Temperature Response of Endo/Exo Crosslinking in Diels-Alder Polymer Networks

We will discuss thermodynamic and kinetic aspects of endo vs. exo isomeric attachments in Diels-Alder (DA) polymer networks. Endo and exo isomer have different dissociation temperatures and the ratio between them plays a crucial role in harnessing DA network’s thermomechanical behaviors. Unlike in solution, where the ratio between endo and exo can be easily adjusted by reaction temperature, the DA reaction in a polymeric matrix is carried out during the cooling process in which the equilibrium of DA reaction sweeps from dissociation to association. DSC analysis showed that reaction temperature had limited impact on the endo/exo ratio in the final crosslinked networks. Instead, the crosslinking density significantly affected the endo/exo ratio, increasing the number of endo attachments. In addition, activation energies, enthalpy, and entropy of the DA reaction were estimated for different DA polymer networks. The results suggest the steric restrictions at the crosslink points and the entropy of crosslinkers play a major role in controlling the thermal response of DA polymers. For instance, when a more flexible crosslinker was used, the dissociation temperature increased from 115 ^oC to 128 ^oC. These findings enable rational tailoring of thermomechanical properties of DA networks.