Fabrication, mechanical properties and failure mechanism of random and aligned nanofiber membrane with different parameters

2.2.1 Preparation of different concentrations PAN nanofibers

The electrospinning apparatus consists of a high voltage power supply, a syringe pump, a stainless steel needle (internal diameter = 0.7mm) and a rotating collector for controlling linear speed (Figure 1A). In this study, different concentrations PAN/DMF solutions were prepared including 8wt%, 10wt%, 12wt%, 14wt% for electrospinning. A static roller with aluminum foil placed below served as a grounded counter electrode. The voltage between the syringe needle and the receiver could be controlled by the high voltage power supply. All kinds of homogenous PAN/DMF solutions transferred to a 10ml plastic syringe respectively. The mass flows were maintained at 1.0 mL/h and applied voltages between the syringe needle and collectorwere set at 18 kV with a tip-to-collector distance of 15 cm. All experiments were performed at room temperature conditions. Figure 2 shows SEM images of different PAN nanofiber membranes.

Figure 1

Schematic of electrospinning and four structures of PAN nanofibers: A) Schematic of electrospinning setup for collecting aligned PAN nanofibers; B) Schematic of 0^∘, 0^∘/90^∘, 0^∘/90^∘/+45^∘ and 0^∘/90^∘/+45^∘/−45^∘ nanofiber membranes.

Figure 2

Preparation and mechanical test of PAN nanofiber membrane: A) Fabricated aligned PAN nanofiber membrane; B) Tensile experiment procedure.

2.2.2 Preparation of aligned PAN nanofibers

The aligned PAN nanofibers were prepared by 12wt% PAN /DMF solution on a rotating drum with speeds from 1000 rpm to 2500rpm in the case of the same other conditions. Figure 3 shows SEM images PAN nanofibers membranes at four kinds of speed.

Figure 3

SEM images of nanofiber membranes with different concentrations; A) 8wt%; B) 10wt%; C) 12wt%; D) 14wt%.

2.2.3 Preparation of aligned PAN nanofibers of four structures

Based on the results of materials with different concentrations and rotary speeds, four kinds of fiber structures for PAN nanofibers were prepared including 0^∘, 0^∘/90^∘, 0^∘/90^∘/+45^∘ and 0^∘/90^∘/+45^∘/−45^∘ (Figure 1B) were prepared by 12wt% PAN/DMF solution on a rotating drum with speeds from 2500rpm in the case of the same other conditions.

3.1.1 Morphological characterization with different concentrations

The uniformity and diameter of PAN nanofiber were adjusted by concentrations of PAN/DMF solutions. Figure 3 shows SEM images of PAN nanofibers at different concentrations. It can be found that the nanofibers have uniform structure without any bead when the concentration of PAN/DMF solution increased 12wt%. The results indicated that increasing concentration lead to an increase in fiber uniformity and higher regular morphology with a decrease of droplets and beaded fibers. This phenomenon is mainly due to the surface tension decreases with the increase of solution viscosity [10]. Besides, the diameter of 8 wt%, 10 wt%, 12 wt% and 14 wt% is about 137±62, 225±61, 370±55, and 520±73 nm, respectively. It can be found that the diameters of PAN nanofibers increase approximately with the solution concentration increases. This is probably attributed to the greater stretching resistance of the solution with increasing solute content [21].

3.1.2 Morphological characterization with different rotary speeds

Electrospinning technique was utilized to prepare aligned PAN nanofibers by using a rotating drum collector that the rotary speed of collector could affect orientation of nanofibers. Figure 4 shows SEM images of PAN nanofiber membranes with concentration 12 wt% at different rotary speeds including 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm. It can be found that increasing rotary speeds lead to an increase in orientation of nanofibers, and the material exhibits the best 0^∘ fiber orientation at rotational speed of 2500rpm. Figure 5A shows that the diameters of fabricated PAN nanofibers with different rotary speeds are 424±28, 208±41, 350±52, and 279±32nm respectively. Figure 5B shows the percentage of angle between nanofibers orientation and 0^∘ direction. It is shown that the percentage of angle 10-0^∘ of 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm is about 35%, 52%, 57%, and 73%, respectively, which further indicates that the orientation of nanofibers increases significantly with increasing rotary speed.

Figure 4

SEM images of aligned PAN nanofiber membranes (12wt%) at different rotary speeds: A) 1000rpm; B) 1500rpm; C) 2000rpm; D) 2500rpm.

