On the nature of the high-frequency relaxation in a molecular glass former:: A joint study of glycerol by field cycling NMR, dielectric spectroscopy, and light scattering
On the nature of the high-frequency relaxation in a molecular glass former:: A joint study of glycerol by field cycling NMR, dielectric spectroscopy, and light scattering
复制标题
DOI：
10.1063/1.2906122
复制
发表时间：
2008-05-07
影响因子：
4.4
通讯作者：
Roessler, E. A.
中科院分区：
文献类型：
--
作者：
Gainaru, C.;Lips, O.;Roessler, E. A.
应用快场循环H-1 NMR弛豫法测定了玻璃形成剂甘油在宽温度（75-360 K）和频率（10 kHz-30 MHz）范围内的自旋-晶格弛豫时间T-1（ω）的色散。弛豫数据用与Ω/T-1（Ω）成正比的磁化率χ”（Ω）来分析，该磁化率与第二级（l=2）分子取向相关函数有关。宽带介电光谱结果表明玻璃化转变温度T-g以上频率温度叠加的有效性。这允许将不同温度的联合收割机NMR数据组合成单个主曲线chi”（ω τ（α）），其以降低的频率ω τ（α）延伸超过15个十年，其中τ（α）是结构α-弛豫时间。将该主曲线与介电谱（l=1）和去偏振光散射（l=2）的相应曲线进行比较。在ω τ（α）1处，在秩l=1和l=2的可分辨性之间反而出现差异。具体而言，在ω τ（α）>>1处，其中磁化率由所谓的过量翼支配，NMR和光散射光谱（均为l = 2）彼此相当一致，并且比介电（l= 1）光谱强约三倍。这是通过假设高频动态仅对应于小角度偏移来解释的。低于T-g，介电和NMR磁化率比较好，并表现出指数温度依赖性。(C)2008年美国物理学会。
Fast field cycling H-1 NMR relaxometry is applied to determine the dispersion of spin-lattice relaxation time T-1(omega) of the glass former glycerol in broad temperature (75-360 K) and frequency (10 kHz-30 MHz) ranges. The relaxation data are analyzed in terms of a susceptibility chi"(omega) proportional to omega/T-1 (omega), related to the second rank (l=2) molecular orientational correlation function. Broadband dielectric spectroscopic results suggest the validity of frequency temperature superposition above the glass transition temperature T-g. This allows to combine NMR data of different temperatures into a single master curve chi"(omega tau(alpha)) that extends over 15 decades in reduced frequency omega tau(alpha), where tau(alpha) is the structural alpha-relaxation time. This master curve is compared with the corresponding ones from dielectric spectroscopy (l=1) and depolarized light scattering (l=2). At omega tau(alpha) 1, there rather appears a difference between the susceptibilities of rank l=1 and l=2. Specifically, at omega tau(alpha)>>1, where the susceptibility is dominated by the so-called excess wing, the NMR and light scattering spectra (both l = 2) rather coincide with each other and are about three times more intense than the dielectric (l= 1) spectrum. This is explained by assuming that the high frequency dynamics correspond to only small-angle excursions. Below T-g, dielectric and NMR susceptibility compare well and exhibit an exponential temperature dependence. (C) 2008 American Institute of Physics.