Effect of Inhomogeneous Structure of Polybenzoxazine on Mechanical and Thermal Properties

Abstract

Polybenzoxazine was one of the most widely employed matrix for advanced composites, due to their low viscosity, good dimensional stability, high glass transition temperature (Tg) and wide molecular design flexibility. To obtain high perfomance resin matrix, a fundamental understanding of the formation of crosslinking network structure and the relationship between structure and properties was essential. Therefore, the blends of benzoxazine precursor with different functionality were designed to achieve various network molecular architectures, and the effects of inhomogeneous structure of polybenzoxazine on mechanical and thermal properties were investigated. The bifunctional benzoxazine precursor (BA-a) based on bisphenol-A, formaldehyde and aniline, and the monofunctional benzoxazine monomer (Ph-a) based on phenol, formaldehyde and aniline were synthesized respectively. The blends of BA-a and Ph-a, in which the mole ratio was 1:0, 2:1, 1:1 and 1:2, repectively, were thermally cured through ring-opening reaction to obtain polybenzoxazines with various network structures. The fracture surface morphology of various polybenzoxazines was observed by atomic force microscopy (AFM). The hard phase with highly crosslinking density was dispersed in the soft phase with slightly crosslinking density, which led to the generation of inhomogeneous structure of polybenzoxazine. Dynamic mechanical thermal analysis (DMTA) of carbon fiber reinforced polybenzoxazine showed two glass transition temperatures (Tg), which corresponded to the soft phase and hard phase, respectively.With increasing the mole ratio of Ph-a, the increase of hard phase resulted in the enahncement of flexural modulus of polybenzoxazine, whereas the tensile and flexural strength of polybenzoxazine decreased due to the reduction of the crosslinking density of soft phase. Derivative thermogravimetric (DTG) analysis exhibited three major degradation steps, which characterized the decomposition, weight-loss and charring, respectively. Thermogravimetric analysis (TGA) showed that the onset degradation temperature and char yield at 850 oC increased with the increase of Ph-a mole ratio, indicating higher thermal stability and lower decomposition rate, which was attributed to the increase of hard phase with highly crosslinking density.