Proton exchange membranes by radiation grafting of styrene onto FEP films. IV. Evaluation of the states of water

The nature of water in proton exchange membranes, obtained by radiation grafting of styrene onto poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) films and subsequent sulfonation, was investigated by differential scanning calorimetry. It was found that the membranes contained three different types of water, which were categorized as freezing-free, freezing-bound, and nonfreezing water. The degree of grafting had a considerable influence on the relative ratios of these three types of water molecules. The nonfreezing water per ion-exchange site ratio was found to be independent of the degree of grafting. However, freezing-free and freezing-bound water per ionic site ratios increased with increasing graft levels. A correlation between the water uptake and the structural changes occurring in the membrane as a function of the degree of grafting was established. © 1995 John Wiley & Sons, Inc.     

Proton exchange membranes prepared by simultaneous radiation grafting of styrene onto poly(tetrafluoroethylene‐co‐hexafluoropropylene) films. I. Effect of grafting conditions

The simultaneous radiation grafting of styrene onto poly(tetrafluoroethylene‐co‐hexafluoropropylene) (FEP) films was studied at room temperature. The effects of grafting conditions (type of solvent, irradiation dose, dose rate, and monomer concentration) were investigated. The degree of grafting was found to be dependent on the investigated grafting conditions. The dependence of the initial rate of grafting on the dose rate and the monomer concentration was found to be of 0.5 and 1.3 orders, respectively. The results suggest that grafting proceeds by the so‐called front mechanism in which the grafting front starts at the surface of the film and moves internally toward the middle of the film by successive diffusion of styrene through the grafted layers. Some selected properties of the grafted films were evaluated in correlation with the degree of grafting. We found that the grafted FEP films possess good mechanical stability, which encourages their use for the preparation of proton exchange membranes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 220–227, 2000     

Proton exchange membranes prepared by simultaneous radiation grafting of styrene onto poly(tetrafluoroethylene‐co‐hexafluoropropylene) films. II. Properties of sulfonated membranes

Proton exchange membranes were prepared by radiation‐induced grafting of styrene onto commercial poly(tetrafluoroethylene‐co‐hexafluoropropylene) films using a simultaneous irradiation technique followed by a sulfonation reaction. The resulting membranes were characterized by measuring their physicochemical properties such as water uptake, ion exchange capacity, hydration number, and proton conductivity as a function of the degree of grafting. The thermal properties (melting and glass transition temperatures) and thermal stability of the membrane were also investigated using differential scanning calorimetry and thermal gravimetric analysis, respectively. Membranes having degrees of grafting of 16% and above showed proton conductivity of the magnitude of 10⁻² Ω⁻¹ cm⁻¹ at room temperature, as well as thermal stability at up to 290°C under an oxygen atmosphere. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2443–2453, 2000     

Proton-exchange membranes by radiation grafting of styrene onto FEP films. II. Mechanism of thermal degradation in copolymer membranes

The thermal degradation of radiation-grafted and sulfonated FEP-g-polystyrene membrane has been investigated in the temperature range 50–650°C in an inert gas atmosphere by thermogravimetry. The evolved gaseous products were analyzed by FTIR in combination with mass spectrometry to correlate weight losses at different temperatures with molecular composition of the copolymer membrane. On the basis of these results, a simple degradation model has been established. © 1994 John Wiley & Sons, Inc.     

Cation exchange membranes by radiation‐induced graft copolymerization of styrene onto PFA copolymer films. III. Thermal stability of the membranes

Thermal stability of cation exchange, PFA‐g‐polystyrene sulfonic acid membranes prepared by radiation‐induced graft copolymerization of styrene onto PFA films followed by sulfonation was studied by thermal gravimetric analysis (TGA) and oven heat treatment. The tested samples included original and grafted PFA films as reference materials. All the membranes showed multistep decomposition patterns due to dehydration, desulfonation, dearomatization, and decomposition of the PFA matrix. Investigations of the individual decomposition behaviors showed that the weight loss strongly depends upon the degree of grafting. However, the decomposition temperatures were found to be independent of the degree of grafting. The loss in some selected membrane properties such as ion exchange capacity and water uptake was found to be function of the degree of grafting, temperature, and the time of heat treatment. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1877–1885, 2000     

Preparation and characterization of a proton‐exchange membrane by the radiation grafting of styrene onto polytetrafluoroethylene films

