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Handspinning Enabled Highly Concentrated Carbon Nanotubes with Controlled Orientation in Nanofibers

View Article: PubMed Central - PubMed

ABSTRACT

The novel method, handspinning (HS), was invented by mimicking commonly observed methods in our daily lives. The use of HS allows us to fabricate carbon nanotube-reinforced nanofibers (CNT-reinforced nanofibers) by addressing three significant challenges: (i) the difficulty of forming nanofibers at high concentrations of CNTs, (ii) aggregation of the CNTs, and (iii) control of the orientation of the CNTs. The handspun nanofibers showed better physical properties than fibers fabricated by conventional methods, such as electrospinning. Handspun nanofibers retain a larger amount of CNTs than electrospun nanofibers, and the CNTs are easily aligned uniaxially. We attributed these improvements provided by the HS process to simple mechanical stretching force, which allows for orienting the nanofillers along with the force direction without agglomeration, leading to increased contact area between the CNTs and the polymer matrix, thereby providing enhanced interactions. HS is a simple and straightforward method as it does not require an electric field, and, hence, any kinds of polymers and solvents can be applicable. Furthermore, it is feasible to retain a large amount of various nanofillers in the fibers to enhance their physical and chemical properties. Therefore, HS provides an effective pathway to create new types of reinforced nanofibers with outstanding properties.

No MeSH data available.


Stress-strain curves measured with the various CNT concentrations (a) single electrospun nanofibers; (b) single handspun nanofibers; (c and d) tensile strength and Young’s modulus of single electrospun and handspun nanofibers extracted from the stress-strain curves.
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f3: Stress-strain curves measured with the various CNT concentrations (a) single electrospun nanofibers; (b) single handspun nanofibers; (c and d) tensile strength and Young’s modulus of single electrospun and handspun nanofibers extracted from the stress-strain curves.

Mentions: To investigate the effect of the fabrication method on the nanofibers’ mechanical properties, Young’s modulus and the tensile strength of a single nanofiber were measured from strain-stress curves of the electrospun and handspun samples as the concentration of CNTs was varied (Fig. 3 and Table 1). The electrospun PVAc nanofibers without fillers had a tensile strength and a Young’s modulus of 13.2 ± 5.0 and 450 ± 49 MPa, respectively, while those of the handspun PVAc nanofibers were 23.2 ± 10.1 and 530 ± 31 MPa, respectively, corresponding to ~180 and 120% increases, respectively, when the fabrication method was changed from ES to HS. This indicated that the mechanical stretching force in the HS process induced the alignment of the polymer chains. The important aspects that affect the strength of nanofibers are the alignment of polymer chains along the axis of the fibers and the degree of crystallinity. In our previous report11, it was found that the HS process produced more stretched fibers than the ES process. However, the degree of crystallinity was not affected critically by the spinning method, i.e., electrospinning or handspinning, due to their relatively low crystallinity. Thus, the HS process is beneficial in that it enhances the tensile strength of the fibers by inducing polymer chain alignment, which the ES process cannot do. Also, the incorporation of CNTs should have a significant effect due to the resulting enhancement of the mechanical properties of the nanofibers. Increasing the amount of CNTs in the PVAc matrix dramatically increased Young’s modulus and tensile strength (Fig. 3c and d). Note that we were unable to use our apparatus to characterize electrospun nanofibers with CNT concentrations greater than 2 wt% or handspun nanofibers with CNT concentrations greater than 10 wt% due to their poor mechanical properties. Using the ES method, 0.5 wt% loaded PVAc/CNT nanofiber had higher tensile strength and modulus, i.e., 26.3 ± 12.1 MPa and 690 ± 103 MPa, respectively, than the PVAc nanofiber. However, when the concentration was increased to 1 wt%, the tensile strength decreased to 21.5 ± 9.2 MPa, while the modulus increased to 740 ± 127 MPa. Because CNT aggregates were formed at concentrations greater than 1 wt%, as confirmed in the electron microscopic studies (Figures S2 and S4), the tensile force was not distributed evenly along the fiber; rather, it was focused on local points where the CNT aggregates were located in the nanofiber, leading to lower tensile strength. However, the handspun nanofibers did not exhibit the degradation of tensile strength at CNT concentrations less than 2 wt%. The tensile strength and the modulus were 33.4 ± 13.9 MPa and 780 ± 90 MPa, respectively, when the concentration of CNTs was 0.5 wt%, and increasing the concentration of CNTs from 1 wt% to 2 wt% resulted in an increase in the tensile strength from 50.9 ± 18.3 to 64.1 ± 19.1 MPa and an increase in the modulus from 830 ± 87 to 1000 ± 71 MPa. Young’s modulus of the samples with CNT concentrations of 5 and 7 wt% increased to 1110 ± 83 and 1280 ± 102 MPa, respectively, indicating the nanofibers became more rigid even though the tensile strengths decreased to 32.7 ± 16.2 and 20.6 ± 10.4 MPa, respectively. When the CNT concentration was increased from 5 to 7 wt%, the breaking point of a single nanofiber changed 70 to <10%. This indicated that the nanofibers cannot withstand the application of an elongation force, which means the tensile strength was inversely proportional to the concentration of the filler33. In comparison to the conventional ES method, the HS method enhanced the mechanical properties of the nanofibers by a simple change in the fabrication method. The ES method could not be used to make highly-concentrated CNTs in PVAc fiber, but the HS method, in which the concentration of CNT could be increased, achieved a Young’s modulus that was 1.8 times was greater than that of the ES method. At the same CNT concentration for both methods, the tensile strength 2.4 times greater for the HS method, making it a powerful tool to attain strong nanofibers when the orientation of the CNTs is important.


