Cooling Rate Effects on Morphological, Optical, and Piezoelectric Properties of Melt-Recrystallized PVDF-TrFE Thin...

Polyvinylidene fluoride (PVDF) and its copolymers have recently garnered significant attention for their applications in sensors, actuators, and energy-harvesting devices due to their strong piezoelectric properties, flexibility, chemical resistance, ease of processing, and optical transparency. Consequently, PVDF and its copolymers are increasingly utilized as optical piezoelectric elements in consumer electronics, human-machine interfaces (HMI), and wearable sensors and actuators. Among these materials, the copolymer PVDF-TrFE (polyvinylidene fluoride-trifluoroethylene) is particularly notable for its superior piezoelectric properties, ferroelectric stability, and thermal stability. PVDF-TrFE thin films are predominantly produced through solution casting or doctor-blading methods, followed by annealing to enhance their piezoelectric properties. However, the impact of annealing that involves melt-recrystallization and cooling rates on the piezoelectric, optical, and mechanical properties remains inadequately understood. This study explores the doctor-blading process of PVDF-TrFE using a PVDF-TrFE, Dimethylformamide (DMF), and acetone-based solution, followed by annealing above its melting temperature and cooling at varying rates. PVDF-TrFE films annealed and cooled at slower rates (1-2 °C/min) demonstrated a higher β-phase content but exhibited haziness and poor optical transparency. In contrast, films rapidly cooled by immersion in water displayed a lower β-phase content but maintained high transparency. A comprehensive analysis of the surface properties of these films, coupled with a correlation of surface roughness with a light scattering model, indicated that the optical transparency of both films was predominantly influenced by large micrometer-scale surface roughness. These findings could provide valuable insights into developing surface treatment processes to optimize both optical and piezoelectric properties in PVDF-TrFE thin films.