Post-irradiation time effects on the graft of poly(ethylene-alt-tetrafluoroethylene) (ETFE) films for ion exchange membrane application

Grafting of styrene followed by sulfonation onto poly(ethylene-alt-tetrafluoroethylene) (ETFE) was studied for synthesis of ion exchange membranes. Radiation-induced grafting of styrene onto ETFE films was investigated after simultaneous irradiation (in post-irradiation condition) using a ⁶⁰Co source. The ETFE films were irradiated at 20 kGy dose at room temperature and chemical changes were monitored after contact with styrene for grafting. The post-irradiation time was established at 14 days when the films were remained in styrene/toluene 1:1 v/v. After this period the grafting degree was evaluated in the samples. The grafted films were sulfonated using chlorosulfonic acid and 1, 2-dichloroethane 20:80 (v/v) at room temperature for 5 h. The membranes were analyzed by infrared spectroscopy (FTIR), differential scanning calorimeter (DSC), thermogravimetric measurements (TG) and degree of grafting (DOG). The ion exchange capacity (IEC) of membranes was determined by acid–base titration and the values for ETFE membranes were achieved higher than Nafion^® films. Preliminary single cell performance was made using pure H₂ and O₂ as reactants at a cell temperature of 80 °C and atmospheric gas pressure. The fuel cell performance of ETFE films was satisfactory when compared to state-of-art Nafion^® membranes.     

Radiation-induced grafting of perfluorinated vinyl ether into fluorinated polymer films

We report herein the first successful grafting of perfluorinated vinyl ether monomer into base polymer films by simultaneous radiation method. 2-Bromotetrafluoroethyl trifluorovinyl ether (BrTFF) could be grafted into poly(ethylene-co-tetrafluoroethylene) (ETFE) films by γ-rays irradiation at room temperature. The grafting yield increased linearly with an increase in the dose up to 1400 kGy. The required dose for a satisfactory grafting yield, such as 20%, was as high as ca. 400 kGy probably due to low polymerization reactivity of fluorinated monomers. However, the solvent and catalyst had no positive influence for improving the grafting yield. FTIR spectra and SEM-EDS testified that BrTFF was successfully grafted into ETFE films homogeneously in the perpendicular direction. The thermal analysis of the grafted films further indicated no phase separation between poly(BrTFF) grafts and ETFE films, probably owing to high compatibility of the fluorinated grafts and base polymers.     

Viscoelastic phase diagram of fluorinated and grafted polymer films and proton‐exchange membranes for fuel cell applications

The influence of temperature and moisture activity on the viscoelastic behavior of fluorinated membranes for fuel cell applications was investigated. Uncrosslinked and crosslinked ethylene tetrafluoroethylene (ETFE)‐based proton‐conducting membranes were prepared by radiation grafting and subsequent sulfonation and their behavior was compared with ETFE base film and commercial Nafion^® NR212 membrane. Uniaxial tensile tests and stress relaxation tests at controlled temperature and relative humidity (RH) were carried out at 30 and 50 °C for 10% < RH < 90%. Grafted films were stiffer and exhibited stronger strain hardening when compared with ETFE. Similarly, both uncrosslinked and crosslinked membranes were stiffer and stronger than Nafion^®. Yield stress was found to decrease and moisture sensitivity to increase on sulfonation. The viscoelastic relaxation of the grafted films was found to obey a power‐law behavior with exponent equal to ?0.04 ± 0.01, a factor of almost 2 lower than ETFE, weakly influenced by moisture and temperature. Moreover, the grafted films presented a higher hygrothermal stability when compared with their membranes counterparts. In the case of membranes, a power‐law behavior at RH < 60% was also observed. However, a markedly different behavior was evident at RH > 60%, with an almost single relaxation time exponential. An exponential decrease of relaxation time with RH from 60 s to 10 s was obtained at RH ≥ 70% and 30 °C. The general behavior of grafted films observed at 30 °C was also obtained at 50 °C. However, an anomalous result was noticed for the membranes, with a higher modulus at 50 °C when compared with 30 °C. This behavior was explained by solvation of the sulfonic acid groups by water absorption creating hydrogen bonding within the clusters. A viscoelastic phase diagram was elaborated to map critical conditions (temperature and RH) for transitions in time‐dependent behavior, from power‐law scaling to exponential scaling. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1139–1148     

Thermal behavior of poly(vinylbenzyl chloride)‐grafted poly(ethylene‐co‐tetrafluoroethylene) films

