Effect of co-electrospinning system on morphology and oil adsorption of helical nanofibers

Abstract

In this study, inspired by cucumber tendrils, nanofibers with helical morphology are fabricated in co-electrospinning systems with different spinneret configurations. Poly(m-phenylene isophthalamide) (Nomex) and thermoplastic polyurethane are chosen as the two components in co-electrospinning. Using simulation and experimental methods, the electric field distribution and the morphology of nanofibers from the three co-electrospinning systems are analyzed. The helical nanofibers generated from the off-centered spinneret co-electrospinning system possess a helical shape with larger curvature and show higher tensile strain. In addition, the helical nanofibers show an oil adsorption capacity up to 79.3 g g⁻¹ and high oil retention (91%). This study may help control helical fiber morphology and extend the applications of helical materials.

Keywords

Helical nanofibers co-electrospinning system miscibility mechanical property oil adsorption

Helical structures, one of the most fundamental geometrical shapes, are fascinating and elaborate structures that are widely present in nature.^1
–3 The fabrication of functional materials with a helical structure, which is bioinspired by coiled plant tendrils, wool and microscopic DNA in nature,^4
–6 has been given particular attention. Many materials utilize fibers with helical shapes to achieve multiple functions and superior mechanical properties.⁷ With the combination of helical structure and micro/nanofibers, micro/nano-scaled helical fibers possess the advantages of a high specific surface area, high porosity, good flexibility and resilience,⁸ which has aroused wide concern. Recently, helical micro/nanofibers have shown great potential in applications of filtration,⁹ adsorption,¹⁰ tissue engineering,^11,12 and flexible electronics.^13
–16 The large-scale preparation of helical micro/nanofibers involves various techniques, including mechanical twisting,¹² three-dimensional (3D) printing,¹⁷ microfluidic spinning,^2,3,15 melt blown blends,^18,19 electrospinning,^20
–24 and so on.

Electrospinning is a versatile and effective technique for producing helical fibers with a diameter ranging from nanometers to several micrometers. Silva et al. reviewed a series of helical shapes obtained from different electrospinning processes.²⁵ Some researchers^26
–29 have reported that micro/nano-scaled helical fibers, fabricated by conventional electrospinning of a single polymer solution, were formed by the buckling instability of the electrospinning jets. Other researchers^20
–23,30 have proposed that bicomponent helical micro/nanofibers were fabricated by co-electrospinning of two polymers with distinct mechanical behaviors. Generally, side-by-side and core-shell electrospinning techniques are represented in the fabrication of bicomponent helical fibers. Lin et al. reported that helical nanofibers were prepared from an elastomeric polyurethane and a thermoplastic polyacrylonitrile by side-by-side electrospinning.³⁰ Chen et al. reported that helical fibers were generated by side-by-side and off-centered core-shell electrospinning of rigid and flexible polymers, that is, rigid poly(m-phenylene isophthalamide) (Nomex), polylactide, and polysulfonamide, and flexible thermoplastic elastomer polyurethane (TPU).²⁰ Zhang et al. reported that adopting an off-centered core-shell electrospinning technique, a type of hierarchically structured fiber with helical morphology and a porous core was constructed by combining cellulose acetate and an appropriate flexible polymer component.⁹ However, studies on the fabrication of helical micro/nanofibers using other techniques are rare, and the controllability of the helical shape (i.e. curvature) still remains a challenge.

Combining the properties of two polymers, bicomponent fibers with improved properties are fabricated and used in many textile-related areas. Subiah et al. developed bicomponent fibers with ordered insulating and conducting segments for electronic skin (e-skin) or electronic textiles (e-textiles).³¹ Yang et al. reported that bicomponent fibers were used for antistatic fabric in textiles.³² Asare et al. utilized bicomponent fibers in fire-resistant or flame-retardant textiles.³³ Lee et al. proposed bicomponent fibrous multilayer nonwovens as sound absorption textiles.³⁴ The properties of the polymeric blend will commonly depend on the properties of its polymeric components and the miscibility of the polymers.³⁵ For the fabrication of helical fibers, an interfacial interaction introduced by the bicomponent is the basic condition. Two polymers with different physical behaviors give rise to the stress difference of the fiber segments in jets, and thus endow the fibers with curvature to build helical structure. The miscibility of two components, as one of the most important factors, determines the phase-to-phase cohesiveness of two polymer phases.³⁶ For partially miscible polymers, there is actually a third region between the two phases, that is, the interphase (transition/boundary) layer where two types of polymer chains can mutually diffuse. The thickness of the interface layer mainly depends on the miscibility of the two polymers, which greatly affects the physicochemical and mechanical properties of polymer blends. Therefore, the morphology and the mechanical properties of bicomponent fibers can be affected. As is known to all, intermolecular interaction and the bicomponent ratio can influence the miscibility of the polymer phases. However, the effects of the miscibility of two polymers on the morphology and properties of helical fibers have been not systematically investigated in previous studies.

In this research, three types of spinnerets, termed ‘online blending spinneret’, ‘off-centered spinneret’, and ‘two spinnerets’, are introduced into co-electrospinning systems to fabricate TPU/Nomex helical nanofibers. The electric field distribution for co-electrospinning systems was analyzed. The fiber morphology and mechanical properties of helical fibers from co-electrospinning systems were investigated, and the degree of miscibility of two components was characterized. In addition, the oil adsorption performances of helical nanofibers were evaluated. This research paves the way for controlling the fiber morphology of helical fibers, and expands helical fibers in the field of oil adsorption.