Radiation‐induced simultaneous grafting of styrene onto polytetrafluoroethylene (PTFE) films and the subsequent sulfonation in the chlorosulfonic acid/dichloroethane were investigated. The effects of the main radiation grafting conditions, such as the type of solvents, irradiation dose, dose rate, the styrene concentrations, etc., on the degree of grafting (DOG) were studied. To elucidate the influence of both the grafting and sulfonation conditions on the properties of the PTFE‐g‐polystyrene‐sulfonic acid (PSSA) membranes, the sulfonation conditions, including the sulfonation temperature and the concentration of the ClSO₃H with respect to the DOG, were systematically evaluated. The grafted and sulfonated membranes were characterized by FTIR–ATR spectra, ion‐exchange capacity (IEC), water uptake, thickness measurement, etc. The as‐prepared PTFE‐g‐PSSA membranes in this work showed a good combination of a high IEC (0.85–2.75 meq g^?1), acceptable water uptake (8.86–56.9 wt %), low thickness, and volume expansion and/or contraction. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1415–1428, 2006     

Pervaporation membranes prepared by radiochemical grafting of N-vinylpyrrolidone onto films

The pervaporation performances of membranes obtained by radiation grafting of N-vinylpyrrolidone onto poly(tetrafluoroethylene), polyethylene, polybutene, and poly(dimethylsiloxane) films were studied using a water–dioxane azeotropic mixture. The influence of the following parameters was examined: grafting ratios, radiation dose rates, nature of the base films, poly(tetrafluoroethylene) film-making methods, and structural transition. An attempt has been made to interpret the results in terms of variations in the number, size, and density of the grafted domains which form the diffusion paths for the permeating molecules. Based on this interpretation, a general rule is proposed for selecting adpated methods to prepare efficient pervaporation membranes.     

Preparation and properties of cationic membranes obtained by radiation grafting of methacrylic acid onto PTFE films

Cationic membranes were prepared by direct radiation grafting of methacrylic acid (MAA) onto poly(tetrafluoroethylene) (PTFE) films followed by alkaline treatment to confer ionic character in the graft copolymer. The complete inhibition of homopolymerization of MAA by using ammonium ferreous sulfate (Mohr's salt) failed. However, the addition of 0.5 wt % FeCl₃ to the monomer solution effectively inhibited the homopolymerization process and higher grafting yield was obtained. It was found that the graft polymerization proceeded successfully in presence of methanol/water mixture (30/70 wt %), and much higher degrees of grafting were obtained as compared with those in the presence of other diluents used here. The influence of irradiation atmosphere (air, N₂ gas, and vacuum) on the grafting process was investigated. The dependence of the grafting rate on MAA concentration was found to be of orders 2.9 and 0.72 in the presence of 0.5 wt % Mohr's salt or 0.5 wt % FeCl₃, respectively. This grafting system proceeds by the front mechanism. Investigation of mechanical properties, electrical conductivity, and swelling behavior of the grafted films revealed that such a copolymer could be acceptable in practical use as a cation-exchange membrane.     

Proton exchange membranes by grafting of styrene–acrylic acid onto FEP by preirradiation technique. I. Effect of synthesis conditions

Grafting of styrene–acrylic acid (sty–AA) polymer onto fluorinated ethylene propylene copolymer (FEP) was carried out by preirradiation of FEP with γ‐rays. Effect of reaction conditions on the degree of grafting such as total radiation dose, monomer concentration, temperature, and time of grafting has been studied. From the study, it is seen that the degree of grafting is strongly dependent on the reaction conditions. The rate dependence of 0.53 (～0.5) and 1.03 (～1) have been obtained with respect to total dose and monomer concentration, respectively indicating that radiation induced grafting follows free radical polymerization. The activation energy for the grafting in the temperature range of 60–90°C is found to be 33 kJ/mol. The results indicate that the grafting takes place by the front mechanism where the grafting starts at surface and slowly proceeds inwards by diffusion of the monomer through the swollen grafted chains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2572–2577, 2003     

Preparation and properties of membranes obtained by radiation induced grafting of 4-vinylpyridine onto polytetrafluoroethylene films

The preparation and properties of membranes obtained by radiation induced grafting are described, using vinylpyridine (VPy) on a polytetrafluoroethylene (PTFE) matrix. Subsequent quaternarization of the pyridine groups improves greatly the hyperfiltration properties of these membranes, considering both flux and rejection, and they may be developed for desalination purposes.The dependence of grafting yields on irradiation times was studied as well as the thermal and chemical properties of the membranes obtained. Flux and rejection to NaCl of quaternarized VPy/PTFE membranes was measured as a function of various chemical and physical properties.     

