Preparation And Properties Of Programmable Liquid Crystal Elastomers Containing Siloxane Bonds

Liquid crystal elastomers,a kind of liquid crystal polymer with low cross-linking degree,are capable of achieving large and reversible deformation while exposed to heat,light,electric and magnetic field,etc.Siloxane-based LCEs(Si-LCE)exhibit promising potential for application in soft robots,flexible actuators,biomimetic systems,and other fields,owing to their unique characteristics brought by siloxane bonds,including low glass transition,high failure strain,low surface energy,and excellent thermal stability.Alignment is one of the crucial factors for achieving reversible deformation of LCEs.Different from the traditional alignment methods,using dynamic covalent bonds(DCBs)to align the mesogens is easy to operate and not limited by the crosslinking degree,while giving numerous favorable properties to the materials at the same time,such as self-healing,recyclability,and programmability.However,the traditional Si-LCE typically lack the ability for dynamic reactions due to the high stability of siloxane bonds.Even though siloxane bonds could undergo exchange reaction at high temperatures with the assistance of base or acid catalysts,the introduction of the additional catalysts increases the complexity of the operation and reduces the stability of the LCE materials.In this dissertation,Michael addition reaction was used to prepare Si-LCE with dynamic siloxane bonds through a dual self-catalytic strategy,and the ability of dynamic exchange reaction of Si-LCE was further improved by introducing dynamic hindered urea bonds and dynamic boronic ester bonds.The main contents of this dissertation are as follows:1.A novel and simple dual self-catalytic strategy for producing Si-LCE with dynamic covalent siloxane bonds was designed and proposed.A readily available compound-1,3-bis(aminopropyl)tetramethyldisiloxane(BATS)acts as chain extender and crosslinker for elastomers and provides dynamic covalent bonds(DCBs)as well.More importantly,amino groups of BATS play the role of dual self-catalyst,not only accelerating the thiol-Michael addition reaction and aza-Michael addition reaction simultaneously during producing Si-LCE,but also promoting dynamic covalent siloxane bonds exchange reaction at an elevated temperature after the formation of the cross-linked network.The presence of dynamic siloxane bonds endows Si-LCE with excellent programming,self-healing,and recycling properties,while the hydrogen bonding formed during the reaction enhances their mechanical properties.Furthermore,a highly efficient organic photothermal reagent YHD796 was synthesized for the preparation of photo responsive Si-LCE.The results show that the fully cured polydomain Si-LCE can be easily aligned,programmed,self-healed,and recycled by utilizing the dual self-catalytic strategy.Meanwhile,the addition of YHD796 endows Si-LCE with excellent photo-responsive properties.A series of multi-functional soft actuators were fabricated by simple welding method,showing a local fast response and precise control by light and heat.Furthermore,soft robots fabricated with a bilayer structure of Si-LCE can perform crawling and coiling actuation controlled by nearinfrared(NIR)light facilely.These results provide a new solution for the exploitation of multi-functional liquid crystal elastomers with fast response to various stimulus.2.A diacrylate-functionalized monomer containing dynamic hindered urea bonds(DA-HUB)was synthesized to create programmable LCEs with dynamic hindered urea bonds(Si-LCE-HUB)through a self-catalytic Michael addition reaction.The results show that,the incorporation of hindered urea bonds further enhances the dynamic exchange reaction ability of Si-LCE,resulting in superior programming,self-healing,and recycling properties with higher self-healing and recycling efficiency at lower temperatures.Moreover,the formation of hydrogen bonding within the system contributes to the excellent mechanical and stability properties of LCE actuators,enabling long-term stability.Additionally,the addition of spiropyran imparts photochromic properties to the material,allowing for the preparation of liquid crystal elastomers with reversible photochromism.By leveraging dynamic hindered urea bonds,bilayer LCE films with differentially oriented structures were created,exhibiting varying bending angles at different temperatures.The thermally induced bending and photochromic properties of the bilayer films were further utilized to fabricate a biomimetic triangular plum flower,enabling simulation and control of the blooming process and color-changing behavior of petals through heat and light.3.A diamino monomer containing dynamic boronic ester bonds(DAB)was synthesized and combined with the monomer containing dynamic siloxane bond to prepare LCEs with dual dynamic covalent bonds(Si-LCE-B).Meanwhile,a synchronous differential orientation strategy was developed to create multilayer LCEs with varied alignment properties based on the difference in bond energy of the dual dynamic covalent bonds.The results demonstrate that boronic ester bonds enhance LCEs with remarkable self-healing,programming,and processing characteristics.Additionally,by exploiting the difference in bond energy of the dual dynamic covalent bonds,the alignment of the multi-layer LCE films can be independently controlled.The shape and reversible deformation ability of the samples can be conveniently regulated by adjusting the temperature and heating time during the orientation process.Moreover,incorporating azobenzene to introduce photo-responsive properties,in combination with the differentially oriented structure of multilayer LCE films,enables the preparation of a range of flexible actuators with photo-reversible and thermosreversible deformations.