Dynamic Polymer Network Based On Transesterification

Shape memory polymers(SMPs)are a class of smart shape-shifting materials,which could recover to the permanent shapes from temporary shapes.The shape of the SMP plays a vital role in the functionalization,therefore the development of fabrication methodology of complex shapes is significantly desired.However,the elastic nature of SMPs requires a topological constant network structure for polymeric materials,which severely limits its shape complexity.The reason lies in the absence of an intrinsic relaxation mechanism.Dynamic covalent bonds offer a route for stress relaxation in networks via the reversible reaction induced segments exchange.Herein we developed a polycaprolactone network with thermally distinct mechanical behavior.The shuffling of bonds is sufficiently fast at high temperature to trigger the plasticity and significantly depressed at moderate temperature to exert no impact on the intrinsic elasticity of the network.Thus the network could sustain shape memory cycles the permanent and temporary shapes of which could both be altered,as we termed thermadapt shape memory.With origami and kirigami techniques,we achieved complex shape-shifting involving self-folding and hierarchical shape.To introduce the spatial heterogeneity into network,we synthesized photobase as controllable catalyst releasing agents which is able to realize a distribution of catalyst via tunable irradiation.Therefore we could spatially define the distribution of elastic and plastic behavior of the network.The material under pre-strain with such heterogeneous mechanical behavior could undergo mechanical stability induced shape-shifting to relieve the internal stress as a result of partial stress relaxation.While the network above would not alter the topology with bond exchange,we designed a network with dynamic polycaprolactone branches whose molecular weight could be decreased once the bonds are activated.Upon tuning the extent of chain transfer reaction of the branches,we accomplished programming of the topology and physical property of the network,including modulus,crystallinity and transition temperature.Thus we could fabricate numerous polymeric materials with distinct distribution of property with one single stem material.Through collaborated mechanical design,we prepared metamaterials with negative poisson ratio and achieved toughening of the kirigami based metamaterials with softened areas for suppressing crack propagation.