Degradable Schiff base benzoxazine thermosets with high glass transition temperature and its high-performance epoxy alloy: Synthesis and properties

Benzoxazines have been widely used in many fields, such as the aerospace, electronics, and automotive industries; however, these resins exhibit nonrecyclable nature and limited strength (for their brittleness). Introducing reversible covalent bonds in benzoxazines is expected to address these issues. Herein, two-benzene-ringconjugated Schiff-base benzoxazine was successfully synthesized using a designed strategy. Using this strategy, possible side reactions, such as the transimination of the Schiff-base, were prevented when preparing the target product via the traditional Mannich reaction. The cured benzoxazine could chemically degrade under acidic conditions and it undergoes controllable degradation when the temperature, acidity, and solvent are changed. The synthesized benzoxazine forms noticeable liquid crystals when heated to its melting point. Its glass transition temperature also reaches 265 degrees C and the char yield at 800 degrees C is 60.8%, considerably higher than that of traditional bisphenol A-aniline based benzoxazine (BA-a). Upon blending with epoxy (EP), the mechanical properties of the copolymers are significantly improved. With increasing benzoxazine content, the tensile strength of the copolymers increases from 63.6 to 95.2 MPa, an increase of similar to 50%. The rigid Schiff-base structure is uniformly incorporated into the crosslinked network of EP and the EP curing agent via a transimination reaction during curing, thereby significantly improving the tensile modulus and strength of the composite material.