Synthesis and characterization of poly (dihydroxybiphenyl borate) with high char yield for high-performance thermosetting resins

Highlights: • PDDB with excellent thermal resistance and a high char yield was synthesized. • The structure, solubility, and thermal stability of PDDB were investigated. • PDDB exhibits high glass transition temperature and high char yield. • The thermal stability of PDDB increases with an increase in boron content. • Boron oxide and boron carbide are formed during the pyrolysis of PDDB. The objective of the current work is to synthesize novel boron-containing polymers with excellent thermal resistance, and reveal the structure and the reason for the high char yield. Thus, poly (dihydroxybiphenyl borate) (PDDB) with a more rigid molecular chain, was successfully synthesized using 4,4′-dihydroxybiphenyl and boric acid. Structural characterizations of the prepared PDDB were performed via NMR, FTIR, XPS, and XRD analyses. The results reveal that PDDB consists of aromatic, PhOB and BOB structures as well as a small number of boron hydroxyl and phenolic hydroxyl groups. PDDB shows good solubility in strong polar solvents, which is of great importance for the modification of thermosetting resins. TGA combined with DSC were employed to evaluate the thermal properties of PDDB, and increases in the glass transition temperature (T_g) and char yield were observed with increased boron content. T_g and char yield of PDDB (800 °C, nitrogen atmosphere) reached up to 219 °C and 66.5%, respectively. PDDB was extensively characterized during pyrolysis to reveal the high char yield of PDDB. As briefly discussed, the boron oxide and boron carbide that formed during pyrolysis play a crucial role in the high char yield of PDDB, which reduces the release of volatile carbon dioxide and carbon. This research suggests that PDDB has great potential as a novel modified agent for the improvement of the comprehensive performance of thermosetting resins to broaden their applicability in the field of advanced composites.