Ngai K L
CNR-IPCF, Largo Bruno Pontecorvo 3, Pisa I-56127, Italy.
J Phys Chem A. 2021 Apr 8;125(13):2759-2763. doi: 10.1021/acs.jpca.1c00761. Epub 2021 Mar 24.
A longstanding problem in the Brillouin light scattering (BLS) study of polymers is the relaxation times τ() being more than an order of magnitude shorter than the α-relaxation times τ() determined by dielectric, depolarized light scattering (DLS), and molecular dynamics simulations. In tackling the problem, τ() was identified with the relaxation time τ() of the primitive relaxation in the coupling model, which can be calculated from τ() and the stretch exponent β of the Kohlrausch correlation function for the α-relaxation.. The problem was solved by finding that indeed τ() is in good agreement with τ(). A recent work performed the neutron spin echo study of the structural α-relaxation of the network ionic liquid ZnCl and found the same anomaly as polymers. The α-relaxation time τ() from neutron spin echo (NSE) as well as the α-relaxation time τ() from DLS of ZnCl are much longer than τ() from BLS obtained before by several research groups. The finding of the same anomaly in ZnCl and polymers with very different chemical and physical structures offers an opportunity to critically test the explanation given before. The test was carried out by calculating the primitive relaxation times τ() and τ() from τ() and τ(), respectively, in zinc chloride. Good agreements of τ() from BLS with τ() and τ() were found and thus the explanation given for polymers remains valid for ZnCl. The test was extended to glycerol by comparing τ() with τ() and τ() calculated from the α-relaxation time τ() and τ() from incoherent and coherent neutron scattering, respectively. There is good agreement between τ() and τ() in glycerol. There is also semiquantitative agreement of τ() with τ() from dielectric spectroscopy as well as τ(). Thus, the explanation for polymers is verified in the two very different glass formers, ZnCl and glycerol, and it is an advancement in the application of BLS to study the dynamics of glass formers.
在聚合物的布里渊光散射（BLS）研究中，一个长期存在的问题是弛豫时间τ（）比通过介电、去偏振光散射（DLS）和分子动力学模拟确定的α弛豫时间τ（）短一个多数量级以上。在解决这个问题时，τ（）被确定为耦合模型中原始弛豫的弛豫时间τ（），它可以根据τ（）和α弛豫的科尔劳施相关函数的伸展指数β来计算。通过发现τ（）确实与τ（）吻合良好，这个问题得到了解决。最近的一项工作对网络离子液体ZnCl的结构α弛豫进行了中子自旋回波研究，发现了与聚合物相同的异常现象。来自中子自旋回波（NSE）的α弛豫时间τ（）以及ZnCl的DLS的α弛豫时间τ（）比之前几个研究小组通过BLS获得的τ（）长得多。在化学和物理结构非常不同的ZnCl和聚合物中发现相同的异常现象，为严格检验之前给出的解释提供了一个机会。通过分别从氯化锌中的τ（）和τ（）计算原始弛豫时间τ（）和τ（）来进行测试。发现BLS的τ（）与τ（）和τ（）吻合良好，因此之前对聚合物给出的解释对ZnCl仍然有效。通过将τ（）分别与从不相干和相干中子散射得到的α弛豫时间τ（）和τ（）计算得到的τ（）和τ（）进行比较，将测试扩展到了甘油。甘油中τ（）与τ（）吻合良好。τ（）与介电谱的τ（）以及τ（）也有半定量的吻合。因此，在两种非常不同的玻璃形成体ZnCl和甘油中验证了对聚合物的解释，这是BLS在研究玻璃形成体动力学应用方面的一个进展。
