Elaboration and characterisation of PDMS-HTiNbO5 nanocomposite membranes

Poly(dimethylsiloxane) (PDMS)-HTiNbO₅ nanocomposite membranes with various HTiNbO₅ nanofiller content were prepared by melt intercalation. WAXS diffraction measurements and TEM observations have suggested that the HTiNbO₅ mineral was exfoliated in the PDMS matrix. The influence of the filler in the membrane was evaluated by water diffusion, gas permeation (CO₂, N₂, O₂, ethane and ethylene), toluene pervaporation and by CO₂ sorption measurements.A filler content of only 2 wt.% in PDMS-HTiNbO₅ nanocomposite membranes slows down the water diffusion significantly, and a filler content of 5 wt.% reduces also the permeability of the films for toluene. The addition of a filler content up to 10 wt.% do not significantly influences the gas permeability (P) except for CO₂. The PDMS matrix appears to be highly permeable and, therefore, a decreasing effect on P is only marked for a very high HTiNbO₅ content. This effect is more pronounced for CO₂, the P value of which decreases by 80% when the amount of nanofiller is 40 wt.%. The sorption measurements show that the interaction between CO₂ and PDMS is weak (isotherms agree with Henry’s law). The filler decreases the solubility of CO₂ in the films (S = 7.94 × 10⁻³ and S = 5.44 × 10⁻³ cm³ STPcm⁻³ film cmHg⁻¹ for PDMS and PDMS-HTiNbO₅ 40 wt.%, respectively).     

Novel anion-exchange organic–inorganic hybrid membranes: Preparation and characterizations for potential use in fuel cells

A new series of supported anion-exchange organic–inorganic hybrid membranes were prepared by quaternizing the copolymer of vinylbenzyl chloride (VBC) and γ-methacryloxypropyl trimethoxy silane (γ-MPS) and then applying a sol–gel reaction to the copolymer and monophenyltriethoxysilane (EPh). The membranes were characterized for potential use in fuel cells. The results show that the physicochemical properties, including ion-exchange property, hydrophilicity, and thermal/chemical stability, can be easily controlled by adjusting the quaternization extent of the copolymer and the dosage of EPh. The hybrid membranes have relatively strong alkali resistance, high temperature tolerance (thermal degradation temperature in air, Td, in the range of 250–300 °C), high tensile strength (TS) and elongation at break (E_b). The hydroxyl ion conductivity is in the range of 2.27–4.33 × 10⁻⁴ S/cm.     

Anti-trade-off in dehydration of ethanol by novel PVA/APTEOS hybrid membranes

Novel organic–inorganic hybrid membranes were prepared through sol–gel reaction of poly(vinyl alcohol) (PVA) with γ-aminopropyl-triethoxysilane (APTEOS) for pervaporation (PV) separation of ethanol/water mixtures. The membranes were characterized by FTIR, EDX, WXRD and PALS. The amorphous region of the hybrid membranes increased with increasing APTEOS content, and both the free volume and the hydrophilicity of the hybrid membranes increased when APTEOS content was less than 5 wt%. The swelling degree of the hybrid membranes has been restrained in an aqueous solution owing to the formation of hydrogen and covalent bonds in the membrane matrix. Permeation flux increased remarkably with APTEOS content increasing, and water permselectivity increased at the same time, the trade-off between the permeation flux and water permselectivity of the hybrid membranes was broken. The sorption selectivity increased with increasing temperature, and decreased with increasing water content. In addition, the diffusion selectivity and diffusion coefficient of the permeants through the hybrid membranes were investigated. The hybrid membrane containing 5 wt% APTEOS has highest separation factor of 536.7 at 50 °C and permeation flux of 0.0355 kg m⁻² h⁻¹ in PV separation of 5 wt% water in the feed.     

