Stretchable and Transparent High-k Polymers for Wearable Acoustic Devices

Abstract

As wearable electronics and soft robotics advance toward invisible and skin-integrated interfaces, there is a growing demand for functional materials that simultaneously possess high dielectric constant (high-k), mechanical stretchability, and optical transparency. However, conventional ferroelectric polymers such as poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) are inherently stiff and brittle. While recent slight crosslinking strategies have successfully imparted stretchability, they often suffer from a critical optical trade-off due to the formation of conjugated bonds during the reaction that leads to severe discoloration, and the scattering from crystalline domains causes haze. Herein, we present a materials design strategy to develop a transparent and stretchable high-k material by combining non-chromogenic crosslinking chemistry with kinetic control of crystallization. We synthesized a hydroxylated PVDF-TrFE and crosslinked it with polyethylene glycol diglycidyl ether (PEGDGE). Unlike conventional diamine-based routes that induce dehydrofluorination and coloration, our approach utilizes epoxide ring-opening reactions to form saturated ether linkages, effectively preventing the formation of light-absorbing conjugated double bonds. Furthermore, a melt- quench process was employed to suppress the growth of large spherulites, thereby minimizing light scattering and securing optical clarity. The resulting composite film exhibited superior electromechanical properties compared to the pristine polymer. In mechanical tests, the crosslinked film demonstrated robust elasticity, maintaining an elastic recovery of over 60 % even under 50 % strain, whereas the non-crosslinked counterparts showed plastic failure. Furthermore, the crosslinked film exhibited enhanced relative permittivity approximately 30 % improvement while retaining sufficient dielectric performance for practical actuator applications. Finally, we fabricated a stretchable and transparent loudspeaker to demonstrate practical utility. The device exhibited stable sound pressure level (SPL) performance across the audible range, with a maximum SPL deviation of less than 10 %. This work provides a versatile solution for resolving the conflict between mechanics and optics in high-k polymers, paving the way for next-generation invisible soft electronics.