Experimental

Materials

TPU (1.20 × 10³ kg m⁻³; Desmopan DP 2590A) was supplied by Bayer Materials Biology Technology Co., Ltd., Germany. Nomex (1.38 × 10³ kg m⁻³) staple fibers were obtained from Shanghai Xiangrun Trading Co., Ltd., China. N,N-Dimethylacetamide (0.937 g/mL at 25°C), N,N-dimethylformamide (0.944 g/mL at 25°C), and tetrahydrofuran (0.889 g/mL at 25°C) were purchased from Sigma-Aldrich Co., Ltd., USA. Lithium chloride anhydrous (LiCl) was obtained from Shanghai Macklin Biochemical Co., Ltd., China. All chemicals were used as received without further purification.

Co-electrospinning

TPU solution with a concentration of 14 wt% was prepared by dissolving the TPU pellets in N,N-dimethylformamide/tetrahydrofuran solvent (4:1 v/v). Nomex was dissolved in N,N-dimethylacetamide with the addition of 1.8 wt% LiCl to form 12 wt% Nomex solution. The addition of LiCl was to improve the conductivity of the Nomex solution. The two solutions were stirred separately for 12 h to form homogeneous solutions.

Three kinds of co-electrospinning systems with different spinneret configurations were designed, and the schematics are illustrated in Figure 1. Figure 1(a) shows the co-electrospinning system with an online blending spinneret. Figure 1(b) shows the co-electrospinning system with an off-centered core-shell spinneret in which the inner needle is eccentrically inside the outer one. Another type of co-electrospinning system with two spinnerets is shown in Figure 1(c). It is observed that one needle A is perpendicular to the collector and the other needle B is perpendicular to needle A; 20 kV voltage was supplied to the off-centered spinneret and the online blending spinneret. The distance from these two types of spinnerets to the corresponding collectors was 15 cm. For the two spinnerets co-electrospinning system, the applied voltage of needle A was 15 kV, while that of needle B was –5 kV. The perpendicular distance between needle A and the collector was 15 cm. In detail, the horizontal distance and vertical distance between needle A and needle B are set to 7 cm and 6 cm, respectively. All experiments were carried out at room temperature with a relative humidity of 40∼60%. Figure 1(d) shows the molecular structures of the components involved in the TPU/Nomex fiber membranes.

Figure 1.

Schematic illustration of co-electrospinning systems with different spinneret configurations; (a) the online blending spinneret; (b) the off-centered spinneret; and (c) the two spinnerets; (d) molecular structures of the Nomex and TPU components.

Characterization

The fiber morphology and structure were observed by field emission scanning electron microscopy (SEM; SU8010, Hitachi, Japan) and transmission electron microscopy (TEM; JEM-2100, JEOL, Japan). The chemical composition of the membrane was analyzed using Fourier transform infrared spectroscopy (FTIR, Nicolet6700, USA). The miscibility of polymers was analyzed by differential scanning calorimetry (DSC; DSC 4000, Shanghai, China). Mechanical properties were tested by a high strength and high modulus fiber tensile strength tester (XQ-1C, Shanghai, China) at constant temperature and humidity laboratory. The thickness of the fiber membrane was measured by a thickness gauge (YG-141N, Nantong, China). The electric field intensity and distribution were analyzed by Ansoft Maxwell software (ANSYS Inc., USA) by means of the finite element method according to the real dimensions, location and electrical conductivity of the set-up in the co-electrospinning system. The wettability of the fiber membrane was tested using a contact angle goniometer (OCA15EC, Germany).

The oil adsorption test was conducted by immersing fiber membranes in oils and organic solvents at room temperature. Two types of oils (colza oil and motor oil) and three types of organic solvents (n-hexane, heptane, and dichloromethane) were employed in the test. After 60 min adsorption, the wet sorbent was taken out and drained for 2 min, and the weights of the sample before and after adsorption were recorded. The oil adsorption capacity for the sorbent was determined by equation (1):

Q = \frac{m_{s} - m_{0}}{m_{0}}

(1)

where Q is the oil adsorption capacity (g/g), m_s (g) is the total mass of wet sorbent after 2 min drainage, and m₀ (g) is the initial mass of sorbent before adsorption. The oil retention percentage for the wet sorbent was obtained from equation (2):

\begin{array}{l} P= \frac{Q_{f}}{Q_{m}} \times 100 % \end{array}

(2)

where P is the oil retention percentage (%), Q _m is the maximum oil adsorption capacity (g/g) and Q _f is the oil adsorption capacity (g/g) after 30 min drainage.

Results and discussion

Electric field simulation

The electric field plays a vital role in fiber morphologies during the electrospinning process.³⁷ For comparison, the electric fields of co-electrospinning systems with three types of spinnerets were simulated. Figure 2 and Figure 3 show the electric field simulation results for the three co-electrospinning systems. The color map indicates the electric field distribution. Figure 2(a–d) show that for all co-electrospinning systems, the electric field concentrates on the surrounding area of the spinnerets, while the electric field intensity at the collector is rather weak. Figure 2(e) shows the overall trend is that the electric field intensity decreases along the z-axis from the spinneret to collector, finally decreases to 0 at the position of about z = 10 mm. It is worth mentioning that, for the co-electrospinning system with two spinnerets, the electric field intensity increases at first then decreases as a result of the vector superposition of the electric field formed by the positive and negative high voltage. As shown in Figure 3, the local electric field distributions in the x–y plane at the position close to the spinneret (z = 1 mm) are different in co-electrospinning systems. The sharp edge of the spinneret generates a strong electric field, which is not considered in this case. The electric field distributions around the spinnerets along the x-axis are shown in Figure 3(a–c), and the intensities of the electric field are shown in Figure 3(d–f). The electric field intensity distribution created by the online blending spinneret is relatively symmetric (Figure 3(a) and (d)). An asymmetric electric field is created by the off-centered spinneret due to its inner needle with eccentric configuration (Figure 3(b) and (e)). For the two spinnerets co-electrospinning system, an asymmetric electric field with the lowest electric field intensity is shown around the two spinnerets (Figure 3(c) and (f)). The asymmetry of the electric field is an important factor that leads to the deflective jet path, which may influence the morphologies of fibers.

