Design And Preparation Of Thermoplastic Polyimide Resins With Low Cte And High Transmittance

As a special engineering plastic,aromatic polyimide（PI）shows excellent mechanical properties,high and low temperature resistance and irradiation resistance,making it one of the ideal materials in the field of high-temperature optical materials.However,the imide and aromatic rings in the molecule chain make the chain segments less mobile and prone to charge transfer between the chains,resulting in poor thermoplastic processability and optical transmittance of PI resin,and dark colour of the products.The molecular structure design approach to improve the thermoplastic processability of PI often leads to a reduction in thermal dimensional stability,making it difficult to maintain the stability of device performance during changes in service temperature.To achieve a balance between thermoplastic processability,optical property and thermal dimensional stability of PI resins,in this project,a series of thermoplastic polyimide resins have been prepared by adjusting the chemical composition and sequence structure of molecular chains,and their properties have been systematically characterised and analysed.Firstly,in order to prepare thermoplastic PI resins with high optical transmittance,flexible monomers 1,3-bis（4-aminophenoxy）benzene（TPER）and 4,4’-（4,4’-isopropyldiphenoxy）bis（phthalic anhydride）（BPADA）were used to ensure low melt viscosity,2,2’-bis（trifluoromethyl）diaminobiphenyl（TFMB）and cyclobutene tetracarboxylic dianhydride（CBDA）were used to improve optical properties.Adjusting the molar ratio of dianhydride and the type of diamine to synthesis PI resins.The properties of PI resins have been systematically investigated.The PI resins based on TFMB show excellent properties:the T_g is 224-233℃,the T_5%is 522-533℃.The TFMB-based polyimide resins have good melt flowability and the lowest melt viscosity can reach 664 Pa·s.The PI sheets were prepared using a hot pressing process and the optical properties,thermal dimensional stability and mechanical properties of PI sheets were tested:the sheet is nearly colourless and transparent,the transmittance of polyimide sheets can reach 77%at 450 nm and 87%at 650 nm.The CTE is 51.45-60.86 ppm/K.To reduce the coefficient of thermal expansion of thermoplastic PI resins while maintaining their good processing properties,a series of random copolyimide resins were prepared using 4,4’-biphenyl ether dianhydride（ODPA）and 4,4’-diaminodiphenyl sulfone（DDS）as flexible components,3,3’,4,4’-biphenyltetracarboxylicdianhydride（BPDA）and2,2’-bis（trifluoromethyl）diaminobiphenyl（TFMB）as rigid components.Adjusting the molar ratio of rigid and flexible monomers and determining the ratio which achieve a balance between thermoplastic processability and thermal dimensional stability.A series of block copolyimide resins with different rigid and flexible block lengths were then prepared based on the determined ratio.The properties of copolyimide resins have been systematically investigated:the glass transition temperature is above 257℃and the 5%weight loss temperature is above 550℃.The lowest melt viscosity can reach 675 Pa·s and the CTE of PI sheets is 39.87-49.57 ppm/K.With the same ratio of rigid and flexible monomers,copolyimide resin with suitable rigid-flexible block lengths exhibit better melt flow,thermal dimensional stability and mechanical properties.XRD and DMA tests show that the block copolyimide has a more regular molecular chain structure and tight interchain stacking,reducing the effect of secondary relaxation on the free volume in the glassy state.Based on the idea that"a regular,flexible molecular chain structure could reduce the melt viscosity and coefficient of thermal expansion of PI resins",a series of polyimide resins were respectively prepared using 4,4’-diaminodiphenyl ether（4,4’-ODA）,3,4’-diaminodiphenyl ether（3,4’-ODA）and1,3-bis（4-aminophenoxy）benzene（TPER）as diamine monomer,polymerization with 4,4’-biphenyl ether dianhydride（ODPA）to synthesis polyimide resin,The properties of PI resins have been systematically investigated.The glass transition temperature is 207-253℃,the 5%weight loss temperature is above 580℃,the CTE is 45.91-50.57 ppm/K and the lowest melt viscosity can reach 300 Pa·s.The 4,4’-ODA system shows a higher melt viscosity and CTE.The results of XRD and DMA tests show that the TPER resin has a crystalline structure and that the 3,4’-ODA resin has a tighter interchain stacking structure than the 4,4’-ODA system,which effectively inhibits secondary relaxation in the glassy state.