Preparation And Electromagnetic Shielding Effectiveness Of Shape Memory EVA Fiber Film Composite

The rapid development of the Internet of Things has promoted the upgrading of electronic devices.More miniaturized and integrated electronic components on the flexible printed circuit（FPC）of advanced electronic device will inevitably generate more electromagnetic radiation.Therefore,the investigation of electromagnetic interference（EMI）shielding materials is of great and profound significance.Conductive polymer fiber composites（CPFCs）can make up for shortcomings of metal materials,such as high density,easy corrosion,difficult preparation and high cost,and also could overcome disadvantages of intrinsic conductive polymer materials,for example,poor designability of structure and performance.More importantly,CPFCs possess light weight,ultrathin,strong shielding effectiveness and wide shielding band,which are expected to be a new generation of superior EMI shielding materials.Shape memory polymers are intelligent materials in which the initial shape and temporary shape can be mutually transformed in response to external stimuli.If shape memory polymer is chosen as the substrate,that is,EMI shielding material is given programmability and recyclability.EMI shielding material can be adhered to irregular or bumpy shielding surfaces by external stimuli.Moreover,EMI shielding material can be recovered to its initial shape by external stimuli,saving cost and reusable.Together with the shape memory effect（SME）,EMI shielding materials will develop towards the directions of intelligence and multi-functions.In this study,ethylene-vinyl acetate copolymer（EVA）fiber with favorable microscopic morphology is selected as the core layer,polydopamine（PDA）as the middle layer,and powerful conductive Ag layer as the shell layer.The composite takes advantage of core-shell structure and porous structure,which improves the overall level of reflection,absorption and multiple reflection of electromagnetic waves.What’s more,the composite has excellent electrical/thermal-driven SME,which solves the existing problems of FPC electromagnetic shielding film such as higher limitation,greater disposal rate,and larger space occupancy.The main work and findings are as follows:（1）Highly cross-linked EVA fiber membrane is prepared by electrospinning and UV cross-linking.The influences of solvent ratio,feed rate,spinning voltage and solution concentration on the microscopic morphology of fiber are studied,and the impact of solute ratio on the mechanical properties of fiber is investigated.The optimal experimental conditions for the preparation of EVA fiber membrane are finally determined as follows:solute ratio of 100:3:1,solvent ratio of 7:3,feed rate of 2.0 ml/h,spinning voltage of 16 k V,and solution concentration of 10%.In addition,EVA fiber membrane exhibits excellent thermal-driven SME with shape fixity and shape recovery ratios of 97%and 98%,respectively.（2）EVA@PDA@Ag fiber membrane with superior electrical conductivity and excellent electromagnetic interference shielding effectiveness（EMI SE）is prepared by dopamine self-polymerization and chemical Ag plating successively.The adjustability of Ag NO₃concentration,glucose concentration and reaction temperature on the conductivity of EVA@PDA@Ag fiber membrane are investigated.When the Ag NO₃ concentration is 10 g/L,the glucose concentration is 20 g/L,and the reaction temperature is 40 ^℃,the conductivity of EVA@PDA@Ag fiber membrane is as high as 2.5×10⁵ S/m.Since conductivity is closely related to EMI SE,its EMI SE is as high as 86.75 d B.The conductivity and EMI SE of EVA@PDA@Ag-10-40 ^℃ fiber membrane after ultrasonication,folding,and stretching are kept at high levels,with retention rates of 98%and 99.5%,respectively,which indicates that the fiber membrane is a favorable EMI shielding material with excellent conductivity stability and superior EMI SE stability.（3）The unique segment structure of EVA,the outstanding electrical conductivity and Joule heating property of Ag layer confer excellent electrical/thermal-driven SME to EVA@PDA@Ag-10-40 ^℃ fiber membrane.The initial shape can be programmed at 0.55 V,and 50%strain sample with a 97%shape fixity ratio exhibits EMI SE of 50.91 d B,which blocks99.9998%electromagnetic waves.The deformed shape can be recovered at 80 ^℃,and electrical conductivity as well as EMI SE of recovered sample with a 98%shape recovery ratio are almost the same as the initial value,2.0×10⁵ S/m and 87.06 d B,respectively,which can consume99.9999997%electromagnetic waves.An applied idea is proposed based on the excellent electrical/thermal-driven SME and EMI SE of EVA@PDA@Ag-10-40 ^℃ fiber membrane.When the electronic device is assembled,the fiber membrane subjected to external stimuli is not only freely deformed according to the size of FPC,but also fully attached to the dense electronic components on the FPC,meanwhile,its EMI SE remains at a relatively high level.Once electronic device is abandoned,the shape and EMI SE of fiber membrane can be recovered by applying external stimuli again,which has strong recyclability.In summary,EVA@PDA@Ag-10-40 ^℃ fiber membrane has the characteristics of thin and light,high flexibility,high conductivity,strong EMI SE,wide shielding band,programmable,and recyclable,which provides a new direction for the optimization of current FPC electromagnetic shielding film.