Effects of Isohexide Stereochemistry on Vinylogous Urethane Covalent Adaptable Networks

The starch-derived isohexides, with their unique structures of two fused tetrahydrofuran rings in a cis conformation, have been exploited to prepare covalent adaptable networks (CANs) and to tailor and understand their structure-property relationships, in pursuit of replacing oil-based thermosets. Here, dynamicity was achieved through vinylogous urethane chemistry, rigidity via the use of the starch-derived isomeric building blocks isosorbide, isomannide, and isoidide, and flexibility through the amines utilized. Similar to what is known for thermoplastics, depending on the isomer chosen, thermal stability and mechanical properties could be tailored to some extent. The distance between cross-links was ruled by the amines employed, and when this distance was long enough to allow sufficient chain mobility, stereochemical effects on mechanical performance were observed. The CAN structures all display thermoset properties, and as a consequence of the incorporated dynamic bonds, they were mechanically reprocessable. Based on the CANs structural design, i.e., isohexide isomer and amine structure, tensile strengths (σ_b) ranging from 1.57 to 19.1 MPa, glass transition temperatures (T _g) ranging from 20 to 114 °C, and thermal stabilities (T _d,5%) between 200 and 305 °C were achievable. Mechanical reprocessing was proven, and no mechanical performance decay was observed after two reprocessing cycles. This provides important information on the structure-property relationship of CANs from starch-derived building blocks, and consequently, how material properties can be tailored depending on the targeted application.