Effect of an Ultra‐Low Content of Reactive Polyhedral Oligomeric Silsesquioxane on Structure and Thermal Properties of Polycyanurate/Polybenzoxazine Hybrid Nanocomposites

Abstract

In this work, a comprehensive investigation of the co‐curing behavior, chemical transformations during network formation, and thermal properties of dicyanate ester of bisphenol E (DCBE)/benzoxazine (BZ) (75/25 wt.%) systems modified with epoxycyclohexyl‐functionalized polyhedral oligomeric silsesquioxane (ECH‐POSS) was carried out to establish the role of the reactive nanofiller in the structure and properties of hybrid organic–inorganic polycyanurate (PCN)/polybenzoxazine (PBZ)/ECH‐POSS nanocomposites. DSC revealed a complex three‐stage curing process and demonstrated that ECH‐POSS selectively accelerated the cyanate ester component, while only weakly affecting BZ polymerization. At 2 wt.% ECH‐POSS, the curing onset decreased from 113°C to 91°C, the main exothermic peak shifted from 241°C to 220°C, the induction period decreased by ∼28%, and the reaction rate at a fixed heating time increased more than threefold. FTIR and 1H/13C NMR analyses provided evidence for chemical coupling between DCBE and BZ through the formation of triazine, iminocarbonate, and carbamate structures. Incorporation of only 0.025–0.5 wt.% ECH‐POSS dramatically increased the glass transition temperature (Tg) of hybrid nanocomposites by 14°C–21°C, reaching 214°C at 0.025 wt.% ECH‐POSS. They retained high thermal stability and char yields of 43%–44% at 790°C. These results demonstrate that reactive ECH‐POSS effectively allows for combining lower‐temperature curing with substantial Tg enhancement, providing a promising strategy for the design of high‐performance thermosetting nanocomposites.