Pervaporation of ethanol-water mixtures through temperature-sensitive poly(vinyl alcohol-g-N-isopropyacrylamide) membranes

Novel temperature-sensitive membranes have been synthesized by grafting poly(N-isopropyacrylamide) (poly(NIPAAm)) onto a poly(vinyl alcohol) (PVA) backbone using hydrogen peroxide-ferrous ion as initiator. Due to the grafting of poly(NIPAAm), the hydrophilic/hydrophobic balance and the polarity of the pendent groups within the membranes are modified. Significant temperature sensitivity of the grafted membranes is observed close to the LCST of linear poly(NIPAAm) in the pervaporation processes for ethanol-water separation. Both the pervaporation and sorption selectivities for water show a maximum value in the vicinity of 30–32°C for an ethanol content of 75 and 80%. The temperature sensitivity of the grafted membranes also depends on the ethanol concentration. The maxima of pervaporation and sorption selectivities disappear when the ethanol content is lower than 75% because the much larger degree of swelling reduces the size screening effect of the membranes.     

高稳定性化学交联聚乙烯醇/聚乙烯吡咯烷酮碱性固体电解质膜

碱性固体电解质膜的稳定性是影响其在电化学领域应用的一个重要因素.本文在前期研究工作的基础上,通过直接共混和化学交联修饰制备出了聚乙烯醇/聚乙烯吡咯烷酮(PVA/PVP)碱性聚合物电解质膜.采用傅里叶变换红外(FTIR)光谱、热重分析(TGA)、扫描电镜(SEM)和交流阻抗等方法详细考察了复合膜的分子结构、热稳定性、化学稳定性、氧化稳定性和尺寸稳定性.红外光谱结果表明,PVP成功地混入聚合物基体中,在1672cm-1处表现出来自于PVP第I带C襒O的强吸收峰.TGA结果表明,提高掺杂的KOH溶液浓度对PVA/PVP碱性膜的热稳定性没有明显影响.SEM分析结果表明,复合膜经高温、高浓度碱(80℃,10mol·L-1)处理后,其断面结构仍致密均匀,未出现类似小孔等膜降解情况,此时膜电导率(1.58×10-3S·cm-1)相比室温相同碱液时提高91.5%,表明PVA/PVP膜具有很好的耐碱化学稳定性.同时,PVA/PVP碱性膜表现出良好的抗氧化性,在60℃的3%和10%H2O2溶液中处理均没有观察到明显的质量损失,150h后仍能保持原膜质量的89%和85%.此外,由于膜内形成致密的内互交联网络结构,复合膜在水中800h之后也表现出很好的同向性和电导率稳定性.     

Gas transport property of polyallylamine–poly(vinyl alcohol)/polysulfone composite membranes

Polyallylamine (PAAm) was synthesized by free radical polymerization and characterized by Fourier transform infrared resonance (FT-IR) spectroscopy, hydrogen nuclear magnetic resonance (¹H NMR) spectroscopy and differential scanning calorimetry (DSC). The composite membranes were prepared by using PAAm–poly(vinyl alcohol) (PVA) blend polymer as the separation layer and polysulfone (PSF) ultrafiltration membranes as the support layer. The surface and cross-section morphology of the membrane was inspected by environmental scanning electron microscopy (ESEM). The gas transport property of the membranes, including gas permeance, flux and selectivity, were investigated by using pure CO₂, N₂, CH₄ gases and CO₂/N₂ gas mixture (20 vol% CO₂ and 80 vol% N₂) and CO₂/CH₄ gas mixture (10 vol% CO₂ and 90 vol% CH₄). The plots of gas permeance or flux versus feed gas pressure imply that CO₂ permeation through the membranes follows facilitated transport mechanism whereas N₂ and CH₄ permeation follows solution–diffusion mechanism. Effect of PAAm content in the separation layer on gas transport property was investigated by measuring the membranes with 0–50 wt% PAAm content. With increasing PAAm content, gas permeance increases initially, reaches a maximum, and then decreases gradually. For CO₂/N₂ gas mixture, the membranes with 10 wt% PAAm content show the highest CO₂ permeance of about 1.80 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ KPa⁻¹ and CO₂/N₂ selectivity of 80 at 0.1 MPa feed gas pressure. For CO₂/CH₄ gas mixture, the membranes with 20 wt% PAAm content display the highest CO₂ permeance of about 1.95 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ KPa⁻¹ and CO₂/CH₄ selectivity of 58 at 0.1 MPa feed gas pressure. In order to explore the possible reason of gas permeance varying with PAAm content, the crystallinity of PVA and PAAm–PVA blend polymers was measured by X-ray diffraction (XRD) spectra. The experimental results show an inverse relationship between crystallinity and gas permeance, e.g., a minimum crystallinity and a maximum CO₂ permeance are obtained at 20 wt% PAAm content, indicating that the possibility of increasing CO₂ permeance with PAAm content due to the increase of carrier concentration could be weakened by the increase of crystallinity.     

