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

Abstract

In the present communication, dielectric relaxation investigations on three interesting supercooled plastic crystalline substances, i.e., isocyanocyclohexane (ICNCH), cyanocyclohexane (CNCH), and 1-cyanoadamantane (CNADM) are reported. All of these have the main dipole moment situated in their side group- C[Triple Bond]N or- N[Triple Bond]C. Differential scanning calorimetry (DSC) was also employed as a supporting technique. Glassy crystal were easily formed in the first two samples by slowly cooling the plastic phase, but in CNADM it was formed by rapidly quenching the room temperature plastic phase. In addition to the so called alpha process that can reasonably be described by a Havriliak-Negami (HN) shape function, a secondary (or beta) relaxation process is found in all the materials. The beta process in CNADM has an activation energy (DeltaE(beta)) of about approximately 13.8+/-1 kJmol, and is present even in the corresponding ordered crystalline phase, i.e., in its monoclinic phase. On the other hand, the magnitude of DeltaE(beta) in both the isomers of cyanocyclohexane, i.e., ICNCH and CNCH, is similar and is about 21.1 and 23.4 kJmol, respectively. Unlike CNADM, the cyclohexane derivatives are capable of exhibiting additional intramolecular process due to chair-chair conversion (i.e., in addition to the rotational motion of the side group- C[Triple Bond]N or- N[Triple Bond]C). Therefore, the secondary process of these systems is compared to that occuring in the binary liquid glass formed by dispersing a small quantity of these dipolar liquids in nearly nonpolar orthoterphenyl (OTP). Measurements were also made in the supercooled binary mixures of other cyclohexyl derivatives like cyclohexylchloride and cyclohexylbromide with OTP which lack a flexible side group. The sub-T(g) relaxation process exhibited in all these cases have almost similar activation energy as in case of pure ICNCH and CNCH. These observations together with the fact that the activation energy for this process is much below that of chair-chair conversion which is about 43 kJmol leads us to the conclusion that sub-T(g) relaxation process in the binary mixtures is JG type, and perhaps beta relaxation process in phase I of ICNCH and CNCH is also similar. With the help of semiemperical calculations of the dipolemoments for the axial and equitorial confirmers, it is concluded that the process associated with the chair-chair may not be dielectrically very active and, hence, should be relatively weaker in magnitude. The beta process in CNADM has an activation energy (DeltaE(beta)) of about 13.8+/-1 kJmol, and is present even in the corresponding ordered crystalline phase indicating that it may not be characteristic of the glass formation of phase I. The molecular structure of CNADM is such that it does not possess other intramolecular degrees of freedom of the type equitorial to axial (or chair-chair) transformation. Our experimental finding that JG relaxation for CNADM dispersed in glassy OTP matrix is about 31 kJmol, indicating that the well ...

Secondary relaxation dynamics in rigid glass-forming molecular liquids with related structures

“…4(b). 19,22,23,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] It was noted before that the aforementioned relation of activation energy to T g is not universal. 41 Many true JG β-relaxations, not shown in Fig.…”

Broadband Dielectric Spectroscopic, Calorimetric, and FTIR‐ATR Investigations of D‐Arabinose Aqueous Solutions

“…On the other hand, the higher-frequency process, called the fast process, was explained either as a local secondary (Johari-Goldstein) relaxation of water molecules or as due to the reorientational motion of confined water molecules. The relaxation time of the slower process usually follows a Vogel-Fulcher-Tammann (VFT) [22] temperature dependence like typical glass-forming systems where the dynamics becomes frozen approaching low temperatures, and is quite similar to the well-known primary (a) relaxation observed in supercooled systems [23][24][25] above the glass transition temperature range. On the other hand the fast process shows a temperature dependence that is Arrhenius-like below the glass transition of the system, thus indicating that in the low-temperature range the reorientational motion of water molecules could be in some way restricted by the glassy matrix.…”

Effects of internal molecular degrees of freedom on the thermal conductivity of some glasses and disordered crystals

“…On further cooling a glass transition appears at approximately 135 K as proven by calorimetry and dielectric spectroscopy. 8,[34][35][36][37][38] An additional, small heat-capacity jump 39 also appears at around 55 K, which has been attributed to the freezing of the interconversion between the axial and equatorial conformers. 40 A stable low-temperature phase II results by annealing of phase I.…”