| Shape memory polyimides(SMPIs),as a type of extremely high temperature resistant special engineering plastic,have received extensive and in-depth theoretical and experimental research for their high mechanical properties,solvent resistance,high flame retardancy and excellent shape memory properties,which have high application value in aerospace engineering,microelectronic devices,high temperature sensors and actuators,etc.The current research on shape memory polyimide mainly focuses on the optimization of molecular structure design,chemical-physical cross-linking and the regulation of the interaction force of macromolecular chains to achieve enhanced performance.Nevertheless,the deficiencies of pure polyimides,such as low thermal conductivity and single stimulus response,limit their wide application in high-tech fields and can no longer meet the rapidly developing scientific and technological requirements.Consequently,it is necessary to introduce multifunctional fillers to fabricate higher performance and high value-added polyimide composites to achieve fast shape recovery and multi-stimulus response,and further expand their intelligent applications under complex environments in most fields.In this paper,the design and preparation of high-temperature high-performance shape memory polyimide was the main line of the study,with the idea of achieving multi-stimulus response and multi-shape memory.After that,the effects of different cross-linking agent contents and different filler contents on the mechanical properties,thermal properties and shape memory properties of polyimide composites were systematically investigated.1.Initially,the three-dimensional thermoset network was obtained by using triamine as chemical cross-linking points to connect the linear thermoplastic molecular chains.A series of thermosetting polyimides with different degrees of crosslinking were synthesized by adjusting the amount of crosslinking agent.The properties of thermoplastic and thermosetting polyimide were compared and the impact of the cross-linking agent contents on their overall properties was analyzed.The sample with the best comprehensive performance was selected to explore the intrinsic mechanism of polyimide shape memory so as to provide the best matrix material for the design and preparation of high-performance polyimide composites.2.Next,graphene oxide with excellent dispersion as a reinforcing filler was introduced into the thermosetting polyimide matrix,and a series of polyimide composites with different filler contents were prepared.The mechanical properties of the composites were tested and found to be greatly enhanced,with tensile strength up to 134 MPa,strain at break reaching44.1%and toughness rising to 41.7 MJ/m~3 at a low GO content(1 wt%).The composites also have excellent shape memory properties,with a shape fixation rate greater than 99%and a shape recovery rate greater than 98%.The shape restoration behavior of the composites under NIR light irradiation was investigated using the ability of GO with photothermal conversion,which can induce the fixation of temporary shape and then completely recover to the original shape within 7 s by sequential NIR irradiation.Subsequently,an experimental demonstration of the application of the shape memory response of composite materials under photothermal stimulation conditions was performed.3.Then,an amino-capped polyimide oligomer was designed and synthesized for the preparation of polyhexahydrotriazine cross-linked structures with paraformaldehyde through the reaction of amino and aldehyde groups.Moreover,the combination of dynamic covalent bonding of the low molecular weight polyimide and hexahydrotriazine resulted in mechanical properties comparable to those of high molecular weight polyimides.The effects of different treatment temperatures on the thermal stability and dynamic thermo-mechanical properties of the precursor half-crosslinked dynamic network(HDCN)and fully crosslinked network(PHT)formed by the reaction of amino capped polyimide with paraformaldehyde were further discussed.The comprehensive properties of the composite films prepared by adding different GO contents were tested,and it was found that the introduction of GO greatly improved the mechanical properties and shape memory properties.In particular,the shape memory recovery rate increased from 73.7%to 92.6%.4.Finally,bilayer gradient polyimide composites were synthesized based on graphene oxide reinforced monolayer composite films,and the mechanical properties,thermal properties and shape memory properties of the bilayer gradient composites were studied.Furthermore,the triple-shape memory of the PI-PHT-01,PI-PHT-03 and PI-PHT-05 samples with two glass transition temperatures were examined.The qualitative and quantitative tests on the triple-shape memory process revealed that PI-PHT-05 had the best performance,with calculated fixation rates of 96.1%and 91.2%and recovery rates of 107.7%and 55.5%,respectively.The variation in the internal stresses of selected materials with temperature was analyzed,and it was found that the magnitude of the stresses at low and high temperatures was the key to determining the fixation and recovery rates of the materials. |
