At low humidity (<30 RH%), water molecules begin to fill the intermolecular spaces in the membrane. At high humidity (80 RH%) the membrane swells; the free volume size increases.

No abstract

A casting method becomes used for the preparation of the polyvinyl alcohol (PVA)/TiO2 nanocomposite. Characterization of the casting membranes was done using Fourier transform infrared spectrometer (FTIR), thermal gravimetric analysis, and AC impedance method. Surface morphology, element analysis, and distributions were investigated by scanning electron microscopy/energy dispersive X‐ray analysis (SEM/EDX). Positron annihilation spectroscopy was used to examine the microstructure of the membranes under study, while their crystallinity was checked by wide‐angle X‐ray diffraction (WAXD). The FTIR spectra suggest a strong interaction between TiO2 and PVA nanoparticles, whereas WAXD results indicated that the amalgamation of TiO2 nanoparticles into the PVA matrix lowers the main crystallinity of PVA. The free volume size for PVA/TiO2 with cross‐linker decreases suggest that the filling of the cavities by Ti3+ and O− ions as well as complex formation, that is, lead to a lower value of permeability. Because of their lower cost, higher thermal and lower permeability, the cross‐linked PVA/TiO2 membrane was considered to use in alkaline direct methanol fuel cells.

No abstract

The proton conductivity mechanism in per-fluorinated sulfonic acid/PTFE copolymer Fumapem ® membranes for polymer electrolyte membranes has been investigated. Three samples of Fumapem ® F-950, F-1050 and F-14100 membranes with different ion exchange capacity 1.05, 0.95, and 0.71 meq/g, respectively, were used in this study after drying. The o-Ps hole volume size (VF V,Ps) was quantified using the positron annihilation lifetime technique while the proton conductivities (σ) were measured using LCR Bridge as function of temperature. It was found that as the ion exchange capacity increases, the proton conductivity increases and the free volume expands. Temperature dependences of proton conductivity and the o-Ps hole volume size (VF V,Ps) reflect the glass transition temperature of the membrane. A good linear correlation between the reciprocal of the o-Ps hole volume size (1/VF V,Ps) and log(σ) + ∆Ea/2.303kBT , (where ∆Ea is the activation energy, T is the absolute temperature and kB is the Boltzmann constant) at different temperatures indicate that the ionic motion in dry Fumapem ® is governed by the free volume. A linear relationship between the critical hole size γV * i and the ion exchange capacity was also achieved.