Plazanet M, Bartolini P, Torre R, Petrillo C, Sacchetti F
European Laboratory for Non-Linear Spectroscopy and Dipartimento di Fisica, University di Firenze, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy.
J Phys Chem B. 2009 Jul 30;113(30):10121-7. doi: 10.1021/jp901406v.
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.
利用外差检测瞬态光栅光谱法监测了声波在水合Nafion膜中的传播。在室温下，随着含水量的增加，声速降至低于纯Nafion和纯水各自声速的值。这种违反直觉的效应可以通过对由水和Nafion聚合物组成的有效介质中的声速进行简单计算来解释。冷却时，样品中会发生相分离，并观察到冰的形成（M. Pineri等人，《电源杂志》2007年，第172卷，第587 - 596页）。这种相变通过信号中观察到的第二个声波来表征。声音传播和X射线衍射证实了膜表面形成了结晶冰，加热时会可逆地融化。监测样品中形成的冰量随温度的变化，它代表了该转变的一个序参量。这个参数遵循指数为0.5的幂律，表明所观察到的过程具有临界性质。
