|Ph.D Student||Greenfeld Israel|
|Subject||polymer Network Dynamics during Electrospinning and its|
Effect on the Fibers Nanostructure:
Modeling, Simulation and Experiments
|Department||Department of Mechanical Engineering||Supervisor||Professor Zussman Eyal|
|Full Thesis text|
The high strain rate extensional flow of a semi-dilute polymer solution can cause substantial stretching and disentanglement of the polymer network. The research presented in this thesis encompasses a theoretical and experimental investigation of the effects of electrospinning, a flow governed by high strain rate and rapid evaporation, on the dissolved polymer network, and on the nanostructure and mechanical properties of the resulting nanofibers.
Electrospun polymeric nanofibers, typically 50 to 1000 nanometers in diameter, exhibit unique mechanical properties. Specifically, below a certain crossover diameter, dependent on the polymer molar mass, the nanofiber elastic moduli begin to rise sharply. Understanding the mechanisms behind these phenomena is of interest for improving the mechanical, electrical and optical properties of nanofibers, and can lead to novel applications in engineering and life sciences.
In this study, modeling of the dynamic evolution of the entangled polymer network in an electrospinning jet predicted substantial longitudinal stretching and radial contraction of the network, a transformation from an equilibrium state to an almost fully-stretched state. This prediction was verified by fast X-ray phase-contrast imaging of electrospinning jets, which revealed a noticeable increase in polymer concentration at the jet center, within a short distance from the jet start. The model was expanded to semi-flexible conjugated polymer chains, and scanning near field optical microscopy of electrospun nanofibers of such electrically and optically active polymers revealed that the network conformation effectively remains after jet solidification.
Hence, at high flow strain rates, the resulting fiber nanostructure is that of a dense core with axially aligned macromolecules, surrounded by an amorphous boundary. Such molecular and supramolecular structures can account for the increase of the elastic moduli at small fiber diameters. Furthermore, polymer entanglement loss in consequence of network stretching under very high strain rates, evidenced in jet fragmentation and appearance of short nanofibers, reduces the fiber diameter and enhances the homogeneity and alignment of the nanostructure, potentially improving the elastic properties even more.
The thesis reviews and discusses the relevant literature in the fields of polymer physics, polymer mechanical properties, electrospinning, and nanofibers. A description of the methods used in the research, combining theoretical, simulation, and experimental work, is provided. The results are presented in five publications, which summarize the research in a streamlined fashion, and then discussed.