Strip twisted electrospun nanofiber yarns: Structural effects on tensile properties
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Nanofiber yarns with controlled twist levels were prepared by twisting a narrow fibrous strip cut directly from electrospun nanofiber mats. The effects of fiber morphology, diameter and orientation, as well as the yarn twist level on the yarn tensile properties were examined. For the yarns made from randomly oriented fine uniform nanofibers (e.g., diameter 359 nm) and beaded nanofibers, the tensile strength increased with increasing the yarn twist level. Higher fiber diameter (e.g., 634 nm) led to the tensile strength having an initial increase and then decrease trend. The modulus increased with the twist level for all the yarns studied. However, the elongation at break increased initially with the twist level and subsequently decreased. The orientation of aligned fibers within the fiber strip greatly influenced the yarn tensile properties. When the fibers were oriented along the fiber length direction, both tensile strength and modulus were the largest.
I. INTRODUCTION
Nanofibers produced by electrospinning have attracted extensive attention because of their enormous potential in the diverse application areas.1–3 They are often used in the forms of randomly oriented nonwoven fiber webs because of the easy production. However, nanofiber nonwovens are deficient in mechanical strength, which limits their application in areas such as bioscaffolds. Yarns are typically interlocked fibrous bundles forming a basic building block to construct complicated fibrous architectures. The interlocked fibrous structure also assists in improving mechanical property. Indeed, efforts to produce nanofiber yarns directly from electrospinning process have been made over the years. By directly electrospinning nanofibers onto a water reservoir and subsequently withdrawing the freshly formed nanofiber web from the water surface, continuous nanofiber bundles have been prepared.4 Without using liquid reservoir, continuous nanofiber bundles were prepared by combining nanofibers spun out of two electrospinning nozzles with opposite charges5 or based on a self-bundling principle induced by a grounded needle.6 Short and twisted nanofiber yarns were also prepared by collecting aligned nanofibers between two grounded parallel disks while rotating one of the disks to add twist7 or using a rotating metal hemisphere and a stationary metal rod as the collector.8 Recently, a novel electrospinning setup has been developed to directly produce highly twisted continuous nanofiber yarns from a rotating funnel where
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.295 J. Mater. Res., Vol. 27, No. 3, Feb 14, 2012
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oppositely charged nanofibers were deposited.9 Most of the works, however, focused on the preparation, morphology, and structure of nanofiber yarns. The mechanical properties and the influence of various parameters have been less investigated, despite the fact that mechanical property is essential for practical applications. As with conventional yarns, fiber diameter, fiber uni
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