▎ 摘　　要

NOVELTY - Multilayer porous polyimide composite film has porous multilayer structure, and graphene and nano-silver are uniformly distributed in polyimide framework, where the composite film has density of less than 0.32 g/cm3, porosity of more than 56%, electrical conductivity of up to 3.5 S/cm, the electromagnetic shielding performance is up to 37 dmm, the proportion of electromagnetic absorption performance to total shielding performance is higher than 85%, and the elongation at break is up to 125%. USE - The composite film is used as electromagnetic shielding material in telecommunication application. ADVANTAGE - The composite film ensures density of less than 0.32 g/cm3, porosity of more than 56% and electrical conductivity as high as 3.5 S/cm3, electromagnetic shielding performance as high as 37 dB, electromagnetic absorption performance of more than 85% of total shielding performance and elongation at break as high as 125%. DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method for preparing the multilayer porous polyimide composite film, involving (a) uniformly mixing graphene with nano-silver, adding polar solvent, performing ultrasonic treatment for 20-90 minutes to obtain graphene/nano-silver mixed suspension with concentration of 0.1-1 wt.%, adding polyimide and pore-forming agent to suspension, and mechanically stirring at 100-300 rpm for 1-3 hours to obtain polyimide copolymer, pore-forming agent, graphene and nano-silver mixed solution, where the mass ratio of graphene and nano-silver is 1:(0.3-3), the pore-forming agent comprises polyethylene glycol, polyvinylpyrrolidone, hydroxymethyl cellulose, carboxymethyl cellulose, polyacrylamide and/or polyacrylic acid, the polar solvent comprises N-methyl pyrrolidone, N, N-dimethylformamide and/or N,N-dimethylacetamide, the mass ratio of graphene, pore-forming agent and polyimide block copolymer is 1:(10-50):(80-200), (b) pouring polyimide copolymer, pore-forming agent, graphene and nano-silver mixed solution to glass plate, scraping liquid film with 100-1000 mum using wet film preparation device, leaving still for 5-30 minutes, soaking glass plate and liquid film on surface of glass plate in mixed solution of ethanol and water for 0.5-3 hours, peeling obtained film from glass plate, and continuously soaking in deionized water for 1-3 days to obtain polyimide composite wet film, where the volume ratio of ethanol and water is 1:(4-9), and (c) placing obtained polyimide composite wet film on ultra-clean workbench for 1-4 hours, draining surface water and placing in vacuum oven at 80-150degrees Celsius for 5-24 hours, and naturally cooling to room temperature to obtain multilayer porous polyimide composite film.