Heat Capacity of N-(o-Hydroxy-p-Methoxybenzylidene)-p-Butylaniline: A Glassy Nematic Liquid Crystal

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Abstract
The heat capacity of N-(2-hydroxy-4-methoxybenzylildene)-4′-butylaniline(2,4-OHMBBA) with a purity of 99.393 mole % was measured between 13 and 375 K. The Melting point T m was 314.30 K, and the enthalpy and the entropy of fusion were 22.405 kJ mol−1 and 71.425 J K−1 mol−1, respectively. The temperature of transition from nematic to isotropic liquid was 33.65 ± 0.1 K, and the enthalpy and the entropy changes due to this mesophase transition were 887.4 ± 4.9 J mol−1 and 2.690 ± 0.015 J K−1 mol−1, respectively. A second-order character has been postulated for the mesophase transition. By cooling the nematic liquid crystal at a rate of − 12.2 K min−1 a frozen-in state of the stable nematic phase, i.e. a glassy state, was realized. The glass transition temperature Tg was 204 K and the heat capacity jump at T g was Δ C = 107 J K−1 mol−1. The activation enthalpy for the glass transition was estimated to be Δ H = 75 kJ mol−1. The residual entropy of the glassy state at O K was determined to be 12.69 ± 0.16 J K−1 mol −1. The enthalpy of the glassy state at O K was 11.38 ± 0.04 kJ mol−1 higher than that of crystal. The term Glassy liquid crystal is proposed here to indicate a glassy state of this kind. To elucidate the relationship between molecular structure and liquid crystallinity 4 different 2,4-OHMBBA isomers were synthesized. None of them showed liquid crystallinity. From differential thermal analyses it was found that three of them exhibit a glassy state of isotropic liquid: 2,3-OHMBBA (T g = 230 K; T m = 320 K; Δ H = 85 kJ mol−1), 2,5- OHMBBA (218 K; 321.7 K; 90 kJ Mol−1), and 4.3-OHMBBA (265 K; 357.7 K; 73 kJ Mol−1). 3.4-OHMBBA gave no glassy state but a solid-solid transition at 333 K was noted. The material melted at 349 K. 3,4-OHMBBA obtained from the melt exhibited a transiton between metastable crystalline phases at 216 K.