Preparation And Properties Of N-substituted Bisbenzimidazole Polyimide Films With High Tg And Low CTE

In recent years,with the development of flexible,lightweight,wearable and highly integrated electronic components,it is a challenging task to develop and prepare polymer films with ultra-high temperature resistance and very low linear coefficient of thermal expansion(CTE),and to produce electronic components such as flexible displays,organic light emitting diode devices and thin film transistors(TFT).This requires the polymer film to have extremely high heat resistance.For example,in the production of flexible cover,the reflow soldering technology briefly exceeds 250℃,and in the production of TFT,it will exceed 400℃.Among many polymer films,the glass transition temperature(Tg)of which can exceed 250℃ is already water chestnut.In addition,it is required that the CTE value of polymer film be low.Polyimide(PI)is a polymer that can meet such performance requirements.Its Tg is over 300℃ and its dimensional stability is good.PI has outstanding advantages such as excellent mechanical properties,chemical corrosion resistance,excellent electrical properties,self-extinguishing,etc.,and is widely used in aerospace,high-speed rail vehicles,nuclear power and many other fields.In order to have a good application in flexible display,it is necessary to further improve its Tg and reduce its CTE value.From the chemical structure analysis,when the content of rigid units in the polyimide molecular chain increases,the polyimide molecular chain shows a rigid/rod-like structure,so that the molecular chain segment is difficult to move,and the polyimide material will show a higher Tg and a lower CTE value.If there is strong hydrogen bonding and conjugation between polyimide molecular chains,the polymer molecular chains will be packed more tightly,which will further restrict the movement of polyimide molecular chains,which will further increase Tg and lower CTE value.Polybenzimidazole imides(PBIIs)is a high-performance material which introduces imidazole ring(BI)units into polyimide.Generally speaking,it is obtained by polycondensation of diamine monomer containing imidazole ring units and dianhydride monomer.Polybenzimidazole imide has the advantages of both polybenzimidazole and polyimide,and can make up for the shortcomings of polyimide in some aspects.This is mainly due to the strong nucleophilicity and intermolecular hydrogen bond of imidazole ring unit,which not only increases the rigidity of polyimide molecular chain,but also increases the intermolecular interaction.Although the intermolecular charge transfer(CT)is enhanced and appears brown,it also enhances the Tg of polyimide and reduces the CTE value.On this basis,the linear diamine monomer was obtained by introducing the bis-benzimidazole unit into the diamine monomer,and then the bis-benzimidazole unit was modified by N-substituent,which improved the film-forming properties of bis-benzimidazole diamine and 1,2,4,5-pyromellitic dianhydride(PMDA).The main work of this paper is as follows: Two kinds of diamine monomers containing bisbenzimidazole units,namely,1-methyl-2,2’-bisbenzimidazole-5,5’-diamine(Me DABZ)and 1-phenyl-2,2’-bisbenzimidazole-5,5’-diamine(Ph DABZ),were prepared by molecular design.Benzimidazole diamines and 4,4’-diaminodiphenyl ether(ODA)in different molar ratios(100:0;75:25;50:50;25:75;0:100)and PMDA,and some series of polybenzimidazole imide films were obtained by two-step thermal imidization.The modification of bisbenzimidazole unit by Nsubstituent increases the distance between molecular chains of polybenzimidazole imide and improves its film-forming property.Because the bisbenzimidazole unit and PMDA are rigid units,the synthesized polybenzimidazole imide molecular chain is a rigid rod-shaped molecule,which makes the movement of the chain segment difficult.The prepared polybenzimidazole imide film has ultra-high Tg,extremely low CTE value and excellent mechanical properties.In addition,the intermolecular hydrogen bond and conjugation restrict the movement of polymer molecular segments,which further enhances the Tg of polybenzimidazole imide film and reduces the CTE value.The properties of polybenzimidazole imide films were systematically evaluated by TGA,DMA,TMA and universal testing machine.The synthesized polybenzimidazole imide film has extremely high heat resistance,and the 5% weight loss temperature exceeds 530℃.With the decrease of ODA content,the 5% weight loss temperature increases.Polybenzimidazole imide films also have ultra-high Tg,all above 430℃,which also increases with the decrease of ODA content.The Tg of 100 Me DANZ/0ODA and 100 Ph DABZ/0ODA are as high as 483℃ and 500℃,which is because the molecular chain of 100 Ph DABZ/0ODA is linear,and the movement of molecular chain is more difficult because of the large steric side groups such as phenyl.The CTE values of these series of polybenzimidazole imide films are also excellent,all of which are below 20ppm/K,and even lower than 0.4ppm/K and 9.3ppm/K for 100 Me DANZ/0ODA and 100 Ph DABZ/0ODA.The CTE values of 100 Me DANZ/0ODA are lower because of the smaller molecular spacing and the difficulty of chain segment movement.The mechanical properties of these films are also excellent.With such excellent comprehensive properties,these polybenzimidazole imide films are a powerful potential stock in the fields of flexible circuit,flexible display,TFT and organic light emitting diode devices.