Synthesis and properties of two series of H-bonded and non-H-bonded thermotropic liquid crystal monomers

Abstract

Two series of monomers, namely n-1-bromo-\\[4-(4-nitrophenylazo)phenyloxy]alkanes (Bn, n2, 3, 4, 5, 6, 8, 10) and N - n -\\[4-(4-nitrophenylazo)phenyloxy]alkyl diethanolamines (C n, n 3, 5, 6, 10), were synthesized and characterized. Their thermal behaviour was studied by differential scanning calorimetry (DSC), wide angle X-ray diffractometry (WAXD) and polarizing optical microscopy (POM) equipped with a hot stage. The results showed that the B n s (n6) exhibit monotropic nematic liquid crystalline behaviour; no liquid crystalline phase was found for the B ns (n6), while for the C ns, enantiotropic smectic liquid crystallinity for n 5, 6, 10 was seen, and for n 3 monotropic smectic phases were found. This different phase behaviour between B n and C n compounds is attributed to their different end groups. The FTIR analysis of C n indicated that there exists an intermolecular hydrogen bond between hydroxy groups, so that more stable liquid crystalline phase are formed. The effect of the length of the flexible chain on the thermal behaviour is also discussed.