Particle-Size-Dependent Nanomechanical Behavior of Fe₃O₄-Reinforced Electrospun PVDF Nanofibers

Poly(vinylidene fluoride) (PVDF) is a semi-crystalline polymer with tunable mechanical, piezoelectric, and chemical resistance properties. PVDF nanofibers have found tremendous applications in sensing, energy storage, and bio-scaffold applications. Yet, the nanoparticle-induced nanomechanical attributes of such nanofibers remain underexplored. Here, we fabricate PVDF and PVDF-Fe₃O₄ nanofibers leveraging electrospinning and investigate the nanoscale mechanics using atomic force microscopy (AFM). To elucidate the mechanical response, we reinforce PVDF with 20 and 50 nm sizes of Fe₃O₄ nanoparticles. Our results indicate that reducing particle size alters the fiber morphology, surface roughness, distribution of crystalline-amorphous regions, and local modulus of the nanofiber composites. Additionally, we observe load-dependent modulus behavior, in which increasing the load reduces the local modulus across all samples. Among the nanofibers, the 20 nm system showed the highest modulus (2.78 MPa) at 500 nN compared to the neat PVDF or in 50 nm systems. To explain such behavior, we leverage some micromechanical models (Voigt, Reuss, Halpin-Tsai, Mori-Tanaka, Guth-Gold) to theoretically calculate the composite modulus, but they fail to replicate the experimental results. To explain the particle-size-induced mechanics, we develop a theoretical model that accounts for AFM contact mechanics and nanoparticle size inclusions. Our proposed model produced mean absolute percentage errors of 9.29% and 15.26% for the 20 nm and 50 nm systems, respectively. While other micromechanical models fail, our model provides a logical solution to the nanoparticle-induced modulus response in PVDF nanofiber systems.