The effect of the degree of grafting (DOG) on the thermal behavior of poly(vinylbenzyl chloride)‐grafted poly(ethylene‐co‐tetrafluoroethylene) (ETFE‐g‐PVBC) films was investigated by differential scanning calorimetry (DSC), X‐ray diffraction (XRD), dynamic mechanical analysis ( DMA), FT‐IR, and thermogravimetric analysis (TGA) instruments. Several ETFE‐g‐PVBC films with various degrees of grafting, including 10, 24, 41, 60, and 94%, were prepared using a radiation grafting technique. The DSC and XRD results of the ETFE‐g‐PVBC films revealed that the crystallinity of the films decreased as the DOG increased. The DMA and FT‐IR results of the films indicated that a crosslinking reaction occurred at temperatures above 250 °C. In the thermal properties of the grafted films, an increase in the DOG led to an increase in the decomposition temperature. The activation energy (E_a) of the thermal decomposition was calculated using Kissinger's equation from TGA results. The E_a value of the PVBC graft chain was found to increase as the DOG increased, indicating that the crosslinking reaction of ETFE‐g‐PVBC films increased with an increase in the DOG during the thermal degradation process. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 517–525     

TEMPO addition into pre-irradiated fluoropolymers and living-radical graft polymerization of styrene for preparation of polymer electrolyte membranes

We prepared proton exchange membranes (PEMs) by 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO)-mediated living-radical graft polymerization (LRGP) of styrene into fluoropolymer films and subsequent sulfonation. Poly(vinylidene fluoride) (PVDF) and poly(ethylene-co-tetrafluoroethylene) (ETFE) films were first irradiated and then treated with TEMPO solutions in various solvents. TEMPO addition was confirmed by the test of styrene grafting into TEMPO-treated films at 60 °C, at which the LRGP never proceeds. This test enabled us to differentiate the LRGP from the conventional graft polymerization. In order to gain a deep insight about TEMPO-addition reaction, the TEMPO-penetration behavior into the base polymer films was examined by a permeation experiment and computer simulation. Xylene and dioxane were appropriate solvents for the complete introduction of TEMPO into PVDF and ETFE films, respectively. Then, the LRGP of styrene was performed based on the fully TEMPO-capped films at 125 °C with various solvents. By using an alcoholic solvent, the degree of grafting was enhanced and it reached a maximum of 38%. This grafted film was sulfonated to prepare a PEM showing an ion exchange capacity of 2.2 meq/g and proton conductivity of 1.6×10^?1 S/cm.     

Fabrication and characterization of supported dual acidic ionic liquids for polymer electrolyte membrane fuel cell applications

In this study, we proposed an innovative and versatile method for preparation of highly stable and conductive supported ionic liquid (IL) membranes for proton exchange fuel cell applications. Novel covalently supported dual acidic IL membranes were prepared by radiation induced grafting of 4-vinyl pyridine (4-VP) onto poly(ethylene-co-tetrafluoroethylene) (ETFE) film followed by post-functionalization via sequential treatments with 1,4-butane sultone and sulfuric acid to introduce pyridinium alkyl sulfonate/hydrogen sulfate moieties. The advantage of our approach lies in grafting polymers with highly reactive functional groups suitable for efficient post-sulfonation. The membranes displayed better swelling and mechanical properties compared to Nafion 112 despite having more than 3 times higher ion exchange capacity (IEC). The proton conductivity reached superior values to Nafion above 80 °C. Particularly, the membrane with ion exchange capacity of 3.41 displayed a proton conductivity of 259 mScm⁻¹ at 95 °C. This desired conductivity value is attributed to the high IEC of the membranes as well as dissociation of the hydrophobic ETFE polymer and hydrophilic pyridinium alkyl sulfonate groups. Such appealing properties make the supported IL membranes promising for proton exchange membrane fuel cells (PEMFC).     

Fe2+ Catalyzed Synthesis of Radiation Grafted Functional Membranes and Application in Fuel Cells and Ion Recovery

Summary: 1-Vinylimidazole is grafted onto Poly(ethylene-alt-tetrafluoroethylen (ETFE) by means of pre-irradiation technique with electron beam doses between 50 and 150 kGy. The degree of grafting is significantly increased to values of up to 115% by addition of 20-50 mmol/L of an Fe²⁺ salt that facilitates the decomposition of the peroxide structures within the irradiated films. The obtained mechanically stable and flexible ETFE-graft-poly(1-vinylimidazole) membranes exhibit a homogenous distribution of the graft polymer for degrees of grafting greater than 50 wt.-% and show thermal stability up to 250 °C. The applicability of these membranes as anhydrous proton conducting material when doped with phosphoric acid and the complexation capability for Cu²⁺ ions is demonstrated.     

