The Preparation And Properties Of P(VDF-TrFE) Based Electro Wetting Dielectric Materials

EWOD(electrowetting-on-dielectric)is a promising technique based on an apparent contact angle modulation controlled by the applied electric field,which has gained a complete attention in micro scale systems,including lab-on-a-chip devices,optical waveguides,electronic displays and digital microfluidics.The insulating dielectric film between electrode and liquid drop plays a critical role in EWOD.It prevents electrolysis of droplets which are often conductive in nature.Apart from the insulating property,the film should possess hydrophobic properties and low actuation voltage.Generally,the initial contact angle of film should be more than 100° for obtaining obvious electrowetting response under electric field.To reduce actuation voltage which is very important for ensuring long-term reliability and stability of devices,a dielectric with a high permittivity is required.Poly(vinylidene fluoride)P(VDF)-based polymer and copolymer,widely known to exhibit high energy storage with high polarization,is showing its promising properties as the most suitable dielectric for electrowetting applications.In this paper,P(VDF-TrFE)was chosen as the polymer matrix,due to its relatively high permittivity(?r?10)and large contact angle(over 100°).TiO2 nanowires and BaTiO3 particles were imposed to increase the dielectric constant of P(VDF-TrFE)matrix.Single-layer composite film was obtained by solution mixture and spin-coating process.In order to optimize the electrowetting behaviors,linear PMMA polymer was introduced into P(VDF-TrFE)matrix to fabricate compact organic blended films.(1)TiO2 nanowires synthesized via hydrothermal method were incorporated into P(VDF-TrFE)and PDMS matrix,respectively.The introduction of inorganic fillers significantly improves the dielectric properties of the composites,but also inevitably increases the dielectric loss caused by defects.P(VDF-TrFE)based composites shows better properties and thus P(VDF-TrFE)was chosen as the polymer matrix for further research.(2)In order to improve the dielectric properties,a-terpineol treated BaTiO3 nanoparticles with less agglomeration were incorporated into P(VDF-TrFE)matrix.The dielectric behavior and hydrophobic property of these composites were studied.Typically,the composite with 25 vol.%surface treated BaTi03 nanoparticles reveals dielectric constant of 46.5,three times of that of pure P(VDF-TrFE),and a lower dielectric loss of 0.091 at 103 Hz compared to the composite filled with commercial BaTiO3 nanoparticles.Further,the composite film exhibits high static contact angle(109.9°)with excellent AC electrowetting response,which demonstrates the great potential of such single-layer composite film for low-voltage electrowetting devices.(3)P(VDF-TrFE)/PMMA(PVT/PMMA)blended films are synthesized through a facile solution-blending method to further optimize the electrowetting properties of the films.Typically,Teflon-covered P(VDF-TrFE)/PMMAl blended film containing 20 wt.%PMMA exhibits a high dielectric constant of 13 with low dielectric loss(-0.05)at 100 Hz,and a large initial contact angle of 122°.The van der Waals forces and hydrogen bonding in neighboring P(VDF-TrFE)and PMMA chains,together with the suppressed free volume(or defects)are all critical to the significantly enhanced electrical properties.Electrowetting responses with an obvious contact angle modulation of 50° in air and low contact angle hysteresis combining theoretical calculation demonstrate that it is promising.Furthermore,with an energy density of 11.8 J/cm3,approximately the double of that of pure P(VDF-TrFE),PVT/PMMA blend films turn out to be superior materials for both practical low-voltage electrowetting devices and high energy storage applications.