Thermodynamic aspects of the glass transition phenomenon. II. Molecular liquids with variable interactions

As a contribution to the understanding of the thermodynamics of the glass transition phenomenon a series of molecules having the same steric character, but differing in the strength and nature of intermolecular interactions, has been investigated. The series is based on systematic changes of substituents on disubstituted benzene ring compounds, the simplest example of which is meta-xylene. Meta-isomers are chosen in each instance because of their greater tendency to supercool. In particular, m-fluoroaniline cannot be crystallized at ambient pressure. The principal measurements performed were of heat capacity and enthalpy change, using the technique of differential scanning calorimetry, and these have been examined in the light of literature data on the liquid viscosities and some recent data for dielectric relaxation. As the strength of hydrogen-bonding interactions between the ring substituents on adjacent molecules increases, the glass transition temperature T-g increases by almost 100 degrees from the lowest value in the series, 122.5 K, for m-fluorotoluene. Empirical rules involving T-b/T-m and T-g/T-m are found wanting. The important thermodynamic characteristic of the glass transition, viz., the change in heat capacity at the glass transition, Delta C-p, remains approximately constant until the -OH substituent is introduced, whereupon a new element appears. This is a specific component of Delta C-p which appears at temperatures above an initially small jump at T-g. It is well accounted for by the addition of a two-state H-bond breaking component (with the usual H ...-OH bond energy) to the total excess heat capacity. The liquid ground state (or Kauzmann) temperature T-K assessed from thermodynamic data acquired in this study, falls 20%-30% below the glass transition temperature. From the limited transport data available, these liquids appear to be quite fragile in character implying that the phenyl group influence dominates the hydrogen bond factor which has often seemed responsible for decreased fragility. In the case of cresol the hydrogen bonding apparently produces dielectric/shear relaxation anomalies of a character previously only seen in certain aliphatic monoalcohols. (C) 1999 American Institute of Physics. [S0021-9606(99)50909-4].