Highly thermally stable polyhedral oligomeric silsesquioxane based on diacetal-functionalized polybenzoxazine nanocomposites

An increasing number of high-performance industries are prioritizing the use of polymeric materials with exceptional thermal stability to support long-term advancements toward a more sustainable future. In this study, we first synthesized an allyl-functionalized benzoxazine (BZ) with a diacetal structure by reacting 4,4′-(2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diyl)diphenol (ACE) with allylamine in the presence of paraformaldehyde to obtain ACE-BZ monomer. Highly thermally stable organic-inorganic benzoxazines were subsequently prepared through a hydrosilylation reaction of ACE-BZ with double-decker silsesquioxane (DDSQ) and octakis(dimethylsilyloxy)octasilsesquioxane (ODMS-POSS), yielding DDSQ-ACE-BZ and POSS-ACE-BZ polymer networks, respectively. The chemical structures of ACE-BZ, DDSQ-ACE-BZ, and POSS-ACE-BZ were confirmed using FTIR, ¹H NMR, and ¹³C NMR spectroscopy; respectively. The thermal curing peaks, ring-opening polymerization (ROP) behavior, and thermal stability properties of the ACE-BZ, DDSQ-ACE-BZ, and POSS-ACE-BZ were analyzed using differential scanning calorimetry (DSC), FTIR and thermogravimetric analysis (TGA). After thermal curing, the thermal stability (T_d10 ,char yields) of poly(ACE-BZ), poly(DDSQ-ACE-BZ), and poly(POSS-ACE-BZ) were (435 °C, 46 wt%), (544 °C, 75 wt%), and (510 °C, 74 wt%), respectively. Notably, poly(DDSQ-ACE-BZ) demonstrated superior thermal stability compared to poly(POSS-ACE-BZ), primarily attributed to the inherently higher thermal stability of the rigid DDSQ moiety relative to POSS. Based on our current understanding, the DDSQ-based polybenzoxazine resin discussed in this study demonstrates the highest thermal stability that has been documented so far.