Radiation-grafted proton exchange membranes based on co-grafting from binary monomer mixtures into poly(ethylene-co-tetrafluoroethylene) (ETFE) film

In this study, proton exchange membranes (PEMs) based on a poly(ethylene-co-tetrafluoroethylene) (ETFE) film were synthesized through the graft copolymerization of styrene and VTMS (vinyltrimethoxysilane), or styrene and TMSPM (3-(trimethoxysilyl) propyl methacrylate) binary monomer systems using a simultaneous irradiation method. The prepared membranes with the similar degrees of grafting were investigated by measuring ion exchange capacity, proton conductivity, water uptake, chemical stability, and dimensional stability. The results indicate that the silane-crosslinked proton exchange membrane (PEM) has not only lower water uptake and dimensional change but also high proton conductivity at low humidity condition compared to non-crosslinked poly(ethylene-co-tetrafluoroethylene)-g-poly(styrene sulfonic acid) (ETFE-g-PSSA). Also, the chemical stability of silane-crosslinked fuel cell membranes was more improved than that of non-crosslinked fuel cell membrane.     

Membranes obtained by preirradiation grafting of acrylic acid onto poly(tetrafluoroethylene–perfluorovinyl ether)

Some properties of the membranes obtained by preirradiation grafting of acrylic acid onto poly(tetrafluoroethylene-perfluorovinyl ether) copolymer (PFA) films have been investigated. The dimensional change caused by grafting and swelling behavior, water uptake, electrical conductivity, and mechanical properties of the grafted films were found to increase as the grafting proceeds. The influence of the preparation conditions (such as preirradiation dose, monomer concentration, grafting temperature, and film thickness) on those properties was studied. These properties were found to be dependent mainly on the degree of grafting regardless of grafting conditions, except at higher monomer concentration (>40 wt %). The electric conductivity and mechanical properties for the membranes obtained at higher AAc concentrations were lower than those obtained at lower ones. Analysis by x-ray microscopy of the grafted films revealed that the grafting begins at the part close to the film surface and proceeds into the central part with progressive diffusion of monomer to give finally homogeneous distribution of the electrolytes in the whole bulk of the polymer. The membranes show good electrochemical and mechanical properties which make them acceptable for practical use as cation-exchange membranes.     

Optimization strategies for radiation induced grafting of 4-vinylpyridine onto poly(ethylene-co-tetraflouroethene) film using Box-Behnken design

The radiation induced grafting of 4-vinylpyridine (4-VP) onto poly(ethylene-co-tetrafluoroethene) (ETFE) was optimized using the Box-Behnken factorial design available in the response surface method (RSM). The optimized grafting parameters; absorbed dose, monomer concentration, grafting time and reaction temperature were varied in four levels to quantify their effect on the grafting yield (GY). The validity of the statistical model was supported by the small deviation between the predicted (GY=61%) and experimental (GY=57%) values. The optimum conditions for enhancing GY were determined at the following values: monomer concentration of 48 vol%, absorbed dose of 64 kGy, reaction time of 4 h and temperature of 68 °C. A comparison was made between the optimization model developed for the present grafting system and that for grafting of 1-vinylimidazole (1-VIm) onto ETFE to confirm the validly and reliability of the Box-Behnken for the optimization of various radiation induced grafting reactions. Fourier transform infrared (FTIR), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to investigate the properties of the obtained films and provide evidence for grafting.     

Pre-Irradiation Grafting of Styrene into Modified Fluoropolymers

Summary: The grafting of styrene into commercially available fluoropolymer films by the pre-irradiation method has been investigated. Poly(tetrafluoroethylene) (PTFE), poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), poly(tetrafluoroethylene-co-perfluoropropylvinyl ether) (PFA) and poly(tetrafluorethylene-co-ethylene) (ETFE) were chosen as the base polymer material. The influence of the base material, the pre-irradiation dose, and the storage time between the irradiation and the grafting step on the yield of grafting was examined. The base materials were pre-treated by irradiation in the molten state under oxygen-free conditions in order to create branches and cross-links. The effect of pre-treatment on the yield of grafting was studied.     

