Simultaneously Strong and Tough Ultrafine Continuous Nanofibers

Abstract

Strength of structural materials and fibers is usually increased at the expense of strain at failure and toughness. Recent experimental studies have demonstrated improvements in modulus and strength of electrospun polymer nanofibers with reduction of their diameter. Nanofiber toughness has not been analyzed; however, from the classical materials property trade-off, one can expect it to decrease. Here, on the basis of a comprehensive analysis of long (5-10 mm) individual polyacrylonitrile nanofibers, we show that nanofiber toughness also dramatically improves. Reduction of fiber diameter from 2.8 μm to ∼100 nm resulted in simultaneous increases in elastic modulus from 0.36 to 48 GPa, true strength from 15 to 1750 MPa, and toughness from 0.25 to 605 MPa with the largest increases recorded for the ultrafine nanofibers smaller than 250 nm. The observed size effects showed no sign of saturation. Structural investigations and comparisons with mechanical behavior of annealed nanofibers allowed us to attribute ultrahigh ductility (average failure strain stayed over 50%) and toughness to low nanofiber crystallinity resulting from rapid solidification of ultrafine electrospun jets. Demonstrated superior mechanical performance coupled with the unique macro-nano nature of continuous nanofibers makes them readily available for macroscopic materials and composites that can be used in safety-critical applications. The proposed mechanism of simultaneously high strength, modulus, and toughness challenges the prevailing 50 year old paradigm of high-performance polymer fiber development calling for high polymer crystallinity and may have broad implications in fiber science and technology.

Electrospinning of recycled PET to generate tough mesomorphic fibre membranes for smoke filtration

“…This could be explained by a higher degree of molecular alignment when thinner bres were produced in the electrical eld, due to the increased bre stretching. 26 The strain at break is usually reduced when the strength and modulus increase, but, the results contradicted this trade-off with a positive correlation between strength and strain at break, as shown in Fig. 4, i.e.…”

Rotary jet spinning review – a potential high yield future for polymer nanofibers

“…Measuring the angle of deflection from the cantilever arm and fiber extension provides an and fiber diameter in these fibers. Although fiber modulus generally increases with decreasing fiber diameter this effect is typically only observed for diameters below ~250 nm, 126 which is much lower than the 1.4 μm of the fibers tested by Tan et al Arinstein et al, 181 for example, showed that a reduction in diameter of electrospun PA 6,6 fibers lead to a considerable increase in mechanical properties of these fiber due to improved molecular orientation and chain confinement (Figure 15).…”

“…As a simple example, a 10 μm diameter microfiber has the same cross sectional area as 10,000 nanofibers with diameter 100 nm -resulting in much more surface area to interact with a composite matrix to aid in energy absorption processes as mentioned above. 125 Papkov et al 126 found that by reducing the diameter of electrospun polyacrylonitrile (PAN) fibers from 2.8 μm to ~100 nm increased the elastic modulus from 0.36 to 48 GPa, with the largest increase in fibers below 250 nm (see Figure 15). This increase was also commented on by Yao et al 8 in their review of high strength and high modulus electrospun nanofibers, where it is noted that this is not the only method of achieving increased mechanical properties.…”

Lignin-based Carbon Fibers

“…69,106 It is now also known that electrospun PAN nanofibers with very small diameters in the 100 nm range exhibit remarkable mechanical tensile strength, elongation at break, and corresponding toughness due to pronounced size effects. 107 Production of strong carbon nanofibers from electrospun PAN has also been reported, 108 along with numerous reports of PAN-based carbon nanofibers being used as electrodes in batteries and supercapacitors and other applications. 103 Electrospinning of Alcell lignin using a coaxial spinneret system was the first electrospinning study focused on lignin to be published in journal literature.…”

“…It should also be noted that decreasing the diameter of lignin-based carbon nanofibers to diameter less than 100 nm is a very interesting area for further exploration given the recent exciting findings on size effects in electrospun PAN fibers. 107 Papkov and coworkers recently reported that as-spun PAN nanofibers with ∼100 nm diameters have exceptional ductility and toughness along with tensile strengths up to 1.75 GPa and tensile moduli of 48 GPa. Furthermore, a PAN-based carbon nanofiber carbonized at 800 • C with diameter of 108 nm was recently reported to possess tensile strength of 7.3 GPa and tensile modulus of 262 GPa.…”