Modification and characterization of semi-crystalline poly(vinyl alcohol) with interpenetrating poly(acrylic acid) by UV radiation method for alkaline solid polymer electrolytes membrane

Semi-crystalline poly(vinyl alcohol) was modified by UV radiation with acrylic acid monomer to get interpenetrating poly(acrylic acid) modified poly(vinyl alcohol), PVAAA, membrane. The stability of various PVAAA membranes in water, 2 M CH₃OH, 2 M H₂SO₄, and 40 wt% KOH aqueous media were evaluated. It was found that the stability of PVAAA membrane is stable in 40 wt% KOH solution. The PVAAA membranes were characterized by differential scanning calorimetry, X-ray diffraction, and thermogravimetry analysis. These results show that (1) the crystallinity in PVAAA decreased with increasing the content of poly(acrylic acid) in the PVAAA membranes. (2) The melting point of the PVAAA membrane is reduced with increasing the content of poly(acrylic acid) in the membrane. (3) Three stages of thermal degradation were found for pure PVA. Compared to pure PVA, the temperature of thermal degradation increased for the PVAAA membrane. The various PVAAA membranes were immersed in KOH solution to form polymer electrolyte membranes, PVAAA-KOH, and their performances for alkaline solid polymer electrolyte were conducted. At room temperature, the ionic conductivity increased from 0.044 to 0.312 S/cm. The result was due to the formation of interpenetrating polymer chain of poly(acrylic acid) in the PVAAA membrane and resulting in the increase of charge carriers in the PVA polymer matrix. Compared to the data reported for different membranes by other studies, our PVAAA membrane are highly ionic conducting alkaline solid polymer electrolytes membranes.     

聚乙烯醇/聚乙烯吡咯烷酮碱性复合膜的制备及其性能

通过在不同浓度KOH溶液中进行掺杂,制备出了聚乙烯醇/聚乙烯吡咯烷酮(PVA/PVP)碱性聚合物电解质膜.详尽考察了膜的组成、微观结构、热稳定性、离子电导率和甲醇吸收率.结果表明,PVA与PVP两者具有较好的相容性,当m(PVA)∶m(PVP)=1∶0.5时,膜断面致密、均匀,未发生大尺度相分离.PVP的混入可以极大提高复合膜的电导率和热稳定性.当m(PVA)∶m(PVP)=1∶1时,复合膜的电导率可达2.01×10-3 S.cm-1.PVA/PVP/KOH膜的甲醇吸收率随温度的升高没有明显变化,100℃时其甲醇吸收率仅为同条件下Nafion 115膜的1/4.这表明该复合膜有望作为一种新型的碱性直接甲醇燃料电池用固体电解质膜且可提高膜的使用温度.     

Water-swollen cation-exchange membranes prepared using poly(vinyl alcohol) (PVA)/poly(styrene sulfonic acid-co-maleic acid) (PSSA-MA)

A water-swollen type of poly(vinyl alcohol) (PVA)/poly(styrene sulfonic acid-co-maleic acid) (PSSA-MA) cation-exchange membrane was prepared and characterized in terms of its electrochemical properties including ion-exchange capacity (IEC), electrical resistance, and transport number, etc. PVA/PSSA-MA membranes exhibited low electrical resistance and highly swelling property. In spite of 2–4 times higher water swelling ratio (WSR) than that of CMX (Tokuyama Corp., Japan), the transport number of the prepared membrane was comparable to that of the commercial membrane (t_n>0.93). Moreover, the electric resistance of PVA/PSSA-MA membrane was measured as low as 1.0–1.5 Ω cm². Further, in this study, interrelation between the membrane characteristics and crosslinking was investigated, and the result exhibited that the crosslinking degree is one of major factors affecting the ion transport through a water-swollen ion-exchange membrane (IEM).     