Biomimetic membrane arrays on cast hydrogel supports

Lipid bilayers are intrinsically fragile and require mechanical support in technical applications based on biomimetic membranes. Tethering the lipid bilayer membranes to solid substrates, either directly through covalent or ionic substrate-lipid links or indirectly on substrate-supported cushions, provides mechanical support but at the cost of small molecule transport through the membrane-support sandwich. To stabilize biomimetic membranes while allowing transport through a membrane-support sandwich, we have investigated the feasibility of using an ethylene tetrafluoroethylene (ETFE)/hydrogel sandwich as the support. The sandwich is realized as a perforated surface-treated ETFE film onto which a hydrogel composite support structure is cast. We report a simple method to prepare arrays of lipid bilayer membranes with low intrinsic electrical conductance on the highly permeable, self-supporting ETFE/hydrogel sandwiches. We demonstrate how the ETFE/hydrogel sandwich support promotes rapid self-thinning of lipid bilayers suitable for hosting membrane-spanning proteins.     

Synthesis and light gas separations in cross-linked gemini room temperature ionic liquid polymer membranes

Cross-linkable gemini room temperature ionic liquids (GRTILs) were synthesized and photo-cross-linked into thin films. The resultant polymer membranes were tested for their permeabilities to CO₂, N₂, CH₄ and H₂. Permeabilities for each gas were found to be much lower when compared to previously reported poly(RTIL) membranes, mainly as a result of highly restricted diffusion. Separation factors were similar to previously studied poly(RTIL) membranes. CH₄ and N₂ fluxes were small enough to consider these membranes as “barrier” films to the transport of those gases. Poly(GRTILs) may have use in applications where flow of those gases is not desirable.     

Novel UV‐induced photografting process for preparing poly(tetrafluoroethylene)‐based proton‐conducting membranes

A novel process comprising the UV‐induced photografting of styrene into poly(tetrafluoroethylene) (PTFE) films and subsequent sulfonation has been developed for preparing proton‐conducting membranes. Although under UV irradiation the initial radicals were mainly generated on the surface of the PTFE films by the action of photosensitizers such as xanthone and benzoyl peroxide, the graft chains were readily propagated into the PTFE films. The sulfonation of the grafted films was performed in a chlorosulfonic acid solution. Fourier transform infrared and scanning electron microscopy were used to characterize the grafted and sulfonated membranes. With a view to use in fuel cells, the proton conductivity, water uptake, and mechanical properties of the prepared membranes were measured. Even through the degree of grafting was lower than 10%, the proton conductivity in the thickness direction of the newly prepared membranes could reach a value similar to that of a Nafion membrane. In comparison with γ‐ray radiation grafting, UV‐induced photografting is very simple and safe and is less damaging to the membranes because significant degradation of the PTFE main chains can be avoided. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2624–2637, 2007     

Perfluorinated anion exchange membranes: Preparation and preliminary tests of dialysis

Using the preirradiation technique a kinetic study of the grafting of the 4-vinyl pyridine (V₄P) and an aliphatic ammonium monomer (ALAM) onto the copolymer film of ethylene–tetrafluoroethylene (ETFE) has been performed. The influence of dose, temperature, and concentration of monomer, reticular agent, and inhibitor were investigated. The results are discussed on the basis of the interactions between monomer diffusibility and viscosity of the medium. The characteristics of some membranes were determined. Their applicability to the recovery of acid by dialysis is demonstrated.     

Chemically stable hybrid polymer electrolyte membranes prepared by radiation grafting,sulfonation, and silane‐crosslinking techniques

To prepare a crosslinked hybrid polymer electrolyte membrane (PEM) with high chemical stability, a silane monomer, namely p‐styryltrimethoxysilane (StSi), was first grafted to poly(ethylene‐co‐tetrafluoroethylene) (ETFE) film by γ‐ray preirradiation. Hydrolysis‐condensation and sulfonation were then performed on the StSi‐grafted ETFE (StSi‐g‐ETFE) films to give them crosslinks and proton conductibility, respectively. Thus, a crosslinked proton‐conducting hybrid PEM was obtained. The crosslinks introduced by the silane‐condensation have an inorganic ? Si? O? Si? structure, which enhance the chemical and thermal stabilities of the PEM. The effect of the timing of the hydrolysis‐condensation (before or after sulfonation) and the sulfonation method (by chlorosulfonic acid or H₂SO₄) on the properties of the resulting hybrid PEMs such as ion‐exchange capacity, proton conductivity, water uptake, chemical stability, and methanol permeability were investigated to confirm their applicability in fuel cells. We conclude that the properties of the new crosslinked hybrid StSi‐grafted PEMs are superior to divinylbenzene (DVB)‐crosslinked styrene‐grafted membranes. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5559–5567, 2008     

