No abstract

For the first time, we report the effects of elevated temperatures, from 80 to 100 °C, on the changes in the states of water and ion–water channels and their correlation with the proton conductivity of Nafion NR212, which was investigated using a Fourier transform infrared spectroscopy study. Experimentally, three types of water aggregates, protonated water (H + (H 2 O) n ), nonprotonated hydrogen (H)-bonded water (H 2 O···H 2 O), and non-H-bonded water, were found in Nafion, and the existence of those three types of water was confirmed through ab initio molecular dynamics simulation. We found that the proton conductivity of Nafion increased for up to 80 °C, but from 80 to 100 °C, the conductivity did not increase; rather, all of those elevated temperatures showed identical conductivity values. The proton conductivities at lower relative humidities (RHs) (up to 50%) remained nearly identical for all elevated temperatures (80, 90, and 100 °C); however, from 60% RH (over λ = 4), the conductivity remarkably jumped for all elevated temperatures. The results indicated that the amount of randomly arranged water gradually increased and created more H-bonded water networks in Nafion at above 60% RH. From the deconvolution of the O–H bending band, it was found that the volume fraction f i ( i =each deconvoluted band) of H-bonded water for elevated temperatures (>80–100 °C) increased remarkably higher than for 60 °C.

SPEEK is known to possess high proton conductivity at high water content, being comparable with other popular membranes used in fuel cells, such as Nafion and sulfonated polyethersulfone (SPES). However, much less is known about its fundamental properties, including the status of proton dissociation and spectral features. In this work, the properties of two model molecules of SPEEK, M1 (20 atoms), M2 (50 atoms) and their hydrated systems, M1 + nH2O and M2 + nH2O (n = 1-9), have been investigated using static electronic structure calculations and the ab initio molecular dynamics (MD) method. Optimized structures for all of the systems and the trajectories of M1 + nH2O (n = 3-6) at finite temperatures have been computed using density functional theory at the B3LYP level of theory. Proton dissociation has been discussed in detail, especially for n = 3 and n = 4. In addition, the infrared spectra of SPEEK and its hydrated systems have been studied using a combination of theory and experiment. The characteristic bands of SPEEK and the surrounding water clusters have been assigned with emphasis on their relationship with the degree of proton dissociation. We have found that the hydronium ion stretching modes, which appear in the 2000-3000 cm(-1) region in static electronic structure calculations, are not observed experimentally. This discrepancy is explained by the stationary structure and the temperature effect.

The effects of water on the changes in morphology of sulfonated poly(phenylene sulfide) (SPPS) hydrocarbon polymer electrolyte membranes (PEM) with an ion exchange capacity (IEC) of 0-2.0 mequiv/g are investigated using small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). Wide-angle X-ray scattering (WAXS) was used to characterize the effect of direct sulfonation on the changes in membrane crystalline structure, and it was found that the crystallinity and crystallite domain size decrease and the volume of the amorphous region in the SPPS membranes increases with increasing IEC. The experimental data have been fitted to the Porod law for approaching the analysis of the sharpness of the polymer/water interface, development of the proton channel, or dispersion of water in the hydrated membranes. Porod plots showed positive deviation which revealed that the polymer/water interface in the hydrated SPPS membrane is not smooth but diffused and a well-developed proton channel does not form in the membrane.

ABSTRACT:Poly(vinylidene fluoride/trifluoroethylene)single crystalline (SC) films, which were prepared by uniaxial stretching followed by crystallization in the paraelectric phase, are highly double-orientated, that is, the orthorhombic c-axis, and the axes perpendicular to the (110)/(200) planes of fully extended chain crystals, are preferentially oriented parallel to the stretching axis and normal to the film surface, respectively. The thickness dependence of orientation distributions of these axes in SC films were evaluated using wide-angle X-Ray diffraction. The results revealed that the orientation factors of the c-axis and the axes perpendicular to the (110)/(200) planes increase with decreasing film thickness. It is suggested that the preferential orientation of the axes perpendicular to the (110)/(200) planes to the direction parallel to the film normal, which is developed in the hexagonal phase, is very high near the film surface, but it becomes gradually weaker as the distance from the surface increases. The formation mechanism of the orientational order is discussed, and a model for the preferential orientation distribution of the crystal planes is proposed.KEY WORDS P(vinylidene fluoride/trifluoroethylene) Copolymer/ Single Crystalline Film / Double Orientation/ Orientation Distribution/ Crystallization/ Much attention has been paid so far to the copolymers of vinylidene fluoride and trifluoroethylene, P(VDF/ TrFE), due to their prominent ferroelectricity, strong piezoelectricity and widespread applications to various piezoelectric devices, especially to ultrasonic transducers. These properties are highly dependent on the orientation of the crystal axes, and hence the properties can often be tailored by controlling the orientation of the polymer chains. Therefore it is of considerably practical and scientific interest to understand the orientation mechanism of P(VDF/TrFE) copolymer films.Ohigashi et al. 1 first found that a single crystalline (SC) film of P(VDF/TrFE) is formed by crystallization of a uniaxially stretched film in the hexagonal (paraelectric) phase under the condition that both surfaces of the film are made free from inhomogeneous stresses arising from contact with solid materials. In this highly crystallized film, the orthorhombic [001] axis and the axes perpendicular to the (110) and/or (200) planes of fully extended chain crystals are preferentially oriented parallel to the stretching axis and normal to the film surface, respectively. Owing to such unique structure, the SC films have mechanical and piezoelectric properties much superior to those oflamellar crystalline films. 1 ' 2 The preferential orientation of chain molecules in fluoro-polymers is very sensitive to the boundary condition at the film surface as in P(VDF/TrFE). Wittmann and Smith 3 investigated friction-transfer thin layers of a single crystal-like film of polytetrafluoroethylene (PTFE). The observed chain orientation and crystal perfection were very high. They found that the PTFE thin layer has ability to induce...