Poly(vinylalcohol)/poly(ethyleneglycol)/poly(ethyleneimine) blend membranes - structure and CO2 facilitated transport

Poly(vinylalcohol) (PVA)/poly(ethyleneimine) (PEI)/poly(ethyleneglycol) (PEG) blend membranes were prepared by solution casting followed by solvent evaporation. The effects of the blend polymer composition on the membrane structure and CO₂/N₂ permeation characteristics were investigated. IR spectroscopy evidenced strong hydrogen bonding interactions between amorphous PVA and PEI, and weaker interactions between PVA and PEG. DSC studies showed that PVA crystallization was partially inhibited by the interactions between amorphous PVA and PEI blend, in which PEG separated into nodules. The CO₂ permeability decreased with an increase in CO₂ partial pressure in feed gas, while the N₂ permeability remained constant. This result indicated that only CO₂ was transported by the facilitated transport mechanism. The CO₂ and N₂ permeabilities increased monotonically with the PEI content in the blend membranes, whereas the ideal selectivity of CO₂ to N₂ transport showed a maximum. When CO₂ is humidified, its permeability through the blend membranes is much higher than that of dry CO₂, but the change in permeability due to the presence of humidity is reversible.     

Poly(vinyl alcohol)/poly(methyl methacrylate) blend membranes for pervaporation separation of water + isopropanol and water + 1,4-dioxane mixtures

Pervaporation (PV) separation of water + isopropanol and water + 1,4-dioxane mixtures has been attempted using the blend membranes of poly(vinyl alcohol) (PVA) with 5 wt.% of poly(methyl methacrylate) (PMMA). These results have been compared with the plain PVA membrane. Both plain PVA and PVA/PMMA blend membranes have been crosslinked with glutaraldehyde in an acidic medium. The membranes were characterized by differential scanning calorimetry and universal testing machine. Pervaporation separation experiments have been performed at 30 °C for 10, 15, 20, 30 and 40 wt.% of feed water mixtures containing isopropanol as well as 1,4-dioxane. PVA/PMMA blend membrane has shown a selectivity of 400 for 10 wt.% of water in water + isopropanol feed, while for water + 1,4-dioxane feed mixture, membrane selectivity to water was 104 at 30 °C. For both the feed mixtures, selectivity for the blend membrane was higher than that observed for plain PVA membrane, but flux of the blend membrane was lower than that observed for the plain PVA membrane. Membranes of this study are able to remove as much as 98 wt.% of water from the feed mixtures of water + isopropanol, while 92 wt.% of water was removed from water + 1,4-dioxane feed mixtures at 30 °C. Flux of water increased for both the feed mixtures, while the selectivity decreased at higher feed water concentrations. The same trends were observed at 40 and 50 °C for 10, 15 and 20 wt.% of water mixtures containing isopropanol as well as 1,4-dioxane feed mixtures, which also covered their azeotropic composition ranges. Membrane performance was studied by calculating flux (J_p), selectivity (), pervaporation separation index (PSI) and enrichment factor (β). Permeation flux followed the Arrhenius trend over the range of temperatures investigated. It was found that by introducing a hydrophobic PMMA polymer into a hydrophilic PVA, the selectivity increased dramatically, while flux decreased compared to plain PVA, due to a loss in PVA chain relaxation.     

燃料电池用聚乙烯醇基碱性聚合物电解质膜的制备及其性能

通过在不同浓度KOH溶液中进行掺杂,制备出了聚乙烯醇(PVA)、聚乙烯醇/聚乙烯吡咯烷酮(PVA/PVP)和聚乙烯醇/聚乙二醇二甲醚(PVA/PEGDE)碱性聚合物电解质膜详细考察了膜的外观形貌、微观结构、热稳定性、离子电导率和化学稳定性等.结果表明,PVA与PVP以及PEGDE具有很好的相容性,所制备的复合膜断面致密...     

