Glass Transition Phenomena in Two-Component Plastic Crystals:  Study of Hexasubstituted Benzenes

Abstract

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.

Study of secondary relaxation in disordered plastic crystals of isocyanocyclohexane, cyanocyclohexane, and 1-cyanoadamantane

“…[48][49][50][51][52][53][54] From Table III, it is interesting to note that the value of the exponent "r" is located in a narrow range of 9-12, as found in many of the supercooled liquids 19,26,55 and plastic crystals. 13,14,[22][23][24] Recently, Colby 56 has given a physical interpretation of this high exponent value, where it is related to the size of the cooperatively rearranging region.…”

“…͑1͒. 23 Unlike in other glassy systems, the ␤ process in these two samples is asymmetric and could be represented by Eq. ͑1͒.…”

“…4. The temperature dependence of ␣ process follows the critical power law 22,23,26 given by the equation below, FIG. 3.…”

“…On one hand, alcohols demonstrate multiple relaxation processes 21 attributable to intramolecular motions such as the -OH group relaxation, while on the other in glassy crystals of hexasubstituted bezenes, no clearly resolvable ␤ process is observed. [22][23][24] In this context, 1-cyanoadamantane ͑CNADM͒ ͑C 10 H 15 CN, T g = 168.4 K͒ and cyanocyclohexane ͑CNCH͒ ͑C 6 H 11 CN, T g = 135.6 K͒ in which the main dipole is situated in the side group -C w N, and isocyanocyclohexane ͑ICNCH͒ ͑C 6 H 11 NC, T g = 131.3 K͒ with the main dipole connected to the cyclohexyl ring as -N w C should prove to be interesting; the corresponding activation energy of the ␤ process should reveal if the process is effected by the type of side group of the molecules.…”

Double Primary Relaxation in a Highly Anisotropic Orientational Glass-Former with Low-Dimensional Disorder

“…The phase behaviour of this molecule, which forms a layered structure of triangular planes, has been extensively studied. 19,23,24,[34][35][36][37][38][39][40][41] A glass transition was observed by adiabatic calorimetry at approximately 185 K, which was assigned to the freezing of the orientational disorder within the hexagonal planes. 37 Similar results were found by Correia et al by DSC and thermally stimulated currents, with a reported transition temperature of 191 K. 38,39 Here, we show by means of broadband dielectric spectroscopy (BDS) that such phase is characterized by a double primary (collective) relaxation which is related to the low-dimensional character of the system and which results in a spectral landscape reminiscent of α relaxations characterized by an excess wing.…”