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 a...