Synthesis and mesophase characterization of novel methacrylate based thermotropic liquid crystalline monomers and their polymers

Abstract

A new series of side chain methacrylate monomers with a three phenyl ring core connected by an ester with a terminal alkoxy chain and hexamethylene spacer are synthesized by a multistep synthetic route. The corresponding polymers are realized by free radical solution polymerization. Two structurally similar mesogens are also synthesized for use as models to study the influence of the methacrylic unit on the mesophase characteristics. The molecular structure of the intermediates, monomers as well as the polymers is unambiguously confirmed by Fourier-transform infrared (FT-IR), solution ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy and by elemental analysis. The mesophase characteristics of all of the monomers and polymers are determined by hot-stage optical polarized microscopy (HOPM) and differential scanning calorimetry (DSC). These investigations revealed the existence of enantiotropic nematic (N), smectic A (S_A) and smectic C (S_C) mesophases. Furthermore, for a representative monomer and polymer, the presence of the smectic phase is confirmed by variable temperature X-ray diffraction (XRD) where a characteristic layer ordering is noticed. The molecular weight of the polymers is determined by gel permeation chromatography (GPC) and the values are found to be typically in the range of 2.0 × 10³ to 3.7 × 10³. The mesogenic polymers are also found to be stable up to 320 °C by thermogravimetric analysis (TGA).