Preparation And Properties Of Multifunctional And Multi-responsive Shape Memory Polyimides

Shape memory polyimides have attracted much attention from researchers due to the excellent thermal stability,corrosion resistance,irradiating resistance,superior mechanical properties.However,most studies focus on the dual shape memory effects for the polyimides,but multiple shape memory effects are rarely reported.Besides,the reported multi-shape memory polyimides are always synthesized by copolymerization.In addition,materials are shown limited strain due to the strong interchain interactions,and the maximum strain of shape memory polyimides was no more than 200%.Moreover,all of shape memory polyimides are thermal induced,that is heated environmental temperature over to the transition temperature.There are no reports of other stimuli to induce the shape memory polyimides to date.To sum up,in this paper we will focus on the following three points: 1.Multiple shape memory polyimides with low dielectric constant;2.Multiple shape memory polyimides with extremely high strain;3.Electricity-,light-induced shape memory polyimides.Firstly,from the designation of polymer structure perspective,we selected the long chains diamines containing trifluoromethyl and dianhydrides containing trifluoromethhyl to execute polymerization reactions.A series of polyimides with low crosslink density were obtained by crosslinking with non-coplanar crosllinkers.Swelling property of crosslinked polyimides was investigated,and the crosslinking densities of the materials were calculated by the linear fitting of the stress-strain relationship according to the Mooney-Rivlin formula.The synergistic effect of physical and chemical crosslinks endowing polyimides with shape memory effects,and the relationships between crosslinked densities,thickness of film and shape memory effect was discussed.The broad glass transition temperature intervals of polyimides suggesting that cross linked polyimides have multiple shape memory effect.The presence of trifluoromethyl effectively decreased the interchain interactions,hence to decrease the dielectric constant.In addition,these fluorinated polyimides showed low water absorption due to the presence of fluorine which can reduce the polarization.Secondly,multiple shape memory polyimides with extremely high strain were studied.We selected kinked diamines containing trifluoromethyl and dianhydrides containing trifluoromethhyl to execute polymerization reactions.A series of polyimides with low crosslink density were obtained by crosslinking with non-coplanar crosllinkers.Swelling property of crosslinked polyimides was investigated,and the crosslinking densities of the materials were calculated by the linear fitting of the stress-strain relationship according to the phenomenon theory.Analyzing the influence on shape memory by crosslink densities,the results showed that polyimides showed the best shape memory effects when the content of crosslinker is 0.5%,and the strain as high as 460%.In addition,based on the study of triple shape memory properties,we found that the higher temperature value of twice deformation temperatures,the better shape memory performance of the materials.Moreover,we also investigated this series of low crosslinking density polyimides for the shape memory cycle stability,stress relaxation,thermal properties,mechanical properties,dielectric properties and water absorption.Finally,we have studied on the electricity and thermal induced shape memory polyimides.Based on the research of two chapters before,we selected 0.5% crosslinker polyimides with kinked structures to prepare polyimides composites.Electric conductivity and paste-strip electric conductivity of polyimides composites using different conductive substrate,different length-diameter ratio of silver nanowires(AgNW),different preparation methods and different spinning times of AgNW are studied.The results show that silver plating polyimide films and AgNW/AA/PI which prepared by combining AgNW network on the surface of polyacrylate film has good conductivity,but the conductivity of PI/Ag was disappeared entirely after 1000 times paste-stripping,however,the sheet resistance of AgNW/AA/PI was only increased about 23% after 1000 times paste-stripping.The electroconductive properties of polyimide films treated with plasma were less conductive and conductive properties disappeared after pasting and stripping.In addition,the polyimides which prepared by polyamide acid directly dropped to the surface of the silver nanowire are not conductive.Furthermore,with good conductive properties of polyimide composite film has carried on the electrically induced shape memory performance test,the material in the case of electricity can quickly return to the permanent shape.In addition,due to the silver nanowires with infrared absorption effect,the silver nanowires/polyimide composite film series of infrared laser induced shape memory effect has carried on the systematic study,and the different length to diameter ratio of silver nanowires and preparation methods of silver nanowires/polyimide composite film series to shape memory behavior were studied.