Effect Of Silane Coupling Agent Functionalized KTN On Dielectric Properties Of PI-based Composite Films

Polymer-based nanocomposites have been widely used in modern electronic equipment such as capacitors,sensors and electromagnetic interference shielding,due to their good machinability,mechanical flexibility and high breakdown strength.Usually,in order to obtain polymer matrix composites with high dielectric constant,nanofillers are added into the polymer matrix.However,with the content of filler increasing,nanoparticles are tend to agglomerate,which will lead to the increase of dielectric loss and the decrease of breakdown strength of the composite film.In order to solve those problems,the surface of KTa_0.5Nb_0.5O₃（KTN）was modified by silane coupling agent containing amino,phenyl,isocyanate and epoxy cyclohexyl functional groups.The modified KTN was used as nanofiller and polyimide（PI）was used as matrix to prepare KTN/PI nanocomposite films.The effects of different silane coupling agents on the dielectric properties of KTN/PI composite films were studied,and the regulation law of different functional groups on the dielectric properties of composite films were discussed.The multilayer composite film was designed and prepared,the influence of the composite films’structure on the dielectric properties of the overall composite material was studied.Firstly,the KTN/PI composite model was established,and the interaction energies between hydroxyl,amino,phenyl,isocyanate,3,4-epoxycyclohexyl functionalized KTN nanofillers and PI were calculated by molecular dynamics simulation.The effects of different functional groups of KTN on hydrogen bond density and interface parameters of PI matrix composites were analyzed.The results showed that the order of interaction energy between KTN and PI with different functional groups on the surface was isocyanate group,amino group,benzene ring,3,4-epoxycyclohexyl group and hydroxyl group.Among them,the interaction energy between PI and KTN with isocyanate group on the surface was 53.70×10^-4kcal·mol^-1·（?）^-3,which was 1.43 times as much as that between unmodified one.Compared with other functional groups,the highest hydrogen bond density between KTN and PI containing isocyanate group is 2.516×10^-4（?）^-3,and the interface overlap parameter between KTN and PI is 2.936×10^-2.On account the hydrolysis of isocyanate group,and the formation of amino and carboxyl groups,the stronger hydrogen bond with PI matrix will generate,as well as the isocyanate group will get higher interaction energy.Secondly,KTN nanoparticles were modified by four silane coupling agents containing amino group,benzene ring,isocyanate group and 3,4-epoxycyclohexyl group,respectively.The corresponding KTN-NH₂/PI,KTN-C₆H₅/PI,KTN-NCO/PI and KTN-C₆H₉O/PI composite films were prepared,and the dielectric properties of the composite films were studied.The results showed that the loss tangent of KTN-NH₂/PI,KTN-C₆H₅/PI,KTN-NCO/PI and KTN-C₆H₉O/PI composite films were lower than it of KTN/PI composite films.The dielectric loss was reduced.At10~6Hz,the loss tangent of KTN/PI composite film with doping amount of 3-11 wt%was lower than 0.017.Among them,the minimum loss tangent of KTN-NH₂/PI with11 wt%doping content was 0.0061（at 10~4Hz）.The breakdown strength（350.882k V/mm）of KTN-NCO/PI with 3 wt%doping content was the highest,which was2.16 times as much as KTN/PI.This is due to the high interaction between the functional groups on the modified filler surface and PI,which improves the compatibility,reduces the electric field distortion of the composite membrane and improves the breakdown strength of the composite membrane.Finally,the layer structure of the composite film was designed.The double-layer and three-layer polymer-based nanocomposite films were prepared by layer-by-layer casting method,which improved the breakdown strength of the whole film material.Compared with the single-layer composite films,due to the introduction of pure PI insulating layer,the carrier transport was inhibited by the barrier between pure PI and KTN/PI layers in the multilayer system.The breakdown strength of the bilayer,which is composed of pure PI and KTN-NCO/PI with 11 wt%doping content,can reach 297.786 k V/mm.The breakdown strength of the three-layer film,whose 11 wt%KTN-NCO/PI is the intermediate layer and pure PI is the outer layer can reach 372.709 k V/mm.It is 2.80 times as much as the KTN-NCO/PI monolayer film under the same doping amount.