Glass transition dynamics and boiling temperatures of molecular liquids and their isomers

The relation between a dynamic and a thermodynamic temperature, glass transition T_g and boiling point T_b, is investigated for various glass-forming liquids, with emphasis on monohydroxy alcohols. As is well known, T_b and T_g are positively correlated across a large variety of liquids. However, we found that the same quantities show a negative correlation within an isomeric series, i.e., T_b decreases with increasing T_gfor different isomers of the same chemical formula. For the alcohol series, C_nH_2n+1OH with 3 ≤ n ≤ 10, a master curve of the negative T_g - 7_b correlation is obtained if the temperatures are normalized to the respective values of the n-alkanols. This T_g - T_b, dependence of isomeric liquids is linked to entropic effects and responsible for much of the scatter of the correlation observed for a large number of molecular organic glass-formers with 45 < T_g < 250 K. Dielectric relaxation is measured for three groups of isomers: (a) 3-methoxyl-l-butanol and 2-iso-propoxyethanol, (b) 1,4-, 1,2-, and 2,4-pentanediol, and (c) di-n- and di-iso-butyl phthalate. Two key parameters of the dynamics, fragility m and stretching exponent β, are found to be indistinguishable within isomers of moderately different T _gs. Larger fragility differences are readily expected with pronounced structural change, but no systematic trend is observed within an isomer series. The results provide a useful tool for assessing T_g, m, and β for marginal glass formers on the basis of their isomers.