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Document Type:Latin Dissertation
Language of Document:English
Record Number:53296
Doc. No:TL23250
Call number:‭3392235‬
Main Entry:Mohammad Naraghi
Title & Author:Processing dependent mechanical behavior and molecular structure of electrospun polymeric nanofibersMohammad Naraghi
College:University of Illinois at Urbana-Champaign
Date:2009
Degree:Ph.D.
student score:2009
Page No:176
Abstract:Polymeric nanofibers are versatile building blocks for a range of hierarchical structures, such as filtration systems, tissue scaffolds and nanocomposites. Among the methods that are available for the fabrication of polymeric nanofibers, electrospinning stands out because of the control that it provides in terms of nanofiber structure, length and alignment. In this dissertation research, new experimental methods were developed to investigate the mechanics of polymeric nanofibers as a function of strain rate and electrospinning conditions. The goal was to generate relations between the nanofiber molecular structure, the fabrication parameters and the resulting mechanical behavior. The novel experimental methods of this research employed surface micromachined tools and Microelectromechanical Systems (MEMS) to quantify the mechanical behavior of ductile polyacrylonitrile (PAN) nanofibers (MW = 150k) at a wide range of strain rates and at small and large deformations that were not possible with existing tools for nanoscale experimentation. The experimental method of this thesis takes advantage of the tenfold increase in strain resolution measurements from optical images compared to conventional in situ measurements, which is provided by Digital Image Correlation (DIC) that enabled the measurement of nanofiber extensions considerably smaller than 50 nm, simply by using optical microscopy imaging. In terms of the effect of electrospinning parameters on the nanofiber structure and its properties, it was found that, for a given average electric field intensity, long electrospinning distances improved the molecular alignment in the nanofibers and hence their strength and elastic modulus. This improvement was found to take place in the last segments of the polymer jet flow near the nanofiber collector. On the other hand, higher electric field intensity favored the formation of a graded nanofiber cross-section with a sharp discontinuity in local stiffness. A direct consequence of this nanofiber structure is the formation of periodic surface ripples during axial stretching at room temperature. Strain rate experiments at 2.5·10-4 -2.5·10-2 s-1 showed that this process of nanofiber rippling is owed to the fragmentation of the hard nanofiber skin, which provides periodic sites for localization of strain in the ductile nanofiber core. Loading at high strain rates (>100 s-1) further supported the argument of a clearly defined core-skin structure in the particular nanofibers as complete debonding between the nanofiber core and the skin was observed. On the other hand, proper selection of the electric field intensity allowed for nanofibers that deformed at large strains and in a homogenous manner. The thinnest nanofibers (200 nm diameter) had 6-7 times higher elastic stiffiress and strength compared to the thickest nanofibers (800 nm diameter). Reasons for this size effect were sought in the increased surface energy in thinner nanofibers and the molecular alignment in thinner specimens. It was established that the contribution of surface energy was too small to explain the experimental data. Fourier Transform Infrared (FTIR) spectroscopy data pointed to a connection between molecular alignment with the nanofiber axis and the elastic modulus and strength of the thin nanofibers. The strain rate sensitivity of PAN nanofibers was investigated at rates between 2.5·10-4 s-1 to 200 s-l. The elastic stiffness and the mechanical strength of the nanofibers increased monotonically with the strain rate but only at strain rates between 2.5·10-2 s-1 to 200 s-1. The tensile strength at the slowest strain rate 2.5·10-4 s-1 was the same as that at 2.5·10-2 s-1 potentially because of the competition between plasticity induced orientation hardening and the material creep/stress relaxation. The nanofiber ductility virtually did not change between 2.5·10-2 s-1 and 200 s-1, contrary to nanofibers loaded at the slowest rate (2.5·10-4 s-1), which demonstrated significantly higher ductility accompanied by material dilatation and void formation. The variation of the mechanical properties with the applied strain rate was also present in nanofibers prone to the formation of periodic surface ripples, with the only difference that in those nanofibers, a strain rate of 2.5·10-4 s-1 resulted in rather uniform nanofiber deformation and unexpectedly high strengths compared to the same nanofibers loaded at faster strain rates. The formation of voids in the nanofiber core and the increase in the nanofiber volume at strains higher than 200% showed that, regardless of the fabrication conditions, the structure of PAN nanofibers is graded with a molecularly oriented outer layer and a more ductile core where PAN has increased free volume. Thus, small polymer jet diameters and high jet velocities during electrospinning result in rapid solvent desorption and the formation of a distinct nanofiber skin, whereas at lower jet velocities a graded molecular distribution develops from the nanofiber surface to its core. The high strain rate sensitivity of the PAN nanofibers implies enhanced creep behavior. Therefore, creep experiments were conducted with individual nanofibers. In agreement with the strain rate experiments and the aforementioned nanofiber diameter size effects, the creep compliance of thinner nanofibers (200-300 nm) was more than two times smaller than that of the thicker nanofibers (600-800 nm), which was attributed to the higher molecular alignment in the thinner nanofibers. Finally, a semi-empirical model was proposed to capture the viscous response of PAN nanofibers. At small viscoelastic deformations, the model is composed of a set of linear springs and dashpots, appropriately assembled to capture the strain rate sensitive elastic modulus and creep behavior of the nanofibers. At large viscoplastic deformations, the model is composed of a Langevin spring and an Eyring's dashpot that capture the strain rate sensitive yield stress and the orientation hardening observed in the experimental stress-strain curves.
Subject:Social sciences; Applied sciences; Nanomechanics; Viscoelasticity; Electrospinning; Nanofilms; Viscoplasticity; Polymeric nanofibers; Aerospace engineering; Mechanical engineering; Nanotechnology; 0548:Mechanical engineering; 0538:Aerospace engineering; 0652:Nanotechnology
Added Entry:I. Chasiotis
Added Entry:University of Illinois at Urbana-Champaign