Synthesis of high-molecular-weight benzoxazines having azomethine linkages in the main-chain and the properties of their thermosetting resins

Abstract

High-molecular-weight benzoxazines having azomethine linkages in the main-chain were synthesized by a two-step method consisting of 1) the synthesis of benzoxazine-containing dialdehydes from diamines, formaldehyde, and hydroxybenzaldehyde and 2) the polymerization of the dialdehydes and diamines. Free-standing precursor films were easily obtained by casting the solutions of the benzoxazines on glass plates, followed by drying. By the thermal curing of the precursor films up to 240 °C, brown-colored, transparent, and tough polybenzoxazine films were obtained. The toughness was confirmed by the tensile test; the tensile strength (sb) and elongation at break (eb) of the polybenzoxazine films were 41–67 MPa and 2.2–4.9%, respectively, which were higher than those of the typical bisphenol A-aniline type polybenzoxazine, PB-a, film (sb = 30 MPa and eb = 1.1%). The glass transition temperatures of the polybenzoxazines derived from the high-molecular-weight benzoxazines were determined to be over 280 °C by dynamic mechanical analysis, which was approximately 100 °C higher than that of PB-a. The polybenzoxazines also showed a high thermal stability as evidenced by the 5% weight loss temperatures in the range of 379–391 °C observed via thermogravimetric analysis.