A preliminary study on the flow-induced crystallization phenomenon in 3D printing of polyvinylidene fluoride

Fused Filament Fabrication (FFF) is a widely used additive manufacturing (AM) technique, renowned for its versatility, affordability, and ease of use. It involves the layer-by-layer deposition of extruded thermoplastic filaments to build three-dimensional objects. In the context of high-performance semicrystalline polymers, FFF, which is also known as 3D printing, presents both opportunities and challenges due to the unique properties of these materials. A notable opportunity lies in the ability to orient and align polymer chains. During the 3D printing process, both the shear flow within the nozzle and the velocity gradients induced by deposition can significantly deform the polymer microstructure. This deformation, termed flow-induced crystallization (FIC), mitigates kinetic barriers to crystallization and directs the resultant morphology. This phenomenon of enhanced oriented crystallization could be crucial for the production of piezoelectric devices of Polyvinylidene fluoride (PVDF). PVDF is a thermoplastic semi-crystalline polymer distinguished by its polymorphism, with multiple crystalline phases that significantly impact its properties and applications. The piezoelectric effect of PVDF is closely linked to the β-phase content, morphology, and alignment, all of which are influenced by processing conditions. This study aims to investigate the potential and limitations of flow-induced crystallization for producing PVDF specimens with high β-phase content. A Design of Experiments (DoE) methodology was employed to examine the effects of two factors, printing speed and extrusion temperature, on various response variables. These response variables were identified through comprehensive characterization analyses. Post-printing, measurements such as the total crystallinity and the melting temperature were obtained via differential scanning calorimetry (DSC), while the β-phase percentage was assessed using Fourier Transform Infrared Spectroscopy (FTIR). ANOVA analysis of the DSC results indicated that extrusion temperature is the critical parameter, positively influencing total crystallinity. Conversely, the melting temperature was found to increase as the extrusion temperature decreased. Furthermore, statistical analysis of the FTIR results reinforced the significance of extrusion temperature on the crystallization phenomenon, revealing that the β-phase content increased with decreasing extrusion temperature. From these findings, it can be inferred that low extrusion temperatures could decrease system entropy due to the alignment of polymer chains induced by the material flow. Additionally, a combination of increased extrusion temperature and low printing speed promotes the nucleation and growth of crystals. However, this condition diminishes the likelihood of achieving a microstructure characterized by a high β-phase percentage.