Figure 2.

Electric field distribution of co-electrospinning systems. Electric field distribution in the center plane along the z-axis (and x-axis) for (a) the online blending spinneret; (b) the off-centered spinneret; (c) the two spinnerets (needle A); and (d) the two spinnerets (needle B) and (e) electric field intensity for the three spinneret configurations along the z-axis at their centerlines of spinnerets. It is noted that the intensity for the two spinnerets is simulated at needle A along the z-axis.

Figure 3.

Electric field distribution of co-electrospinning systems. Electric field distribution in the center plane along the x–y plane for (a) the online blending spinneret; (b) the off-centered spinneret; and (c) the two spinnerets. Electric field intensity for (d) the online blending spinneret; (e) the off-centered spinneret; and (f) the two spinnerets (needle A) along the x–y plane at z = 0 mm.

Morphology and structure of helical fibers

The morphology and structure of the electrospun TPU/Nomex fibers fabricated from three types of co-electrospinning systems are shown in Figure 4. Nano-scaled fibers with a 3D helical shape can be generated in the three co-electrospinning systems, which are shown in Figure 4(a–c). The fiber membrane shows loose and fluffy coils and some straight fibers (Figure 4(a)), their average fiber dimeter is approximately 152 ± 41 nm. Figure 4(b) shows some fibers are present in forms of tight coils while other fibers are straight, and the average diameter is approximately 165 ± 38 nm. Figure 4(c) shows most fibers are straight, and a small number of fibers with a helical shape are formed. The average diameter of fibers is 250 ± 98 nm. It is noted that these helical structures produced by Nomex and TPU from co-electrospinning systems are different from the the buckling fibers with much larger coil diameters. Helical structures are expected to endow micro/nano-scaled fiber membrane with better membrane structural characteristics (i.e. larger specific area, larger porosity, and better resilience).

Figure 4.

Scanning electron microscopy (SEM) images of thermoplastic elastomer polyurethane (TPU)/poly(m-phenylene isophthalamide) (Nomex) helical fibers fabricated from (a) the online blending spinneret; (b) the off-centered spinneret; (c) the two spinnerets in the co-electrospinning system. (d) Schematic diagram of straight line and helical curves with different helix curvature. The helix radius (r) and helix pitch (p) are shown in the schematic diagram. (e) Helix curvatures of helical fibers fabricated from different spinnerets.

In order to describe quantitatively the geometric variables of the helical structure, curvature k of the helix was introduced, which was derived from the report of Gerbode et al.⁴ on the study of cucumber tendril helices. The curvature k of the helix is determined by the following equation (3):

k = \frac{r}{r^{2} + {(\frac{p}{2 π})}^{2}}

(3)

where r is the helix radius, p is the helix pitch. The helix radius and helix pitch in the helical structure is illustrated in Figure 4(d). These parameters are calculated from the SEM images, and the average curvature and corresponding standard deviation are obtained from 20 helical fibers.

The strategy of bending is to obtain longer length in a limited space. Compared with a straight line, a helix obtains the larger length in the same space. Similarly, for helical nanofibers with the same fiber diameter, the larger the helix curvature k is, the longer fiber length is obtained in the same space (Figure 4(d)). In general, fibers with large helix curvature show more compact helical structure and possess better helical shape.

The helix curvature k values of fibers generated from the three types of co-electrospinning systems are shown in Figure 4(e). The curvature of TPU/Nomex fibers fabricated from the off-centered co-electrospinning system are higher, middle from the online blending co-electrospinning system, and lower from the two spinnerets co-electrospinning system, illustrating that the combination of Nomex and TPU can produce fibers with the tightest spring-like structure from the off-centered co-electrospinning system.

Effects of interfacial interaction of TPU and Nomex on helical fiber morphologies

In co-electrospinning systems, the interfacial interaction arising from the longitudinal compressive stress differential of TPU and Nomex is fundamental for the formation of helical structures. Hydrogen bonding, miscibility of polymer phases, and the interphase layer play a vital role in interfacial interaction between the two components.

Successful fabrication of the TPU/Nomex fiber membranes is confirmed by FTIR spectra in Figure 5. All the characteristic peaks of TPU and Nomex appear in the spectra curves of the TPU/Nomex fiber membranes. Hydrogen bonding occurs between TPU and Nomex, which is verified by the shift of peaks at 1456 cm⁻¹ (the bending vibration of N-H and the stretching vibration of C-N),³⁸ 1535 cm⁻¹ (the bending vibration of N-H),^39,40 1648 cm⁻¹ (the stretching vibration of C = O),³⁹ and 1727 cm⁻¹ (the stretching vibration of –COO).⁴¹ During co-electrospinning, when stretched, the tightly bonded TPU and Nomex jet is the basis for generating helical TPU/Nomex fibers. The presence of the hydrogen bond increases the interfacial interaction between TPU and Nomex layers, thus contributing to the generation of helical fibers.