Solid polymer electrolyte composite membranes for olefin/paraffin separation

Ethylene and propylene are produced in larger quantities than any other organic compound. Production of these olefins requires separation of the olefins from the corresponding paraffins. Distillation is currently used but this is an extremely energy-intensive process due to the very low relative volatility of the components. Previous studies have shown that facilitated transport membranes can have high selectivity for olefin/paraffin separation. However, four problems have limited the commercial application of facilitated transport membranes: (i) poor mechanical stability, (ii) the difficulty in preparing thin, high-flux composite membranes, (iii) the requirement of a water-vapor-saturated feed to provide mobility for the olefin-selective carrier, and (iv) poor chemical stability due to carrier poisoning. Solid polymer electrolytes are a novel class of facilitated transport membranes for olefin/paraffin separation. These membranes solve the first three problems listed above. Solid polymer electrolyte membranes are based on rubbery, polyether-based polymers containing a dissolved olefin-complexing metal salt. Solid polymer electrolyte composite membranes made from poly(ethylene oxide) loaded with silver tetrafluoroborate showed an ethylene/ethane selectivity of up to 240 and an ethylene permeance of 8×10⁻⁶ cm³(STP)/cm² s cmHg with a dry feed gas mixture.     

Plasticized microporous poly(vinylidene fluoride) separators for lithium‐ion batteries. II. Poly(vinylidene fluoride) dense membrane swelling behavior in a liquid electrolyte—characterization of the swelling kinetics

Some microporous poly(vinylidene fluoride) (PVdF) separators for lithium‐ion batteries, used in liquid organic electrolytes based on a mixture of carbonate solvents and lithium salt LiPF₆, were characterized by the study of the swelling phenomena on dense PVdF membranes. Various aspects of the kinetics of the carbonate solvents and the solvent mixture sorption in dense PVdF slabs were studied at different temperatures. Non‐Fickian behavior, characterized by S‐shaped sorption curves, was highlighted, and a salt effect, which resulted in two‐stage sorption, was studied. Diffusion coefficients and activation energies were calculated for the Fickian portions of the sorption curves, that is, at short times and low swelling ratios. A strong influence of the different interaction parameters was shown for the swelling kinetics. This study proved that the swelling of microporous PVdF membranes could be considered instantaneous. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 544–552, 2004     

Sorption characteristics and separation of tellurium ions from aqueous solutions using nano-TiO2

Titanium dioxide nanoparticles (nano-TiO₂) were employed for the sorption of Te(IV) ions from aqueous solution. A detailed study of the process was performed by varying the sorption time, pH, and temperature. The sorption was found to be fast, equilibrium was reached within 8 min. When the concentration of Te(IV) was below 40 mg L⁻¹, at least 97% of tellurium was adsorbed by nano-TiO₂ in the pH range of 1-2 and 8-9. The sorbed Te(IV) ions were desorbed with 2.0 mL of 0.5 mol L⁻¹ NaOH. The sorption data could be well interpreted by the Langmuir model with the maximum adsorption capacity of 32.75 mg g⁻¹ (20 ± 0.1 °C) of Te(IV) on nano-TiO₂. The kinetics and thermodynamics of the sorption of Te(IV) onto nano-TiO₂ were also studied. The kinetic experimental data properly correlated with the second-order kinetic model (k₂ = 0.0368 g mg⁻¹ min⁻¹, 293 K). The overall rate process appeared to be influenced by both boundary layer diffusion and intra-particle diffusion. The mean energy of adsorption was calculated to be 17.41 kJ mol⁻¹ from the Dubinin-Radushkevich (D-R) adsorption isotherm at room temperature. Moreover, the thermodynamic parameters for the sorption were estimated, and the ΔH⁰ and ΔG⁰ values indicated the exothermic and spontaneous nature of the sorption process, respectively. Finally, Nano-TiO₂ as sorbent was successfully applied to the separation of Te(IV) from the environmental samples with satisfactory results (recoveries >95%, relative standard deviations was 2.0%).     

Hydroxyethyl Starch-g-Poly-(N,N-dimethylacrylamide-co-acrylic acid): An efficient dye removing agent

Hydroxyethyl Starch-g-Poly-(N,N-dimethylacrylamide-co-acrylic acid) was synthesized by solution polymerization technique using potassium peroxydisulfate (K₂S₂O₈) as the initiator at 90 °C. The synthesized graft copolymer was characterized by FTIR, NMR (both ¹H and ¹³C) Spectroscopy, molecular weight determination by GPC, TGA/DTG and SEM analysis. Biodegradation study was carried out by enzymatic hydrolysis. The number of carboxylic acid groups incorporated into the polymer was calculated by measuring neutralization equivalent (N.E) of the graft copolymer titrimetrically. The synthesized graft copolymer was used as the adsorbent for the removal of Malachite green, a cationic dye from its aqueous solution. The operating variables studied were the amount of adsorbent, solution pH, contact time, temperature and the initial dye concentration. The adsorption data were used to fit in the pseudo-first order and pseudo-second order rate equation in order to investigate the sorption mechanism. Equilibrium isotherm was analyzed using the Langmuir and the Freundlich isotherms. In the present investigation it was found that the adsorption kinetics followed a pseudo second order kinetics for the studied dye concentration range. The negative value of free energy change indicates the spontaneous nature of the adsorption and also suggesting a chemisorption process.     

