Mikael Paronen

Structural investigation of radiation grafted and sulfonated poly(vinylidene fluoride), PVDF, membranes

Radiation grafted and sulfonated poly(vinylidene fluoride), PVDF, nmembranes have been studied by thermal analysis and X-ray diffraction to ndetermine the changes in membrane crystallinity and structure during npreparation. Commercial PVDF films were irradiated with an electron beam, ngrafted with styrene and finally sulfonated. Both the crystallinity and nthe size of the crystallites of PVDF decrease in the grafting reaction. A nfurther decrease in crystallinity is observed in the sulfonation reaction. nThe residual crystallinity of PVDF was considerable (10–20%) even in nmembranes which had been subjected to severe reaction conditions. These nresults can be explained by assuming that the grafting takes place mainly nin the amorphous region of the PVDF, and close to the surfaces of the ncrystals, but that grafts do not penetrate into the crystals. The proton nconductivity of the grafted and sulfonated PVDF membranes reached values ncomparable to those of Nafion membranes.

Structure and properties of sulfonated poly [(vinylidene fluoride)–g-styrene] norous membranes porous membranes

The possibility of developing polyfvinylidene fluoride (PVDF)-based membranes as proton conductors has been investigated. Using electron beam radiation grafting and functionalisation, the composition of the membranes can be controlled. Membranes have been prepared from porous films of PVDF, and their structure and properties have been studied with Raman spectroscopy, wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), swelling tests, and by impedance spectroscopy. The grafting reaction of styrene, initiated in the amorphous regions and at the surfaces of the crystallites in the partly crystalline PVDF matrix, is very efficient, and high degrees of grafting can be achieved. Grafts are formed both from C–H and C–F branch pointsl Sulfonation can be accomplished to ca. 100%, and occurs mainly at the para-position of the phenyl rings. The hydration number was found to be independent of the degree of grafting, degree of sulfonation and crystallinity. The overall crystallinity decreased in the structure with the degree of grafting and sulfonation, partly owing to the diluting effect of the grafts, partly owing to efficient penetration of the grafts in the crystallites. The conductivity increased with the content of sulfonic acid groups and, in particular, with decreasing crystallinity. In the case of fully sulfonated membranes, a levelling-out of the conductivity was found at a degree of grafting of around 200%. Conductivities up to 120 mS cm–1 at room temperature were achieved.

The state of water and the nature of ion clustersin crosslinked proton conducting membranes of styrene grafted and sulfonatedpoly(vinylidene fluoride)

Proton conducting membranes were prepared by irradiation grafting nwith styrene followed by sulfonation on matrices of poly(vinylidene fluoride), nPVDF. Membranes crosslinked with divinylbenzene and/or bis(vinylphenyl)ethane nwere compared to non-crosslinked membranes. The ion conductivity of the ncrosslinked membranes is lower than that of the non-crosslinked membranes. nThis is due partly to the very inefficient sulfonation of the crosslinked nmembranes below the graft penetration level, which in turn leads to a low nwater uptake at low degrees of grafting. The graft penetration level is lower nin crosslinked membranes than in non-crosslinked membranes. This leads nto a more compact structure of the crosslinked grafts within the matrix. The nlower ion conductivity in the crosslinked membranes is therefore partly also ndue to restricted mobility of the ion clusters necessary for ion and water ntransport in the membranes.

Phase separation and crystallinity in proton conducting membranes of styrene grafted and sulfonated poly(vinylidene fluoride)

A series of proton exchange membranes have been prepared by the preirradiation grafting method. Styrene was grafted onto a matrix of poly(vinylidene fluoride) (PVDF) after electron beam irradiation. Part of the samples was crosslinked with divinylbenzene (DVB) or bis(vinylphenyl)ethane (BVPE). Subsequent sulfonation gave membranes grafted with poly(styrene sulfonic acid) and marked PVDF-g-PSSA. It was found that the intrinsic crystallinity of the matrix decreased in both the grafting and the sulfonation reaction in all the membranes. The graft penetration and the ion conductivity are influenced strongly by the crosslinker. The ion conductivity is considerably lower in crosslinked membranes than in noncrosslinked ones. Generally, the mechanical strength decreases with crosslinking. The membranes show a regular phase separated structure in which the sulfonated grafts are incorporated in the amorphous parts of the matrix polymer. The phase separated domains are small, of the order of magnitude of 100–250 nm. These were resolved on transmission electron micrographs and on atomic force images but could not be resolved with microprobe Raman spectroscopy.