Figure 5.

Fourier transform infrared spectroscopy (FTIR) curves of thermoplastic elastomer polyurethane (TPU), poly(m-phenylene isophthalamide) (Nomex), and TPU/Nomex blends. TPU/Nomex fibers fabricated from the online blending spinneret, off-centered spinneret, and two spinnerets systems, which are named ‘TPU/Nomex-1’, ‘TPU/Nomex-2’, and ‘TPU/Nomex-3’ in the figure, respectively.

The miscibility of two components (TPU and Nomex) in co-electrospinning systems is evaluated by DSC analysis. The DSC results are shown in Figure 6. It should be mentioned that the mixing time of two polymer phases plays a role in the degree of miscibility during the fiber formation process. When a certain degree of mixing at molecular scale is formed between the two polymers in the blending system, there exists mutual diffusion between the polymer chains. At this time, the two glass transition temperatures (Tgs) are close to each other, and the degree of proximity depends on the degree of mixing at the molecular scale, that is, the miscibility. Greater chain rigidity leads to higher Tg of a polymer.⁴² According to that principle, the miscibility of TPU and Nomex in three co-electrospinning systems is compared. The Tg values of polymers are shown in Table 1. The Tgs of TPU and Nomex are –72.9°C and 303.3°C, respectively (Figure 6(a) and (b)), illustrating the flexibility of TPU chains and the rigidity of Nomex chains. As shown in Figure 6(c–e), there exist two Tg values (Tg₁ and Tg₂) for TPU/Nomex polymer, in which Tg₁ is close to the Tg of TPU, whereas Tg₂ is close to the Tg of Nomex. Two Tg values for TPU/Nomex indicate that a partially miscible two-phase dispersed system is present in the TPU/Nomex blend. The smaller difference between Tg₁ and Tg₂ shows the larger degree of proximity, and thus the larger degree of miscibility for two polymers. From the difference value between Tg₁ and Tg₂ (Table 1), it is concluded that the miscibility degree of TPU/Nomex fabricated in the online blending co-electrospinning system is the maximum, middle in the off-centered co-electrospinning system, and smaller in the two spinnerets co-electrospinning system.

Figure 6.

Differential scanning calorimetry (DSC) thermograms of (b) thermoplastic elastomer polyurethane (TPU); (c) poly(m-phenylene isophthalamide) (Nomex); and TPU/Nomex fibers fabricated from (d) the online blending spinneret; (e) the off-centered spinneret; and (f) the two spinnerets.

Table 1.

Glass transition temperatures of TPU, Nomex, and TPU/Nomex fibers fabricated from the online blending spinneret, the off-centered spinneret, and the two spinnerets

Tg (°C)/Polymers	TPU	TPU/Nomex (online blending spinneret)	TPU/Nomex (off-centered spinneret)	TPU/Nomex (two spinnerets)	Nomex
Tg₁	–72.9	–32.5	–35.7	–37.2	/
Tg₂	/	224.9	279.9	282.5	303.3
Tg₂–Tg₁	/	257.4	315.6	319.7	/

Tg: glass transition temperature; TPU: thermoplastic elastomer polyurethane; Nomex: poly(m-phenylene isophthalamide).

Based on the fact that the fibers fabricated from different co-electrospinning systems show different helix curvature, it is concluded that the appropriate miscibility of Nomex and TPU tends to endow fibers with a larger helix curvature. The partial miscibility prevents the slippage of the two components and ensures the generation of interfacial interaction between the two components, which are the requirements for helical fiber formation. Comparison of the curvature values demonstrates that a relatively larger or lesser degree of miscibility between two polymer phases are not conducive to the formation of tight and compact helical fibers. It is speculated that the degree of miscibility between two components in various co-electrospinning systems is related to the different interfacial interaction, and thus influences the morphology of TPU/Nomex fibers. TEM images of helical fibers fabricated from the three co-electrospinning systems are shown in Figure 7.

Figure 7.

Transmission electron microscopy (TEM) images of helical nanostructures generated from (a) the online blending spinneret; (b) the off-centered spinneret; and (c) the two spinnerets co-electrospinning systems.

During the fiber formation process, two polymer phases in the form of solutions contact with each other. Then the macromolecular chain segments of the two polymers mutually diffuse to form an interphase layer with a two-phase continuous structure. The thickness of the interphase layer is largely determined by the degree of miscibility of polymers at the molecular scale, which will affect the interfacial interaction, and thus affect the morphology of fibers. A thinner interphase layer may result in the failure of the fabrication of composite fibers, due to the lesser interaction between two components. However, the thicker interphase layer can increase the adhesion of two polymer phases, which is also bad for the formation of helical fibers.

Figure 8 depicts the interphase layers in helical fibers from co-electrospinning systems. A schematic diagram of two polymer phases (TPU and Nomex) in helical nanofibers generated from the online blending spinneret, the off-centered spinneret, and the two spinnerets co-electrospinning systems are shown in Figure 8(a), (b) and (c), respectively. In this study, a 3D helical structure with larger curvature is fabricated by TPU/Nomex in the off-centered electrospinning system, which resulted from the appropriate interphase layer between partially miscible polymer phases. A schematic diagram of completely mixed polymer phases (TPU and Nomex) is shown in Figure 8(d). Figure 8(e) shows that a completely mixed TPU/Nomex solution using a single spinneret can only be electrospun into fibers with a straight shape.

