Preparation Of Shape Memory Polymer Materials Based On Unconventional Crystalline Phases

Shape memory polymers(SMP)are smart responsive materials that can be programmed into the arbitrary temporary shapes and recover to the permanent shapes under external stimuli,such as heat,light,eclectric field,magnetic field,etc.Because of these advantages of light weight,large deforming stain,simple programming process and adjustable modulus,SMP has shown many high-value-added applications in aerospace,biomedicine,4D printing,flexible robots and other fields in the past three decades.In order to further simplify the processing technologies and broaden application scenarios of SMP,researchers provide a variety of design ideas.In previous studies,more attention has been paid to regulating the thermal behaviors of polymers through the optimal design of chemical structures,which gives SMP excellent shape memory performances and diverse shape morphing paths.However,thes methods inevitably involve many elaborate chemical reactions and cumbersome experimental processes,which increases the difficulty,time and cost of samples preparation.On the other hand,the chemical structures are not easily changed after being designing.This is not conducive to the materials’reprogramming and recycling.As one of the most commonly used SMP,the crystallization behaviors of semicrystalline polymers are affected by many physical conditions.Based on this,we propose programming physical structures of SMP to regulate their crystallization behaviors and optimize shape memory behaviors.In this work,we firstly use the cocrystallization behavior of polymers to regulate the switching temperature(Tsw)of SMP.Tsw is a key parameter governing the service condition of SMP.However,tuning Tsw of SMPs often requires sophisticated synthesis or intricate processing.Herein,we report a simple yet effective strategy to prepare the SMP with tunable Tsw and good reconfigurability by using the cocrystalline polyesters as the reversible phase.The cocrystallizable copolyesters with rearranged sequences were prepared by the transesterification of mixed polyester diols and then photo-cross-linked to achieve the SMP networks.Cocrystallization of copolymer blocks endows the SMP networks tunable melting point and relatively high crystallinity,affording the network good shape fixing and recovery ability at the body temperature.Besides,the dynamic nature of transesterification enables the network good shape reconfigurability,allows for the easy processing of SMPs with complicated shapes.The reconfigurable SMPs capable of actuating at the body temperature show great potentials for use as biomedical devices.Shape programming polymer materials from planar sheets to three-dimensional(3D)geometries is appealing in many advanced devices.One of typical approaches is programming various Tm into one polymer sampel to coupling different shape memory behaviors.Herein,we report a simple yet effective approach that allows programable shape morphing of semicrystalline polymer by light-induced polymorph pattern.Two regions with different polymorphs show greatly distinct melting temperatures(Tm),moduli,and shape recovery abilities at a specified temperature.The resulting 3D geometries can be controlled by the applied strain and the spatial distribution of polymorphs.Due to the remarkable moduli difference,2D sheets with well-designed polymorph patterns can transform into concaveconvex structures used for "mortise-tenon" joints,which can be assembled into load-bearing devices.The materials with polymorph patterns are reprogrammable,due to the reversible nature of melting/crystallization process.Our strategy can be generalized to other semicrystalline polymers with polymorphism and can also expand unconstrained shape programming of morphing materials.Different crystal phases of polymorphic polymers are not only able to show distinct physical properties,but also always change with each other.Typically,PB preferentially crystallizes the metastable crystal form-Ⅱ with kinetic priority from melt,and then gradually tansforms to the thermodynamic stable crystal form-Ⅰ during annealing.The spontaneous nonequilibrium structural transformation from a higher free energy state to a lower energy one is the essential mechanism of constructing self-evolving materials.Therefore,a new principle for developing the self-evolving materials from commercialized polymorphic polyolefin via programmable crystal transition is proposed.The self-evolving materials can encode information of pattern and morphing by metastable crystal phase.Dynamically,such phase transforms to the stable crystal phase so that the encoded information self-evolves with time,displaying the autonomous characteristic.Moreover,this process can be interrupted at arbitrary time through the solvent-induced recrystallization.These advantages have been demonstrated by fabricating an edible period indicator and imitating sophisticated huamn body language.It is believed that this work may inspire future researches on self-evolving materials based on the non-equilibrium process of dry materials.However,it is challenging to prepare stress-free 2W-SMPs with good actuation performance and shape programmability from single-component semicrystalline polymers.Herein,we demonstrate a straightforward and universal strategy for preparing 2W-SMPs through self-nucleated crystallization(SNC)of semicrystalline polymers.SNC enables the formation of two types of crystals in the 2W-SMPs,annealed and primary crystals,which function as the skeleton phase and actuation phase,respectively.We achieved a high reversible actuation strain of 17.6%and a good reprogrammability of the SNC-treated polymer networks.Complex shape transformations were obtained,and smart devices were fabricated from the SNC-treated networks by using a locally designed folding and kirigami structure.The SNC strategy provides a generalized approach to improve the 2W-shape memory behavior of semicrystalline polymers.In conclusion,we show how to regulate the physicochemical properties of polymers by changing their crystallization behaviors,so as to optimize their shape memory performances.The strategy allowing programming physical structurse usually avoids complex chemical reactions or preparation processes.The nature reversible melting-crystallization behaviors also endow the materials repeated programming and recycling.The above results can be generalized to other semicrystalline polymer systems and have good application prospects due to the commonly used raw materials.