Structure and acoustic properties of hydrated nafion membranes.
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.
Plazanet
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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
Bartolini
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Torre
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Petrillo
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Sacchetti
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