Figure 8.

Schematic illustration of the interphase layer between two components of thermoplastic elastomer polyurethane (TPU)/poly(m-phenylene isophthalamide) (Nomex) fibers fabricated from (a) the online blending co-electrospinning system; (b) the off-centered co-electrospinning system; (c) the two spinnerets co-electrospinning system; (d) the traditional single-spinneret electrospinning system, in which the TPU/Nomex solution is completely mixed before electrospinning. (e) Scanning electron microscopy (SEM) images of traditional single-spinneret electrospun TPU/Nomex fibers. The TPU/Nomex solution is completely mixed before electrospinning and (f) Typical stress–strain curves of TPU/Nomex fiber membranes produced from different electrospinning systems.

The mechanical properties of fiber membranes prepared from the three co-electrospinning systems are investigated. The tensile stress–strain curves of the TPU/Nomex fibers are shown in Figure 8(f). It is observed that the fibers fabricated from the off-centered spinneret have the larger strain at break (76%) than those from the online blending spinneret (51%) and two spinnerets (23%). When stretched, the fibers with a larger curvature tend to have a larger deformation, and thus exhibit higher strain at break. The fibers generated by the traditional electrospinning of fully mixed TPU/Nomex solution exhibit tensile stress of 0.62 MPa. The fibers produced from the two spinnerets co-electrospinning system show a larger tensile stress (0.85 MPa) at break than those from the off-centered co-electrospinning system (0.76 MPa) and online blending co-electrospinning system (0.66 MPa). These results indicate that compared with traditional single-spinneret electrospinning, the co-electrospinning systems tend to improve the tensile strength of fibers, and generate helical fibers with better mechanical properties. Among the co-electrospinning systems, the fibers produced by the off-centered electrospinning system have relatively good mechanical properties.

Effects of process parameters on helical fiber morphologies

The mechanism of the helical fiber formation during co-electrospinning may be very complex. Except for the electric force and surface tension force, the elastic force is involved in the formation of helical fibers. Due to the viscoelasticity difference between TPU and Nomex components, a stress difference (net elastic force) arises in the interface of the components, which introduces interfacial interaction between the two components and thus endows the fibers with curvature to generate helical structure. The net elastic force is affected by the relative amount of elastomeric TPU and the rigid Nomex.²¹ Therefore, the amount of TPU and Nomex is investigated to explore the geometry variables (i.e. curvature) of helical structures for co-electrospinning systems.

For not considering the effect of the electric field, the same off-centered spinneret is employed. The off-centered spinneret is composed of inner and outer needles. As an important factor that influences the relative amount of the two components, the mixing time of two solutions at the spinneret is slightly changed with the relative positions of the inner and outer needles in the spinneret. As illustrated in Figure 9(a), three off-centered spinnerets are named spinneret A1, spinneret A2, and spinneret A3, respectively. Spinneret A2 is the off-centered spinneret used in the previous experiment. For spinneret A1, the inner needle contracts inward relative to the outer needle, while the inner needle extends outward relative to the outer needle for spinneret A3. Figure 9(b–d) shows that helical fibers can be obtained from three types of off-centered co-electrospinning systems. The curvature values for helical nanofibers are shown in Figure 9(e). It is observed that among the three off-centered co-electrospinning systems, the helical fibers produced by spinneret A2 exhibit the higher helix curvature (Figure 9(c)).

Figure 9.

(a) Schematic illustration of the off-centered spinnerets with various spinneret configurations. Scanning electron microscopy (SEM) images of helical fibers fabricated from (b) spinneret A1; (c) spinneret A2; (d) spinneret A3. (e) Helix curvatures of helical fibers fabricated from different spinnerets.

As another factor that influences the relative amount of the two components, the flow rate of TPU and Nomex solution is investigated to explore the morphology of the helical structure. Figure 10 shows the flow rate of TPU and Nomex on helical structures. For co-electrospinning systems, the same off-centered spinneret is utilized, which is illustrated in Figure 10(a). Three flow rate ratios of TPU/Nomex of 2:1, 1:1, and 1:2 (v/v) are used in the experiment. The SEM images of helical fibers are shown in Figure 10(b–d). The curvature values of three types of helical fibers are shown in Figure 10(e). It is observed that helical fibers fabricated with a relatively larger amount of TPU component (TPU/Nomex of 2:1 v/v) possess greater helix curvature, which may result from the larger net elastic force exerted by the TPU component.

Figure 10.

(a) Schematic illustration of the off-centered spinneret. Scanning electron microscopy (SEM) images of helical fibers fabricated from the off-centered spinneret at a thermoplastic elastomer polyurethane (TPU)/poly(m-phenylene isophthalamide) (Nomex) flow rate of (b) 2:1; (c) 1:1; (d) 1:2 and (e) Helix curvatures of helical fibers fabricated from the off-centered spinneret at different TPU/Nomex flow rates.

Figure 11 further confirms the effect of the TPU component on the formation of helical structures, and a modified online blending spinneret is used. As illustrated in Figure 11(a), there is a 2 cm channel for mixed TPU/Nomex solution before reaching the spinneret. Three flow rate ratios of TPU/Nomex of 2:1, 1:1, and 1:2 (v/v) are utilized in the experiment. Figure 11(b–d) shows the SEM images of helical fibers fabricated from the online blending spinneret, and the curvature values of helical fibers generated from different TPU/Nomex flow rates are shown in Figure 11(e). Again, it is observed that the larger amount of TPU component contributes to the greater helix curvature, which is consistent with the result in Figure 10. In addition, compared with the helix curvatures of helical fibers from the online blending co-electrospinning system, the curvature values from the off-centered co-electrospinning system are relatively higher, which is in accordance with the data in Figure 4(d).

