the space-charge model. Functionalized membranes with collapsed grafted polymer hydrogel layer thicknesses of a few nanometers exhibited almost zero values of the zeta potential estimated from the trans-membrane streaming potential measurements. This was caused by a "hydrodynamic screening" of surface charge by the neutral hydrogel. Very pronounced changes in permeability as a function of temperature were measured for PET membranes with grafted polyNIPAAm layers, and the effective layer thickness in the swollen state--here up to approximately 300 nm--correlated well with the degree of functionalization. The subtle additional effects of solution ionic strength on the hydrodynamic layer thickness at 25 degrees C were different from the effects for the base PET membranes and could be explained by a variation in the degree of swelling, resembling a "salting-out" effect. Overall, it had been demonstrated that the functionalized capillary pore membranes are well suited for a detailed and quantitative evaluation of the relationships between the synthesis, the structure, and the function of grafted stimuli-responsive polymer layers.     

Radiation-induced grafting of styrene onto ultra-high molecular weight polyethylene powder and subsequent film fabrication for application as polymer electrolyte membranes: I. Influence of grafting conditions

Ultra-high molecular weight polyethylene (UHMWPE) powder was irradiated by gamma rays using a ⁶⁰Co source. Simultaneous and pre-irradiation grafting was performed in air and in inert atmosphere at room temperature. The monomer selected for grafting was styrene, since the styrene-grafted UHMWPE could be readily post-sulfonated to afford proton exchange membranes (PEMs). The effect of absorbed radiation dose and monomer concentration in methanol on the degree of grafting (DG) is discussed. It was found that the DG increases linearly with increase in the absorbed dose, grafting time and monomer concentration, reaching a maximum at a certain level. The order of rate dependence of grafting on monomer concentration was found to be 2.32. Furthermore, the apparent activation energy, calculated by plotting the Arrhenius curve, was 11.5 kJ/mole. Lower activation energy and high rate dependence on monomer concentration shows the facilitation of grafting onto powder substrate compared with film. The particle size of UHMWPE powder was measured before and after grafting and found to increase linearly with increase in level of grafting. FTIR-ATR analysis confirmed the styrene grafting. The grafted UHMWPE powder was then fabricated into film and post-sulfonated using chlorosulfonic acid for the purposes of evaluating the products as inexpensive PEM materials for fuel cells. The relationship of DG with degree of substitution (DS) of styrene per UHMWPE repeat unit and ion exchange capacity (IEC) is also presented.     

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.     

Formation of pore‐filled microfiltration membranes using a combination of modified interfacial polymerization and grafting

A novel method of fabricating pore‐filled membranes has been developed by coating a hydrophobic polyolefinic microfiltration membrane with polyethylenimine (PEI) to produce a chemically active surface. An evaporative coating technique was used to coat the internal surface of a polyolefinic membrane with PEI in chloroform (CHCl₃). The coated PEI was then crosslinked by naphthalene‐1,4‐disulfonyl chloride in carbon tetrachloride (CCl₄). The incorporation of the PEI coating changed the properties of the base membrane from hydrophobic to relatively hydrophilic; the nitrogens could be titrated, and the coating could not be washed out. 4,4′‐Azo‐bis(4‐cyanovaleryl chloride), a heat‐sensitive radical source, was incorporated into the coated membrane. The coated membrane, with incorporated azo compound, was heated to 75 °C in the presence of acrylic acid, 4‐(vinylpyridine), or styrene to give the corresponding grafted, pore‐filled membranes. The membranes exhibited hydraulic fluxes and pH valve effects consistent with molecular brushes grafted within the pores of the membranes. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 242–250, 2002     

