Sami Hietala

Thermal stability of styrene grafted and sulfonated proton conducting membranes based on poly(vinylidene fluoride)

The thermal stability of styrene grafted and sulfonated poly(vinylidene fluoride), PVDF-g-PSSA, proton conducting membranes has been studied using thermal gravimetric analysis in combination with mass spectrometry and thermochromatography. The matrix polymer, PVDF, and the non-sulfonated counterpart, PVDF-g-PS, were studied as reference materials. It was found that the degradation of the PVDF-g-PS membrane proceeds in two steps starting atca. 340 °C with the evolution of degradation products typical of polystyrene. The PVDF-g-PSSA membranes are stable to around 270 °C even in a strongly oxidising atmosphere. The degradation starts with the simultaneous evolution of water and sulfur dioxide. The polystyrene grafts start decomposing at 340 oC in the PVDF-g-PSSA membranes. Thus the membranes are suitable for tests in electrochemical applications at elevated temperatures.

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

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

Sorption and diffusion of methanol and water in PVDF-g-PSSA and Nafion® 117 polymer electrolyte membranes

Sorption and diffusion properties of poly(vinylidene fluoride)-graft-poly(styrene sulfonic acid) (PVDF-g-PSSA) and Nafion® 117 polymer electrolyte membranes were studied in water/methanol mixtures. The two types of membranes were found to have different sorption properties. The Nafion 117 membrane was found to have a maximum in-solvent uptake around 0.4 to 0.6 mole fraction of methanol, while the PVDF-g-PSSA membranes took up less solvent with increasing methanol concentration. The proton NMR spectra were recorded for membranes immersed in deuterated water/methanol mixtures. The spectra showed that the hydroxyl protons inside the membrane exhibit resonance lines different from the resonance lines of hydroxyl protons in the external solvent. The spectral features of the lines of these internal hydroxyl groups in the membranes were different in the Nafion membrane compared with the PVDF-g-PSSA membranes. Diffusion measurements with the pulsed field gradient NMR (PFG-NMR) method showed that the diffusion coefficient of the internal hydroxyl groups in the solvent immersed Nafion membrane mirrors the changes in the diffusion coefficients of hydroxyl and methyl protons in the external solvent. For the PVDF-g-PSSA membranes, a decrease in the diffusion coefficient of the internal hydroxyl protons was seen with increasing methanol concentration. These results indicate that the morphology and chemical structure of the membranes have an effect on their solvent sorption and diffusion characteristics.

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 with styrene followed by sulfonation on matrices of poly(vinylidene fluoride), PVDF. Membranes crosslinked with divinylbenzene and/or bis(vinylphenyl)ethane were compared to non-crosslinked membranes. The ion conductivity of the crosslinked membranes is lower than that of the non-crosslinked membranes. This is due partly to the very inefficient sulfonation of the crosslinked membranes below the graft penetration level, which in turn leads to a low water uptake at low degrees of grafting. The graft penetration level is lower in crosslinked membranes than in non-crosslinked membranes. This leads to a more compact structure of the crosslinked grafts within the matrix. The lower ion conductivity in the crosslinked membranes is therefore partly also due to restricted mobility of the ion clusters necessary for ion and water transport 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.

Water-Resistant, Transparent Hybrid Nanopaper by Physical Cross-Linking with Chitosan