Figure 11.

(a) Schematic illustration of the online blending spinneret with a 2 cm channel for the mixed thermoplastic elastomer polyurethane (TPU)/poly(m-phenylene isophthalamide) (Nomex) solution. Scanning electron microscopy (SEM) images of helical fibers fabricated from the online blending spinneret at a TPU/Nomex flow rate of (b) 2:1; (c) 1:1; (d) 1:2 and (e) Helix curvatures of helical fibers fabricated from the off-centered spinneret at different TPU/Nomex flow rates.

Oil adsorption performance

The properties such as hydrophobicity-oleophilicity, good oil-water selectivity, and high oil adsorption capacity are required for an ideal sorbent material for oil sorption.⁴³ The surface wettability of various helical fiber membranes produced from the three co-electrospinning systems, including the online blending spinneret, off-centered spinneret, and two spinnerets systems, was tested, and the result is shown in Figure 12(a). The result shows the hydrophobicity of three types of helical fiber membranes. The fiber membranes fabricated from the off-centered co-electrospinning system show the highest water contact angle of 131.5°, while, successively, from the online blending co-electrospinning system (125.8°) and from the two spinnerets co-electrospinning system (117.6°). The fibers with the larger curvature tend to possess better 3D structure and thus larger fiber surface roughness. According to the relationship between surface roughness and wettability revealed in the model of Wenzel,⁴⁴ surface roughness can increase the wettability of the solid surface due to the helical micro/nanostructures on the rough surface, and a certain amount of air is retained in a lower space. When a water droplet contacts the fibers, the air acts as an air cushion to support the water droplet, and the fibers exhibit more hydrophobic properties. Therefore, the fiber membrane with a relatively larger curvature prepared from the off-centered co-electrospinning system has the highest water contact angle.

Figure 12.

Oil adsorption performance of thermoplastic elastomer polyurethane (TPU)/poly(m-phenylene isophthalamide) (Nomex) fiber membranes. (a) Contact angles of water droplets on helical fiber membranes produced from different co-electrospinning systems. (b) Removal process of colza oil (red liquid) from oil/water mixture using the fiber membrane. (c) Adsorption capacities as functions of time for fibrous adsorbent with motor oil. (d) Adsorption capacities of fibers fabricated from different co-electrospinning systems. (e) Adsorption capacities of the fibrous adsorbents for different oils. (f) Oil retention of fibers fabricated from different co-electrospinning systems and (g) Schematic diagram of oil retention for helical fibers with different helix curvatures.

The oil–water selectivity of helical fiber membranes was investigated. The oil adsorption process is shown in Supplementary Movie S1. As demonstrated in Figure 12(b), when the as-prepared helical fiber membrane is dipped into the oil and water mixture in a beaker, the oil is quickly adsorbed in a few minutes, indicating the oleophilicity and good oil–water selectivity of the membrane.

The synergistic effect of the hydrophobic-oleophilic property and the membrane pore makes helical nanofibers with an oil adsorption capability. The oil adsorption process is described as follows: first, the oil wets the fiber surface. Due to the hydrophobic-oleophilic property of fibers, the oil rapidly spreads out on the surface, and penetrates into the pores of the fiber membrane under the capillary action. Finally, the oil fills the entire porous fiber membrane. According to the adsorption capacity over time (Figure 12(c)), the oil adsorption process should be a progressively kinetic adsorption process.

Taking the advantages of the oleophilic property of TPU/Nomex and the nanoscale helical structures of fiber membranes, we evaluate the oil adsorption performance of fiber membranes. The oil adsorption capacities for helical fiber membranes from three co-electrospinning systems are compared and shown in Figure 12(d). As shown, fibers fabricated from the off-centered spinneret exhibit higher oil sorption capacity (79.3 g/g) than that from the online blending spinneret (63.4 g/g) and the two spinnerets (20.8 g/g). The oil adsorption capacity of helical fibers is related to their hydrophobic/oleophilic property.

To explore the maximum adsorption capacity of the helical fiber membrane, various oils and organic solvents were used in the experiment. The helical fibers show the highest adsorption capacity (79.3 g/g) for motor oil adsorption (Figure 12(e)), which is related to the largest viscosity of motor oil (Table 2). In addition, the above adsorption experiments show that the helical fiber membrane possesses a considerable competitiveness of adsorption capability as compared with other oil adsorbents reported in Teng et al.,¹⁰ Lin et al.,⁴³ Lu et al.,⁴⁵ Wu et al.,⁴⁶ and Krasian et al.⁴⁷ (Table 3).

Table 2.

Physical properties of the oils used for adsorption

Oils	Viscosity (MPas)	Density (g cm^–3)
Motor oil	270	0.91
Colza oil	50	0.92
n-Hexane	0.41	0.66
n-Heptane	0.33	0.68
Dichloromethane	0.44	1.33

Table 3.

