Covalent Adaptable Networks with Associative Siloxane Exchange Enabled by Amide-Based Internal Catalysis: Designing for Reprocessability and Extrudability by Increasing the Cross-Link Density

Replacing non-recyclable thermosets with covalent adaptable networks (CANs) that recover cross-link density after reprocessing will reduce waste and contribute to a circular polymer economy. Many CANs undergoing associative dynamic exchange require catalysis. External catalysis often leads to harmful effects, e.g., increased creep, accelerated material aging, and catalyst leaching. Herein, internally catalyzed siloxane dynamic chemistry is demonstrated resulting from amides covalently linked through alkyl chains to siloxanes. Small-molecule studies show the formation of exchange products resulting from the reaction of two amide-containing siloxane molecules. From the rubbery plateau modulus, each siloxane-exchange-based CAN exhibits a cross-link density that is temperature-invariant, or nearly so, characteristic of associative CANs. The alkyl length in the siloxane-containing monomer tunes the network cross-link density. Cross-link density recovery after reprocessing is achieved, with the required reprocessing time and temperature decreasing with increasing cross-link density. Stress relaxation is also faster with increasing cross-link density. The faster dynamics and reprocessability with increasing cross-link density arise because associative exchange is second order in siloxane (i.e., cross-linker) concentration. Capitalizing on this, the melt extrusion of the highest cross-link density CAN is demonstrated, achieving the same cross-link density in extruded and compression-molded CANs. Using identical conditions, the next-highest cross-link density CAN is not extrudable.