Effects of the curing atmosphere on the structures and properties of polybenzoxazine films

The effects of four different curing atmospheres, i.e., static air, circulating air, nitrogen and vacuum, on the polymerization mechanism, chemical structure, hydrogen bonding, mechanical property, thermal property, water contact angle, etc., of the polybenzoxazine films were systematically studied. It was found that curing in air caused more oxidation and decomposition, generating more benzoquinones, carbonyl groups and iminium ions in the resultant polybenzoxazine films. Consequently, the films cured in air were less cross-linked, darker in color and more brittle. The films cured in static air (SA) showed tensile strength and elongation at break of 12.59 MPa and 2.16%, respectively, which were 61.24 MPa and 2.74% lower than those of the films cured in nitrogen (N2). Moreover, it was demonstrated that more intramolecular rigid –OH···N hydrogen bonding was formed in the films cured in air, which was believed to be the fundamental reason for the lower chemical cross-linking density and poorer toughness. Nevertheless, these films showed higher char yield due to the formation of more thermally stable groups in oxidation process. Further investigation on chemical structures, hydrogen bonding and water contact angles of the upper and lower surfaces of the films revealed that the upper surfaces were more inclined to be oxidized and decomposed during curing and more intramolecular hydrogen bonding (–OH···N HB) was formed on upper surfaces, which led to increased water contact angles.