Comparison of maximum adsorption capacities of TPU/Nomex fibers with other oil adsorbents

Reference	Adsorbents	Adsorbates	Adsorption capacity
Teng et al.¹⁰	Melt-blow PP	Oils	30–60 g/g
Liu et al.⁴⁵	SiO₂ nanofibers	Oils	34–45 g/g
Wu et al.⁴⁶	PS-CNTs nanofibers	Oils	70–120 g/g
Krasian et al.⁴⁷	PLA/hybrid materials	Oils	17–23 g/g
Lin et al.⁴³	PS/PU nanofibers	Oils	35–65 g/g
	This work	Oils	20–80 g/g

PP: polypropylene; SiO₂: silicon dioxide; PS: polystyrene; CNTs: carbon nanotubes; PLA: polylactide; PU: polyurethane.

In addition, the oil retention performances of the fibers were tested. The traditional single-spinneret electrospun rod fibers have an oil retention of 60%, while the nano-scaled helical fibers generated from three co-electrospinning systems exhibit the maximum oil retention of 91% (Figure 12(f)). The nano-scaled helical fibers help keep the adsorbed liquids and preventing oil dripping. Under the same spiral radius condition, the helical structure with the larger curvature endows the fibers with an advantage in oil retention, which is illustrated in Figure 12(g). Therefore, as expected, TPU/Nomex helical fibers can be considered as a good oil adsorbent, and the large helix curvature tends to endow fibers with good oil retention.

Conclusions

In summary, we fabricated TPU/Nomex helical fibers using three types of co-electrospinning systems, and the helix curvature of helical fibers was investigated. The appropriate partial miscibility between two polymer phases facilitates the generation of helical fibers with a large helix curvature. Flexible TPU components are of great importance to form helical fibers. The oil sorption experiments show the good sorption performance of TPU/Nomex fibers, and the helical fibers with a large curvature exhibit good adsorption capacity and oil retention. This study provides new insight into controling fiber morphology by changing the spinneret configuration in co-electrospinning systems, and can expand the application of helical fibers in oil adsorption.

Footnotes

Declaration of competing interests

The author(s) declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the National Natural Science Foundation of China (No. 12172087), the Fundamental Research Funds for the Central Universities and Graduate Student Innovation Fund of Donghua University (No. CUSF-DH-D-2021011).

ORCID iD

Yongchun Zeng

References

Liu

Zhang

Kim

, et al. Helical metallic micro- and nanostructures: fabrication and application. Nanoscale 2014; 6: 9355–9365.

Shang

, et al. Bioinspired helical microfibers from microfluidics. Adv Mater 2017; 29: 1605765.

Xie

Liu

, et al. Bioinspired microfibers with embedded perfusable helical channels. Adv Mater 2017; 29: 1701664.

Gerbode

Puzey

McCormick

, et al. How the cucumber tendril coils and overwinds. Science 2012; 337: 1087–1091.

Ehrmann

Non-toxic crosslinking of electrospun gelatin nanofibers for tissue engineering and biomedicine – a review. Polymers 2021; 13: 1973.

Wang

Song

Cui

, et al. Silk fibroin H-fibroin/poly(epsilon-caprolactone) core-shell nanofibers with enhanced mechanical property and long-term drug release. J Colloid Interface Sci 2021; 593: 142–151.

Tao

Cheng

Shi

, et al. Applications of chitin and chitosan nanofibers in bone regenerative engineering. Carbohydr Polym 2020; 230: 115658.

Zhao

Zheng

Zeng

YC.

Design of a novel co-electrospinning system with flat spinneret for producing helical nanofibers. J Polym Sci, Part B: Polym Phys 2019; 57: 1496–1505.

Zhang XM, Chen JW, Zeng YC. Construction of helical nanofibers from cellulose acetate and a flexible component. Cellulose 2019; 26: 5187–5199.

10.

Teng

Wahid

Zeng

YC.

Zein/PVDF micro/nanofibers with improved mechanical property for oil adsorption. Polymer 2020; 188: 122118.

11.

Cheng

, et al. Controlled fabrication of bioactive microfibers for creating tissue constructs using microfluidic techniques. ACS Appl Mater Interfaces 2016; 8: 1080–1086.

12.

Islam

Laing

Wilson

, et al. Fabrication and characterization of 3-dimensional electrospun poly(vinyl alcohol)/keratin/chitosan nanofibrous scaffold. Carbohydr Polym 2022; 275: 118682.

13.

Zhu

Zhou

Liu

, et al. Highly sensitive, ultrastretchable strain sensors prepared by pumping hybrid fillers of carbon nanotubes/cellulose nanocrystal into electrospun polyurethane membranes. ACS Appl Mater Interfaces 2019; 11: 12968–12977.

14.

Ren

Zhou

Wang

, et al. Highly stretchable and durable strain sensor based on carbon nanotubes decorated thermoplastic polyurethane fibrous network with aligned wavelike structure. Chem Eng J 2019; 360: 762–777.

15.

Yang

Guo

MY.

Bioinspired polymeric helical and superhelical microfibers via microfluidic spinning. Macromol Rapid Commun 2019; 40: 1900111.

16.

Guo

Sun

, et al. Microfluidic generation of microsprings with ionic liquid encapsulation for flexible electronics. Research 2019; 2019: 6906275.

17.

Farahani

Chizari

Therriault

Three-dimensional printing of freeform helical microstructures: a review. Nanoscale 2014; 6: 10470–10485.

18.

Huang

Meng

, et al. Fabrication of helical microfibers from melt blown polymer blends. J Polym Sci, Part B: Polym Phys 2018; 56: 970–977.

19.

Huang

Zeng

YC.

Effects of compatibilizer and airflow field on the formation of helical microfibers via melt blowing. J Polym Sci, Part B: Polym Phys 2019; 57: 1423–1433.

