Exploration Of Non-conventional Polymer Processing Methods Using Dynamic Covalent Polymer Networks

In nature,delicate geometries are essential for many intriguing functions of biological systems.Increasingly,advanced devices of complex shapes are also highly pursued.Traditional processing techniques of polymer materials usually undergo a molding and demolding process,which makes it fail to meet this demanding requirement.The freedom offered by 3D printing has opened up unprecedented opportunities through additive manufacturing.Introducing time as an additional dimension,the recently emerged so-called 4D printing has brought more potential for shape-shifting.From the perspective of material design,adaptive properties(e.g.plasticity)have been achieved by introducing dynamic covalent linkages into polymer networks,which allow permanent reshaping in the solid-state in a mold-free fashion via network topological rearrangement.Though the above emerging 3D fabrication methods bring many new opportunities,the printing speed,material diversity,and accessible shapes are still the bottlenecks.Intending to solve these problems,we propose new molecular design strategies to regulate the dynamic performance of polymers and explore unique opportunities for the above non-traditional fabrication methods.Solid-state plasticity relies on dynamic bond exchange to allow topological rearrangement and shapeshifting.To satisfy diverse application requirements,it is highly desirable to design a dynamic network for which the topological rearrangement can be activated in a tunable temperature range.However,such tunability is usually narrow with only one type of dynamic bond.In this work,two sets of dynamic bonds are incorporated into a hybrid network.By changing the bond ratio,networks with highly tunable topological rearrangement kinetics are obtained.Combining this characteristic with dynamic welding facilitates the fabrication of multi-material 3D complex shapes.Unlike the plasticity that requires continuous imposing of an external force,4D printing allows the materials to spontaneously morph over time into a preset 3D shape.One of the important branches is to introduce spatial stress contrast within a 2D sheet,and further use the release of stress to achieve 2D-to-3D transformation.Therefore,the 3D shape can be achieved by printing a single-layer film.While gaining printing speed,the accessible shapes are quite limited using this type of 4D printing.In this work,we propose a concept of modular 4D printing that combines modular assembly with 4D printing.By introducing the dynamic covalent bonds into the material system,4D printed structures of diverse thermal/mechanical properties can be assembled in a modular fashion by interfacial dynamic bond exchange.Consequently,complex 3D objects can be produced.This method can also realize the assembly of modules of different material properties,allowing fabrication of multi-responsive shape-shifting meta-devices.In the above-mentioned plasticity or modular 4D printing process,the dynamic bond exchange allows the material to reconfigure the permanent shape and/or achieve efficient welding,but the network structure and the associated material properties have remained unchanged.If the network topologies can be tailored in the exchange process,it is possible to regulate its mechanical performance.This idea may overcome a notable limitation in photo-curable 3D printing,that is,the intrinsic coupling of the printing process and material performance.It would provide an effective way to obtain high-performance materials without compromising the printablility.Accordingly,in this work,dynamic covalent bonds are incorporated into the photosensitive precursors.The printed materials can undergo network rearrangement under certain external stimuli,thereby allowing regulation of the ultimate material performances independent of the printing process.