Mechanical strength of single strand electrospun fiber

Mechanical characterization of single strand small-diameter fiber is of particular interests due to the size constrain that theoretically limits the amount of defects and thus potentially showing much better mechanical properties. However, determining properties such as tensile strength and strain of the fiber is challenging due to the difficulty in handling the fine fibers without compromising its mechanical characteristics. Atomic force microscopy (AFM) is the most commonly used instrument to investigate the mechanical properties of single strand small diameter fibers due to its ability to measure light forces although determination of the mechanical characteristics is by three-point bend analysis.

The mechanical properties of several single strand electrospun fibers have been tested using AFM. Single strand nanofibers from natural materials such as collagen type I [Carlisle et al 2010] and fibrinogen [Carlisle et al 2009] has been tested using the AFM. Carlisle et al (2010) study on electrospun collagen type I nanofibers found that the maximum stress decreased as the fiber radius increased. The initial modulus of 2.8 GPa recorded for the individual electrospun collagen fiber falls within the dry bending modulus of collagen type I fibrils from bovine Achilles tendon.

Tensile properties of small diameter fibers in the submicron dimension have been determined using mechanical tensile testers. Tan et al (2005) study on electrospun polycaprolactone (PCL) fiber (dimeter from 1 µm to 1.7 µm) showed that its ultimate tensile stress increases with smaller diameter but strain at break was reduced. At strain at break of 200%, the tensile strength of the PCL fiber was about 40 MPa and tensile modulus at 120 MPa. Such value is comparable to or lower than gravity spun fibers (diameter of about 100 µm) cold drawn to 500% extension. Drawing is commonly used to improve the strength and modulus of conventional fibers. Similarly, drawing on electrospun fibers can be done through the use of a rotating drum collector for collecting the fibers during electrospinning. Inai et al (2005) tested the effect of take-up velocity of a rotating disc collector on electrospun poly-L-lactide (PLLA). With a higher take-up velocity of 630 m/min the tensile strength of single strand PLLA nanofibers were increased from 89 MPa to 183 MPa. The increment in tensile strength and modulus was attributed to higher molecular orientation as a result of a greater take-up velocity. However, the tensile modulus and strength was lower than melt-spun fibers with similar take-up velocity.

Rigid chain polymers such as aramid may show greater improvement in its mechanical properties when directly spun into small diameters fibers compared to flexible chain polymers as rigid chain polymer fibers do not undergo solid state drawing process. Yao et al (2015) was able to test the mechanical property of single strand poly(p-phenylene terephthalamide) (p-aramid) and a Young's modulus and tensile strength of 59 GPa and 1.1 GPa respectively was obtained. Young's modulus and mechanical strength was found to increase dramatically with reducing fiber diameter. Young's modulus of 2.1 µm diameter fiber at 59 GPa was close to commercial p-aramid fibers of Standard Twaron 1000 or Kevlar 29 fibers (60-80 GPa) but tensile strength at 1.1 GPa was just half that of commercial p-aramid fibers. This may be due to the presence of defect or flaw in the fiber. High strength polyimide fibers have also been fabricated through electrospinning of its precursor, polyamic acid and conversion. The resultant nanofiber showed tensile strength and modulus that falls within the range of values in conventional polyimide fibers and well above most other type of electrospun fibers. This is probably due to greater chain alignment along the nanofiber axis determined by X-ray diffraction analysis [Chen et al 2008]. With polyphenylene sulfide (PPS), Fan et al (2019) was able to reduce the fiber diameter from about 20 µm (meltblown or spun bonded) to less than 8.0 µm in diameter using melt electrospinning. A cold crystalization peak was detected in the melt electrospun fiber but none was found in commercial staple fibers. Compared to commercial PPS staple fiber, melt electrospun PPS fibers exhibited 10 times higher storage modulus, better tensile strength and more than 400% elongation at break. Commercial staple PPS fibers are known to be brittle due to its slower cooling process. It has been shown that a reduction in the fiber diameter by increasing the nozzle tip to collector distance in the melt electrospinning setup, the tensile strength increases [Fan et al 2019].

The mechanical properties of fibers are influenced by many factors such as molecular weight, crystallinity, molecular arrangement and defects. As the diameter of fibers get reduced, any small differences in the factors may have greater impact on the fiber's mechanical properties. Gu et al (2005) showed that with changes in the electrospinning applied voltage, there is a significant increase in the elastic modulus of polyacrylonitrile (PAN) fibers from 5.72 GPa to 27 GPa. This may be due to a combined effect of greater molecular alignment in the fibers electrospun using a high voltage and a reduction in the fiber diameter (from 635 nm to 560 nm).

Material Description	Tensile Stress	Modulus	Test Method	Reference
Collagen Type I Average diameter: 302 nm	Max. stress: 25 MPa	0.2 to 8.0 GPa. Average 2.8 GPa	Three-point bend test using lateral force AFM	Carlisle et al 2010
Fibrinogen fibers Average diameter: 416 nm	Max. stress: 22 MPa	Relaxation modulus: 7.2 MPa	AFM	Carlisle et al 2009
Polyacrylonitrile Average diameter: 635 nm Voltage: 18 kV		Elastic modulus: 5.72 GPa	AFM	Gu et al 2005
Polyacrylonitrile Average diameter: 560 nm Voltage: 22 kV		Elastic modulus: 27 GPa	AFM	Gu et al 2005
Polycaprolactone Diameter 440 - 1040 nm	Yield stress: 9 MPa	Total tensile modulus: 62 MPa	AFM	Baker et al 2016
poly(p-phenylene terephthalamide) Average diameter: 2.1 µm	Max. stress: 1.1 GPa	59 GPa	Mechanical Tensile Tester	Yao et al 2015
polyimide [poly(p-phenylene biphenyltetracarboximide) Average diameter: 300 nm	Max. stress: 1.7 GPa	76 GPa	Mechanical Tensile Tester	Chen et al 2008
Polyamic acid Average diameter: 300 nm	Max. stress: 766 MPa	13 GPa	Mechanical Tensile Tester	Chen et al 2008
Poly-L-lactide Take-up velocity: 63 m/min Diameter: 890 nm	Max. stress: 89 MPa	1 GPa	Mechanical Tensile Tester	Inai et al 2005
Poly-L-lactide Take-up velocity: 630 m/min Diameter: 610 nm	Max. stress: 183 MPa	2.9 GPa	Mechanical Tensile Tester	Inai et al 2005

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