Degradation of polymer mixtures. IV. Blends of poly(vinyl acetate) with poly(vinyl chloride) and other chlorine-containing polymers

The degradation of the binary polymer blends, poly(vinyl acetate)/poly(vinyl chloride), poly(vinyl acetate)/poly(vinylidene chloride) and poly(vinyl acetate)/polychloroprene has been studied by using thermal volatilization analysis, thermogravimetry, evolved gas analysis for hydrogen chloride and acetic acid, and spectroscopic methods. For the first two systems named, strong interaction occurs in the degrading blend, but the polychloroprene blends showed no indication of interaction. In the PVA/PVC and PVA/PVDC blends, hydrogen chloride from the chlorinated polymer causes substantial acceleration in the deacetylation of PVA. Acetic acid from PVA destabilizes PVC but has little effect in the case of PVDC because of the widely differing degradation temperatures of PVA and PVDC. The presence of hydrogen chloride during the degradation of PVA results in the formation of longer conjugated sequences, and the regression in sequence length at high extents of deacetylation found for PVA degraded alone is not observed.     

Properties of a novel thermal sensitive polymer based on poly(vinyl alcohol) and its layer‐by‐layer assembly

Structurally modified poly(vinyl alcohol) (PVA) was prepared as novel thermally sensitive polymers by partially acetalyzing and/or ionizing the commercially available PVA. Their aqueous solutions experience completely reversible polymer aggregation and dissolution above and below the lower critical solution temperature (LCST), respectively. The LCST of a partially acetalyzed PVA (APVA) can be readily controlled by the degree of acetalysis or the molecular weight of the starting PVA. Introduction of a small amount of cationic group onto the APVA backbone increases the LCST obviously, while the LCST is highly sensitive to NaCl concentration. Then APVA and cationic APVA multilayers are assembled on rayon to make a thermal responsive fiber. The atomic force microscopy (AFM) images of the surface reveal the increment of roughness stimulated by temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Time-dependence of pervaporation performance for the separation of ethanol/water mixtures through poly(vinyl alcohol) membrane

To clarify the cause of time-dependent separation behavior, the pervaporation performance with operating time through pure poly(vinyl alcohol) (PVA) membrane and glutaraldehyde (GA) cross-linked PVA membranes was investigated. The results showed that the water concentration in the permeate for the air-side surface of the PVA membrane increased dramatically from 92.2 to 95.7% in about 110 min and then remained almost unchanged. However, the water selectivity for the glass-side surface did not change with operating time. Similar results were observed for the GA cross-linked PVA membranes. Furthermore, the contact angle of water on the air-side surfaces of those membranes decreased with the time of contact with the feed. These results revealed that this dynamic pervaporation process was mainly attributable to the reconstruction of hydroxyl groups at the air-side surfaces of PVA membranes in response to the change of their surrounding medium during pervaporation. The reconstruction at the glass-side surface of the membrane did not occur because of the preferential localization of hydroxyl groups at the interface between the membrane and the glass plate during film formation of PVA solution. The above conclusion was further confirmed by the following results. The water concentration in the permeate through PVA membranes with the air-side surface facing the feed reached equilibrium more quickly with increasing operation temperature or decreasing degree of cross-linking, which was consistent with the fact that the rate of surface reconstruction accelerated with the increase of temperature or the decrease of the degree of cross-linking.     

Crosslinking of poly(vinyl alcohol) using functionalized gold nanoparticles

We report on the preparation of a new class of polymer hydrogels obtained through the chemical crosslinking reaction of poly(vinyl alcohol) (PVA) and functionalized gold nanoparticles. Carboxylic group functionalized gold nanoparticles were synthesized, dispersed in a PVA matrix and allow to react with the hydroxyl groups of PVA at high temperature. FT-IR and swelling experiments carried out on the cross-linked samples confirmed that the crosslinking reaction took place. This is the first time, to our knowledge, that functionalized nanoparticles are used as chemical crosslinking agents.     

