title: Structure and acoustic properties of hydrated nafion membranes.
abstract: The propagation of acoustic waves in water-hydrated Nafion membrane has been monitored using heterodyne-detected transient grating spectroscopy. At room temperature, upon increasing the water content, the speed of sound drops to a value lower than the respective velocities of sound in pure Nafion and pure water. This counterintuitive effect can be explained by a simple calculation of the sound velocity in an effective medium made of water and Nafion polymer. Upon cooling, a phase separation occurs in the sample, and the formation of ice is observed (M. Pineri et al. J. Power Sources 2007, 172, 587-596). This phase transition is characterized via a second acoustic wave observed in the signal. Sound propagation and X-ray diffraction confirm the formation of crystalline ice on the membrane surface, that reversibly melts upon heating. The amount of ice that forms in the sample is monitored as a function of temperature and represents an order parameter for the transition. This parameter follows a power law with an exponent of 0.5, indicating the critical nature of the observed process.
authors.affiliation: European Laboratory for Non-Linear Spectroscopy and Dipartimento di Fisica, University di Firenze, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy. marie.plazanet@ujf-grenoble.fr
authors.forename: M
authors.initials: M
authors.lastname: Plazanet
authors.forename: P
authors.initials: P
authors.lastname: Bartolini
authors.forename: R
authors.initials: R
authors.lastname: Torre
authors.forename: C
authors.initials: C
authors.lastname: Petrillo
authors.forename: F
authors.initials: F
authors.lastname: Sacchetti