Dynamic Ablative Networks: Shapeable Heat-Shielding Materials.

Thermoset materials sacrifice recyclability and reshapeability for increased chemical and mechanical robustness because of an immobilized, cross-linked polymeric matrix. The robust material properties of thermosets make them well-suited for applications such as heat-shielding materials (HSMs) or ablatives where excellent thermal stability, good mechanical strength, and high charring ability are paramount. Many of these material properties are characteristic of covalent adaptable networks (CANs), where the static connectivity of thermosets has been replaced with dynamic cross-links. This dynamic connectivity allows network mobility while retaining cross-link connectivity to permit damage repair and reshaping that are traditionally inaccessible for thermoset materials. Herein, we report the synthesis of hybrid inorganic-organic enaminone vitrimers that contain an exceptionally high weight percent of polyhedral oligomeric silsesquioxane (POSS)-derivatives. Polycondensation of ß-ketoester-containing POSS with various diamine cross-linkers led to materials with facile tunability, shapeability, predictable glass transition temperatures, good thermal stability, and high residual char mass following thermal degradation. Furthermore, the char materials show notable retention of their preordained shape following decomposition, suggesting their future utility in the design of HSMs with complex detailing.

High-performance polyimine vitrimers from an aromatic bio-based scaffold.

Bio-based vitrimers represent a promising class of thermosetting polymer materials, pairing the recyclability of dynamic covalent networks with the renewability of non-fossil fuel feedstocks. Vanillin, a low-cost lignin derivative, enables facile construction of polyimine networks marked by rapid exchange and sensitivity to acid-catalyzed hydrolysis. Furthermore, the aromatic structure makes it a promising candidate for the design of highly aromatic networks capable of high-performance thermal and dimensional stability. Such properties are paramount in polymeric thermal protection systems. Here, we report on the fabrication of polyimine networks with particularly high aromatic content from a novel trifunctional vanillin monomer prepared from the nucleophilic aromatic substitution of perfluoropyridine (PFP) on a multi-gram scale (>20 g) in high yield (86%). The trifunctional aromatic scaffold was then crosslinked with various diamines to demonstrate tunable viscoelastic behavior and thermal properties, with glass transition temperatures (Tg) ranging from 9 to 147 °C, degradation temperatures (5% mass loss) up to approximately 370 °C, and excellent char yields up to 68% at 650 °C under nitrogen. Moreover, the vitrimers displayed mechanical reprocessability over five destruction/healing cycles and rapid chemical recyclability following acidic hydrolysis at mild temperatures. Our findings indicate that vitrimers possessing tunable properties and high-performance thermomechanical behavior can be easily constructed from vanillin and electrophilic aromatic scaffolds for applications in heat-shielding materials and ablative coatings.