Effects of the presence/absence of hydroxyl or amine groups in mono/di-substituted benzenes on the α and β structural relaxation properties in liquids and glasses

Dielectric relaxation properties of six benzene-derivative liquids were studied in the temperature range above 80 K and in the frequency range (0.1 Hz)-(1 MHz); 3-isopropylphenol and 2-phenyl-1-propanol with hydroxyl group within their substituents, 3-isopropylaniline and dl-1-phenylethylamine with amine group, and isopropylbenzene and sec-butylbenzene only with alkyl group. Heat capacities were additionally measured of sec-butylbenzene in the range 10-300 K. The β relaxation process as well as the α one was found to be present in all the substances. A striking contrast was found between the substituent dependences of α and β glass transition temperatures Tgα and Tgβ. While the Tgα was considerably high in the substances with hydroxyl group, a little lower in those with amine group than with hydroxyl group, and relatively low in those with only alkyl group as the substituent, the Tgβ was essentially the same being in the range 73-77 K irrespective of the presence/absence of hydroxyl or amine groups within the substituents of a limited size of isopropyl group. In 2-phenyl-1-propanol and sec-butylbenzene the substituents of which have hydroxyl or methyl groups, respectively, extruding beyond the size, the bulkiness appears to raise the Tgβ to 100-105 K. The character is discussed based on the heterogeneous structure originating from short-range ordering in molecular aggregation.