The Preparation And Properties Of PDMS/P(VDF-TrFE) Based Electrowetting Dielectric Materials

Electrowetting on dielectric(EWOD)is one of the most effective methods to control micro droplets by changing the interfacial tension.It has been widely used in biochip,optical device,display,on-chip laboratory and other fields.The dielectric layer between electrode and droplet plays an important role in EWOD devices.The dielectric film with high dielectric constant can effectively inhibit the electrolysis of the conductive droplets on its surface,and increase the range of the contact angle of the dielectric layer when the external electric field is applied.Moreover,the high dielectric constant is very important to reduce the driving threshold voltage and improve the reliability of the device.The low surface energy and low wettability of dielectric layer is another key factor to ensure the convenient flow of microfluidics on the film surface.Thus,dielectric layer with high permittivity and hydrophobic surface is very critical for the low voltage electrowetting devices.In this paper,PDMS and P(VDF-TrFE)were blended into a dielectric wetting film in the previous work.PS microspheres with nano particle size were added into the film to form micro bulges to improve the hydrophobic properties and the dielectric properties of the composite film to a certain extent.Furthermore,in order to optimize the contact angle and dielectric properties and reduce the roughness,experiments are carried out in two directions.One is to replace the preparation method of PS microspheres,reduce the particle size and doping amount of PS microspheres,and make a dielectric wetting film of three organic compounds.The other is to load Au nanoparticles on the PS microspheres,and successfully prepare a perfectly coated PS@Au filler,and then control the amount of filler added to make a composite film with high dielectric properties.(1)From the perspective of constructing the micro convex structure on the surface of the composite organic substrate and improving the hydrophobicity of the composite film,the PS microspheres with a diameter of about 250nm were synthesized by emulsion polymerization and combined with the PDMS/P(VDF-TrFE)substrate(mass ratio 2:3).The dielectric constant of the composite film containing 23wt%PS reaches21 at 100Hz,which is 3 times that of PDMS/P(VDF-TrFE)(mass ratio:2/3).The significant increase of dielectric constant is attributed to the ferroelectric polarization of P(VDF-TrFE)and the enhancement of interface polarization caused by PS addition.Meanwhile,the composite film with 23wt%PS has an initial contact angle of 125°due to the existence of rough surface.(2)In order to further improve the dielectric properties and reduce the surface roughness of the composite film to better realize its application value,PS microspheres with small particle size were prepared by hydrothermal method,and then compounded with PDMS/P(VDF-TrFE)matrix to explore the change of dielectric properties and surface roughness.PS microspheres with 40nm,80nm and 120nm were prepared by controlling the hydrothermal temperature.It is found that the dielectric constant of 5wt%40nmPS composite film is 29 at 100Hz,which is 4 times of PDMS/P(VDF-TrFE)(mass ratio:2/3),and its dielectric loss is kept at a lower value(0.11).(3)In order to further improve the dielectric properties,PS@Au filler was prepared by electroless plating,and the mass ratio of PS:Au was 10:1.The composite with PDMS/P(VDF-TrFE)matrix was used to analyse the influence of different doping amount on the hydrophobic and dielectric properties.The results show that the permittivity of the 5wt%PS@Au doped composite film is equal to that of the 23wt%PS composite film(~22),and the high permittivity is achieved at the low doping level.At the same time,due to the satellite structure design of PS@Au,the Au nanoparticles are more evenly dispersed,and the incorporation of PS@Au has also successfully hindered the movement of polymer molecular chain,making the dielectric loss much lower than that of pure PS doped composite films(<0.2@10~6Hz),while maintaining better insulation performance.