Preparation And Performances Of Electrical Memory Materials Based On Terminated Hyperbranched Polyimides

As a kind of high performance polymers,aromatic polyimides have been applied in aerospace,microelectronics and nuclear industries,due to excellent comprehensive performance,such as good thermal stability,mechanical property and chemical stability.Recently,polyimides have become one of the most widely studied organic resistive random access memory materials,due to their special molecular structure,which leads to simple molecular design and synthesis.However,current researches mostly focus on linear polyimides,which are facing the problems of insolubility and infusibility due to the rigid backbone structure and chain packing.Therefore,it is necessary to consider the balance between the memory performance and processability,which will confine the research.These problems are expected to be solved by introducing hyperbranched polyimides,due to their excellent solubility without insignificant damage to the superior performance of polyimides.Moreover,another advantage is the existence of abundant terminals of hyperbranched polyimides,which provides convenient reaction for modification.Therefore,compared with linear polymer,performances of hyperbranched polymer can not only be adjusted by monomer and main chain design,but also be optimized by terminal modification.In this thesis,departuring from molecular design and choosing hyperbranched polyimides as the research object,we have studied the relationship between the structure of terminals and the memory performances of polymers from different perspective.In chapter 3,we studied the change of the memory performance of the polymers after termination with electroactive moiety.We designed and synthesized a tetra-amine without electron-donating ability,which was used in polycondensation with several dianhydrides,followed by termination with ferrocene moiety and chemical imidization,to give a series of ferrocene-terminated hyperbranched polyimides.It has been found that terminated polymers showed WORM behavior with threshold voltages of 2.2~3.2V and-2.6~-3.3V,and ON/OFF current ratios of 10~4 level,while the non-modified polymer acted as an insulator.The introduction of ferrocene moiety played a key role in the memory performance for polymers.Subsequently,mechanism analysis found that the memory performance can be attributed to the hole trapping of the ferrocene moiety under an applied electrical field,forming a conduction pathway.It was found that the charge transfer was dominated by the ohmic model and space-charge-limitted-current model.In chapter 4,we studied the influence of tiny variation of terminal structure on the memory performance.Three monoamine monomers based triphenylamine moiety were synthesized and used to terminate the polyamic acid which was derived from the tetra-amine and 6FDA,giving three hyperbranched polyimides with different terminal structure.All the devices based on the synthesized polymers exhibited volatile SRAM behavior.The threshold voltages decreased(-2.24 V,-1.64 V and-1.33 V)and ON/OFF current ratio increased(10~4,10~4 and 10~6)with the electron-donating ability of the terminal enhanced.Mechanism analysis through molecular simulation and theoretical calculation demonstrated that the influence of the terminal structure on the molecular orbital level of the polymers were in consistent with the anticipation and the memory properties were attributed to the charge transfer between the triphenylamine-based moieties and dianhydride.Moreover,the stability of the charge-transfer complex was strongly depended on its dipole moment and it became more stable with the dipole moment increasing(3.10 D,1.28 D and 4.20 D),resulting in the retention time increasing of the ON state of device(11 min,5 min and 19 min).Therefore,adjustment of the memory performance can be accomplished by tiny variation of the terminal structure of hyperbranched polyimides.In the chapter 5,we studied the effect of the large conjugate porphyrin terminal and its metal complexation on the memory performance of hyperbranched polyimides.Monoamine tetraphenylporphyrin was synthesized and used to terminate the polyamic acid derived from tetra-amine and 6FDA,resulting in porphyrin moiety terminated hyperbranched polyimide.Subsequently,the polymer was complexed with Zn ion to give Zn-complexation hyperbranched polyimide.Memory devices based on the tetraphenylporphyrin terminated hyperbranched polyimide exhibited WORM behavior with threshold voltage of-1.86 V and ON/OFF current ratio of 10~6,which was attributed to the charge transfer between the porphyrin moiety and 6FDA and the porphyrin moiety could facilitate the stability of the charge transfer complex.However,devices based on the Zn conplexed hyperbranched polyimide exhibited SRAM behavior with threshold voltage of-1.75 V and ON/OFF current ratio of 10~5,due to the internal electrode derived from Zn ion,which facilitated the charge transfer and lowered the threshold voltage while made the charge transfer complex unstable.Therefore,the memory type of hyperbranched polyimide could be tailored by the adjustment of terminal structure.In chapter 6,we studied the memory performance of the binary terminated hyperbranched polyimide.Ferrocene and porphyrin moieties were introduced simultaneously to give the binary terminated polymer which then complex with Zn ion to give another polymer.Device based binary terminal polymer exhibited non-volatile ternary memory performance which was attributed to the field-induced charge transfer and carrier capture derived from porphyrin and ferrocene moieties,respectively.However,in the ternary memory performance of device based on Zn complex binary terminated polymer,the intermediate ON state was volatile due to the internal electrode role of Zn.Therefore,multiple level memory performance of hyperbranched polyimide can be achieved by reasonable design of the type of terminal.