|M.Sc Student||Steiner Gelman Noga|
|Subject||The Local and Global Molecular Orientation of Electrospun|
|Department||Department of Polymer Engineering||Supervisor||Professor Eyal Zussman|
Electrospinning is a unique approach using electrostatic forces to produce fine fibers, with diameters ranging from a few nanometers to several micrometers, from polymer solutions or melts. The mechanical and thermodynamic properties of electrospun polymer nanofibers show a remarkably irregular pattern in comparison with their bulk-related properties. These properties vary sharply when the object scale drops below a critical threshold (e.g., fiber diameter). They are commonly attributed to confinement of non-equilibrium supramolecular structures, formed during electrospinning when the polymer solution jet is exposed to rapid and significant elongation (up to 105 accompanied by a strain rate of the order 103 sec−1). Such extreme processing conditions results in a non-equilibrium stretched state of nanofiber polymer matrix, which can be “frozen” due to massive solvent evaporation and polymer solidification. Polymer solution parameters, process parameters and ambient parameters affect the electrospinning process and the fibers morphology and uniformity.
In this study the microstructure of electrospun poly(hexano-6-lactam) nanofibers (also known as nylon 6 or polyamide 6) with diameter ranging between 40-260 nm was examined as a function of the polymer solution viscosity (polymer solution parameter), flow rate, and the strength of the electrostatic field (process parameters). Single fibers were studied using Selected Area Electron Diffraction (SAED) and Small-Angle X-ray Scattering (SAXS), and compared with cast film and non-woven fiber mat that were analyzed using Wide-angle X-ray Scattering (WAXS) and SAXS.
The diffraction patterns of the fibers show existence of the two known phases, α and γ. SAXS measurements imply that the polymer chain does not form a lamellar structure in the fibers. Both diffraction methods indicated that the crystals are aligned with the long molecular direction parallel to the nanofiber axis. Interestingly, when examining a single fiber using SAED, a distribution of orientation with a sub group of crystalline showing a highly preferred orientation was observed. The degree of orientation was studied from the diffraction arcs. Those results can lead to the proposed mechanism describing the development of the crystal formation in the electrospun nanofiber. Solution viscosity and diffusion are the key parameters affecting the crystal alignment in the fiber. The initial solution viscosity effects the diffusion rate and the final fiber diameter. Lower initial solution viscosity results with higher crystal orientation in the core of the electrospun nanofiber. Furthermore, at the solidified nanofiber shell, crystallization occurs as a result of cold drawing. During diffusion the solvent diffuses through those crystals. The viscosity of the diffusive solvent effects its diffusion rate. Thus, higher initial solution viscosity results with higher orientation at the electrospun nanofiber shell. In comparison, WAXS analysis shows the cast film is rich with α-phase while SAED analysis suggests the presence of α-phase solely. SEM images can serve as proof of spherulites existence in the cast films. Moreover, from the comparison between nonwoven mats (bundles of nanofibers) to cast film, it was found that the amorphous phase in the former has a degree of order in comparison to the latter. The crystalline size has barely changed as function of fiber diameter.