Influence of spacer and flexible chain on polymorphism in complementary hydrogen bonded liquid crystal dimers, SA:nOBAs

Abstract

Higher homologues of hydrogen bonded (HB) liquid crystal (LC) dimers, viz., nOBA:SA:nOBAs, abbreviated as SA:nOBAs are prepared by the soft-covalent interaction between aliphatic non-mesogenic succinic acid and mesogenic n-p-alkoxy benzoic acids for n=10, 11 and 12. Spectroscopic study confirms intermolecular, complementary and alternating type of HB. LC phase structures exhibited by HBLCs are characterized by polarized optical microscopy, calorimetry, capacitance and tilt angle measurements. Nematic, direct SmC, SmX, Re-Entrant SmC and SmG phases grow sequentially by cooling isotropic liquid. Enhancement of smectic-C phase stability is realized through the RE mechanism for n=10 and 12. Direct SmC and SmCRE phases are separated by SmX phase exhibiting finger print texture implying helical order. Chirality of SmX phase is studied by capacitance reflecting induced polarization. Order parameter θ grows asymptotically in tilted SmCRE. θ(T) in direct SmC agrees with critical field prediction. Spontaneous tilt θsp in SmCRE is analyzed. Cooling of direct SmC causes frustration and leads to the growth of SmX. Cooling of SmX decouples the helical order to grow as SmCRE. Molecular conformation, electrostatic, steric factors and large spontaneous tilt value of θ in SmCRE are addressed by a molecular model. Influence of length of spacer moiety, flexible end chain and complementary HB for the Re-Entrant SmC and chiral SmX phase structure is discussed in the wake of results in other HBLCs.