Pretransitional effects at the isotropic-mesomorphic phase transitions in the TBAA series

Thermal microscopy (TM), differential scanning calorimetry (DSC) and dilatometric p(t) experiments were carried out to characterize the phases and to study the nature of mesomorphic transitions in two higher homologues of terephthalylidene-bis-p-n-alkylanilines, TBnAs, namely terephthalylidene-bis-p-n-tetradecylaniline, TB14A and terephthalylidene-bis-p-hexadecylaniline, TB16A. Results infer the first order nature of the isotropic-smectic C (I-S c ) and smectic C-smectic I (S c -S I ) transitions, while the smectic I-smectic F (S I -S F ) transition is found to be second order. Pretransitional effects are estimated through α eff in the vicinity of mesomorphic fluctuation dominated non-linear regions (FDNLR), of the isotropic to mesomorphic transitions in TBnA homologous compounds and the relative strength of growth of FDNLR is discussed at the S I N, S I S A and S I S C phase transitions.