Synthesis of novel allylamine-fluorene based benzoxazine and its copolymerization with typical benzoxazine: curing behavior and thermal properties

Abstract

In this paper, 2,7-dihydroxy-9,9-bis(4-aminophenyl)-fluorene (BADHF) monomer was synthesized via direct condensation reaction catalyzed by methylsulfonic acid. A novel allylamine-based benzoxazine monomer containing fluorene rings and polymerizable groups (t-BF-sa-a) was obtained using the o-hydroxy-benzyl amine method combined with a one stage Mannich condensation reaction. The results showed that the introduction of fluorene rings and polymerizable groups and the increase of the number of oxazine rings in the polymer network structure can result in an obvious increase in the crosslinking density of the polymers. Therefore, t-BF-sa-a polybenzoxazine (poly(t-BF-sa-a)) exhibited a higher glass transition temperature (T_g) and better thermal stability. The synthesized t-BF-sa-a monomer was used to modify traditional phenol-aniline-based mono-functional (P-a) and bisphenol A-aniline-based bifunctional (BA-a) benzoxazines. Due to the introduction of heat-resistant fluorene rings and the increase of the crosslinking degree of the copolymers, the copolymers (poly(P-a/t-BF-sa-a) and poly(BA-a/t-BF-sa-a)) showed excellent thermal properties. Among them, the T_g, T₅, T₁₀, and Y_c of poly(P-a/t-BF-sa-a) were increased by 34 °C, 58 °C, 35 °C, and 8%, respectively, which are even higher than those of bisphenol fluorene aniline-based polybenzoxazine. Furthermore, due to the long alkyl chain in the t-BF-sa-a monomer, the E′ values of the copolymers were decreased significantly, which greatly improved the shortcomings of traditional polybenzoxazines, such as high brittleness and poor toughness.