Twist-based cooling of polyvinylidene difluoride for mechanothermochromic fibers

Abstract

• High-efficiency cooling was realized by changing twist in a PVDF fiber. • Reversible crystal structure transformation was found during cooling. • An inverse twistocaloric cooling was realized for a heterochiral PVDF coil. • A strain sensitive mechanothermochromic fiber was demonstrated. Developing new cooling strategies of solid-state cooling materials is highly desired in order to meet the high energy demand and new application scenarios for multicaloric cooling. Polyvinylidene difluoride (PVDF) is a typical ferroelectric polymer that shows electrocaloric effect, and general strategies for improving the cooling performance of PVDF-based polymers include material composition design, device optimization, etc . In this work, cooling was realized by changing twist of PVDF fibers. Cooling of −1.1 K was achieved by twist removal from a PVDF fiber, and −3.0 K cooling was realized by stretch release of a self-coiled PVDF fiber. During twist insertion and removal of the PVDF fiber, reversible transformation of crystal structure from orthorhombic to monoclinic phase was observed, which was ascribed as the origin of such twist-based cooling effect. Both coil stretching and releasing can realize cooling by using opposite and the same chirality of fiber twisting and coiling, respectively. A strain sensitive mechanothermochromic fiber was demonstrated to show different colors during stretching and releasing of a PVDF coil. The current research provides new era for improving the performance and finding new applications for PVDF-based cooling.