Synthesis and properties of aligned all-aromatic liquid crystal networks

Abstract

A series all-aromatic main-chain liquid crystalline (LC) polymers based on 6-hydroxy-2-naphtoic acid (HNA), biphenyl-4,4′-dicarboxylic acid (BA), resorcinol (RS), and a cross-linkable ethynyl functionality, 4,4′-(1,2-ethynediyl)bisbenzoic acid (EBA) were synthesized using a one-pot melt condensation method. In contrast with reactive LC oligomers where reactive functionalities were placed at the chain terminus, the reactive ethynyl group is now used as a cross-linking functionality placed in the polymer main-chain. Differential scanning calorimetric analysis reveals that these polymers can be cured at temperatures above 350°C in the nematic melt and exhibit excellent thermal properties, that is, temperature at 5% weight loss > 465°C, and reaches a maximum after cure glass transition temperature (T _g) of 167°C. When only 5 mol% of BA was replaced with EBA, a slightly cross-linked nematic network, HNA/BA/RS/EBA-5, was obtained and thin films thereof could be stretched by 400% strain above T _g in four consecutive steps. A maximum order parameter $⟨\overline{P_2}⟩$ of 0.54 could be achieved, and this moderately aligned LC network displays a room temperature storage modulus (E′) of 29 GPa, a tensile strength of 330 MPa, a tensile modulus of 7 GPa, and an elongation at break of 7%. Cross-linking also improved the transverse properties of the nematic films. E′ of HNA/BA/RS/EBA-5 is 1.0 GPa at 150°C after being stretched by 400% versus 0.16 GPa for the non-cross-linked reference polymer. An affine deformation model was used to calculate E′ using $\overline{P_2}$ , and the predicted values are in close agreement with the experimental results.