Sub-Tg relaxations due to dipolar solutes in nonpolar glass-forming solvents.

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]