Thermotropic polyesters from 2,5-furandicarboxylic acid and vanillic acid : synthesis, thermal properties, melt behavior, and mechanical performance

In this work, we address the synthesis of novel aromatic–aliphatic biobased polyesters showing thermotropic behavior in the melt. Successful incorporation of different biobased monomers such as 2,5-furandicarboxylic acid (2,5-FDCA), suberic acid (SuA), and vanillic acid (VA) in thermotropic liquid crystalline polymers (TLCPs) is made possible by performing synthesis at low temperatures. The chemical structures, molecular weights, phase transitions, thermal behavior, and mechanical performance of the synthesized polymers are studied using polarization optical microscopy, WAXD, DSC, TGA, DMTA, solid-state NMR spectroscopy, rheology, and tensile tests. It is shown that the incorporation of the rigid, aromatic 2,5-FDCA moiety enhances the formation of blocky copolymers, whereas the VA moiety tends to decrease the block formation. However, when combined, nonblocky TLCPs containing 2,5-FDCA and VA with high aromatic content can be obtained. These materials show a low temperature transition from the crystalline to the nematic phase, and stable nematic phases up to 300 °C and higher. Furthermore, in such polymers, the 2,5-FDCA and VA moieties require more thermal energy to become mobile compared to the phenyl rings in hydroxybenzoic acid, hydroquinone, and 4,4'-biphenol. Mechanical analysis shows that the performance of these polymers is correlated to their crystallinity. Surprisingly, a higher crystallinity results in ductile behavior, whereas a lower crystallinity results in a higher modulus, a higher stress at break, and a lower strain at break.