Preparation And Properties Of Triphenylamine-based Polyimide Containing Electroactive Units

Electrochromism is a phenomenon of reversible color change after electrochemical oxidation or reduction.Electrochromic materials are widely used in the fields of smart windows,electronic papers,automobile anti-glare rearview mirrors,flexible electronic displays due to their rich color,high electrical conductivity and color rendering after power failure.Electrochromic materials are plentiful and diverse.In addition to transition metal oxides,organic small molecules and conductive polymers,D-A conjugated polymers and triphenylamine electrochromic materials are become one of the most researched electrochromic materials.High-performance polymers such as polyimide,polyamide,poly(aryl ether)sulfone are used in various fields of practical life due to advantages of their strong mechanical strength,excellent thermal properties and chemical resistance and so on.More importantly,the modification of the skeleton structure of high performance polymers by introducing functional groups that can impart functions to the polymers without affecting their properties has greatly enhanced the application prospects of high performance polymers.However,the rigid molecular chain structure and the strong intermolecular forces make these polymers less soluble and processable,which severely limits their secondary processing and film preparation.To solve the above problems,flexible groups,distorted non-coplanar structures,and bulky side groups are often introduced to the polymer backbone through structural modifications.In this paper,two new“donor-acceptor”type electroactive polyimides were designed by molecular design using"triphenylamine-benzimidazole-derived diamine"as the donor unit and the imide ring in the dianhydride as the acceptor unit.Introduction of bulk non-coplanar groups into the synthesized diamine monomers to improve the solubility and processing properties of the polymers.Due to its excellent electroactive and hole-transporting ability,electron-rich triphenylamine is often used as a donor unit,which is endowed with electrochromic properties due to the redox nature of its central nitrogen atom.By the above-mentioned synthesis,we inserted benzimizazole derivatives as protecting groups in the para-position of triphenylamine and introduced them into the molecular chain backbone of polyimide,As electron-donating groups,the benzimidazole derivatives can reduce the oxidation and onset potentials of the new triphenylamine-containing polyimide structure,thus imparting its excellent electrochromic properties.It provides ideas and solutions for future research on high performance and easy to process electrochromic materials,and the specific research of this paper is as follows:By molecular design,two novel benzimidazole-derived dinitro intermediates were synthesized:1,2-diphenyl-N,N'-bis-4-nitrophenyl-5-amine-benzimidazole and 4-nitro-4'-nitrophenyl-4''-1-phenyl-benzimidazolyl-phenyl-aniline(named as TPA-BIA-NO₂ and TPA-BIB-NO₂,respectively)and diamine monomers:1,2-diphenyl-N,N'-di-4-aminophenyl-5-amine-benzimidazole and 4-amino-4'-aminophenyl-4''-1-Phenyl-benzimidazolyl-phenyl-aniline(named as TPA-BIA-NH₂ and TPA-BIB-NH₂,respectively).Two newly synthesized diamine monomers and commercial dianhydride 6FDA were prepared into polyimides(named TPA-BIA-PI and TPA-BIB-PI,respectively)by chemical imine method polymerization,and the successful preparation of dinitro intermediates,diamide monomers and polyimide structures were confirmed by characterization means of nuclear magnetic and infrared spectroscopy.The solubility,characteristic viscosity,relative molecular weight,thermal stability and UV absorption of polyimide were also characterized by UV viscometer,GPC,DSC,TGA,and UV-visible spectrometer.Subsequently,polyimide films were prepared on ITO conductive glass substrates by electrostatic spraying method,and the UV absorption and electrochromic properties of polyimide films were systematically investigated by electrochemical workstation,UV-visible spectrometer and the combination of both instruments.The following conclusions were finally obtained:TPA-BIA-PI and TPA-BIB-PI exhibited relatively excellent solubility in common organic solvents due to the presence of the non-coplanar group triphenylamine and the bulk side group trifluorocarbon in 6FDA.In addition,the glass transition temperature(T_g)of TPA-BIA-PI and TPA-BIB-PI are above 310°C,and the thermal decomposition temperature(T_d)of 10%under nitrogen atmosphere are 548°C and 586°C,respectively,and the residual carbon rate at 800°C is around 65%.The above data indicate that the presence of rigid aromatic groups makes the two new polyimides have excellent thermal stability.During the electrochromic test(three-electrode system),the TPA-BIA-PI and TPA-BIB-PI films"yellowish-dark blue"and"yellowish-dark green",respectively by square wave voltage.The coloring time and blenching time of TPA-BIA-PI at 755nm were measured by UV spectrophotometer and electrochemical workstation were 13s and 16s,respectively,and those of TPA-BIB-PI at 762nm were 13.9s and 9.5s,respectively.The comprehensive properties of the two films make them have great development potential in flexible electronic displays,smart windows and other fields.