Emission and surface properties of main-chain type polybenzoxazine with pyridinyl moieties

Abstract

A main-chain type polybenzoxazine, PBz (1), with pyridinyl moieties was successfully synthesized from the Mannich condensation of 4-phenyl-2,6-bis(4-aminophenyl) pyridine (2), paraformaldehyde, and bisphenol A. For the purpose of property comparison, a structurally similar polybenzoxazine, PBz (2), was prepared. Unfortunately, PBz (2) is insoluble in organic solvents due to its rigid structure. In contrast, the pyridinyl group provided solubility for the processing of PBz (1). When the THF dilute solution of PBz (1) was protonated with HCl, a new absorption signal at 458 nm was observed in the UV-vis spectrum. No emission at 500–700 nm occurred in the fluorescence spectrum before protonation of PBz (1). However, upon protonation, the protonated pyridinyl became a stronger acceptor, and then, an emission at 570 nm occurred between the nitrogen of benzoxazine (donor) and protonated pyridinyl groups (acceptor) after being excited at 458 nm. After thermal curing, the thin film of PBz (1) thermoset was flexible, with a T_g value of 261 °C, a coefficient of thermal expansion of 38 ppm °C⁻¹, and 5% decomposition temperature at 414 (N₂) and 419 °C (air). The contact angle is as high as 102°, and the surface energy is as low as 19.6 mJ m⁻².