Morphology and mechanical properties of MWNT/PMIA nanofibers by electrospinning

Materials

Commercial PMIA fibers (tensile strength at breakage was ca. 4.5 cN/dtex, and density 1.37–1.38 g/cm ³ ) were provided by Yantai Spandex Co. Ltd. (China). MWNTs (diameter < 10 nm, length 1–2 µm, purity 95%–98%) were purchased from Shenzhen Nanotech Port Co. Ltd. (China). Anhydrous lithium chloride (LiCl) was supplied by Shanghai Jufeng Chemical Scientific Co. Ltd. (China). Triton X-100 (C₈H₁₇C₆H₄(OCH₂CH₂)_n, n ≈ 10), DMAc and N,N-dimethylformamide (DMF) were purchased from J&K Scientific Co. Ltd. HNO₃ (65%) and H₂SO₄ (95%) were supplied by Shanghai Chemical Reagent Co., Ltd. (China). Tetrahydrofuran (THF) and SOCl₂ were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All the chemicals were of analytical reagent grade.

Functionalization of MWNTs

Firstly, MWNTs were added into the solvent mixture of HNO₃ and H₂SO₄ (v/v = 1:3), and the solution had a 4 h-ultrasonic treatment at room temperature. Then the MWNTs were separated from the mixture solvent by centrifugation and further purified with deionized water until the pH value reached 7, and finally MWNTs were dried in an oven at 90°C. Afterwards, the MWNTs were added into the solution of SOCl₂ in DMF, and stirred for 24 h at 65°C, the MWNTs-COCl were obtained after washing with THF, and then dried. The MWNTs-COCl were put into the solution of Triton X-100 in DMF, and the solution was stirred under N₂ atmosphere at 120°C for 48 h. The MWNTs grafted with Triton X-100 were obtained after washing with DMF, and dried for the further use. ¹⁵

Preparation of spinning solution

LiCl was dried in a vacuum dryer at 120°C for 2 h. Firstly, the dried LiCl was added into DMAc and stirred till it was dissolved completely, and followed by putting PMIA into the solution. The solution was stirred at 90°C until transparent. Meanwhile, MWNTs-Triton X-100 were added into DMAc solvent and ultrasonicated for 1 h. Finally, the two solutions were mixed with different MWNT mass fractions (0%, 0.2%, 0.6%, 1.0%) after 1 h ultrasonic treatment. The mass fractions of LiCl and PMIA in the solution were 3% and 12% respectively.

Electrospinning

The spinning apparatus was a Nanofiber Electrospinning Unit (KATO TECH Co. Ltd., Japan). Spinning parameters were as follows: inner diameter of spinneret 0.45 mm, voltage 23 kV, collecting spinning distance 11 cm, solution flow rate 0.26 ml·h⁻¹, spinneret scan speed 14 cm·min⁻¹, collector rotating speed 6 m·min⁻¹.

Measurements

Spinning solution viscosity and conductivity were measured respectively by AR2000 rheometer (TA Instruments, USA) at 25°C with awl diameter 40 mm, and DDP-220 conductivity meter (Shanghai Precision & Scientific Instrument Co. Ltd., China), each sample was measured five times and the average value was calculated.

The morphologies of the electrospun MWNTs/PMIA nanofiber mats were observed by scanning electron microscopy (SEM) (Hitachi S-4800, Japan). The mean diameter of the fibers was measured by the image analyzing system (Image-Pro Plus 5.0); 100 measurements were taken, then the average value was calculated.

X-ray diffraction (XRD) patterns of powdered samples were obtained by using an X’Pert Pro MPD x-ray diffraction system (PAN analytical, Holand). A voltage of 40 kV and a current of 40 mA using a CuKα radiation (λ = 0.154 nm) were used. The diffraction degree 2θ ranged from 5° to 45°.

Thermal stability of the nanofibers was measured by using an SDT Q600 simultaneous differential scanning calorimeter/thermal gravimetric analyser (DSC-TGA) instrument (TA Instruments, USA). The sample was ramped at 10°C/min from room temperature to 1000°C under a nitrogen atmosphere.

The dispersion and alignment of MWNTs in the MWNT/PMIA nanofibers were inspected by transmission electron microscopy (TEM) (Tecnai G2 F20 S-Twin, 120KV, USA).

The mechanical properties of the nanofiber mats were tested by Instron 3365 mechanical testing machine with a gauge length of 10 mm, crosshead speed of 10 mm/min, and initial tension 0.2 cN. The sample quantity was 10 for each of the nanofiber mats, and the size of each sample is 50 mm × 2 mm. The tensile strength at breakage was calculated by the mean of 10 samples. All of the samples were conditioned in a laboratory environment (20 ± 1°C and 65 ± 3%) for 24 h before testing.

