Preparation And Characterization Of Low-temperature Imidized Transparent Polyimide

Polyimide(PI)molecular chain contains very stable five-memed imide heterocyclic ring and high density aromatic ring,and has a regular structure,excellent heat resistance,dimensional stability,chemical solvent resistance,etc.In recent years,it has been popular in flexible display,electronic and electrical packaging,etc.These applications require high light transmittance of PI.The preparation process of PI material has a great influence on its light transmittance.The traditional two-step process requires imidization of the precursor polyamic acid(PAA)at more than 300?.Such high temperature treatment not only consumes a lot of energy,but also tends to cause the color of the film deeper.Therefore,it is of great significance to explore the low temperature imidization method and process of PI(less than200?)to obtain light color and high transparency PI film.(1)PAA was prepared from 4,4'-diamino-2,2'-bistrifluoromethylbenzene(TFMB)and3,3',4,4'-biphenyltetracarboxylic dianhydride(BPDA).Then,PAA was imidized with organic base imide accelerators triethylamine(Et₃N),benzimidazole(BI),piperazine(PZ),quinoline(QL)and isoquinoline(YQL)respectively.The imidization degree of PAA films after solvent removal at 70?was measured by infrared spectroscopy,so as to explore the order of imidization effect promoted by imide accelerators.Five kinds of PI films were prepared by curing PAA films at 260?for 1 h,and their thermal stability and mechanical properties were compared.The results showed that the order of imidization promotion effect of five imide accelerators was BI>QL?YQL>PZ>Et₃N.Among them,QL and YQL not only have high imidization promotion effect,but also have less adverse effect on PI film properties than Et₃N,BI and PZ.The thermal stability and mechanical properties of the prepared PI-QL and PI-YQL were very similar,which were obviously better than PI-Et₃N,PI-BI and PI-PZ.(2)QL and YQL were used as imide accelerators to explore the conditions of complete imidization of TFMB-BPDA type PAA at the lowest possible temperature,and the thermal stability,mechanical properties and light transmittance of two kinds of low temperature imidized PI films and conventional high temperature imidized PI films were compared.The results showed that the complete imidization can be achieved by adding QL or YQL at twice the molar amount of dianhydride into PAA solution and curing at 200?for 4 h or 5 h.The residual solvent and imide accelerators can be removed by further treatment at 250?under vacuum for 0.5 h.The thermal stability and tensile properties of the two PI films were only slightly decreased compared with those of conventional imidized PI films at high temperature,while the elongation at break was slightly increased,and the light transmittance was obviously better.The light transmittance at 400 nm was increased from 4.5%to 34.4%and17.2%,respectively.In contrast,QL promoted imidization more efficiently than YQL,and the PI film prepared had better transparency.(3)The PAA was produced by the copolycondensation of TFMB,2,2 bis[4-(4-aminophenoxy)phenyl]hexafluoropropane(BDAF)two diamine monomers and4,4'-(hexafluoroisopropene)diphthalic anhydride(6FDA),using QL as the imide accelerator,PI films with excellent transparency and high toughness were obtained.A series of low temperature imidized PI(FPI)films were prepared by changing the molar ratio of TFMB:BDAF to 1:0,0.9:0.1,0.8:0.2,0.7:0.3 and 0.6:0.4.The thermal stability,mechanical properties,solubility and light transmittance of these FPI films were tested and characterized.The results showed that QL can promote the imidization of FPI films at low temperature when the maximum imidization temperature is 200?for 2 h,and this low temperature imidization process can maintain good thermal stability and mechanical properties of FPI films,and give them better solubility and light transmittance.With the increase of the proportion of BDAF,the light transmittance of FPI films decreased slightly,but the toughness and flexibility of FPI films increased significantly.Wherein,the FPI-3 obtained according to the molar ratio of BDAF:TFMB of 3:7 not only has excellent light transmittance,the light transmittance at 400 nm and 450 nm reached 77.5%and 85.8%,respectively,but also the elongation at break increases from 4.3%to 10.8%compared with FPI-0 without BDAF,and it can be kept intact after 180°bending test.It has a good application prospect.