One of the major, but often overlooked, challenges toward high end applications of nanocelluloses is to maintain their high mechanical properties under hydrated or even fully wet conditions. As such, permanent covalent cross-linking or surface hydrophobization are viable approaches, however, the former may hamper processability and the latter may have adverse effect on interfibrillar bonding and resulting material strength. Here we show a concept based on physical cross-linking of cellulose nanofibers (CNF, also denoted as microfibrillated cellulose, MFC, and, nanofibrillated cellulose, NFC) with chitosan for the aqueous preparation of films showing high mechanical strength in the wet state. Also, transparency (∼70-90% in the range 400-800 nm) is achieved by suppressing aggregation and carefully controlling the mixing conditions: Chitosan dissolves in aqueous medium at low pH and under these conditions the CNF/chitosan mixtures form easily processable hydrogels. A simple change in the environmental conditions (i.e., an increase of pH) reduces hydration of chitosan promoting multivalent physical interactions between CNF and chitosan over those with water, resulting effectively in cross-linking. Wet water-soaked films of CNF/chitosan 80/20 w/w show excellent mechanical properties, with an ultimate wet strength of 100 MPa (with corresponding maximum strain of 28%) and a tensile modulus of 4 and 14 GPa at low (0.5%) and large (16%) strains, respectively. More dry films of similar composition display strength of 200 MPa with maximum strain of 8% at 50% air relative humidity. We expect that the proposed, simple concept opens new pathways toward CNF-based material utilization in wet or humid conditions, which has still remained a challenge.

Water sorption and diffusion coefficients of protons and Water in PVDF-g-PSSA polymer electrolyte membranes

Water sorption properties, proton NMR spectra, and diffusion of water and protons in poly(vinylidene fluoride)-graft-polystyrene sulfonic acid (PVDF-g-PSSA) polymer electrolyte membranes were studied. Sorption curves for the membranes with different degrees of grafting in protonated and Na+ form were measured by equilibrating the membranes over saturated salt solutions. The membrane water content was found to be sensitive to changes in relative humidity (RH). The water/sulfonic acid ratio λ for the protonated samples was around 2 at 20% RH and increased to λ ∼ 30 at 100%. Proton NMR, pulsed field gradient proton NMR (PFG-NMR), and impedance measurements were made on membranes with different λ. In the proton NMR spectra only one peak was found, originating from the water in the membrane. The chemical shift of the peak was found to be dependent on the counterion and the water content. The water self-diffusion coefficients DH2O, measured by PFG-NMR, increased with degree of grafting and water content of the membranes. The proton conductivity and the calculated proton mobility decreased more steeply than the DH2O with decreasing water content.

Investigations on the Humidity-Induced Transformations of Salbutamol Sulphate Particles Coated with l-Leucine

PurposeThe crystallization and structural integrity of micron-sized inhalable salbutamol sulphate particles coated with l-leucine by different methods are investigated at different humidities. The influence of the l-leucine coating on the crystallization of salbutamol sulphate beneath the coating layer is explored.MethodsThe coated particles are prepared by an aerosol flow reactor method, the formation of the l-leucine coating being controlled by the saturation conditions of the l-leucine. The coating is formed by solute diffusion within a droplet and/or by vapour deposition of l-leucine. The powders are humidified at 0%, 44%, 65% and 75% of relative humidity and the changes in physical properties of the powders are investigated with dynamic vapour sorption analysis (DVS), a differential scanning calorimeter (DSC), and a scanning electron microscope (SEM).ResultsVisual observation show that all the coated particles preserve their structural integrity whereas uncoated salbutamol sulphate particles are unstable at 65% of relative humidity. The coating layer formed by diffusion performs best in terms of its physical stability against moisture and moisture-induced crystallization. The degree of crystallization of salbutamol in the as-prepared powders is within the range 24–35%. The maximum degree of crystallization after drying ranges from 55 to 73% when the salbutamol crystallizes with the aid of moisture. In addition to providing protection against moisture, the l-leucine coating also stabilizes the particle structure against heat at temperatures up to 250°C.ConclusionIn order to preserve good flowability together with good physical stability, the best coating would contain two l-leucine layers, the inner layer being formed by diffusion (physical stability) and the outer layer by vapour deposition (flowability).

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.

Subcomponent Self‐Assembly: A Quick Way to New Metallogels

1-2-3 gel! Subcomponent self-assembly is introduced as a new design route towards multistimuli-responsive metallogels. It offers a rapid and facile access to supramolecular gels and allows to design smart materials with diverse functional and structural properties by simply exchanging one (or more) of the components. Herein, the exchange of the metal ions is emphasized (see scheme).