Fully-biodegradable poly(3-hydroxybutyrate)/poly(vinyl alcohol) blend films with compositional gradient

Fully-biodegradable bacterial poly(3-hydroxybutyrate) (PHB)/chemosynthetic poly(vinyl alcohol) (PVA) blend films with compositional gradient from one surface to the other surface of the films were prepared by a dissolution-diffusion technique. Three kinds of PVA samples, high- and low-molecular weight atactic PVA and highly syndiotactic PVA (s-PVA), were used in order to investigate the effects of molecular weight and tactic structure on the generation of compositional gradient. The solution of PHB in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), which is also a good solvent for PVA, was cast on the PVA film and then the solvent HFIP was evaporated. By selecting the optimum volume of solvent and the evaporation rate, the PHB/PVA blend film with compositional gradient was obtained. The formation of compositional gradient was confirmed by FT-IR microscopy and ATR-FT-IR analysis. The 50%/50% PHB/s-PVA blend film with a nearly ideal compositional gradient, that is, the composition of PHB (or PVA) in the film changing gradually from 100% at one surface to 0% at the other surface of the film was obtained by casting PHB/HFIP solution on to the s-PVA film. Positional dependence of the absorbance of C==O and OH stretching bands along the film thickness direction for the PHB/S-PVA cast films.     

Fabrication and characterization of anhydrous polymer electrolyte membranes based on sulfonated poly(vinyl alcohol) and benzimidazole

In the present study, a new type of chemically cross-linked polymer blend membranes consisting of poly(vinyl alcohol) (PVA), sulfosuccinic acid (SSA) and benzimidazole (BnIm), as a dopant, at different stoichometric ratios were prepared and used as proton conducting polymer electrolytes. The proton conductivities of the membranes were investigated as a function of blending composition and the temperature. TGA indicated that the blend polymers were thermally stable up to approximately 175°C; differential scanning calorimetry (DSC) results illustrated the homogeneity of the materials. The local chain flexibility of the host polymer increased with BnIm concentration. The methanol permeability values of the membranes were much lower than that of a Nafion-membrane. The proton conductivity of these materials increased with BnIm and SSA concentration and the temperature.     

Relaxation phenomena in poly(vinyl alcohol)/fumed silica affected by interfacial water

Interaction of poly(vinyl alcohol) (PVA) with fumed silica was investigated in the gas phase and aqueous media using adsorption, broadband dielectric relaxation spectroscopy (DRS), thermally stimulated depolarization current (TSDC), infrared spectroscopy, thermal analysis, and one-pass temperature-programmed desorption (OPTPD) mass-spectrometry (MS) methods. PVA monolayer formation leads to certain textural changes in the system (after suspension and drying) because of strong hydrogen bonding of the polymer molecules to silica nanoparticles preventing strong interaction between silica particles themselves. This strong interaction promotes associative desorption of water molecules at lower temperatures than in the case of silica alone. Interaction of PVA with silica and residual water leads to depression of glass transition temperature (T(g)). There are three types of dipolar relaxations at temperatures lower and higher than the T(g) value. A small amount of adsorbed water leads to significant conductivity with elevating temperature.     

Novel nanocomposite pervaporation membranes composed of poly(vinyl alcohol) and chitosan-wrapped carbon nanotube

Novel nanocomposite membranes (PVA–CNT(CS)) were prepared by incorporating chitosan-wrapped multiwalled carbon nanotube (MWNT) into poly(vinyl alcohol) (PVA). To further explore the intrinsic correlation between pervaporation performance and free volume characteristics, molecular dynamics simulation was first introduced to qualitatively analyze the contribution of carbon nanotube incorporation on improving free volume characteristics of the nanocomposite membranes. Secondly, the pervaporation performance of PVA–CNT(CS) nanocomposite membranes was investigated using permeation flux and separation factor as evaluating parameters. For benzene/cyclohexane (50/50, w/w) mixtures at 323 K, permeation flux and separation factor of pure PVA membrane are only 20.3 g/(m² h) and 9.6, respectively, while the corresponding values of PVA–CNT(CS) (CNT content: 1%) nanocomposite membrane are 65.9 g/(m² h) and 53.4. In order to explain the simultaneous increase of permeation flux and separation factor, as well as to check the calculation reliability of molecular dynamics simulation, positron annihilation lifetime spectroscopy (PALS) analysis was employed.     

