Spatiotemporally Controlled Shape Reconfiguration in Diels-Alder Polymer Networks through Selective Dissociation of Crosslinks

Dynamic covalent polymer networks crosslinked via the Diels-Alder (DA) reaction between furan and maleimide can undergo unique solid-state plastic deformation around 60 ~ 80 ^oC, on demand. Such on-demand network plasticity is enabled by the selective dissociation of the two isomers, endo and exo, of the DA adducts. The thermally less stable endo adducts can dissociate and be converted into exo at elevated temperatures between (60 ~80 ^oC). During this process, the Diels-Alder polymer (DAP) networks can undergo rapid stress relaxation and adapt to a new permanent shape. When the endo content is exhausted, the DAP networks can still participate in shape reconfiguration at a much slower rate through dynamic bond reshuffling of the thermally more stable exo adducts. Due to the higher thermal stability of exo adducts (dissociation is above 120 ^oC), the elastic modulus of the DAP network also increases with  the endo to exo conversion. The content of endo isomers in DAP networks can be easily controlled by varying the crosslinking density of the network. When 3D printed into relevant constructs for shape memory behavior, for example in bilayered strips or graded structures, the drastic difference in the thermal response between endo and exo adducts allows for spatiotemporal shape reconfiguration and actuation.