Preparation and Characterization of Poly(Vinyl Alcohol) (PVA)/SiO2, PVA/Sulfosuccinic Acid (SSA) and PVA/SiO2/SSA Membranes: A Comparative Study

Abstract

New organic–inorganic nanocomposites based on PVA, SiO₂ and SSA were prepared in a single step using a solution casting method, with the aim to improve the thermomechanical properties and ionic conductivity of PVA membranes. The structure, morphology, and properties of these membranes were characterized by Raman spectroscopy, small- and wide-angle X-ray scattering (SAXS/WAXS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), water uptake (Wu) measurements and ionic conductivity measurements. The SAXS/WAXS analysis showed that the silica deposited in the form of small nanoparticles (～ 10?nm) in the PVA composites and it also revealed an appreciable crystallinity of pristine PVA membrane and PVA/SiO₂ membranes (decreasing with increasing silica loading), and an amorphous structure of PVA/SSA and PVA/SSA/SiO₂ membranes with high SSA loadings. The thermal and mechanical stability of the nanocomposite membranes increased with the increasing silica loading, and silica also decreased the water uptake of membranes. As expected, the ionic conductivity increased with increasing content of the SSA crosslinker, which is a donor of the hydrophilic sulfonic groups. Some of the PVA/SSA/SiO₂ membranes had a good balance between stability in aqueous environment (water uptake), thermomechanical stability and ionic conductivity and could be potential candidates for proton exchange membranes (PEM) in fuel cells.     

SAXS cluster structure and properties of sPEEK/PEI composite membranes for DMFC applications

Sulfonated poly(ether ether ketone)/polyethyleneimine (sPEEK/PEI) composite membranes were prepared to reduce the water uptake and methanol permeability of highly sulfonated PEEK membranes (> 65%). Incorporation of small amounts of PEI reduced ionic cluster size via electrostatic complex formation between anionic sulfonic groups of the sPEEK and the cationic amine groups of the PEI, and thus affected membrane properties considerably. Ion cluster size decreased with increasing PEI concentration by small angle X-ray scattering pattern. Addition of 1 wt.% of PEI resulted in reduction of water uptake and methanol permeability by 30% at 60 °C and 85% at room temperature, respectively. The thermal and mechanical stabilities were also enhanced by formation of physical cross-linking induced by electrostatic interactions between acid/base polymers. Although proton conductivity was also reduced by PEI incorporation as a part of the sulfonic acid groups involved in ionic complex formation, its effect on proton conductivity was not as strong as on methanol permeability.     

PVA–APTEOS/TEOS Hybrid Sol–Gel Pervaporation Membrane for Dehydration of Ethanol

Novel organic–inorganic hybrid membranes were prepared by co-hydrolysis and co-condensation reactions of aminopropyltriethoxysilane (APTEOS) and tetraethylorthosilicate (TEOS) in poly(vinyl alcohol) (PVA) aqueous solution. These sol–gel reactions make a silica nanoparticles network which cross-links the structure of membranes. As a result, swelling properties of the membranes change due to these cross-linking reactions. The relative molar content of APTEOS to (APTEOS + TEOS) was changed from 0.0 to 0.75. The effect of APTEOS to (APTEOS + TEOS) on morphology as well as pervaporation performance of the membranes for dehydration of ethanol was investigated. It was found out that the hybrid membranes with a certain mass ratio of 1:1 (APTEOS + TEOS to PVA) exhibit higher permselectivity and more permeation flux for dehydration of ethanol aqueous mixtures compared with the PVA–TEOS membranes. Because hydrolysis and condensation reactions of mixed silanes make smaller silica nanoparticles, the permeability properties of the membranes improve in comparison with those of the membranes prepared using a single silane.     

Synthesis and characterization of radiation crosslinked nano silver- polyvinyl alcohol/polyethylene oxide composites

Poly (vinyl alcohol)/poly (ethylene oxide) (PEO/PVA) blends were modified by gamma irradiation in the presence of acrylic acid (AAc) monomer. The modified PVA/PEO blends were then complexed with silver nitrate salt and lithium trifluoromethanesulfonate. Transmission electron microscopy was used to determine the distribution as well as the particle size of the silver nanoparticles (NP) formed in the matrix. The UV–vis absorbance spectra of the prepared grafted nanocomposite membranes confirmed the formation of Ag NP based on their surface plasmon band at 438?nm. The electrical properties of the blended electrolyte polymer films were characterized and discussed.     