Polyvinylidenefluoride-hexafluoropropylene based nanocomposite polymer electrolytes (NCPE) complexed with LiPF3(CF3CF2)3

Merck KGaA, Germany recently tested a new electrolyte salt LiPF₃(CF₃CF₂)₃ (lithium fluoroalkyl phosphate (LiFAP)) for lithium ion power packs and suggested that it can be replaced with commercially used LiPF₆. LiFAP, for the first of its kind, was incorporated into polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) matrix with ethylene carbonate and diethyl carbonate mixtures as plasticizing agents and SiO₂ nanoparticles as filler. Nanocomposite polymer electrolyte (NCPE) membranes were prepared by solvent casting technique. All NCPE membranes were subjected to a.c. impedance, fluorescence and morphological studies. NCPE membranes containing 2.5 wt% of SiO₂ exhibited enhanced conductivity of 1.13 mS cm⁻¹ at ambient temperature. Molecular motion in the polymeric media was supported by fluorescence studies. The percentage of crystallinity and activation energy has also been calculated.     

Polyelectrolyte membranes prepared by dynamic self-assembly of poly (4-styrenesulfonic acid-co-maleic acid) sodium salt (PSSMA) for nanofiltration (I)

In recent years, the layer-by-layer (LBL) self-assembly of polyelectrolyte has attracted much attention for the preparation of nanofiltration (NF) membranes. However, most researchers focused on the homopolymers, few studied on the copolymers for the preparation of NF membranes. In the present work, a series of nanofiltration membranes were prepared by dynamic self-assembly of a copolymer polyelectrolyte containing both weakly and strongly ionized groups, poly (4-styrenesulfonic acid-co-maleic acid) sodium salt (PSSMA), with poly (allylamine hydrochloride) (PAH) and poly (styrenesulfonic acid sodium salt) (PSS) on the modified polyacrylonitrile (PAN) ultra-filtration membranes. The effects of substrate, deposition pH, SS/MA ratio in PSSMA, concentration of the PSSMA and bilayer number on the properties of the NF membranes were investigated. The results indicated that the performances of the NF membranes prepared by dynamic self-assembly process were superior to those prepared by the static self-assembly process. The membranes terminated with PSSMA were negatively charged. Due to the changes of charge density and conformation of PSSMA in different pH conditions, the [PAH/PSS]₁PAH/PSSMA membrane prepared at pH 2.5 showed higher Na₂SO₄ rejection and larger flux than those of the membrane prepared at pH 5.7. The NF membrane [PAH/PSS]₁PAH/PSSMA composed of only two bilayers exhibited 91.4% Na₂SO₄ rejection and allowed solution flux of 28.6 L/m² h at 0.2 MPa. The solution flux increased to 106.6 L/m² h at 0.8 MPa, meanwhile, no obvious decrease in Na₂SO₄ rejection was observed.     

PEG modified poly(amide-b-ethylene oxide) membranes for CO2 separation

In the present work, membranes from commercially available Pebax^® MH 1657 and its blends with low molecular weight poly(ethylene glycol) PEG were prepared by using a simple binary solvent (ethanol/water). Dense film membranes show excellent compatibility with PEG system up to 50 wt.% of content. Gas transport properties have been determined for four gases (H₂, N₂, CH₄, CO₂) and the obtained permeabilities were correlated with polymer properties and morphology of the membranes. The permeability of CO₂ in Pebax^®/PEG membrane (50 wt.% of PEG) was increased two fold regarding to the pristine Pebax^®. Although CO₂/N₂ and CO₂/CH₄ selectivity remained constant, an enhancement of CO₂/H₂ selectivity (∼11) was observed. These results were attributed to the presence of EO units which increases CO₂ permeability, and to a probable increase of fractional free-volume. Furthermore, for free-volume discussion and permeability of gases, additive and Maxwell models were used.     