Handspinning Enabled Highly Concentrated Carbon Nanotubes with Controlled Orientation in Nanofibers
Stress-strain curves measured with the various CNT concentrations (a) single electrospun nanofibers; (b) single handspun nanofibers; (c and d) tensile strength and Young’s modulus of single electrospun and handspun nanofibers extracted from the stress-strain curves.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC5120309&req=5

f3: Stress-strain curves measured with the various CNT concentrations (a) single electrospun nanofibers; (b) single handspun nanofibers; (c and d) tensile strength and Young’s modulus of single electrospun and handspun nanofibers extracted from the stress-strain curves.
Mentions: To investigate the effect of the fabrication method on the nanofibers’ mechanical properties, Young’s modulus and the tensile strength of a single nanofiber were measured from strain-stress curves of the electrospun and handspun samples as the concentration of CNTs was varied (Fig. 3 and Table 1). The electrospun PVAc nanofibers without fillers had a tensile strength and a Young’s modulus of 13.2 ± 5.0 and 450 ± 49 MPa, respectively, while those of the handspun PVAc nanofibers were 23.2 ± 10.1 and 530 ± 31 MPa, respectively, corresponding to ~180 and 120% increases, respectively, when the fabrication method was changed from ES to HS. This indicated that the mechanical stretching force in the HS process induced the alignment of the polymer chains. The important aspects that affect the strength of nanofibers are the alignment of polymer chains along the axis of the fibers and the degree of crystallinity. In our previous report11, it was found that the HS process produced more stretched fibers than the ES process. However, the degree of crystallinity was not affected critically by the spinning method, i.e., electrospinning or handspinning, due to their relatively low crystallinity. Thus, the HS process is beneficial in that it enhances the tensile strength of the fibers by inducing polymer chain alignment, which the ES process cannot do. Also, the incorporation of CNTs should have a significant effect due to the resulting enhancement of the mechanical properties of the nanofibers. Increasing the amount of CNTs in the PVAc matrix dramatically increased Young’s modulus and tensile strength (Fig. 3c and d). Note that we were unable to use our apparatus to characterize electrospun nanofibers with CNT concentrations greater than 2 wt% or handspun nanofibers with CNT concentrations greater than 10 wt% due to their poor mechanical properties. Using the ES method, 0.5 wt% loaded PVAc/CNT nanofiber had higher tensile strength and modulus, i.e., 26.3 ± 12.1 MPa and 690 ± 103 MPa, respectively, than the PVAc nanofiber. However, when the concentration was increased to 1 wt%, the tensile strength decreased to 21.5 ± 9.2 MPa, while the modulus increased to 740 ± 127 MPa. Because CNT aggregates were formed at concentrations greater than 1 wt%, as confirmed in the electron microscopic studies (Figures S2 and S4), the tensile force was not distributed evenly along the fiber; rather, it was focused on local points where the CNT aggregates were located in the nanofiber, leading to lower tensile strength. However, the handspun nanofibers did not exhibit the degradation of tensile strength at CNT concentrations less than 2 wt%. The tensile strength and the modulus were 33.4 ± 13.9 MPa and 780 ± 90 MPa, respectively, when the concentration of CNTs was 0.5 wt%, and increasing the concentration of CNTs from 1 wt% to 2 wt% resulted in an increase in the tensile strength from 50.9 ± 18.3 to 64.1 ± 19.1 MPa and an increase in the modulus from 830 ± 87 to 1000 ± 71 MPa. Young’s modulus of the samples with CNT concentrations of 5 and 7 wt% increased to 1110 ± 83 and 1280 ± 102 MPa, respectively, indicating the nanofibers became more rigid even though the tensile strengths decreased to 32.7 ± 16.2 and 20.6 ± 10.4 MPa, respectively. When the CNT concentration was increased from 5 to 7 wt%, the breaking point of a single nanofiber changed 70 to <10%. This indicated that the nanofibers cannot withstand the application of an elongation force, which means the tensile strength was inversely proportional to the concentration of the filler33. In comparison to the conventional ES method, the HS method enhanced the mechanical properties of the nanofibers by a simple change in the fabrication method. The ES method could not be used to make highly-concentrated CNTs in PVAc fiber, but the HS method, in which the concentration of CNT could be increased, achieved a Young’s modulus that was 1.8 times was greater than that of the ES method. At the same CNT concentration for both methods, the tensile strength 2.4 times greater for the HS method, making it a powerful tool to attain strong nanofibers when the orientation of the CNTs is important.

View Article: PubMed Central - PubMed

ABSTRACT

The novel method, handspinning (HS), was invented by mimicking commonly observed methods in our daily lives. The use of HS allows us to fabricate carbon nanotube-reinforced nanofibers (CNT-reinforced nanofibers) by addressing three significant challenges: (i) the difficulty of forming nanofibers at high concentrations of CNTs, (ii) aggregation of the CNTs, and (iii) control of the orientation of the CNTs. The handspun nanofibers showed better physical properties than fibers fabricated by conventional methods, such as electrospinning. Handspun nanofibers retain a larger amount of CNTs than electrospun nanofibers, and the CNTs are easily aligned uniaxially. We attributed these improvements provided by the HS process to simple mechanical stretching force, which allows for orienting the nanofillers along with the force direction without agglomeration, leading to increased contact area between the CNTs and the polymer matrix, thereby providing enhanced interactions. HS is a simple and straightforward method as it does not require an electric field, and, hence, any kinds of polymers and solvents can be applicable. Furthermore, it is feasible to retain a large amount of various nanofillers in the fibers to enhance their physical and chemical properties. Therefore, HS provides an effective pathway to create new types of reinforced nanofibers with outstanding properties.

No MeSH data available.