Fluorescence Study on the Structure of Ionic Liquid Aggregates in Aqueous Solutions

Although ionic liquids are a relatively novel class of materials, it is well documented that they form micelles through aggregation of cation aliphatic tails. However, anion self-assembly has not yet been reported. In this study, we analyzed the intrinsic fluorescence of p-toluenesulfonate groups (tosylate) as part of the ionic liquid 1-ethyl-3-methylimidazolium tosylate ([emim][TOS]) and p-toluenesulfonic acid (pTSA), in aqueous solution. pTSA was found to have overlapping monomer and excimer emissions for chromophore concentrations from 10⁻³ to 1 M, whereas [emim][TOS], in the same conditions, showed monomer emission slightly broadened by much weaker excimer emission. These different photophysical behaviors of the same chromophore in the two compounds are explained by the formation of ion pairs by [emim][TOS], which can also be inferred from the loss of vibrational structure of the absorption spectra with respect to pTSA. Despite this different behavior regarding ion pairing, anion aggregation was observed in the excitation spectra of both pTSA and [emim][TOS]. While the absorption spectra corresponded to single chromophores, the excitation spectra changed from those characteristic of a single chromophore (below 10⁻³ M) to red-shifted narrow bands (above 0.1 M) typical of J aggregates. Between those concentrations, the excitation spectra split into blue- and red-shifted bands with relative intensities that changed with concentration as the chromophores rearranged in their clusters from head-to-head to head-to-tail aggregates. Differences between the absorption and excitation spectra were ascribed to aggregation-induced fluorescence enhancement.     

新型BrФnsted酸性离子液体的合成与表征

制备了五种对水稳定性好、带-SO3H官能团的磺酸类BrФnsted酸离子液体,用核磁共振(NMR)、红外光谱(IR)、电喷雾质谱(ESI-MS)、热重分析(TG)等表征手段对制备的离子液体进行了表征。结果表明,制备的离子液体与预期设计的结构一致,离子液体纯度大于95%;热重分析发现离子液体具有高的热稳定性和较宽的液态范围,其分解温度均高于300℃;五种离子液体均存在四种离子存在形式,H 可以单独以离子形式存在,并不是通常所认为的仅有两种离子存在形式。另外,研究了离子液体在常用溶剂中的溶解性,发现制备的离子液体易溶于水、甲醇,不溶于乙醚、甲苯和乙酸乙酯。     

Study of Electrical Conductivity of Para-Toluene Sulfonic Acid Doped Polyaniline/Polyester-Based Polyurethane Blends in the S-Band Frequency Range

Conducting polymer blends were prepared using polyaniline doped with para- toluene sulfonic acid (PTSA-PANI) and a polyester polyol-based polyurethane (PU). The morphological, thermal and dielectric properties of the PTSA-PANI/PU blends in the frequency range of 1–5 GHz (S band) were investigated. It was found that the morphology of the samples was affected by the PTSA-PANI loading, resulting in the formation of agglomerates and pathways when above 10 wt%. The thermal stability of the composites was improved with increased PTSA-PANI loading. The electrical conductivity percolation threshold was obtained at 2.5% of PTSA-PANI loading and the electrical conductivity reached the value of 0.13 S/m at a PTSA-PANI loading of 30 wt%. The obtained results for the PTSA-PANI/PU blends prepared indicate a high potential for their successful use in electrical and electromagnetic applications.     

Fabrication and characterization of polymer blends consisting of cationic polyallylamine and anionic polyvinyl alcohol

Thin sheets of polyallylamine (PAAm) and polyvinyl alcohol (PVA) blend were prepared by employing solution casting technique for potential membranes application. The blends were characterized by Fourier transform infrared (FTIR) and ¹H-NMR spectroscopy, scanning electron microscopy, thermogravimetric analysis, ultraviolet–visible spectroscopy, X-ray diffraction, and mechanical properties. The zeta potential, conductivity and rheological properties of PAAm/PVA blends were also studied. The FTIR spectrum reveals that the C–H asymmetric stretching vibration band of PVA at 2,928 cm^?1 disappeared in all the blend samples. Thermal stability of the blend membrane was better than pure polymers. The crystallinity of the PAAm/PVA blends was decreased, which may be due to the entanglement of PAAm in to PVA chains, which is also responsible for the improvement in the mechanical properties of the blends. Zeta potential decreases where as the conductivity increases as a function of temperature. Hydrophilicity is improved by addition of PVA to PAAm, which may be due to hydroxyl group of PVA. The blend solution shows non-Newtonian character of the liquid. By applying shear stress, increase in the effect of rarefaction was observed. The knowledge about the investigated parameters will be of vital importance for use of the blended material in membrane applications, especially where CO₂ separation is in focus. The membrane performance (separation properties) of the PAAm/PVA blended material is, however, not reported in the current article.     

