Ngai K L, Wojnarowska Z, Paluch M
Dipartimento di Fisica, CNR-IPCF, Università di Pisa, Largo Bruno Pontecorvo 3, 56127, Pisa, Italy.
Institute of Physics, University of Silesia in Katowice, 75 Pułku Piechoty 1A, 41-500, Chorzow, Poland.
Sci Rep. 2021 Nov 12;11(1):22142. doi: 10.1038/s41598-021-01191-9.
The studies of molecular dynamics in the vicinity of liquid-glass transition are an essential part of condensed matter physics. Various experimental techniques are usually applied to understand different aspects of molecular motions, i.e., nuclear magnetic resonance (NMR), photon correlation spectroscopy (PCS), mechanical shear relaxation (MR), and dielectric spectroscopy (DS). Universal behavior of molecular dynamics, reflected in the invariant distribution of relaxation times for different polar and weekly polar glass-formers, has been recently found when probed by NMR, PCS, and MR techniques. On the other hand, the narrow dielectric permittivity function ε*(f) of polar materials has been rationalized by postulating that it is a superposition of a Debye-like peak and a broader structural relaxation found in NMR, PCS, and MR. Herein, we show that dielectric permittivity representation ε*(f) reveals details of molecular motions being undetectable in the other experimental methods. Herein we propose a way to resolve this problem. First, we point out an unresolved Johari-Goldstein (JG) β-relaxation is present nearby the α-relaxation in these polar glass-formers. The dielectric relaxation strength of the JG β-relaxation is sufficiently weak compared to the α-relaxation so that the narrow dielectric frequency dispersion faithfully represents the dynamic heterogeneity and cooperativity of the α-relaxation. However, when the other techniques are used to probe the same polar glass-former, there is reduction of relaxation strength of α-relaxation relative to that of the JG β relaxation as well as their separation. Consequently the α relaxation appears broader in frequency dispersion when observed by PCS, NMR and MR instead of DS. The explanation is supported by showing that the quasi-universal broadened α relaxation in PCS, NMR and MR is captured by the electric modulus M*(f) = 1/ε*(f) representation of the dielectric measurements of polar and weakly polar glass-formers, and also M*(f) compares favorably with the mechanical shear modulus data G*(f).
对液体 - 玻璃转变附近分子动力学的研究是凝聚态物理的重要组成部分。通常应用各种实验技术来理解分子运动的不同方面，即核磁共振（NMR）、光子相关光谱（PCS）、机械剪切弛豫（MR）和介电谱（DS）。最近发现，当用NMR、PCS和MR技术探测时，不同极性和弱极性玻璃形成体的弛豫时间不变分布反映出分子动力学的普遍行为。另一方面，通过假设极性材料的窄介电常数函数ε*(f)是德拜型峰和在NMR、PCS和MR中发现的更宽结构弛豫的叠加，对其进行了合理化解释。在此，我们表明介电常数表示ε*(f)揭示了在其他实验方法中无法检测到的分子运动细节。在此我们提出一种解决该问题的方法。首先，我们指出在这些极性玻璃形成体的α弛豫附近存在未解决的乔哈里 - 戈尔茨坦（JG）β弛豫。与α弛豫相比，JGβ弛豫的介电弛豫强度足够弱，以至于窄介电频率色散忠实地代表了α弛豫的动态非均匀性和协同性。然而，当使用其他技术探测相同的极性玻璃形成体时，α弛豫相对于JGβ弛豫的弛豫强度会降低以及它们之间的分离。因此，当通过PCS、NMR和MR而不是DS观察时，α弛豫在频率色散中显得更宽。通过表明极性和弱极性玻璃形成体的介电测量的电模量M*(f) = 1/ε*(f)表示捕获了PCS、NMR和MR中准普遍加宽的α弛豫，并且M*(f)与机械剪切模量数据G*(f)相比也很有利，支持了这一解释。