Radiation grafting of styrene onto poly(vinylidene fluoride) films in propanol: The influence of solvent and synthesis conditions

Electron-beam-irradiated poly(vinylidene fluoride) films were grafted with styrene with propanol or toluene as a solvent. The influence of the synthesis conditions and, more particularly, of the solvent was investigated. In propanol, the order of dependence of the grafting rate is 0.43 on the pre-irradiation dose and 1.2 on the monomer concentration. The activation energy of the grafting reaction in propanol is approximately 73 kJ/mol. Both the initial grafting rate and the saturation degree of grafting are considerably higher in propanol, which is unable to swell polystyrene grafts, than in toluene, which diffuses with styrene through the grafted moiety. The grafting solvent also influences the structure of the membrane: films grafted in propanol have a much reduced elongation at break and a rougher surface. It is suggested that phase-separated polystyrene domains may be larger when grafting is carried out in a styrene–propanol solution.

Water sorption properties of and the state of water in PVDF-based proton conducting membranes

Abstract A new proton conducting material, poly(vinylidene fluoride) (PVDF) grafted with polystyrene (PS) and sulfonated (PVDF–SPS), has been investigated with regard to water uptake and the state of water in the material. The water uptake was estimated for the materials exposed to liquid water, in the temperature range between 22 and 100°C, and to water vapour of different water activities. The water uptake from saturated water vapour was found to be close to that from liquid water. The state of the absorbed water has been studied by Raman spectroscopy. We found that in the porous materials the state of water is similar to that of bulk water whereas in the non-porous samples a significant part of the absorbed water differs from that of the bulk.

Fuel cell performance of proton irradiated and subsequently sulfonated poly(vinyl fluoride) membranes

Abstract Proton irradiated and sulfonated poly(vinyl fluoride) (PVF-SA) membranes have been tested with respect to fuel cell performance and swelling in water. Swelling of the PVF-SA membranes was clearly lower than that of the Nafion® 117 and 112 membranes. In fuel cell tests the performance of the low price PVF-SA was better than Nafion® 117 membranes tested under similar conditions. In contrast, the Nafion® 112 membrane performed better than the PVF-SA membrane. The low ohmic resistivity of Nafion® 112 and the compatibility between Nafion membrane and Nafion impregnated electrodes explain this. PVF-SA membranes suffered from degradation during the fuel cell testing.

Structure of Sulfonated poly(vinyl fluoride)

Sulfonated poly(vinyl fluoride) (PVF-SA) has been made by chemical sulfonation or radiation-induced sulfonation of commercial poly(vinyl fluoride) (PVF) films. The effects of the irradiation treatment and sulfonation on sulfonic acid distribution, crystallinity, state of water, and molecular organization have been examined. The results indicate that proton irradiation and subsequent sulfonation produce a structure that is different from the ones produced by the sulfonation of nonirradiated or electron beam (EB)-irradiated samples. The water uptake is higher in proton-irradiated samples than in the other samples. In addition, the portion of nonfreezing water is highest in proton-irradiated samples. Infrared spectra of the sulfonated samples indicate that a large part of the freezing bound water is associated with the hydrophobic polymer backbone. However, this portion was smaller in the proton-irradiated sample than in the EB-irradiated sample. The proton-irradiated samples had a small-angle X-ray diffraction maximum with a corresponding Bragg spacing of 70 A, which was taken as evidence for the formation of ion–water cluster domains in the proton-irradiated samples. The ion conductivity was slightly lower in nonirradiated and in EB-irradiated membranes than in the proton-irradiated sulfonated samples in which the highest values were 10–20 mS/cm.

State of water in sulfonated poly(vinyl fluoride) membranes : an FTIR study

Abstract The state of water in new types of polymer-based proton conductors has been characterized by FTIR spectroscopy. The materials were produced by sulfonation of poly(vinyl fluoride) (PVF) films with chlorosulfonic acid after electron beam or proton irradiation or without any preirradiation. Spectra in the O–H and O–D stretching regions and in the H–O–H (D–O–D) bending region were analyzed in terms of frequency position and relative intensity of the various components. We found that the state of water in the sulfonated PVF membranes (PVF–SA) membranes differs significantly from that of the bulk water, and both hydrogen-bonded and non-hydrogen-bonded water molecules were detected. It is also found that the materials display the presence of a large number of proton complexes such as H3O+ or H5O2+. The study demonstrates furthermore that proton irradiation makes the material more hydrophilic than electron irradiation, which can be explained by the different nature of the irradiation effects.

Effects of irradiation on sulfonation of poly(vinyl fluoride)

Preparation and analyses of directly sulfonated poly(vinyl fluoride), PVF, membranes have been performed. Electron beam or proton irradiation was used for the production of reactive sites. Ion-exchange capacity, sulfonation efficiency and ion conductivity were analysed. Increases in the absorbed dose and in the linear energy transfer were found to have a promoting effect on the sulfonation rate and sulfonic acid content. In addition, compared with commercial ion-exchange materials, quite high ion-exchange capacities were achieved. The highest measured ionic conductivity of the materials was 20 mS cm–1 , which was achieved with proton-irradiated samples after long sulfonation times. On the basis of the results the proton irradiation is believed to cause the formation of channels with higher sulfonic acid content than those found in other domains of the matrix. The presence of these channels increases the ionic conductivity.