Mechanical Reinforcement of Polybenzoxazole by Carbon Nanotubes through Noncovalent Functionalization

Polybenzoxazole (PBO) reinforced with carbon nanotubes (CNTs) is of great interest in the field of polymer composites because PBO and CNTs possess the highest-level mechanical properties among polymers and filler materials, respectively. However, the insolubility of PBO and the CNTs makes it difficult to develop a CNT/PBO composite in a solvent except for strong acids. In this study, we describe a novel strategy to design and prepare a CNT/PBO composite; namely, a method using a PBO precursor having an excellent solubility in organic solvents. We discovered that a PBO precursor wraps CNTs though a strong interfacial interaction between the CNT surfaces and the PBO precursor that enables the dispersion of the CNTs in organic solvents. By this method, a CNT/PBO precursor composite film was easily obtained from the CNT/PBO composite solution, and by heating to 400 °C, we obtained a CNT/PBO composite film. Mechanical property measurements revealed that the CNT/PBO films exhibited a high tensile strength (151 MPa) and Young's modulus (8.1 GPa) at a CNT loading of 1.7 wt %, corresponding to 130% and 179% increase compared to those of the PBO film, respectively. These values are the highest values ever reported. Wrapping of the CNTs with PBO and the nice dispersion of the CNTs into a PBO matrix are considered to be important steps for the observed reinforcement. Indeed, scanning electron microscopic observations of the cross section of the CNT/PBO film indicated the homogeneous distribution of the isolated CNTs in the PBO film. This is the first example for the preparation of a CNT/PBO precursor film in an organic solvent based on a noncovalent CNT-functionalization.