Cation exchange membranes by radiation‐induced graft copolymerization of styrene onto PFA copolymer films. II. Characterization of sulfonated graft copolymer membranes

PFA‐g‐polystyrene sulfonic acid membranes were prepared by simultaneous radiation‐induced graft copolymerization of styrene onto poly(tetrafluoroethylene‐co‐perfluorovinyl ether) (PFA) film followed by sulfonation. The membrane physico‐chemical properties such as swelling behavior, ion exchange capacity, hydration number, and ionic conductivity were studied as a function of the degree of grafting. Thermal as well as chemical stability of the membranes was also investigated. The membrane properties were found to be mainly dependent upon the degree of grafting. The water uptake, ion exchange capacity, hydration number, and ionic conductivity of the membranes were increased, whereas the chemical stability decreased as the degree of grafting increased. The membranes showed reasonable physico‐chemical properties compared to Nafion 117 membranes. However, their chemical stability has to be further improved to make them acceptable for practical use in electrochemical applications. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1–11, 2000     

Cation exchange membranes by radiation-induced graft copolymerization of styrene onto PFA copolymer films. I. Preparation and characterization of the graft copolymer

PFA-g-polystyrene graft copolymers were prepared by simultaneous radiation-induced graft copolymerization of styrene onto poly(tetrafluoroethylene-co-perfluorovinyl ether) (PFA) films. The effects of grafting conditions such as monomer concentration, dose, and dose rate were investigated. Three solvents, i.e., methanol, benzene, and dichloromethane, were used as diluents in this grafting system. Of the three solvents employed, dichloromethane was found to greatly enhance the grafting process, and the degree of grafting increased with the increase of monomer concentration until it reached its highest value at a styrene concentration of 60 (vol %). The dependence of the initial rate of grafting on the monomer concentration was found to be of the order of 1.2. The degree of grafting was found to increase with the increase in irradiation dose, while it considerably decreased with the increase in dose rate. The formation of graft copolymers was confirmed by FTIR analysis. The structural investigation by X-ray diffraction (XRD) shows that the degree of crystallinity content of such graft copolymers decreases with the increase in grafting, and consequently, the mechanical properties of the graft copolymers were influenced to some extent. Both tensile strength and elongation percent decreased with the increase in the degree of grafting. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2095–2102, 1999     

Preparation of hydrophilic membranes by grafting acrylic acid onto pre-irradiated teflon-FEP films

Grafting of acrylic acid onto pre-irradiated poly(tetrafluoroethylene-hexafluoropropylene) (Teflon-FEP) films was investigated, and the reaction parameters dose, dose rate, monomer concentration and grafting temperature were examined. The results show that the degree of grafting depends on the dose (5-60 kGy), the weight of grafted specimen increased up to 37%. The degree of grafting was found to be independent of the dose rate. The overall activation energies were calculated to be 28.8 and 81.6 kJ mol^-1 for treatments at above and below 50°C, respectively. The swelling of grafted films was measured in water, aqueous potassium hydroxide and methanol. The degree of swelling was found to increase with the length of grafted chains.     

Thermal stability of proton exchange membranes prepared by grafting of styrene into pre-irradiated FEP films and the effect of crosslinking

The thermal stability of crosslinked ion exchange membranes, prepared by pre-irradiation grafting of styrene into FEP films in the presence of divinylbenzene (DVB) as a crosslinker and subsequent sulfonation of the grafted films, was investigated by thermogravimetry. The formation of different fragments with increasing temperature was monitored by FTIR. Pre-irradiation leads to destabilization of the FEP backbone, showing both earlier depolymerization and substantial redistribution of monomer formation between the two main degradation steps. The presence of DVB had clear effects on the thermal degradation pattern of the sulfonated graft copolymer membrane. Desulfonation occurred at a lower temperature whereas dearomatization was observed to shift to higher temperatures.     

Preparation of ion exchange membranes by preirradiation induced grafting of styrene/divinylbenzene into crosslinked PTFE films and successive sulfonation

Ion exchange membranes (IEMs) were prepared by preirradiation induced grafting of styrene with or without divinylbenzene (DVB) into crosslinked polytetrafluoroethylene (RX‐PTFE) films and successively sulfonated by chlorosulfonic acid. The effects of the DVB concentration and solvent on the kinetic of the graft polymerization were studied. The ion exchange capacity (IEC) values of the prepared membranes ranging from 1.5 to 2.8 mequiv/g were obtained. The degree of swelling increased with the increase in the degree of grafting, while higher crosslinking density of both the RX‐PTFE matrices and the grafts suppressed the degree of swelling. The chemical stabilities of the IEMs were tested by recording the weight of the membranes being soaked in hot H₂O₂ solutions. The weight‐time curves of the prepared membranes showed one‐step quick decrease due to the decomposition of the poly(styrene‐sulfonic acid) (PSSA) grafts. Higher crosslinking density in both the RX‐PTFE matrices and the grafts improved the chemical stability of the IEMs. The ionic conductivity of the IEMs increases with the increase in the IEC values. The IEMs with IEC values higher than 2.2 mequiv/g hold the higher ionic conductivity than that of Nafion® 112 membrane. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3587–3599, 2006     