Preparation de membranes mosaiques par greffage radiochimique dans des films de polytetrafluoroethylene

Mosaic membranes were prepared by two successive grafting steps of acrylic acid and 4-vinyl-pyridine into well localized domains of polytetrafluoroethylene films. The films were subjected to X-rays through a grid-shaped shield which activated linear zones of the film 0.5 mm wide, separated by 0.5 mm of unactivated film. The films were then grafted with acrylic acid, neutralized and subjected to a second irradiation through a shield which protected the grafted zones. 4-Vinylpyridine was then grafted into the stripes left unchanged after the first treatment. The following properties of the resulting mosaic membranes were examined: swelling in water, electric conductivity and ion-pair diffusion. The coefficients of ion-pair diffusion were found to be 100–1000 times higher for the mosaic membranes than for carboxylic membranes of the same grafting ratio.     

Pre-irradiation induced grafting of styrene into crosslinked and non-crosslinked polytetrafluoroethylene films for polymer electrolyte fuel cell applications. I: Influence of styrene grafting conditions

Crosslinked and non-crosslinked polytetrafluoroethylene (PTFE) films [RX-PTFE and V-PTFE films, respectively], were irradiated in air at room temperature using γ-rays from a ⁶⁰Co source. The irradiated films were grafted with styrene in liquid phase. The grafting of styrene into PTFE films was proved by FT-IR spectroscopy. The influence of the reaction temperature and pre-irradiation doses on the resulted degree of grafting was discussed. The grafting speed and the degree of grafting were determined by the reaction temperature and pre-irradiation doses. The apparent activation energies were calculated as 39.7 kJ/mol for RX-PTFE films and 59.5 kJ/mol for V-PTFE films. The dependence index on absorbed doses at pre-irradiation for RX-PTFE films is 0.66, and for V-PTFE films it is 1.57. The geometric size changes of the grafted films were measured and discussed. Interestingly, the thickness of the grafted films was strongly influenced by the reaction temperature. The tensile strength and the elongation at break of the non-grafted and grafted RX-PTFE and V-PTFE films were measured. The grafted films then are sulfonated by chlorosulfonic acid for polymer electrolyte fuel cell (PEFC) applications and the highest IEC value gained is over 3. The analysis of the sulfonated films are now in progress.     

预辐射接枝四氟乙烯-乙烯共聚物湿敏膜的制备及特性研究

利用2 MeV电子加速器, 在常温下采用预辐照引发接枝的方法, 在四氟乙烯-乙烯共聚物(ETFE)上接枝丙烯酸(AA)和对苯乙烯磺酸钠(SSS), 制备了一种含羧酸基团和磺酸基团的接枝膜. 傅里叶变换红外光谱(FTIR)分析结果证明了磺酸基团和羧酸基团的成功引入, 并对接枝膜的热力学和化学特性进行了研究. 结果表明, 随着接枝率的增加, 接枝膜的结晶度逐渐降低, 接触角逐渐减小, 接枝膜的亲水性逐渐增强. 利用制备的接枝膜构建了电阻型湿度传感器, 并测定了传感器的电学特性. 在相对湿度(RH)从5%变化到98%时, 传感器电阻线性变化范围接近4个数量级, 具有响应速度快(吸附<1 min, 解吸<2 min), 湿滞小(<2%RH)的特点.     

Radiation grafting of 4-vinylpyridine onto poly-tetrafluoroethylene-perfluorovinylether (TFA) and poly-tetrafluoroethylene-polyethylene (ET) films

Preparation of membranes of poly-(tetrafluoroethylene-perfluorovinylether (TFA) and poly-tetrafluoroethylene-polyethylene (ET) films grafted with 4-vinylpyridine using γ-rays has been carried out. The appropriate reaction conditions were selected. Furthermore, quaternization of the pyridine of the grafted chains was conducted. The effect of monomer concentration on the rate of grafting was also investigated. The order of the grafting rate gram per hour depending on monomer concentration was found to be 0.94 and 1.0 for TFA and ET films, respectively. Some selected properties of the grafted films such as swelling behavior, dimensional stability, mechanical and electrical properties were investigated. The grafted film of TFA and ET showed a marked decrease in elongation with a significant increase in the tensile strength.