Preparation and characterization of proton-conducting sulfonated poly(ether ether ketone)/phosphatoantimonic acid composite membranes

The solid proton conductor, phosphatoantimonic acid, HSbP₂O₈ · H₂O was prepared by ion exchange of the corresponding potassium salt. The composite membranes of SPEEK with up to 40 wt% of HSbP₂O₈ · H₂O were prepared by introducing the solid proton conductor from the aqueous suspension. The composite membranes were characterized using FT-IR, powder X-ray diffraction, SEM, DSC/TGA. Thermal stability of the composite membranes was slightly lower than that of SPEEK. The composite membranes had higher water uptake when compared with SPEEK and the membranes exhibited controlled swelling up to 50 °C. The proton conductivity of the membranes was measured under 100% relative humidity up to 70 °C. The composite membranes showed enhanced proton conductivity up to 20 wt% of HSbP₂O₈ · H₂O and the conductivity was reduced with further increase of HSbP₂O₈ · H₂O loading. A maximum of four-fold increase in proton conductivity at 70 °C was observed for the composite membrane with 20 wt% of solid proton conductor.     

Sorption and pervaporation properties of crosslinked membranes of poly(ethylene oxide imide) segmented copolymer to aromatic/nonaromatic hydrocarbon mixtures

Poly(ethylene oxide imide) segmented copolymer (PEO‐PI) membranes were crosslinked by the chemical reaction between ethylene glycol diglycidyl ether and benzylalcohol groups of diamine moieties in polyimide segments at high temperatures. Sorption and diffusion of penetrants took place in poly(ethylene oxide) segment microdomains. Sorption and desorption behavior of pure vapors such as benzene (Bz), cyclohexane (Cx) and n‐hexane (Hx) was classified as the Fickian diffusion. Sorption isotherms of binary liquid mixtures could be represented by the Flory–Rehner model, but the model overpredicted the sorption amounts of Cx and Hx, leading to small predictions of sorption selectivity α_S for Bz/Cx and Bz/Hx systems. UNIFAC‐FV model fairly well predicted the sorption amounts of aromatic hydrocarbons, but significantly overestimated those of nonaromatic ones, leading to too small predictions of α_S. Pervaporation (PV) behavior of PEO‐PI membranes was governed by sorption behavior followed by membrane swelling. Diffusion coefficient weakly depended on the minimum cross section of a penetrant. The diffusivity selectivity α_D hardly depended on the feed composition and was about 1.4 and 0.75 for Bz/Cx and Bz/Hx, respectively. PV selectivity α_PV was larger for Bz/Hx than for Bz/Cx because of larger α_S. PEO‐PI membranes displayed high specific permeation flux Ql and reasonably high α_PV for aromatic/nonaromatic hydrocarbons; for example, Ql = 60 Kg μm/(m² h) and α_PV = 8 for a feed mixture containing Bz, Tol, Hx, n‐Ot and i‐Ot of 20 wt % at 353 K. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1800–1811, 2000     

Dispersive micro-solid-phase extraction of benzodiazepines from biological fluids based on polyaniline/magnetic nanoparticles composite

In this study, diverse types of Fe₃O₄ nanocomposites modified by polyaniline, polypyrrole, and aniline–pyrrole copolymer were synthesized through chemical oxidative polymerization process for dispersive-μ-solid phase extraction (D-μ-SPE) in the presence of various dopants. The results showed that the nanocomposite modified by polyaniline with p-toluene sulfonic acid as a dopant demonstrated higher extraction efficiency for lorazepam (LRZ) and nitrazepam (NRZ). Also the synthesized magnetic sorbents were characterized. The nanocomposite sorbent in combination with high performance liquid chromatography–UV detection was applied for the extraction, preconcentration and determination of lorazepam and nitrazepam in urine and plasma samples. Different parameters influencing the extraction efficiency including: sample pH, amount of sorbent, sorption time, elution solvent and its volume, salt content, and elution time were optimized. The obtained optimal conditions were: sample pH, 6; amount of sorbent, 5 mg; sorption time, 5.0 min; elution solvent and its volume, 0.5 mM cethyltrimethyl ammonium bromide in acetonitrile, 150 μL; elution time, 2.0 min and without addition of NaCl. The calibration curves were linear in the concentration range of 1–2000 μg L⁻¹. The limits of detection (LODs) were achieved in the range of 0.5–1.8 μg L⁻¹ for NRZ and 0.2–2.0 μg L⁻¹ for LRZ, respectively. The percent of extraction recoveries and relative standard deviations (n = 5) were in the range of 84.0–99.0, 6.1–7.8 for NRZ and 90.0–99.0, 4.1–7.0 for LRZ, respectively. Ultimately, the applicability of the method was successfully confirmed by the extraction and determination of NRZ and LRZ in human urine and plasma samples.     

