Synthesis of Reprocessable Thermoset Polyurethane Toughened by Dangling Chains

A pure hard-segmented thermosetting polyurethane with microphase separation was developed using 4,4'-dicyclohexylmethane diisocyanate (HMDI) and 1,2-hexanediol (HDO) as the reactive monomers for prepolymerization and glycerol (GI) for post-crosslinking. Its mechanical properties and microphase structure were investigated in detail. Through the post-crosslinking process of GI, the branched chain of HDO can act as a dangling chain to enhance the motility of the rigid main chain and act as a plasticizer. Meanwhile, based on the difference between the flexibility of the side chains and the rigid backbone of the crosslinked network, after the post-crosslinking of GI, the dangling chains can form domains, which form a micro-phase separation from the rigid main chain network, and thus enhance the toughness. By adjusting the proportion of HDO, the microphase-separated structure of thermoset polyurethanes can be modulated, thus conferring high toughness and allowing ductile fracture under uniaxial tensile conditions. In addition, by taking advantage of the dynamic behavior of the urethane bonds without introducing additional dynamic units, the large number of urethane bonds and hydrogen bonds formed by the reaction of HMDI with HDO and GI can confer reprocessing properties to the crosslinked network. The results show that the microphase-separated structure imparts a Young’s modulus of up to 540.29 MPa and a toughness of 63.89 MJ/m³ to the pure hard-segmented polyurethane, which can be reprocessed at 160 ℃ for 1 h.