Design and Construction of Furan-Based and Thiophene-Based Salicyladazine Bisbenzoxazine Resins with High Thermal Stability and Tunable Surface Properties

Abstract

Two disubstituted bisbenzoxazine (Bz) monomers are synthesized using furan (Fa) and thiophene (Th) derivatives: bis((3-(furan-2-ylmethyl)-7-ol-3,4-dihydro-2H-benzo[e][1,3] oxazin-6-yl)methylene)hydrazine (BAZ-Fa-BZ) and bis((3-(thiophen-2-ylmethyl)-7-ol-3,4-dihydro-2H-benzo[e][1,3] oxazin-6-yl)methylene)hydrazine (BAZ-Th-BZ). These monomers are synthesized via Mannich condensation of salicylaldazine (1,2-bis(2,4-dihydroxybenzylidene)hydrazine (BAZ─4OH)) and paraformaldehyde (CH₂O)_n, with furfurylamine (FacNH₂) and thiophene-2-methenamine (Th─NH₂), respectively. The chemical structures of BAZ-Fa-BZ and BAZ-Th-BZ are affirmed using Fourier-transform infrared spectroscopy (FTIR) and NMR, respectively. A thorough investigation of the thermal polymerization process of BAZ-Fa-BZ and BAZ-Th-BZ is conducted using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and in situ FTIR spectra (ranging from 25 to 250 °C). Poly(BAZ-Fa-BZ) exhibits superior thermal properties with a thermal decomposition temperature (T_d10) of 402 °C and a char yield of 58 wt% after thermal treatment at 250 °C, along with a lower surface free energy of 28.9 mJ m⁻² compared to poly(BAZ-Th-BZ) (T_d10 = 359 °C, char yield = 48 wt%, and surface free energy = 34.1 mJ m⁻²). Additionally, poly(BAZ-Th-BZ/BAZ-Fa-BZ) blend with a ratio of 1/3 after thermal curing at 250 °C demonstrates the highest T_d10 of 395 °C and a char yield of 60 wt%. Photoluminescence (PL) measurements conducted in the solid state reveal that BAZ-Th-BZ, BAZ-Fa-BZ, and their blends emit green light when excited at a wavelength of 365 nm.