CO2 sorption in poly(ethylene terephthalate) above and below the glass transition

High-pressure CO₂ sorption data in semicrystalline poly(ethylene terephthalate) (PET) are presented for temperatures ranging from 25 to 115°C. The results are described by Henry's law above the glass-transition temperature of PET, while a dual-mode sorption model comprised of a Henry's law and a Langmuir isotherm applies in the glassy state. The disappearance of the Langmuir capacity of the polymer above T_g presumably results from the elimination of regions of localized lower density which are frozen into the glass upon quenching from the rubbery state. Exposure of PET to a high CO₂ pressure produced a systematic variation in the apparent sorption equilibria. Correlation of the Langmuir capacity of PET with the dilatometric parameters of the polymer provides a useful framework for understanding the origin of the Langmuir sorption mode and for interpreting annealing and conditioning effects in glassy polymers.     

Poly(vinyl pyrrolidone-co-vinyl acetate)–cellulose acetate blends as novel pervaporation membranes for ethanol–ethyl tertio-butyl ether separation

The design of high-performance pervaporation membranes for the selective removal of ethanol from ethyl t-butyl ether (ETBE) was performed by using the polymer blending method. Binary blends of cellulose acetate or cellulose triacetate with a specific copolymer, poly(vinyl pyrrolidone-co-vinyl acetate), were studied by pycnometry, differential scanning calorimetry, infrared spectroscopy, solvent–mixture sorption and pervaporation.The sorption extent and especially the permeability of the blend membranes to the ethanol–ETBE azeotropic mixture increases greatly with the copolymer content with quasi-constant and high selectivity. This behavior is attributed to the specific interactions of amide C=O groups (a strong Lewis-base) in the copolymer with ethanol. The resulting high-performance membranes were stable at low temperatures but showed some performance alteration, at temperatures exceeding 80°C, because of copolymer extraction by the solvent mixture. The different behaviors of the same membrane at high and low temperatures were explained in terms of copolymer chain reptation, which was possible in the rubbery state but not in the glassy state. A crosslinking of the two polymers via urethane bonds led to perfectly stable high-performance membranes for the target application. © 1997 John Wiley & Sons, Ltd.     

Polymeric complex membranes for olefin/paraffin separation

Semi-crystalline polyethylene (PE), rubbery silicone rubber (SR) and glassy poly[(l-trimethylsilyl)-l-propyne] (PTMSP) were modified for olefin/paraffin separation. The polymers were first grafted with the acrylic acid (AA) and then incorporated with silver ions for forming the complex membranes such as PE-g-AA-Ag⁺, SR-g-AA-Ag⁺ and PTMSP-g-AA-Ag⁺ The complex membranes were activated by glycerol solvation and subsequently showed high selectivity in olefin/paraffin separation. The silver ion distribution, the kinetics of olefin binding to PE-g-AA-Ag⁺, the gas permeation properties and the sorption behaviors were studied. A novel dry complex membrane for olefin/paraffin separation based on AgClO₄ complexing with PTMSP main-chain was also studied.     