光接枝法制备新型pH开关核孔膜

用表面光接枝方法制备了具有pH开关特性的丙烯酸接枝PET核孔膜 .XPS、SEM和AFM的表征结果证明 ,接枝膜具有非对称结构 ,丙烯酸接枝层主要在膜的向光侧表面 ,膜背侧和膜孔内没有变化 .膜孔周围的接枝链在湿态下沉入膜孔 ,接枝层对膜孔顶部产生封盖 .接枝链的体积随环境pH值的不同而变化 ,从而控制接枝膜的滤过特性 .在低接枝程度时 ,接枝链体积变化对膜孔径的影响小 ,不具有pH开关特性 ,但是接枝提高了膜的亲水性 ,通量得到提高 ;接枝程度达到一定程度时 (如 0 5 %) ,膜孔径受接枝链体积变化的影响显著 ,接枝膜表现出pH开关效应 .在pH大于 5时 ,接枝膜通量恒定 ,基本不受溶液pH值影响 .当pH值小于 4时 ,膜通量随溶液酸性的增加迅速升高 .接枝膜的开关幅度JpH =2 JpH =6 8,随接枝程度的增加而增加 .膜的通量和开关幅度可以通过接枝程度来调节     

Fluorescence correlation spectroscopy studies of diffusion of a weak polyelectrolyte in aqueous solutions

We apply fluorescent correlation spectroscopy (FCS) to investigate solution dynamics of a synthetic polyelectrolyte, i.e., a weak polycarboxylic acid in aqueous solutions. The technique brings single molecule sensitivity and molecular specificity to dynamic measurements of polyelectrolyte solutions. Translational diffusion of Alexa-labeled poly(methacrylic acid), PMAA*, chains was studied in very dilute, 10(-4) mg/ml, solutions as a function of solution pH and ionic strength. The observed changes in diffusion coefficients were consistent with about twofold expansion of PMAA* coils when pH was changed from 5 to 8, and with chain contraction for alkaline metal ion concentrations from 0.01 to 0.1 M. The dependence of the hydrodynamic size of PMAA* chains on the counterion type followed the sequence: Li(+)>Na(+) approximately equal to Cs(+)>K(+). The dependence of translational diffusion on polyacid concentration was weak at the low concentration limit, but chain motions were significantly slower at higher polymer concentrations when PMAA chains overlapped. Finally, measurements of dynamics of PMAA* chains in "salt-free" solutions showed that self-diffusion of PMAA* chains significantly slowed down when PMAA concentration was increased, probably reflecting the sensitivity of PMAA* translational motions to the onset of interchain domain formation. These results illustrate the utility of the FCS technique for studying hydrodynamic sizes of polyelectrolyte coils in response to variation in solution pH or concentration of salt and polyelectrolytes. They also suggest that FCS will be a promising technique for selective observation of the dynamics of polyelectrolyte components in complex polymer mixtures.     

Light Scattering Study of the Antibody–Poly(methacrylic acid) and Antibody–Poly(acrylic acid) Conjugates in Aqueous Solutions

The effect of the conformational state of the polymer coil on the properties of protein–polymer conjugates has been studied for the conjugates of antibody (monoclonal antibody from 6C5 clone against inactivated rabbit muscle glyceraldehyde‐3‐phosphate dehydrogenase; Ab) with poly(methacrylic acid) (PMAA) or poly‐(acrylic acid) (PAA). The pH‐dependencies of molecular properties and structural parameters of aqueous solutions (radius of gyration, intensity of scattered light, hydrodynamic diameter, and polydispersity index) of Ab, PMAA, and PAA and their conjugates, i. e., Ab‐PMAA and Ab‐PAA, have been studied using static and dynamic light scattering techniques. While free Ab aggregates in solution and precipitates at its isoelectric point, the covalent attachment of a charged polymer to Ab prevents its association and shifts the precipitation point towards more acidic values (from pH 5.95 for Ab to pH ˜ 4.8 for Ab‐PMAA). The predominant role of the conformational status of the polymer in the process of conjugate precipitation has been considered. Contrary to the precipitation of Ab‐PMAA, the formation of stable colloidal particles was suggested for Ab‐PAA at pH < 4.8. In the conjugates, polymer chains surround the protein globule in an extremely compact manner while Ab significantly affects the polymer conformation. The essentially larger hydrodynamic radii of conjugates, when compared with their radii of gyration, confirm the strong interaction of conjugates with solvent molecules.