20.

Chen

Hou

, et al. Polymeric nanosprings by bicomponent electrospinning. Macromol Mater Eng 2009; 294: 265–271.

21.

Chen

Hou

, et al. Effect of different bicomponent electrospinning techniques on the formation of polymeric nanosprings. Macromol Mater Eng 2009; 294: 781–786.

22.

Zheng

Zeng

. Fabrication of helical nanofibers via co-electrospinning. Ind Eng Chem Res 2015; 54: 987–993.

23.

Bian

Gong

, et al. Effects of electric field and polymer structure on the formation of helical nanofibers via coelectrospinning. Ind Eng Chem Res 2015; 54: 9585–9590.

24.

Zhu

Zhang

Chen

, et al. Setaria viridis-inspired electrode with polyaniline decorated on porous heteroatom-doped carbon nanofibers for flexible supercapacitors. ACS Appl Mater Interfaces 2020; 12: 43634–43645.

25.

Silva

PES

de Abreu

Godinho

MH.

Shaping helical electrospun filaments: a review. Soft Matter 2017; 13: 6678–6688.

26.

Qiu

Zha

, et al. Production of aligned helical polymer nanofibers by electrospinning. Eur Polym J 2008; 44: 2838–2844.

27.

Shariatpanahi

Zad

Abdollahzadeh

, et al. Micro helical polymeric structures produced by variable voltage direct electrospinning. Soft Matter 2011; 7: 10548–10551.

28.

Shin

Kim

SJ.

Controlled assembly of polymer nanofibers: from helical springs to fully extended. Appl Phys Lett 2006; 88: 223109.

29.

Xin

Huang

Yan

, et al. Controlling poly(p-phenylene vinylene)/poly(vinyl pyrrolidone) composite nanofibers in different morphologies by electrospinning. Appl Phys Lett 2006; 89: 053101.

30.

Lin

Wang

XG.

Self-crimping bicomponent nanoribers efectrospun from polyacrylonitrile and elastomeric polyurethane. Adv Mater 2005; 17: 2699–2703.

31.

Subbiah

Bhat

Tock

, et al. Electrospinning of nanofibers. J Appl Polym Sci 2005; 96: 557–569.

32.

Yang

Chen

, et al. Hierarchically porous N-doped carbon nanofibers derived from ZIF-8/PAN composites for benzene adsorption. J Appl Polym Sci 2021; 138: 50431.

33.

Asare

Hasan

Shahbazi

, et al. A comparative study of porous and hollow carbon nanofibrous structures from electrospinning for supercapacitor electrode material development. Surf Interfaces 2021; 26: 101386.

34.

Lee

Song

Yoon

KB.

Controlled wall thickness and porosity of polymeric hollow nanofibers by coaxial electrospinning. Macromol Res 2010; 18: 571–576.

35.

Kumar

Prabhakar

Prasad

, et al. Compatibility studies of chitosan/PVA blend in 2% aqueous acetic acid solution at 30 degrees C. Carbohydr Polym 2010; 82: 251–255.

36.

Tsebrenko

MV.

Influence of the degree of compatibility of polymers on the rheological properties of a mixture melt and the processes of structure formation. J Eng Phys Thermophys 2002; 76: 552–561.

37.

Meng

Zheng

Xin

BJ.

Surface morphologies of electrospun polystyrene fibers induced by an electric field. Text Res J 2019; 89: 3850–3859.

38.

Deng

Feng

, et al. Study on wettability, mechanical property and biocompatibility of electrospun gelatin/zein nanofibers cross-linked by glucose. Food Hydrocolloids 2019; 87: 1–10.

39.

Fiorentini

Suarato

Grisoli

, et al. Plant-based biocomposite films as potential antibacterial patches for skin wound healing. Eur Polym J 2021; 150: 110414.

40.

Cheng

Yuan

Zeng

, et al. Efficient reduction of reactive black 5 and Cr(VI) by a newly isolated bacterium of Ochrobactrum anthropi. J Hazard Mater 2021; 406: 124641.

41.

Chen

Zhu

Cui

, et al. Fabrication of starch-based high-performance adsorptive hydrogels using a novel effective pretreatment and adsorption for cationic methylene blue dye: behavior and mechanism. Chem Eng J 2021; 405: 126953.

42.

Dudowicz

Freed

Douglas

JF.

The glass transition temperature of polymer melts. J Phys Chem B 2005; 109: 21285–21292.

43.

Lin

Tian

Shang

, et al. Co-axial electrospun polystyrene/polyurethane fibres for oil collection from water surface. Nanoscale 2013; 5: 2745–2755.

44.

Wenzel

RN.

Resistance of solid surfaces to wetting by water. Trans Faraday Soc 1936; 28: 988–994.

45.

Liu

Tang

Zhao

, et al. Superhydrophobic SiO₂ micro/nanofibrous membranes with porous surface prepared by freeze electrospinning for oil adsorption. Colloids Surf, A 2019; 568: 356–361.

46.

Guo

, et al. CNTs reinforced super-hydrophobic-oleophilic electrospun polystyrene oil sorbent for enhanced sorption capacity and reusability. Chem Eng J 2017; 314: 526–536.

47.

Krasian

Punyodom

Worajittiphon

A hybrid of 2D materials (MoS2 and WS2) as an effective performance enhancer for poly(lactic acid) fibrous mats in oil adsorption and oil/water separation. Chem Eng J 2019; 369: 563–575.