Synthesis Of Flexible Branched Polyimide And Kinetics Of Polymerization

With the popularization of electronic devices such as tablet computers,large-screen TVs and automobile meters.,the display quality requirements for screens are also getting higher and higher,and liquid crystal displays with clear display screens,excellent anti-friction performance,and wide viewing angles have gradually entered the public consciousness.In order to satisfy the above requirements,it is necessary to find not only a novel liquid crystal material,but also a liquid crystal alignment film having excellent performance.Polyimide is the material of choice for liquid crystal alignment films due to its excellent heat resistance,corrosion resistance,mechanical strength,and affinity with liquid crystals.The common synthetic method of polyimide is a two-step method,in which the formation of the polyamic acid prepolymer in the first step has a great influence on the processing technology and comprehensive performance of the subsequent film formation.Therefore,studying the polymerization kinetics of polyamic acid,designing and synthesizing polyimide with flexible branching can provide an important experimental basis for the synthesis of high performance polyimide.Based on this,the paper firstly synthesized polyamic acid using pyromellitic dianhydride(PMDA)and N,N-diaminodiphenyl ether(ODA),and characterized the structure of the product by FTIR.The concentration of ODA during the reaction was determined by HPLC.Real-time detection is performed to derive possible autocatalytic reaction mechanisms.The results show that the formation of polyamic acid is an autocatalytic second-order reversible reaction,the formation of intermediate complex is a control step,and the decomposition of the intermediate complex produces a polyamic acid as a fast reaction.At normal temperature,the reaction rate equation derived from the reaction mechanism is r =dA/dt=k1A2-k2(A0-A)+k3A2(A0-A)-k6(A0-A)2,The reaction rate constants are ki = 0.11～0.23 L2·s2/mol2;k2 = 0.0007～0.0024 L·s/mol;k3 = 4.2～6.4 L3·s3/mol3;k6 = 0.05～0.2 L2·s2/mol2.In addition,The flexible branched b-phenylenediamine(L-14)was synthesized from 2,3,4-trifluoronitrobenzene and 4-(4-heptylcyclohexyl)-phenol and the like,and 1,2 A novel flexible branched polyimide liquid crystal vertical alignment agent was synthesized by reacting 3,4-cyclobutane tetracarboxylic acid dianhydride(CBDA).The solubility test results show that the polyimide is not only soluble in a high boiling solvent such as N-methylpyrrolidone,but also has a good solubility in a low boiling solvent such as chloroform;the polyimide film is used as a liquid crystal.The alignment layer was used to prepare a liquid crystal display element,and its pretilt angle,liquid crystal alignment time,steepness,contrast,voltage holding ratio,and relaxation characteristics of accumulated charges were tested.The results show that the pretilt angle is about 89.7°,the pre-tilt angle of the increased wool indentation remains basically the same,the steepness is about 1.73,and the contrast is as high as 16120,which indicates that the orientation performance is excellent,the display picture is clear,and it can adapt to the demand of multiple channels.The liquid crystal orientation time is between 5s and 7s without compression and recovery,indicating that the orientation time of the liquid crystal display element is short,the screen conversion rate is fast,the voltage retention rate is above 99%,and the accumulation charge relaxation characteristic is about 29%.It shows that the electro-optical performance of the liquid crystal display element is good,the afterimage disappears quickly,and the service life is long;changing the ratio of PDA to L-14 in the raw material,it is found that the polyimide viscosity generated when the ratio of L-14 to PDA is 8:2 Moderate,and with the increase of long chain branching,the optical performance and electrophilic performance of liquid crystal display elements are greatly improved.