Preparation And Performance Study Of Multifunctional Liquid Crystal Elastomer Soft Actuators

In recent years,liquid crystal elastomer(LCE),a class of two-way shape memory polymer material with multi-stimuli-responsive deformation and easy functionalization,has a wide range of potential applications in the fields of soft robots,artificial muscle,microfluidic and bionics.However,exploring novel preparation strategies to develop LCE soft actuator systems with multifunctional properties still remains challenging,which limits the further development of smart liquid crystal materials.In order to overcome the above challenge,three research works are carried out in this paper successively: Firstly,a two-step copper-catalyzed azide-alkyne cycloaddition(Cu AAC)click chemistry strategy is applied to prepare the LCE soft actuators with excellent mechanical properties and multi-stimuli-responsive deformation;In addition,the functionalized tetraarylsuccinonitrile(TASN-diene)chromophore with two terminal vinyl groups is used as a crosslinking agent and connected to the LCE network by room temperature thiol-ene click chemistry method to prepare a biomimetic smart soft actuator with the functions of dynamic deformation,crawling movement,tri-stimuli-responsive color change camouflage,self-healing and recyclability;Finally,the poly(propylene fumarate)-backbone liquid crystal polymer(PPF-LCP)materials are designed and prepared,which can be further chemically crosslinked to prepare the liquid crystal elastomer materials with good mechanical properties.In addition,the liquid crystal elastomer materials show excellent biodegradability.The detailed research content will be discussed in the following three parts:(1)LCE,a class of stimuli-responsive polymeric material with reversible shape-morphing capability,exhibits extraordinary application prospects in robotic and intelligent materials and has attracted widespread attention of scientists.However,the exploration of facile and efficient novel preparation protocols for the development of multifunctional two-way shape memory LCE soft actuators remains challenging.In order to break through the challenge,a two-step copper-catalyzed azide-alkyne cycloaddition(Cu AAC)click chemistry strategy is applied to synthesize a new type of main-chain liquid crystal elastomer(MCLCE)material directly from two small molecule monomers bearing terminal dialkyne and diazide units respectively.The azide-alkyne MCLCE materials exhibit highly ordered enantiotropic smectic A phase,fully reversible thermal-actuating behavior and excellent mechanical properties.Furthermore,through doping photothermal dyes,coating electric conductive layers and complex programming protocols,the azide-alkyne MCLCE materials can be fabricated as multifunctional two-way shape memory soft actuators,such as photo-responsive soft actuators,electric-responsive soft actuators and three-dimensional(3D)shape-morphing soft actuators,which can perform reversible contraction/expansion,bending/unbending,helical curling/uncurling and diversified shape motion modes.This facile and efficient Cu AAC in situ polymerization/crosslinking approach might bring a new perspective for the development of two-way shape memory polymeric materials with superior mechanical properties and enhanced shape morphing complexity.(2)However,the LCE soft actuators prepared above only exhibited the functional characteristics of stimuli-responsive deformation.Obviously,the single functional mode cannot meet the requirements of multifunctional properties of materials in practical applications,and they do not possess the functions such as color changing and self-healing,which limits the real-time monitoring and reuse of the material.Therefore,it remains a difficult problem to fabricate a single soft actuator system with stimuli-responsive deformation,crawling motion,color changing,self-healing and recycling functions.In order to resolve the difficulty,a multi-stimuli-responsive tetraarylsuccinonitrile(TASN)chromophore with two terminal vinyl groups as the crosslinker is chemically incorporated into a polysiloxane based LCE network through a facile room-temperature thiol-ene photoaddition click chemistry method to fabricate a multifunctional TASN-LCE soft actuator.The obtained TASN-LCE soft actuators exhibit not only reversible thermal-induced shape morphing,but also reversible color changing behavior in response to heat,mechanical compression and acid vapor exposure,which is due to the generation and recombination of carbon-centered radical species of TASN units in the polymer network.Moreover,due to the existence of dynamic covalent linkages,the TASN-LCE soft actuators also show excellent self-healing,reprogramming and recycling characteristics.As a proof of concept,a starfish-shaped TASN-LCE soft robot is designed and fabricated to mimic the shape morphing,crawling motion,tri-stimuli-responsive color changing camouflage and self-healing behaviors of natural starfish.We hope that such a TASN-LCE actuator system integrated with dynamic distortion,thermo-mechano-vapo-chromic camouflage and self-healing functionalities would pave the way for further development of multifunctional biomimetic soft robotic devices.(3)For decades,liquid crystal polymer(LCP)has attracted extensive attention due to their widespread applications in artificial muscles,engineering plastics and high-modulus fibers,etc.However,most functional materials based on LCP,such as soft actuators and biomimetic devices,contain the nonbiocompatible and nonbiodegradable rigid aromatic mesogenic units,which not only limits their development in the field of biomedicine,but also brings certain chemical pollution to the environment,and this is contrary to the sustainable development strategy of the state and society.Therefore,in view of continuous development strategy,the design and fabrication of LCP materials with excellent biocompatibility,mechanical properties and biodegradability are still challenging.In order to address the challenge,two poly(propylene fumarate)-backbone LCP(PPF-LCP)materials are synthesized by ring opening polymerization(ROP)between two biocompatible cholesterol-based epoxy monomers and maleic anhydride,and a successive isomerization process.The resulting PPF-LCP materials bearing biocompatible cholesterol units exhibit good thermal stability and highly ordered Sm A phase.Moreover,a UV-initiated chemical crosslinking of the unsaturated PPF main chain dramatically strengthens the mechanical performance of the corresponding PPF-LCP materials,and the crosslinked PPF-LCP elastomer materials possess one-way thermal-responsive deformation and good degradability.Therefore,the PPF-LCP material has great advantages in the biological tissue engineering.