Dynamics and morphology development in electrospinning of polymer solutions

In the process of solution (dry, wet or electro) spinning of polymer fibers removal of the solvent is one of the most critical steps to produce dry fibers. In dry spinning as well as electrospinning the solvent is removed through evaporation from the surface of the fiber, whereas in wet spinning it is removed by exchange with a non-solvent. Therefore, morphology of the fiber is directly influenced by the interactions between the polymer and solvent or non-solvent. In other words, the structure development in polymer fibers is a function of spinning conditions and phase equilibria of the system.; The primary objective of the present work is directed to the elucidation of solvent evaporation process and its effect on the structure evolution in fibers spun from polymer solutions. As the solvent concentration decreases, due to evaporation from the surface of the fiber, the system moves from low concentration to high concentration region in the phase diagram. In doing so, it traverses through different coexistence regions of the phase diagram in a matter dependent on the rate of solvent loss. To mimic the morphology development in binary systems comprised of polymer and solvent and ternary systems containing polymer, solvent and nonsolvent, temporal evolution of the concentration order parameter was modeled in the framework of Cahn-Hilliard equation in conjunction with Flory-Huggins (FH) free energy of mixing. It turns out that, the competition between the phase separation dynamics and solvent evaporation rate determines the structure of the spun fiber.; The aforementioned model was subsequently extended to main chain liquid crystalline polymer (MCLCP) solutions. The structure evolution in these systems is influenced by the mesophase transitions of MCLCP as well as the trajectory of the concentration sweeps across the phase diagrams of these systems. A generalized theoretical scheme was developed for the binary phase diagrams of crystal-liquid crystal mixtures by a combination of a phase field model of solidification, FH theory for liquid-liquid mixing and Maier-Saupe theory for nematic and Maier-Saupe-McMillan theory smectic ordering in liquid crystals. Subsequently, a non-equilibrium thermodynamic approach was developed to describe the emergence of fiber morphologies from MCLCP solution undergoing solvent evaporation. Matsuyama-Kato free energy was utilized which incorporated chain-stiffening, combined with Flory-Huggins free energy of mixing. The temporal evolution of concentration and nematic order parameters pertaining to the above free energy density of liquid crystalline polymer solution was simulated in the context of time dependent Ginzburg Landau (TDGL-Model C) theory coupled with the solvent evaporation rate equation under the quasi-steady state assumption.; The morphology development of polymer systems investigated thus far were driven by concentration sweeps under quiescent conditions. In the actual electro-spinning process the fiber morphology is immensely influenced by the fiber spinning conditions such as applied voltage, flow rate, viscosity of the solutions, solubility of polymer solutions among others. The dynamics of electrospinning process was modeled in terms of an array of beads connected by Maxwell's elements in a cylindrical coordinate system through the force balance between Coulombic and viscoelastic forces. The phase separation dynamics was calculated in the framework of Cahn-Hilliard equation in conjunction with solvent evaporation through the fiber surface. The simulations based on the coupling of these two processes revealed in-situ morphology development registering all structural forming process including polymer droplet, interconnected bi-continuous structure, porous structure and solid fiber. The fiber formation dynamics and associated morphology development were demonstrated as a function of applied voltage, electrical charge density and flowrate.