Preparation of poly(vinyl alcohol)–silicone based hybrid membranes for the pervaporation separation of water–acetic acid mixtures

Hybrid membranes were prepared using poly(vinyl alcohol) (PVA) and tetraethylorthosilicate (TEOS) via hydrolysis followed by condensation. The obtained membranes were characterized by Fourier transform infrared spectroscopy, wide-angle x-ray diffraction and differential scanning calorimetry. A remarkable decrease in degree of swelling was observed with increasing TEOS content in membranes and is attributed to the formation of hydrogen bonding and covalent bonds in the membrane matrix. The pervaporation performance of these membranes for the separation of water–acetic acid mixtures was investigated in terms of feed concentration and the content of TEOS used as cross-linking agent. The membrane containing 1 : 2 mass ratio of PVA and TEOS gave the highest separation selectivity of 1116 with a flux of 3.33 × 10^?2kg/m²h at 30°C for 10 mass% of water in the feed. Except for membrane M-1, the observed values of water flux are close to the values of total flux in the investigated composition range, signifying that the developed membranes are highly water selective. From the temperature dependence of diffusion and permeation values, the Arrhenius apparent activation parameters have been estimated. The resulting activation energy values obtained, showing that water permeation is lower than that of acetic acid, suggest that the membranes have higher separation efficiency. The activation energy values calculated for total permeation and water permeation are close to each other for all the membranes except membrane M-1, signifying that coupled-transport is minimal because of the higher selective nature of membranes. The negative heat of sorption values (ΔH _s) for water in all the membranes suggests a Langmuir mode of sorption.     

Characterization of Poly(Vinyl Alcohol) Membranes Cross-Linked by Aldehyde-Substituting Cyclotriphosphazene

A new type of cross-linker, based on cyclotriphosphazene with six aldehyde groups, was used for the cross-linking of poly(vinyl alcohol) membranes. FTIR-ATR analysis indicated that cyclophosphazene reacted with poly(vinyl alcohol) by forming C–O–C bonds. TGA and DTG analysis showed that cross-linking improved the thermal stability. The swelling degree and pervaporation properties of cross-linked PVA membranes were also characterized. With increasing cross-linker concentration, swelling degrees and flux decrease while separation factors increase. Compared with PVA membranes cross-linked by glutaraldehyde, PVA membranes cross-linked by cyclophosphazene exhibited better selectivity and permeation rate.     

Proton-conducting composite membranes from graft copolymer electrolytes and phosphotungstic acid for fuel cells

New organic–inorganic composite membranes based on poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-poly(styrene sulfonic acid) [P(VDF-co-CTFE)-g-PSSA] with embedded phosphotungstic acid (PWA) were prepared. Fourier transform infrared spectra indicated the existence of a specific interaction between P(VDF-co-CTFE)-g-PSSA graft copolymer and PWA particles. PWA nanoparticles were well confined in the polymeric matrix up to 20 wt.%, above which they started to be extracted from the matrix, as revealed by scanning electron microscope analysis. Accordingly, Young’s modulus of membranes also increased with PWA concentration up to 20 wt.%, above which it continuously decreased. Upon incorporation of PWA nanoparticles, the proton conductivity of composite membranes slightly decreased from 0.042 to 0.035 S/cm at room temperature up to 20 wt.%, presumably due to strong interaction between the sulfonic acids of graft copolymer and PWA nanoparticles. The characterization by thermal gravimetric analysis demonstrated the enhancement of thermal stabilities of the composite membranes with increasing concentration of PWA.     

Copper-containing polyvinyl alcohol composite systems: Preparation,characterization and biological activity

The present investigation reports, the complex formation of Cu(II) with polyvinyl alcohol (PVA) and the synthesis of PVA-stabilized Cu₂O particles. This PVA–Cu₂O composite has been prepared via chemical reduction method using PVA–Cu(II) complex as precursor. At first, Cu(II) ions were stabilized in PVA matrix via complex formation with OH groups; subsequently, this PVA–Cu(II) macromolecular complex as precursor reacted with ascorbic acid as reducing agent at pH=12 to prepare PVA–Cu₂O composite. The products were characterized by FTIR, XRD, FE-SEM, HRTEM, Visible Spectroscopy and atomic absorption. In the following, the antibacterial properties of as-prepared composites were examined against Gram-positive (Bacillus thuringiensis) and Gram-negative bacteria (Escherichia coli), and the results showed excellent antibacterial activity of these materials.     

