Preparation and characterization of naphthalene-containing poly (p-phenylene pyridine diimidazole)

Poly(2,5-dihydroxy-1,4-phenylenepyridinediimidazole) (PIPD) is an aromatic heterocyclic liquid crystalline polymer, possessing good thermal-stability and mechanical properties. This work presents a copolymerization approach to further improve PIPD’s mechanical properties by introducing 2,6-naphthalene dicarboxylic acid (NDCA) as a comonomer. The incorporation of naphthalene rings into the PIPD backbone yielded the modified polymer PIPD-NDCA. Series of PIPD-NDCA copolymers were successfully synthesized from 2, 3, 5, 6-tertraaminopyridine-2, 5-dihydroxy terephthalic acid (TD salt) and 2, 3, 5, 6-tertraaminopyridine-2, 6-naphthalenedicarboxylic acid (NA salt) by controlling their mass ratios (NA salt: 0∼20 wt.%). The structures of the related compounds were confirmed by Fourier-transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The copolymer films with 10 wt.% NA exhibited the highest tensile strength of 108.6 MPa, representing a 43.5% enhancement compared to pure PIPD films. Additionally, compared with PIPD film, the thermal stability of PIPD-NDCA film and the tensile strength retention rate after UV accelerated aging for 145 h were also improved.