Proton‐exchange membranes via the grafting of styrene and acrylic acid onto fluorinated ethylene propylene copolymer by a preirradiation technique. III. Thermal and mechanical properties of the membranes and their sulfonated derivatives

Polymer electrolyte membranes were prepared via the grafting of styrene and acrylic acid onto fluorinated ethylene propylene copolymer with a preirradiation technique and subsequent sulfonation. The thermal and mechanical properties of the grafted membranes and their sulfonated derivatives were dependent on the degree of grafting. The grafted membranes showed a two‐step degradation pattern, whereas their sulfonated derivatives showed a three‐step degradation pattern. The glass‐transition temperature and crystallinity percentage of the membranes were determined with differential scanning calorimetry. With an increase in the degree of grafting and sulfonation, the glass‐transition temperature increased, whereas the crystallinity percentage decreased. The tensile strength and elongation decreased with the degree of grafting and sulfonation. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1418–1425, 2005     

Proton exchange membranes by grafting of styrene–acrylic acid onto FEP by preirradiation technique. II. Physicochemical properties of the membrane and its sulfonated derivatives

Poly(tetrafluoroethylene‐co‐haxafluoropropylene) (FEP)‐g‐styrene–acrylic acid and its sulfonated derivative membranes were prepared by graft copolymerization of styrene–acrylic acid onto FEP by using preirradiation of γ‐ray technique followed by sulfonation. The physiochemical properties such as ion exchange capacity, water uptake, ionic resistance of the grafted membranes, and their sulfonated derivatives were studied as a function of degree of grafting. These membranes on sulfonation gave acid base, indicating property. The membranes gave yellow color in acidic medium and purple color in alkali medium. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2318–2325, 2004     

Effect of mineral and organic acids on radiation grafting of styrene onto polyethylene

A study has been made on the effect of mineral and organic acids on the radiation-induced graft polymerization of styrene onto low-density polyethylene films. Effects of different solvents and dose rate of irradiation on the grafting yield were also investigated. The influence of irradiation time and monomer concentration in the presence and absence of acid on the initial rate and grafting yield was studied. The dependence of the grafting rate on monomer concentration was found to be of the order 1.34 (in absence of acid), 0.61 (in presence of HCl), and 1.25 (in presence of oxalic acid). Such acid enhancement of the radiation grafting process is of practical importance from an economical point of view.     

Liquid transport through membranes prepared by grafting of polar monomers onto poly(tetrafluoroethylene) films. III. Steady-state distribution in membrane during pervaporation

Steady-state concentration and composition gradients of pervaporating liquids inside the membranes were measured using stacks of membranes. The experimental results on pure components agree well with a simple mechanism of diffusion down a chemical potential gradient in a nonporous membrane. For mixtures, concentration gradients are similar to those obtained for pure liquids, and composition distribution shows that the selection between the two species does not take place at a given interface in the membrane but is progressive along the diffusional paths through the membrane. Data are also listed to show that solubility and diffusivity enhancements occur for components which are hydrogen bonded with the basic groups of the membrance.     

Cation exchange membranes by radiation‐induced graft copolymerization of styrene onto PFA copolymer films. IV. Morphological investigations using X‐ray photoelectron spectroscopy

Morphological investigations of poly(tetrafluoroethylene‐co‐perfluorovinyl ether) (PFA)‐g‐polystyrene sulfonic acid membranes prepared by radiation‐induced graft copolymerization of styrene onto PFA films followed by sulfonation were performed by X‐ray photoelectron spectroscopy. The analyzed materials included grafted film and sulfonated membrane samples having various degrees of grafting. Original PFA film was used as a reference material. The results of the X‐ray photoelectron spectral analysis show that PFA film undergoes changes in terms of chemical compositions and binding energies of its basic elemental components under the influence of membrane preparation procedure, i.e., grafting and sulfonation. The chemical compositions of the surfaces of the membranes were found to be dependent on the degree of grafting unlike the binding energies of their elemental components (C, F, O, and S), which were found to be independent of the degree of grafting. The atomic ratio of F/C was found to decrease drastically with the increase in the degree of grafting and the membranes were found to have almost pure hydrocarbon structure at the layers close to their surfaces where degradation is suggested to be concentrated. The results of these investigations suggest that the morphology of the membranes plays an important role in the chemical degradation of the membranes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2455–2463, 2000