Investigations on electrical properties of poly(vinyl alcohol) doped with 1-methyl-3-n-decyl-imidazolium bromide ionic liquid

Thin films of poly(vinyl alcohol) (PVA) that are 100–500 μm thick were prepared by solution casting method. Various ratios of 1-methyl-3-n-decyl-imidazolium bromide ionic liquid [MDIM]⁽⁺⁾Br^(?), were used as dopants (plasticizers) to control the conductivity of the PVA thin films. Fourier transform infrared spectroscopy (FTIR) was used to indicate the detailed interaction of PVA with proton of the dopant in the blends. Ac impedance spectroscopy was used to investigate the impedance of the films within a frequency range of 10–10⁶ Hz as a function of temperature between 298 and 425 K. Each film with a precise doping concentration was sandwiched between two stainless–steel electrodes. The results showed that the electrical conductivity can be engineered by controlling the [MDIM]⁽⁺⁾Br^(?) doping concentration. Therefore, those films have potential to be used in flexible and cheap organic device applications.     

Surface studies of radiation grafted sulfonic acid membranes: XPS and SEM analysis

PTFE-g-polystyrene sulfonic acid membranes prepared by radiation-induced graft copolymerization of styrene onto poly(tetrafluoroethylene) (PTFE) films followed by sulfonation reactions were investigated with respect to their morphology and surface chemical properties. The chemical composition of the membranes surfaces was monitored at various degrees of grafting using X-ray photoelectron spectroscopy (XPS). Considerable differences in the concentration of the chemical components of the surfaces were observed despite the predominance of all membranes surfaces by a hydrocarbon fraction originated from the incorporated sulfonated polystyrene grafts. The distribution of the sulfonated polystyrene grafts in membranes having various degrees of grafting was investigated by scanning electron microscopy (SEM). The membranes achieved a homogenous distribution at degrees of grafting of 24% and above. The results of this work suggest that the membranes have a chemically sensitive surfaces and this is most likely to have an impact on their interfacial properties and chemical stability.     

Comparative study of the electrical and dielectric properties of PVA–PEG–Al2O3–MI (M=Na,K, Ag) complex polymer electrolytes

Plasticized nanocomposite polymer electrolytes (PNCPEs) based on poly(vinyl alcohol) (PVA)–MI (M=Na, K, Ag) dispersed with nanofillers Al₂O₃ and plasticized with poly(ethylene glycol) (PEG) have been prepared by solution cast technique. The structural properties of the samples have been characterized using various experimental techniques such as XRD, DSC, FTIR and B-G spectroscopy. The ionic conductivity and dielectric constant of the samples have been estimated using a LCZ meter in a wide temperature range, i.e. from 30 to 170 °C. It has been observed that the presence of water molecules in polymer electrolytes significantly affects the mobility of ionic species. The temperature dependent ionic conductivity shows the Arrhenius type behavior separated in three distinct regions. The ionic transference number for all PNCPE samples is found to be ≈1, which strongly suggests their ionic nature.     

Ecologically friendly xanthan gum-PVA matrix for solid polymeric electrolytes

Two water-soluble and biodegradable polymers: xanthan gum (XG) and poly(vinyl alcohol) (PVA) were used to synthesize ecologically friendly solid polymer electrolyte (SPE) matrices. While XG is a natural polymer, PVA is a synthetic one, but both are colorless and form transparent membranes. To obtain ionic conductivity properties, the samples were doped with acetic acid and characterized by electrochemical impedance spectroscopy (EIS), X-ray diffraction, UV-Vis spectroscopy, and tensile test. The best results of ionic conductivity of 1.97 × 10^?4 and 7.41 × 10^?4 S/cm at room temperature and 80 °C, respectively, were obtained for the sample containing 55 wt% of acetic acid. Moreover, this electrolyte was found to be predominantly amorphous with transmittance in the visible region of 80% and absorbance values below 0.5 between 240 and 375 nm. Tensile test of this sample, applied up to 18 N of maximum force, resulted in strain of 2322% and Young’s modulus of 0.02 MPa. The obtained results showed that these new eco-friendly materials are promising for use as electrolytes in electrochemical devices.     

