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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=N or– N=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 α process that can reasonably be described by a Havriliak–Negami (HN) shape function, a secondary (or β) relaxation process is found in all the materials. The β process in CNADM has an activation energy (ΔEβ) of about ≈13.8±1 kJ/mol, and is present even in the corresponding ordered crystalline phase, i.e., in its monoclinic phase. On the other hand, the magnitude of ΔEβ in both the isomers of cyanocyclohexane, i.e., ICNCH and CNCH, is similar and is about 21.1 and 23.4 kJ/mol, 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=N or– N=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-Tg 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 kJ/mol leads us to the conclusion that sub-Tg relaxation process in the binary mixtures is JG type, and perhaps β 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 β process in CNADM has an activation energy (ΔEβ) of about 13.8±1 kJ/mol, 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 kJ/mol, indicating that the well resolved sub-Tg process in CNADM is due to the small side group, i.e., –C=N and JG relaxation in phase I of CNADM is perhaps not resolvable or too small to be detected. [ABSTRACT FROM AUTHOR]

It is well known that rigid dipolar solutes (in smaller quantity) dispersed in a nonpolar glassy matrix exhibit a sub-Tg (or βs) relaxation due to the solute often designated as Johari–Goldstein (JG) relaxation, which is intermolecular in nature. In this article, we report the results of our study of such a sub-Tg process in a wide variety of dipolar solutes in different glassy systems using dielectric spectroscopy over a frequency range of 20–106 Hz down to a temperature of 77 K. The Tg of these solutions are determined using differential scanning calorimetry. The solvents used in this study are o-terphenyl (OTP), isopropylbenzene (IPB), and methylcyclohexane. In the case of rigid molecular solutes, like mono-halogen benzenes, the activation energy (ΔEβ) of the βs process is found to increase with decreasing Tg of the solvent, with a corresponding decrease in the magnitude of the βs process. In the case of more symmetrical molecular solute, for example, tert-butylchloride, the change in ΔEβ is not very appreciable. These results emphasize the importance of the size of the cage of the host matrix in the relaxation of the solute molecules. We have also studied the sub-Tg relaxation(s) due to some flexible molecular solutes, viz., 1butylbromide, 1hexylbromide, 1butylacetate, and benzylacetate. These solutes in IPB matrix exhibit only one relaxation, whereas in OTP matrix they exhibit an additional sub-Tg process, which may be identified with a JG type of relaxation. These observations lead us to the conclusion that the β process observed in the glassy states of these pure solutes is predominantly intramolecular in nature. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Some hexa-substituted benzenes are known to exhibit molecular rotation in their crystalline phase, but this phenomenon is not very well understood. In this context, we have critically examined two materials, namely pentachloronitrobenzene and 2,3,4,5,6-pentabromotoluene over a wide range of temperatures, using dielectric spectroscopy and differential scanning calorimetry (DSC) to ascertain the nature of the above said relaxation. A relaxation process is found in the crystalline phase, which is slightly non-Arrhenius in its temperature dependence in the former and deviation from Debye behavior has been found in both the samples. This behavior is similar to that of the primary (or α-) relaxation found in glass forming materials. This phenomenon is found to correspond to a steplike (small) change in the specific heat in the DSC measurements indicating that it is a glass transition phenomenon. The sub-T[sub g] relaxation data do not indicate any secondary (or β-) process (of measurable magnitude). However, the same materials when studied in small quantities in the liquid matrix of o-terphenyl (nonpolar) indicated a considerable magnitude for the β-process of molecules even below the liquid T[sub g] of these mixtures. The results are discussed in the context of the glass transition phenomena of the above materials. [ABSTRACT FROM AUTHOR]

We have critically examined the possibility of the existence of yet another process (often referred to as process II in the literature) of much smaller magnitude on the high frequency side in the case of 2-ethyl-1-hexanol and 4-methyl-3-heptanol using the dielectric relaxation technique. We have also studied the mixtures of 2-ethyl-1-hexanol with nonpolar methylcyclohexane and a mixture of 1-bromobutane in 4-methyl-3-heptanol for this purpose. In addition, the differential scanning calorimetry (DSC) has been used to study the structural relaxation in the vicinity of the glass transition temperature using the Tool-Narayanaswami-Moynihan procedure as discussed by Sartor et al. [J. Phys. Chem. 100, 6801 (1996)]. Our results show the presence of a clear process II on the higher frequency side whose freezing-out corresponds to the glass transition event at T[sub g] in the DSC studies. It is suggested that process II is associated with free alcohol molecules and is connected to the structural relaxation in alcohols. The dielectric data have been analyzed to understand the thermodynamics of hydrogen bonding using the model of Dannhauser [J. Chem. Phys. 48, 1911 (1968)]. An attempt has been made to correlate the ratio of relaxation rates of processes I and II and the observed deviation from Debye behavior in the case of alcohols. [ABSTRACT FROM AUTHOR]