Effect of Strain Rate on the Mechanical Behavior of 10-micron Long Polymeric Nanofibers
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0948-B08-03
Effect of Strain Rate on the Mechanical Behavior of 10-Micron Long Polymeric Nanofibers Mohammad Naraghi1, Ioannis Chasiotis1, Yuris Dzenis2, Y. Wen2, and Hal Kahn3 1 Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 61801 2 University of Nebraska-Lincoln, Lincoln, NE, 68588 3 Case Western Reserve University, Cleveland, OH, 44106
ABSTRACT The strain rate mechanical behavior of 12-micron long polymeric nanofibers was investigated. Experiments were carried out by a novel method that employs a MEMS-based leaf spring load cell attached to a polymeric nanofiber that is drawn with an external PZT actuator. The elongation of the fiber and the deflection of the load cell were calculated from optical microscopy images by using Digital Image Correlation (DIC) and with 65 nm resolution in fiber extension. The nanofibers were fabricated from electrospun polyacrylonitrile (PAN) with MW = 150,000 and diameters between 300-600 nm. At strain rates between 0.00025 s-1 to 0.025 s-1 the fiber ductility scaled directly with the rate of loading while the tensile strength was found to vary non-monotonically: At 0.00025 s-1 material relaxations allowed for near-uniform fiber drawing with up to 120% ductility and 120 MPa maximum tensile strength. At the two faster rates the tensile strength scaled with the rate of loading but the fiber ductility was the result of a cascade of localized deformations at nanoscale necks with relatively constant wavelength for all fiber diameters.
INTRODUCTION Electrospinning is a method for mass production of long polymeric nanofibers in a continuous nonwoven form. A jet of a polymer melt or solution is subjected to high flow rates and multiple flow instabilities to be transformed into polymeric nanofibers with diameters significantly smaller than that of the jet [1,2]. This diameter reduction is associated with molecular chain alignment along the fiber axis [3]. The large variability in fabrication conditions and parameters in conjunction with the large surface-to-volume ratio of individual fibers can give rise to a spectrum of mechanical behaviors that may deviate significantly from that of bulk. Thus, there is need for a universal test frame for the mechanical characterization of onedimensional soft nanostructures. To date, several methods have been explored for this purpose including nanoindentation [4,5], bending [5-8], and tension tests [5,9-17]. In general, tension tests are preferred because of the uniform applied stress and the straightforward data analysis and property extraction.
However, submicron scale experimentation requires careful manipulation of fragile samples, for which established methods are not in place yet. So far most tensile testing approaches have incorporated an Atomic Force Microscope (AFM) cantilever as the load sensor [9,11,12]. A drawback of this approach is that the force and displacement sensors are the same, and as a result the relative uncertainty in calculated properties varies with the relative stiffness of the specimen and th
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