Research on the Effect of 5-Aminoindole on Polymerization Behaviors and Mechanical Properties of Poly(Aldehyde Functional Benzoxazine)

The effect of 5-aminoindole (5-AI) on the behaviors and mechanical properties of poly(aldehyde functional benzoxazine) was studied in our research described in this paper. Different proportions of 5-AI were used for curing with aldehyde functional benzoxazine (PHB-a) (2:1, 1:1 and 1:2 mole ratios of PHB: 5-AI). The curing behaviors and thermal properties of the resulting benzoxazine systems were investigated using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and thermal gravimetric analysis (TGA). High efficiency of 5-AI toward ring opening polymerization of the benzoxazines at a low temperature, 70 °C, was achieved. Besides the ring-opening polymerization of the oxazine rings, special reactions between the aldehyde groups of PHB-a and the indole groups of 5-AI occurred, different cross-linking structures formed in all the PHB-a/5-AI systems and the modified benzoxazine resins exhibited better processability and mechanical properties compared to the neat PHB-a resin. When the mole ratio of PHB-a/5-AI was 1:1, a breaking elongation of 2.54% with a failure strength of 72.35 MPa and a char yield of 63.6% at 800 °C under nitrogen atmosphere were achieved, this composition exhibiting the best tensile properties and thermal performance compared to the other PHB-a/5-AI systems. These findings show a new method to improve the mechanical properties of brittle polybenzoxazine resins while maintaining good thermal stability.