Surface morphology

Figure 1 shows SEM images of MWNT/PMIA nanofibers electrospun from the solutions at the MWNT concentrations of 0 wt%, 0.2 wt%, 0.6 wt%, 1.0 wt%, respectively. The results indicated that the nanofibers were uniform and smooth, and with no beads. The average diameter of the nanofibers is shown in Figure 2 and decreased by a small amount with the increase of MWNT contents. Theoretically, the morphology and the diameter of the nanofibers are affected by the polymer solution properties (such as viscosity, electrical conductivity, etc).^3,5 With increasing MWNT concentrations, the spinning solution viscosity shown in Figure 3 did not present significant changes, but the electrical conductivity, as listed in Table 1, was increased, which would cause the reduced diameter. OPEN IN VIEWER

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Figure 2.

The diameter of the nanofibers at different MWNT concentrations.

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Figure 3.

Spinning solution viscosity at different MWNT concentrations.

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Table 1.

Conductivity of spinning solutions at different MWNTs concentrations

Concentration/wt%	0	0.2	0.6	1.0
Conductivity/μs.cm−1	2507 ± 46	2514 ± 45	2616 ± 47	2622 ± 51

Crystal structure

Figure 4 shows the XRD spectra of the commercial PMIA fibers and the electrospinning nanofibers. The crystal cell of PMIA is triclinic. ²⁹ The commercial PMIA fibers show three diffraction peaks at the scattering angle 2θ of ca. 17.2°, 23.6° and 27.3°. However, the electrospinning pure PMIA nanofibers (i.e. MWNTs 0 wt%) exhibited a diffraction peak at the angle 2θ of ca. 24.3°, and a reduced peak intensity, namely, a decrease of the crystallinity of nanofibers. The degenerative peak and weak intensity may be caused by faint stress-induced crystallization during electrospinning. In fact, the commercial PMIA fibers undergo intensive heat drawing at high temperature during its forming process. Since all the electrospinning nanofibers demonstrated the same diffraction peak, the addition of the MWNTs may not change the crystalline structure of nanofibers. OPEN IN VIEWER

Thermal stability

Figure 5 shows TGA curves of the commercial PMIA and the electrospun fibers. For the nanofibers, the release of adsorbed moisture and residual solvents initiates the mass loss at temperatures lower than 120°C. The peak mass loss took place from 240°C to 520°C, remarkably starting at ca. 398.7, 398.9, 399.3 and 399.8°C for the samples with MWNT concentrations of 0, 0.2, 0.6, 1.0 wt%, respectively. This indicated that the dose of MWNTs does not change the initial degradation temperature. However, it was found that there were more mass fraction retentions for the modified version, when compared with that for the unmodified version, at the temperature ranges between 600°C and 800°C. The reason may be that a compact molecular chain exists between PMIA and MWNTs. Jain et al. once shared a similar opinion on the PAN/MWNT fibers. ³⁰ For the commercial PMIA fibers, the initial mass loss is attributed to the adsorbed moisture at temperatures lower than 120°C as well. The main mass loss occurs at 420.8°C, due to the main chain scission of PMIA. The temperatures of perceptual mass loss for the electrospun nanofibers were about 20°C lower than that of the commercial fibers, and the reason may be the same as for the case of crystallinity. OPEN IN VIEWER

The dispersion and alignment of MWNTs in nanofibers

Figure 6 shows the TEM images of the MWNT/PMIA nanofibers. The MWNTs were well dispersed in the fibers and almost fully oriented along the nanofibers. This implies the functionalization of the MWNTs with Triton X-100 does greatly improve the dispersion of MWNTs in the spinning solution. MWNTs are thought to be preferentially aligned along the electric field line during injection. ¹³ OPEN IN VIEWER

The surfaces of electrospun nanofibers were compared for roughness with that of the pure PMIA nanofibers (MWNTs at 0 wt%). This indicates that the addition of MWNTs induces the rough surface of nanofibers. There may be two reasons for this: 1) MWNTs usually possess two states of semiconductive and metallic forms, ³¹ and 2) small fractions of MWNTs are not well embedded in the nanofibers but protruded from the fiber surface.

Mechanical properties

Figure 7 shows the mechanical properties of the electrospun nanofiber mats. The thickness of the nanofiber mats with different MWNT contents (0, 0.2, 0.6 and 1.0 wt%) were 0.058, 0.057, 0.060 and 0.059 mm, respectively. The results indicated that the tensile strength at break has been improved much compared to that of pure PMIA nanofiber mats, for instance, the tensile strength is increased by ca. 73% at the load of 0.2 wt% of MWNTs. The striking improvement occurs at the load 0.6 wt% with the strength increased by ca 86%. In fact, TEM images showed that the MWNTs were well dispersed in the nanofibers and well aligned along with the nanofibers, which would significantly contribute to the mechanical properties. However, the overload of MWNTs, such as 1.0 wt%, leads to the decreased strength due to misalignment and agglomeration in the polymer matrix. ¹⁴ The significant improvements in the strength of the nanofibers can be explained well by the reinforcement of MWNTs in the matrix. OPEN IN VIEWER