Jin Xiao, Guo Yuxing, Feng Shidong, Capaccioli Simone, Ngai K L, Wang Li-Min
State Key Lab of Metastable Materials Science and Technology and College of Materials Science and Engineering, Yanshan University, Qinhuangdao, Hebei 066004, China.
CNR-IPCF, Università di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy.
J Phys Chem B. 2020 Jul 30;124(30):6690-6697. doi: 10.1021/acs.jpcb.0c04444. Epub 2020 Jul 21.
The dielectric spectra of methyl -toluate (MMT) in supercooled liquid and glassy states were measured over wide ranges of temperature at ambient and elevated pressures . We found that the frequency dispersion of the loss peak contributed by the structural α-relaxation is invariant to changes of and , while keeping the loss peak frequency (,) constant. This isochronal superposition property of the α-relaxation holds for different choices of (,). The invariant frequency dispersions for the same (,) are also indicated by the fractional exponent β in the Fourier transform of the Kohlrausch-Williams-Watts (KWW) function. Similarly, the fragility index of MMT keeps approximately constant on varying pressure, largely different from H-bonded glass formers. The secondary βrelaxation at a frequency higher than (,) is found to shift to lower frequencies by elevating pressure in concert with the α-relaxation. The ratio τ(,)/τ(,) is approximately unchanged to variations of and while keeping τ(,) constant. These properties observed in MMT offer experimental evidence of the dynamic correlation between α- and βrelaxations in pure small-molecule glass-formers.
在环境压力和高压下，在很宽的温度范围内测量了甲基苯甲酸甲酯（MMT）在过冷液体和玻璃态下的介电谱。我们发现，由结构α弛豫引起的损耗峰的频率色散对于压力和温度的变化是不变的，同时保持损耗峰频率（T，p）不变。α弛豫的这种等时叠加特性适用于（T，p）的不同选择。相同（T，p）下的不变频率色散也由科尔劳施 - 威廉姆斯 - 瓦特（KWW）函数的傅里叶变换中的分数指数β表示。同样，MMT的脆性指数在压力变化时大致保持不变，这与氢键玻璃形成剂有很大不同。发现在高于（T，p）的频率处的次级β弛豫会随着α弛豫一起通过升高压力而向低频移动。在保持τ（T，p）不变的情况下，τ（T，p）/τ（T₀，p₀）的比值对于压力和温度的变化大致不变。在MMT中观察到的这些特性为纯小分子玻璃形成剂中α弛豫和β弛豫之间的动态相关性提供了实验证据。
