Sustainable Framework of Chitosan–Benzoxazine with Mutual Benefits: Low Curing Temperature and Improved Thermal and Mechanical Properties

Polybenzoxazines (PBzs) are emerging as a highly promising and superior class of thermoset polymers for a variety of applications. However, it remains a significant challenge to substantially lower the ring-opening polymerization (ROP) temperature with an ease in processability. On the other hand, biomacromolecule chitosan (CS) is explored extensively, but its practical applications have been precluded by poor thermal and mechanical properties. Here, we developed a fully biobased copolymer of vanillin benzoxazine (V-fa) monomer with CS, which is effective in providing mutual benefits, effective lowering in ROP temperature of benzoxazine (70 °C), and an enhanced thermal stability of CS (by 85 °C, and with char yield of ∼32%). To understand this unusual lowering in ROP temperature, we investigated the structural interaction mechanism between solvated CS and V-fa using in situ NMR studies. The analysis of fully intercalated co-structure demonstrated that there is a strong preference for ROP over Schiff base reaction. It is anticipated that benzoxazine molecules move within the interplanar distance of CS as supported by powder X-ray diffraction studies. An increase in V-fa content in feed ratio led to a placement of V-fa units from random to a systematic and hierarchical arrangement within the CS framework followed by its subsequent polymerization. The synergistic interactions were further supported by Fourier transform infrared, differential scanning calorimetry, scanning electron micrsocopy, thermogravimetric analysis, and tensile studies. Current work represents preparation of CS benzoxazine copolymers using a low-cost, efficient, and sustainable approach to assist metal-free ROP reaction of Bz to afford low curable temperature processable films. A new strategy is devised for the utility of CS-PBzs copolymers, enabling their extension to innovative applications in cross-domains.