Benzoxazine monomers based on aromatic diamines and investigation of their polymerization by rheological and thermal methods

In the present article with the aim to find new polybenzoxazines with improved thermal properties, new benzoxazine monomers based on phenol and the following aromatic diamines were synthesized: 3,4′-oxydianiline (P-3,4′oda); o-tolidine (P-ot); m-tolidine (P-mt) and 4,4′-(1,3-phenylenedioxy)dianiline (P-tper) and their comparison with the previously known benzoxazines based on 4,4′-diaminodiphenylmethane (P-ddm); 4,4′-oxydianiline (P-4,4′-oda) and 2,2-Bis4-(4-aminophenoxy)phenyl]propane (P-bapp). The dependence of the thermal and rheological characteristics on the structure of benzoxazine monomers based on various diamines was estimated and possible methods for their processing were identified. All the polybenzoxazines obtained in this work have high char yield and reduced flammability. It was found that the structure of the diamine can have a fundamental effect on both the rheological properties and heat resistance of polybenzoxazines. The benzoxazine monomers P-ddm, P-tper and P-4,4′oda retain viscosity up to 1 Pa s. at 110°C for 2 h, the P-tper monomer with a resorcinol bridge has about five times lower viscosity compared to the P-bapp monomer with a bisphenol A bridge. Polybenzoxazines based on the monomers P-ddm, P-mt, P-bapp and P-tper show excellent thermal stability with a temperature of 10% weight loss above 400°C. In particular, T_g of P-3,4′oda and P-mt monomers is relatively high (202 and 239°C, respectively), while P-ot's is unusually low (115°C), which may be caused by the specific effect of the substituents in the aromatic ring of the amine and their position.