Hydrogen bonding and liquid crystallinity of low molar mass and polymeric mesogens containing benzoic acids: a variable temperature Fourier transform infrared spectroscopic study

The phase behaviour and mesomorphism of poly(4-(6-propenoyloxyhexyloxy)benzoic acid) (PPOHBA) and 4-pentyloxybenzoic acid (POBA) is studied using variable-temperature Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction. PPHOBA exhibits a smectic C phase and POBA, a nematic phase. The temperature dependence of the Fermi resonance bands associated with the hydroxyl groups and of the carbonyl stretching region in the FTIR spectra indicates that there is a dynamic equilibrium between monomers and open and closed dimers formed by hydrogen bonding between benzoic acid moieties. The nematic phase observed for POBA is linked to the anisotropic cyclic dimer, while an abrupt increase in the concentration of monomer drives isotropisation. In PPOHBA, hydrogen-bonded supramesogens promote smectic behaviour, while hydrogen-bonded crosslinks stabilise the lamellae. The increased viscosity arising from this dynamic crosslinking is offset by the flexibility of the acrylate backbone and alkyl spacers.