Thermodynamic Study on Simple Molecular Glasses

Abstract

We have combined our recent calorimetric works on vapor-deposited simple molecular glasses and our previous works on ordinary molecular glasses to investigate the universal thermodynamic feature of molecular glasses. Following a brief review for the studies on glass transitions, we describe how the vapor-deposition method is useful for simple molecular glasses. Then, we mention the structure of the adiabatic calorimeter recently developed for vapor-deposited samples. The heat capacities C_p of vapor-deposited carbon tetrachloride (CCl₄), propene (CH₂=CHCH₃) and propane (CH₃CH₂CH₃) are shown. Low-energy excitations characteristic to glasses were observed in all glassy samples. Glass transitions appeared in propene and propane at 56 K and 45 K, respectively; 45 K is the lowest record of Tgs of molecular glasses. The configurational heat capacity ΔC_p and entropy S_c are calculated from the C_p data of ten molecular glasses. The universal feature for the temperature dependence of ΔC_p is demonstrated by scaling temperature using the Kauzmann temperature T_K. The size z* of cooperatively rearranging region (CRR) is also calculated based on the Adam-Gibbs theory. They are increasing with decreasing temperature and frozen at each T_g. The frozen numbers z*(0) are 3-7 molecules and depend on (T_g-T_K)/T_K; the larger (T_g-T_K)/T_K is, the smaller z*(0) becomes. Thus, the present work has demonstrated the universal thermodynamic feature of molecular glasses and indicated the validity of the Adam-Gibbs theory based on the experimental data.