The influence of sulfonated polyethersulfone (SPES) on surface nano-morphology and performance of polyethersulfone (PES) membrane

In this research, firstly sulfonation of polyethersulfone (PES) was carried out and then polyethersulfone (PES)/sulfonated polyethersulfone (SPES) blend membranes were prepared with phase inversion induced by immersion precipitation technique. polyvinylpyrrolidone (PVP, 2 wt% concentration) was added in the casting solution as pore former. SPES was characterized by FT-IR and UV-visible spectra, ion exchange capacity and swelling ratio. The characterization of SPES polymer indicates that the sulfonic acid groups were produced on PES polymer. Also, the prepared PES/SPES blend membranes were characterized by contact angle, AFM, SEM and cross-flow filtration for milk concentration. The contact angle measurements indicate that the hydrophilicity of PES membrane is enhanced by increasing the SPES content in the casting solution. The SEM and AFM images show that the addition of SPES in the casting solution results in a membrane with larger surface pore size and higher sub-layer porosity. The mean pore size of the membrane increased from 98 nm for PES membrane to 240 and 910 nm for 50/50 and 0/100 PES/SPES blend membranes, respectively. The pure water flux and milk water permeation through the prepared membranes are increased by blending PES with SPES. Moreover, the protein rejection of PES/SPES blend membranes was lower than PES membrane.     

Conductivity studies of plasticized proton conducting PVA–PVIM blend doped with NH4BF4

The proton conducting solid-state polymer electrolyte comprising blend of poly(vinyl alcohol) (PVA) and poly(N-vinylimidazole) (PVIM), ammonium tetrafluoroborate (NH₄BF₄) as salt, and polyethylene glycol (PEG) (molecular weight 300 and 600) as plasticizer is prepared at various compositions by solution cast technique. The prepared films are characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy analysis. The conductivity–temperature plots are found to follow an Arrhenius nature. The conductivity of solid polymer electrolytes is found to depend on salt and plasticizer content and also on the dielectric constant value and molecular weight of the plasticizer. Maximum ionic conductivity values of 2.20?×?10^?4 and 1.28?×?10^?4?S?cm^?1 at 30 °C are obtained for the system (PVA–PVIM)?+?20 wt.% NH₄BF₄?+?150 wt.% PEG300 and (PVA–PVIM)?+?20 wt.% NH₄BF₄?+?150 wt.% PEG300, respectively. The blended polymer, complexed with salt and plasticizer, is shown to be a predominantly ionic conductor. The proton transport in the system may be expected to follow Grotthuss-type mechanism.     

Ultrasound-assisted solvent-free synthesis of lactic acid esters in novel SO3H-functionalized Brønsted acidic ionic liquids

Mild and efficient Fischer esterification reactions of lactic acid with a variety of straight chain aliphatic alcohols, cyclohexanol and benzyl alcohol were successfully performed using two novel Brønsted acidic ionic liquids that bear an aromatic sulfonic acid group on the imidazolium or pyridinium cation under ultrasound irradiation. These reactions carried out smoothly with good to excellent conversion rate (78–96%) and satisfactory yields (73–92%) in shorter reaction time (4–6 h) at room temperature when the amount of ionic liquids was 20 mol%. These ionic liquids could be recovered readily and recycled five times without any significant loss in their catalytic activity.     

Synthesis and impedance analysis of proton-conducting polymer electrolyte PVA:NH4F

An attempt has been made to prepare a new proton-conducting polymer electrolyte based on poly(vinyl alcohol) doped with ammonium fluoride (NH₄F) by solution casting technique. The complex formation between polymer and dissociated salt has been confirmed by X-ray diffraction and Fourier transform infrared spectroscopy studies. The highest ionic conductivity has been found to be 6.9?×?10^?6?Scm^?1 at ambient temperature (303 K) for 85PVA:15NH₄F polymer electrolyte. The conductance spectra contain a low frequency plateau region and high frequency dispersion region. The dielectric spectra exhibit the low frequency dispersion, which is due to space charge accumulation at the electrode–electrolyte interface. The modulus spectra indicate non-Debye nature of the material. The highest ionic conductivity polymer electrolyte 85PVA:15NH₄F has low activation energy 0.2 eV among the prepared polymer electrolytes.