Alkaline Zn-air and Al-air cells based on novel solid PVA/PAA polymer electrolyte membranes

High ionic conducting solid polymer electrolyte membranes (SPEM) had been successfully prepared from poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA). The solution casting method yielded highly hydrophilic membranes with uniform structure that were suitable for electrochemical applications. The room temperature ionic conductivity of the alkaline PVA/PAA polymer electrolyte membranes was in the range of 0.142–0.301 S cm⁻¹ depending on the composition. The cyclic voltammetry analysis was carried out using Zn|SPEM|Zn and Al|SPEM|Al cells. The analysis results revealed the excellent electrochemical stability of these newly developed alkaline solid PVA/PAA polymer electrolyte membranes. Metal-air fuel cells were also prepared from the alkaline solid PVA/PAA polymer electrolyte membranes. The electrochemical cell performance was evaluated based on Zn-air and Al-air cells at C/10 and C/5 discharge rates. The experimental results exhibited high percent of utilization for metal powders at room temperature. It was up to 90% for Zn-air cell when assembled with PVA:PAA = 10:7.5 polymer electrolyte membrane and discharged at C/10 rate. The power density could be as high as 50 mW cm⁻² at room temperature. However, the cell percent utilization was reduced to 73% with the same composition electrolyte membrane when C/5 discharge rate was tested.     

Heterotactic poly(vinyl alcohol)

Bulky substituents in vinyl trialkylsilyl ethers and vinyl trialkylcarbinyl ethers led to heterotactic polymers (H = 66%). The polymers were converted into poly(vinyl alcohol) (PVA) and further to poly(vinyl acetate), and tacticity was determined as poly(vinyl acetate). Vinyl triisopropylsilyl ether in nonpolar solvents yielded a heterotactic polymer with a higher percentage of isotactic triads than syndiotactic triads (Hetero-I). Vinyl trialkylcarbinyl ethers in polar solvents gave a heterotactic polymer with more syndiotactic triads than isotactic (Hetero-II). Heterotactic PVA was soluble in water and showed characteristics infrared absorptions. Interestingly, Hetero-I PVA showed no iodine color reaction, but Hetero-II showed a much more intense color reaction than a commercial PVA. The mechanism of heterotactic propagation was discussed in terms of the Markóv chain model.     

Synthesis and characterization of bisulfonated poly(vinyl alcohol)/graphene oxide composite membranes with improved proton exchange capabilities

Composite membranes based on poly(vinyl alcohol) (PVA) and graphene oxide (GO) were prepared by solution-casting method to be used as proton exchange membranes (PEMs) in fuel cell (FC) applications. Bisulfonation was employed as a strategy to enhance the proton conductivity of these membranes. First, a direct sulfonation of the polymer matrix was accomplished by intra-sulfonation of the polymer matrix with propane sultone, followed by the inter-sulfonation of the polymer chains using sulfosuccinic acid (SSA) as a crosslinking agent. Furthermore, the addition of graphene oxide (GO) as inorganic filler was also evaluated to enhance the proton-conducting of the composite membranes. These membranes were fully characterized by scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and tensile tests. Besides, the proton conductivity of these membranes in a fully hydrated state was also analyzed by electrochemical impedance spectroscopy (EIS). The effect of the intra- and inter-sulfonation of the polymer matrix on the structural, morphological, thermal and mechanical properties of the membranes were determined. Increasing the density of sulfonic acid groups in the membranes resulted in a trade-off between a better proton conductivity (improving from 0.26 to 1.00 mS/cm) and a decreased thermal and mechanical stability. In contrast, the incorporation of GO nanoparticles into the polymer matrix improved the thermal and mechanical stability of both bisulfonated composite membranes. The proton conductivity appreciably increased by the combination of bisulfonation and introduction of GO nanoparticles into the polymer matrix. The sPVA/30SSA/GO composite membrane exhibited a proton conductivity of 1.95 mS/cm at 25 °C. The combination of the GO nanoparticles with the chemical bisulfonation approach of PVA allows thus assembling promising proton exchange membrane candidates for fuel cell applications.