Shahin Md, Murthy S S N, Singh L P
School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110 067, India.
J Phys Chem B. 2006 Sep 21;110(37):18573-82. doi: 10.1021/jp063599j.
We have critically examined the relaxation that is known to occur in the crystalline phase of pentachloronitrobenzene (PCNB) and 2,3,4,5,6-pentabromotoluene using dielectric spectroscopy and differential scanning calorimetry (DSC). Within the resolution of our experimental setup, a relaxation process similar to that of the primary (or alpha-) relaxation is found. A slight deviation from Arrhenius behavior is noticed only in the vicinity of the glass transition temperature (T(g)). This deviation and a small steplike change found in the DSC scans at T(g) indicates that the "fragility" of these plastic crystals is rather low. However, in PCNB, the dielectric strength (Deltaepsilon) of the above said alpha-process did not change appreciably with temperature, and, interestingly, a small addition of an impurity such as pentachlorobenzene (PCB) to the molten state of PCNB drastically lowered the dielectric strength and the calorimetric signature of glass transition phenomena in the DSC data at T(g). The room-temperature powder X-ray diffraction measurements in combination with the DSC data in the melting temperature region did not indicate any observable change in the crystalline structure. A residual alpha-process with no significant change in the shape of the dielectric spectrum indicates that the hindrance to the rotational motion of PCNB molecules is caused by the presence of a small number of PCB molecules in the crystalline lattice of PCNB over a certain region. Outside of this region, the original PCNB disordered phase is preserved, which is the origin of the residual alpha-process. With a further increase in PCB concentration, the alpha-process, characteristic of pure PCNB, vanishes, and instead another relaxation develops. This process is explained with the help of a solid-liquid phase diagram of the alpha-process of the plastic phase of 2:1 and 1:2 compound formations, which are stable below 386 +/- 1 and 366 +/- 1 K, respectively.
我们使用介电谱和差示扫描量热法（DSC），对已知在五氯硝基苯（PCNB）和2,3,4,5,6 - 五溴甲苯晶相中发生的弛豫进行了严格研究。在我们实验装置的分辨率范围内，发现了一个与初级（或α - ）弛豫相似的弛豫过程。仅在玻璃化转变温度（T(g)）附近才注意到与阿仑尼乌斯行为略有偏差。这种偏差以及在DSC扫描中在T(g)处发现的一个小的阶梯状变化表明，这些塑性晶体的“脆性”相当低。然而，在PCNB中，上述α过程的介电强度（Δε）随温度变化不明显，而且有趣的是，向PCNB熔融态中少量添加五氯苯（PCB）等杂质，会显著降低介电强度以及DSC数据中T(g)处玻璃化转变现象的量热特征。室温粉末X射线衍射测量结合熔融温度区域的DSC数据，未表明晶体结构有任何可观察到的变化。一个介电谱形状无显著变化的残余α过程表明，PCNB分子旋转运动的阻碍是由PCNB晶格中特定区域内少量PCB分子的存在引起的。在该区域之外，原始的PCNB无序相得以保留，这就是残余α过程的起源。随着PCB浓度进一步增加，纯PCNB特有的α过程消失，取而代之的是另一种弛豫发展起来。这一过程借助2:1和1:2化合物形成的塑性相α过程的固 - 液相图得到了解释，这两种化合物分别在386±1 K和366±1 K以下稳定。
