Polyesters based on hydroxybenzoic acid, hydroquinone, sebacic acid, and suberic acid: Molecular design towards broad nematic range

To achieve a thermotropic liquid crystalline polymer with a relatively low melting temperature and a broad mesophase range, a series of polyesters based on hydroxybenzoic acid (HBA), hydroquinone (HQ), sebacic acid (SEA), and suberic acid (SUA) have been designed and synthesised. The molecular design was based on two concepts. First, the flexible segment of SUA is shorter than that of SEA. Replacement of SEA with SUA units would lead to an increase in the chain rigidity and hence the nematic to isotropic transition temperature. Second, since SEA and SUA have similar reactivity, the partial replacement of SEA with SUA units would randomise the structure and thus cause depression of melting points. DSC shows that the melting temperatures of some HBA/HQ/SEA/SUA polymers are indeed lower than the corresponding HBA/HQ/SEA polymer. Liquid crystallinity of the polymers was examined using polarised light microscopy. This verified that the clearing point of the HBA/HQ/SEA/SUA polymers increases as the SUA concentration increases. At a HBA/HQ/SEA/SUA molar ratio of 60:40:20:20, the crystallites melt completely at ∼200 °C, while the clearing point is still well above 300 °C. This liquid crystalline polyester has great potential to act as a processing aid for conventional thermoplastics with moderate processing temperature windows.