Study On Synthesis And Modification Of Polyimide With High Light Trnsmittance

Polyimide(PI) is a kind of polymer materials whose precursor named polyamic acid is synthesized with the dianhydride and diamine by condensation reaction, and then polyimide is obtained after the imidization process. There are very stable aromatic heterocyclic structures in polyimide molecules. Compared with other polymers, polyimide has excellent radiation resistance, abrasion resistance, dimensional stability of products and so on, meanwhile its heat resistance is more outstanding. As a kind of polymer material with excellent performance, it has been used in a lot of fields.Usually There are imide ring and benzene ring in polyimide backbone, thus the strong intermolecular force exists and results in close packing of the molecular chain. So that polyimide has the following shortcomings:(1) The polyimide film has poor transparency, and its applications are not satisfied;(2)The expensive production of raw materials, resulting in high costs;(3) The poor processing performance. Therefore, facing of the above problems,comprehensively considered about the cost, process, and performance etc. The best solution should be modifing polyimide.There are several methods for modifing polyimide in view of the above problems. For example, modified by introducing the required side groups to obtain desired properties, modified by blending inorganic fillers with polyimide and copolymerization modified by introducing feature structures into the the molecular backbone. However, due to the high cost of raw materials and stringent requirements for synthesis conditions and other shortcomings when using copolymer or functional modification, the applications of polyimide are limited. Compared with structure modification, copolymerization modification or developing a new generation of polymeric materials, the modification with inorganic filler is an economical and effective method. Polyimide/inorganic composite materials can both have the advantages of organic materials and inorganic materials, such as low thermal expansion coefficient, heat resistance and thermal stability at high temperature of the inorganic material and excellent toughness, processability of organic materials. The physical and chemical properties of the materials will be greatly improved when the particle size of inorganic materials reaches the nanometer level. So, to achieve the composite polyimide of high light transmittance and excellent mechanical properties under the premise of maintaining the high heat resistance of polyimide. In this experiment firstly we synthesized the precursor-polyamic acid, and then polyimide powder was added into the polyamic acid solution to prepare composite polyimide. Finally we modified composite polyimide by adding TiO2 coated ZnO particles.Nano TiO2 has excellent optical properties and heat resistance, when added into polymer can improve the optical and thermal properties of materials. ZnO has excellent thermal stability and compatibility with matrix. For ZnO and TiO2 composite powders, its thermal stability and dispersion has been greatly improved. But adding inorganic particles and too large particle size usually causes the decrease of mechanical properties, therefore the smallest particle size of particles need to be prepared. Meanwhile we add different amount of polyimide powder into the polyamic acid solution to prepare PI / PAA polyimide,so that the mechanical properties can be improved.The particle size and the structural characterization of TiO2 coated with ZnO particles can be tested by the particle size analyzer, ransmission electron microscopy(TEM) and X-ray diffraction(XRD); To test transmittance,mechanical properties and heat resistance of composite polyimide using UV-Vis spectrophotometer(UV), electronic universal testing machine(UTM) and thermogravimetric analyzer(TG); To analysis the chemical structure and the microstructure of polyimide by infrared spectroscopy(FTIR) and scanning electron microscope(SEM), and explore the mechanism of the improved light transmittance and mechanical properties.The results show that: When the amount of PI powder gradually increases from 0.2% to 0.5%, the transmittance of PI / PAA polyimide shows a trend of gradually increasing. When the amount of PI powder is 0.4%, PI/PAA polyimide has the highest transmittance of 62%. While the amount of PI powder graduallyincreases from 0.2% to 0.5%, its modulus, tensile strength and elongation at break increase significantly. Especially when the added amount of PI powder is0.4% and 0.5%, the tensile strength and modulus of PI/PAA is much bigger than pure PI. Consider a combination of transmittance and mechanical properties, we choose PI/PAA polyimide with PI powder dosage of 0.4% to be modified by TiO2 coated with ZnO particles. After the modification although the mechanical properties decreases, but it is still much higher than pure polyimide. Meanwhile its transmittance can be up to 70%, and the thermal decomposition temperature is still as high as 500 ℃. The addition of PI powder makes a change of morphology regularity in polyimide, which cause the enhancements of the mechanical properties. TiO2 coated with ZnO particles can improve the transmittance by strengthen the refraction effect.