Molecular design of liquid crystalline poly(ester-amide)s with perfluoroalkyl spacers

Abstract

We have introduced a series of perfluoroalkyl dicarboxylic acids with different lengths of fluorinated aliphatic segments into the 2,6-acetoxynaphthoic acid (ANA) and acetoxy acetanilide (AAA) systems; and their effects on the evolution of liquid crystal texture and liquid crystallinity have been investigated. The perfluoroalkyl dicarboxylic acids are tetrafluorosuccinic acid (TFSA, n = 2), hexafluoroglutaric acid (HFGA, n = 3), perfluorosuberic acid (PFSUA, n = 6) and perfluorosebacic acid (PFSEA, n = 8). Computational results based on the 'RIS' Metropolis Monte Carlo method indicate that the ANA/AAA/perfluoroalkyl system may form thermotropic liquid crystalline polymers (LCPs) because the calculated persistence ratios are greater than 6.42. Computational results also predict that the systems containing even-numbered perfluoroalkyl acids have greater persistence length and molar stiffness than that containing odd-numbered acids. Experiments were carried out using the in situ thin film polymerization technique under a polarizing optical microscope. We observed that systems containing short aliphatic units (n = 2, 3) tend to remain in the LC phase, while systems containing a long aliphatic spacer (n = 8) tend to crystallize during the late stage of the polycondensation reaction. The liquid crystal domain formed in the early stage has a disclination strength S of + 1. Ternary phase diagrams were plotted to show the relationship among monomer structure, composition, anisotropic and crystalline phases. FTIR results confirm the formation of LCPs.