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A systematic crystallization kinetic study using thermal microscopy and diOE erential scanning calorimetry has been carried out on two novel liquid crystalline compounds, DBA : MHB and DBA : ACP. These involve intermolecular hydrogen bonding between 4-n -decyloxybenzoic acid (DBA) and methyl 4-hydroxybenzoate (MHB); and between DBA and 2-amino-5-chloropyridine (ACP). The kinetics experiments were performed from the crystal G phase, which is a common induced kinetophase in both the compounds. Further, the proton donor and acceptor capabilities of the COOH group of DBA towards the OH group of MHB and N atom of ACP were studied in the light of mesomorphism and rate of crystallization. The dimensionality in the crystal growth and the sporadic nucleation were estimated from the Avrami exponent, n . A similar type of crystallization mechanism is predicted to operate for all the crystallization temperatures. The characteristic crystallization time ( t *) at each crystallization temperature is deduced from the individual plots of log t vs. D H (change in enthalpy).

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IntroductionSupramolecular liquid crystals, obtained through hydrogen-bonding of complementary molecules, have extensively been investigated in recent years. Following the well-established examples of liquid crystal formation through dimerization of aromatic carboxylic acids [1,2], several classes of compounds have been reported, including conventional liquid crystals [3][4][5][6][7][8][9], supramolecular polymer networks [10][11][12][13], and ferroelectric liquid crystals [14][15][16][17] formed by the interaction of complementary molecules, the liquid crystalline behaviour of which is crucially dependent on the structure of the resulting supramolecular system. It is now well-established that for the formation of liquid crystalline materials through hydrogen bonding, the complementarity of the interacting components coupled with the directionality of the hydrogen bonds are the main (but not the only) factors contributing to liquid crystallinity [18]. In fact, it has been widely accepted that the proton donor and acceptor capabilities of the atoms of the functional groups involved in the hydrogen bonding contribute remarkably to the occurrence of new phases and their thermal stability. Despite their low bond activation energies, these non-covalent interactions show a profound impact on thermal and physical properties such as melting points, heats of vaporization, mesomorphic behaviour, etc.In continuation of our systematic investigation of Hbonded liquid crystals comprising different proton and acceptor groups [19][20][21][22][23], the present communication deals with the synthesis and phase behaviour of a new series of liquid crystalline systems involving intermolecular hydrogen bonding between p-n-alkoxybenzoic acids (nABA) and p-aminobenzonitrile (ABN). The molecular structure of the ABN:nABA series is represented in Figure 1. New liquid crystalline compounds involving intermolecular hydrogen bonding between mesogenic p-n-alkoxybenzoic acids (nABA) (where n denotes the alkoxy carbon number varying from propylto decyl-and dodecyl-) and p-aminobenzonitrile (ABN) are synthesized. The thermal and phase behaviour of these materials is studied by Thermal Microscopy and Differential Scanning Calorimetry. A detailed IR spectral investigation in solid and solution states confirms the formation of H-bonding between cyano and -COOH groups of ABN and nABA, respectively. Comparative thermal analyses of both free p-n alkoxybenzoic acids and H-bonded complexes suggest the induction of smectic-G phase in all the complexes.

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A new series of mesomorphic complexes formed through intermolecular hydrogen bonding between p-n-alkoxybenzoic acids (where alkoxy denotes chains from propoxy-to decyloxyand dodecyloxy-) and non-mesogenic p-hydroxybutyl benzoate, have been synthesized and characterized by thermal microscopy, di erential scanning calorimetry, infrared spectroscopy (IR) and 1 H NMR studies. A detailed IR spectral investigation in the solid state and in solution suggests that the acid and phenol groups are complementary to each other, each acting as both proton donor and proton acceptor. The results of comparative thermal analyses of both free p-n-alkoxybenzoic acids and H-bonded complexes exhibited an induced crystal smectic G phase in the complexes throughout the series, its thermal range increasing with alkoxy carbon number.

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