Bio-based Benzoxazine-Containing\nPolycyclosiloxanes\nas Flame-Retardant Adhesive Materials

The combination of polybenzoxazines and siloxane-based\npolymers\nhas been exploited to merge the advantages of the two types of polymers\nvia two main methods: (1) synthesizing polybenzoxazine-polysiloxane\ncopolymeric structures and (2) introducing polysiloxane into a polybenzoxazine\nmatrix. The polymer composites demonstrated enhanced mechanical, thermal,\nand hydrophobic properties. Herein, monofunctional furfurylamine-based\nbenzoxazine (FBz) side groups were incorporated into polycyclosiloxane\n(pCS) to prepare prepolymers (pCS-FBzs) via a facile one-pot, two-step\nhydrosilylation reaction. In the polymer structure, the content of\nFBz side groups can be linearly controlled with different substitution\nratios (from 21 to 65 mol %) by controlling the feeding ratios in\nthe second step of the hydrosilylation reaction. Widely tunable <i>T</i><sub>g</sub> values were obtained from −67.8 to\n−1.7 °C with increasing FBz content in the pCS-FBz prepolymer.\nReaction efficiency was determined by using <sup>1</sup>H NMR and\nFT-IR measurements. The curing behaviors of the prepolymers were investigated\nusing the DSC results. Both the Si–H group and the benzoxazine\nring in the prepolymers can undergo thermal cross-linking at 210 and\n243 °C, respectively. Interestingly, the two exothermic peaks\nshifted to lower temperatures in the pCS-FBz prepolymers and merged\ninto a single peak as low as 156 °C for pCS-FBz<sub>21</sub> (21\nmol % FBz content). The other exothermic peaks of each prepolymer\nwere also demonstrated, and possible effects on reducing curing temperatures\nwere discussed. The cured pCS-FBz polymers were found to have outstanding\nthermal stability (<i>T</i><sub>d5</sub> > 420 °C,\nlimiting oxygen index (LOI) > 45, and the char yield >70%) without\nthe addition of any flame-retardant additives as well as strong lap\nshear adhesion. Results indicate that the pCS-FBz polymers are promising\nthermal noncombustible materials with high thermal and physical stability.