Interplay of liquid-liquid and solid-liquid phase separation mechanisms in porosity and polymorphism evolution within poly(vinylidene fluoride) nanofibers

Porosity and polymorphism evolution within poly(vinylidene fluoride) (PVDF) nanofibers is discussed in relation to interplay of liquid-liquid (L-L) and solid-liquid (S-L) phase separation mechanisms. To this end, poly(vinylidene fluoride) (PVDF) solutions composed of nonvolatile solvents, dimethylformamide (DMF) and N-methylpyrrolidone (NMP), are subjected to electrospinning under environmental conditions of constant temperature (T=20 °C) and different levels of relative humidity (RH) ranging from 20 to 80 %. It is demonstrated that bead appearance, fiber diameter, porosity formation and polymorphism evolution is strongly affected by L-L phase inversion. Increasing RH as well as size of L-L miscibility gap in the ternary phase diagram of nonsolvent (water)/solvent (DMF or NMP)/polymer (PVDF) reduces time required to induce L-L demixing as verified by calculated mass transfer pathways. Therefore, bead-free fibers of larger diameters are expected, meanwhile, growth of β-phase crystals is suppressed. This is why fibers electrospun from DMF-based solution contain less β-phase crystals at high values of RH. In contrast, for solutions composed of NMP, jet stretching due to whipping instability plays pivotal role to form fiber structure as a result of delayed L-L demixing with respect to S-L demixing at high RH, 80 %. Furthermore, retarded L-L demixing in NMP-based systems destabilizes fiber formation at low humid environment which can be enhanced by addition a volatile solvent such as acetone. Additionally, more evidence for increment of β-phase formation with increasing working distance (w.d.) at constant RH is provided.