Dipartimento di Fisica and CNISM-CNR, Università de L'Aquila, via Vetoio, 67100 Coppito, Italy.
J Chem Phys. 2012 Sep 7;137(9):094504. doi: 10.1063/1.4748354.
Brillouin light scattering spectra from transverse and longitudinal acoustic waves in liquid and supercooled 3-methylpentane have been collected from room temperature down to 80 K, just above the glass transition. Spectra at different wave vectors have been obtained using 532 nm and 266 nm excitation. We found evidence of a shear relaxation with a characteristic time of 100 s at the glass transition which only partly accounts for the relaxation observed in the propagation and attenuation of the longitudinal modes. The inclusion of a relaxing bulk viscosity contribution with a relaxation time of the order of 10(2) ns at the glass transition is found to adequately reproduce the experimental data including transient grating data at a much lower frequency. A consistent picture of relaxed shear and bulk moduli as a function of temperature is derived. These two quantities are found to be related by a linear relation suggesting that a Cauchy-like relation holds also above the glass transition.
已从室温收集到液体和过冷的 3-甲基戊烷中横波和纵波的布里渊光散射光谱，直至 80 K，刚好高于玻璃化转变温度。使用 532nm 和 266nm 激发获得了不同波矢的光谱。我们发现，在玻璃化转变时，剪切弛豫具有 100s 的特征时间，这仅部分解释了在纵模传播和衰减中观察到的弛豫。发现包括玻璃化转变时松弛体粘度贡献的弛豫时间为 10(2)ns 的松弛体粘度贡献，足以重现实验数据，包括在低得多的频率下的瞬态光栅数据。得出了弛豫剪切模量和体模量随温度变化的一致图像。这两个量之间存在线性关系，表明在玻璃化转变温度以上也存在类似 Cauchy 的关系。
