Investigation of the phase transitions and microstructural features of 6BA/nABA (n = 3, 4) mesogenic complexes

This study investigates the phase transitions and microstructural features of hydrogen-bonded mesogenic complexes formed from mesogenic 4-hexylbenzoic acid (6BA) and non-mesogenic n-aminobenzoic acid (nABA; n = 3, 4). Differential Scanning Calorimetry (DSC) and Polarised Optical Microscopy (POM) confirmed liquid crystalline behaviour in both 6BA/3ABA and 6BA/4ABA complexes, following the phase sequence: Crystal (Cr) ↔ Smectic C (SmC) ↔ Nematic (N) ↔ Isotropic (I). The 6BA/4ABA complex exhibited a wider nematic range (70.6 °C) and an overall mesophase range of 81.9°C. Order parameters and activation energies were derived from DSC data. FTIR analysis confirmed hydrogen bonding between the components, with the resulting complexes forming monomeric structures. X-ray diffraction (XRD) analysis further supported successful complex formation, revealing average basal spacings of 4.38 Å and 4.77 Å for the 6BA/3ABA and 6BA/4ABA complexes, respectively. The crystallite size was larger in the 6BA/3ABA complex (28.67 nm) compared to the 6BA/4ABA complex (24.28 nm), indicating higher crystallinity in the latter. Structural analysis suggested a monoclinic crystal structure for both complexes. These findings demonstrate that the synthesised 6BA/3ABA and 6BA/4ABA complexes, particularly 6BA/4ABA, exhibit favourable thermal stability and extended nematic phase ranges, making them promising candidates for liquid crystal applications.