Parameters governing the piezoelectric properties of PVDF

This thesis investigates the effects of thermal annealing, PLA incorporation, and electrospinning on the crystalline structure, β-phase content, and piezoelectric properties of PVDF-based materials. Thermal annealing treatments were performed at temperatures close to and further from the crystallization peak of PVDF prior to stretching and poling. The crystal size, degree of crystallinity, and β-phase content were characterized before and after stretching, while piezoelectric properties (d33) were measured after poling. Annealing increased both crystal size and degree of crystallinity, with the strongest effect observed for samples melt-annealed near the crystallization temperature. After stretching, both annealed and non-annealed samples exhibited similar β-phase fractions, but the annealed samples retained thicker lamellae. All samples showed comparable d33 values except the one melt-annealed near the crystallization peak, whose d33 decreased by 23% to 30% depending on stretching conditions. This reduction was attributed to larger crystal size and lower chain alignment, which decreased poling efficiency and consequently the piezoelectric response. PVDF was melt-blended with 5% and 40% PLA to obtain beaded and co-continuous morphologies, respectively. The degree of crystallinity remained unchanged upon PLA addition, while the β-phase fraction decreased significantly at 40% PLA content. After poling, the beaded PVDF/PLA sample exhibited a d33 value 34% to 45% lower than pure PVDF depending on the stretching conditions. This reduction was attributed to differences in permittivity between PVDF and PLA, which reduced the effective electric field in the PVDF phase during poling. The co-continuous blend exhibited no measurable piezoelectric response due to predominant α-phase formation. Electrospinning of PVDF promoted a high β-phase content and enhanced chain orientation due to the large stretching ratio inherent to the process, leading to a d33 value 30% higher than that of the stretched film. However, an additional poling step was still required to activate piezoelectricity. Incorporation of 5% PLA into electrospun PVDF completely suppressed piezoelectricity, likely due to small PLA domains that reduced the local electric field in PVDF, prevented proper orientation of PVDF chains and lamellae during stretching, and dramatically increased PVDF/PLA interfacial area, promoting strong interactions and stiffening of the material that hindered chain rotation during poling. Overall, this work demonstrates that achieving high piezoelectric performance in PVDF-based systems requires not only a high β-phase fraction, but also fine control of crystal size, chain orientation, and electric field distribution during poling.