Preparation And Electrical Bistable Memory Effects Of Hyperbranched Polyimides

In recent years,polyimides have attracted growing attentions in the filed of polymer memory devices due to their outstanding electrical properties,excellent thermal stability and chemical resistance.At present,most polyimides used for memory devices are linear structure,but their rigid backbones and strong interaction packing always give rise to poor solubility.The molecular design of linear polyimides should take into account their solubility and heat resistance,which limit the structure diversity of linear polyimides to some extent.Hyperbranched polyimides(HBPIs)combine the properties of hyperbranched polymers with polyimides,which not only have good organic solubility but also have excellent heat resistance.In addition,the unique molecular structures of HBPIs endow them with rich structural flexibility.Therefore,the preparation of HBPIs through a reasonable molecular design and studying the relationship between its molecular structure and storage performance are of great significance for the future developments of polymer memory materials.Herein,HBPIs were chosen as the research objects to study the relationship between their molecular structure and memory performance from different angles,such as the branched structure,linear link length and terminal group.In chapter 1,we investigated the push-pull electron effect in the branched structures of HBPIs on their storage performance.A novel triamine monomer containing multi-triphenylamine moieties was designed and synthesized.A series of HBPIs with different branched structures were synthesized by reacting the triamine monomer with various dianhydrides.Memory devices based on these HBPIs showed various memory characteristics,such as electrical insulator,volatile static random access memory(SRAM)and nonvolatile write once read-many-times memory(WORM).Molecular simulation results suggested that the bistable memory behaviors of HBPIs were dominated by the charge transfer mechanism.The hyperbranched structure is beneficial to improve the charge transfer efficiency,giving rise to low switching voltages.Moreover,the electron-withdrawing abilities and dipole moments of HBPIs could be tuned through adjusting the acceptor moieties,which will have an influence on the stability of charge transfer complexes,realizing the regulation of memory performance.In chapter 2,we investigated the relationship between the linear link lengths of HBPIs and their memory characteristics.A triphenylamine-contained diamine monomer was synthesized.A series of hyperbranched polyimides with different linear chain lengths were prepared by tuning the copolymerization ratio.The thermal performances of HBPIs increased with the growth of linear chain length.Memory devices were fabricated by using these HBPIs as the active layers.Memory characteristics of these HBPIs tuned from volatile SRAM to nonvolatile WORM with the growth of linear chain length.Molecular simulation results suggested that the growth of linear chain length will increase the twist of polymer conformation.The twisted conformation will create a barrier for the back charge transfer and stabilize the charge separation state,leading to nonvolatile memory performance.Therefore,the memory behaviors of HBPIs can be controlled by adjusting the linear chain length.In chapter 3,we investigated the relationship between the terminal groups of HBPIs and their memory characteristics.A series of hyperbranched polyimides with different terminal groups were synthesized by using the triamine monomer as electron-donor,6FDA as electron-acceptor,phthalic anhydride(PA),1,8-Naphthalic anhydride(NA)and 3,4-perylenedicarboxylic anhydride(PDA)as endcapping reagents.HBPIs endcapped with NA and PDA exhibited enhanced thermal stability and photoelectric performance.Memory device based on PA-HBPI was a SRAM in type.Compared to PA endcapped HBPI,NA and PDA endcapped HBPIs exhibited enhanced nonvolatile WORM behaviors with low switch voltages.Molecular simulation results suggested that end-capping of HBPIs by using large conjugated structure is helpful to promote and stabilize the charge transfer process,which will further enhance the storage performance of HBPIs.