The influence of inert gases on the high-pressure phase equilibria of EH-copolymer/1-hexene/ethylene-mixtures

The influence of the addition of several inert compounds on the phase equilibria of poly(ethylene-co-1-hexene) copolymer solutions in 1-hexene and ethylene was determined at the conditions of the industrial high-pressure copolymerization process. Using a high-pressure autoclave we measured cloud point pressures in dependence on temperature and mixture composition. The temperature was varied in the range of 393 to 493 K. We present the results for quasi-ternary mixtures consisting of a poly(ethylene-co-1-hexene) copolymer (EH), ethylene and an inert compound (helium, nitrogen, CO₂, methane, ethane, propane, n-butane). Also the quasi-quaternary system EH-copolymer/ethylene/1-hexene/nitrogen is discussed. Experiments were carried out at 1-hexene:ethylene mass ratios between 0 and 85:15. Helium, nitrogen, methane and carbon dioxide lowered the copolymer solubility. The antisolvent effect decreases in the order helium>nitrogen>methane>CO₂. In contrast, ethane, propane and especially n-butane were found to act as cosolvents.     

Evaluation of sorption of uranium onto metakaolin using X-ray photoelectron and Raman spectroscopies

Metakaolin prepared from a natural clay mineral ore of aluminium kaolinite is a promising low cost and high activity aluminosilicate material that has been investigated for studying the sorption behavior of uranium. Here, metakaolin was characterized using X-ray photoelectron spectroscopy (XPS) and the effects of pH, contact time and initial metal ion concentration on its sorption behavior were studied. The sorption process was found to initially be rapid (∼60% at time 0 min) but became slower with time; equilibrium was established within 24 h (∼80% sorption). The data were applied to study the kinetics of the sorption process. The Langmuir and Dubinin-Radushkevich (D-R) sorption isotherms were used to describe partitioning behavior for the system at room temperature. The binding of metal ions was found to be pH dependent, with optimal sorption occurring at pH 5. The retained metal ions were eluted with 5 mL of 0.1 M HNO₃. Raman spectroscopy and XPS were used to evaluate the sorption mechanism of U(VI).     

Controlled synthesis of AB2 amphiphilic triarm star-shaped block copolymers by ring-opening polymerization

This paper describes the synthesis of a novel amphiphilic AB₂ triarm star-shaped copolymer with A = non-toxic and biocompatible hydrophilic poly(ethylene oxide) (PEO) and B = biodegradable and hydrophobic poly(ε-caprolactone) (PCL). A series of AB₂ triarm star-shaped copolymers with different molecular-weights for the PCL block were successfully synthesized by a three-step procedure. α-Methoxy-ω-epoxy-poly(ethylene oxide) (PEO-epoxide) was first synthesized by the nucleophilic substitution of α-methoxy-ω-hydroxy-poly(ethylene oxide) (MPEO) on epichlorohydrin. In a second step, the α-methoxy-ω,ω′-dihydroxy-poly(ethylene oxide) (PEO(OH)₂) macroinitiator was prepared by the selective hydrolysis of the ω-epoxy end-group of the PEO-epoxide chain. Finally, PEO(OH)₂ was used as a macroinitiator for the ring-opening polymerization (ROP) of ε-caprolactone (εCL) catalyzed by tin octoaote (Sn(Oct)₂). PEO-epoxide, PEO(OH)₂ and the AB₂ triarm star-shaped copolymers were assessed by ¹H NMR spectroscopy, size exclusion chromatography (SEC) and MALDI-TOF. The behavior of the AB₂ triarm star-shaped copolymer in aqueous solution was studied by dynamic light scattering (DLS) and transmission electron microscopy (TEM).     

Attractive double-layer forces and charge regulation upon interaction between electrografted amine layers and silica

Chitosan (CS) and poly(acrylic acid) (PAA) were crosslinked by an ionic gelation method to form super absorbent polymers (SAPs). CS and PAA form amide bonds between the amino and carboxyl groups. The CS-PAA copolymers were synthetically engineered by varying the feed ratios of the prepolymer units. The copolymer materials possess tunable sorption and mucoadhesive properties with a backbone structure resembling proteinaceous materials. The sorption properties of the copolymers toward methylene blue (MB) in aqueous solution were studied using UV-Vis spectrophotometry at ambient pH and 295 K. The copolymers showed markedly varied interactions with MB, from physisorption- to chemisorption-like behavior, in accordance with their composition, surface area, and pore structure characteristics. The sorption isotherms were evaluated with the Sips model to provide estimates of the sorption properties. The sorbent surface area (271 and 943 m²/g) and the sorption capacity (Q_m = 1.03 and 3.59 mmol/g) were estimated for the CS-PAA copolymer/MB systems in aqueous solution.