Figure 5

Fiber diameters and angular percentage of PAN nanofiber membranes (12wt%) at different rotary speeds: A) Diameters; B) Percentage of angle 10-0^∘ and 90-10^∘ with 0^∘ direction.

3.1.3 Morphological characterization with different structures

Figure 6 shows SEM images of fabricated four kinds of aligned nanofiber membranes with fiber structure of 0^∘, 0^∘/90^∘, 0^∘/90^∘/+45^∘ and 0^∘/90^∘/+45^∘/−45^∘. It can be found that the nanofiber membrane has uniform and different fiber orientation structure. From Figure 6A, it can be found that the nanofiber membrane has good 0^∘ fiber orientation structure; From Figure 6B, the material has a good fiber orientation of 0^∘ and 90^∘, and is formed by overlapping; From Figure 6C, the nanofiber membrane stacked together and oriented along 0^∘, 90^∘ and +45^∘ directions; From Figure 6D, the material is oriented in four directions of 0^∘, 90^∘, +45^∘, −45^∘, and exhibits the characteristics of transverse isotropy.

Figure 6

SEM images of PAN nanofiber membranes with different structures; A) 0^∘ B) 0^∘/90^∘ C) 0^∘/90^∘/+45^∘ D) 0^∘/90^∘/+45^∘/−45^∘.

3.2.1 Mechanical properties with different concentrations

Figure 7 shows stress-strain curves of PAN nanofiber membranes with different concentrations. It can be found that four curves show non-linear feature and the curves go up gradually with the increase of the concentration in the preliminary stage. The slope and rigidity of membranes increase with the increase of the concentration form the stress-strain curves because the fiber content and loaded capacity increase gently with the increase of the concentration. Besides, slope of stress-strain curve of 8wt% concentration is less than other three kinds of membranes. This is because the bearing capacity of 8wt% concentration membrane is affected significantly by lots of beads formed in the nanofibers.

Figure 7

Tensile stress-strain curves with different concentrations

From Table 1, the average tensile strengths of 8wt%, 10wt%, 12wt% and 14wt% are about 1.63±0.13MPa, 4.72±0.87MPa, 6.32±1.46MPa, 6.88±1.59MPa, respectively and the modulus are 0.069, 0.221, 0.233 and 0.298GPa. Compared with the results at 8wt%, the strength at 10wt%, 12wt% and 14wt% increased by 189.57%, 287.73% and 322.09%, respectively, and the modulus increased by 220.28%, 237.68% and 331.88%. The reason is the homogeneous and diameters of the fibers are important factors affecting the tensile properties and modulus of PAN nanofiber membranes. As the PAN concentration increases, the more homogeneous and larger diameters for fibers are obtained in the nanofiber membranes, in turn boost the tensile properties.

3.2.2 Mechanical properties with different rotary speeds

Figure 8 shows stress-strain curves of PAN nanofiber membranes with different rotational speeds. It can be observed that the curves almost linearly increase and then the nonlinear feature near the peak, but with different slopes. When the curves have attained the peak stress, the load of all curves drops gradually, showing the plastic fracture feature. It is mainly because the 0^∘ fibers in the shear plane dislocate and are torn under tensile and shear stress cou pling which causes gradual damage to the material. It can also be found that the slope and rigidity of membranes rise gradually with the increase of rotational speed. This is attributed to the improvement orientation and increased content of the fibers along the loading direction.

Figure 8

Tensile stress-strain curves with different rotary speeds

From Table 1, comparing the properties at the 1000rpm, with that elevated 1500rpm, 2000rpm and 2500rpm, the corresponding strength are 25.56±3.19, 28.78±2.99, 30.75±3.86 and 36.50±4.11MPa, and increased by 12.60%, 20.31%, 42.80%. Meanwhile, the modulus at 1000rpm, 1500rpm, 2000rpm and 2500rpm are 0.401, 0.522, 0.988 and 1.146GPa and increase 30.17%, 146.38%, 185.78%. It can be found that the strength and modulus increase significantly with increasing the rotary speeds. The reason is that the main load-carrying fibers increase in the direction of tension with the increase of rotary speed.

Therefore, the rotary speed is also an important factor affecting the tensile properties of PAN nanofiber membranes, and the performance can be improved significantly by increasing rotary speed.

3.2.3 Mechanical properties with different structures

Figure 9 shows stress-strain curves of 0^∘, 0^∘/90^∘, 0^∘/90^∘/+45^∘ and 0^∘/90^∘/+45^∘/−45^∘ PAN nanofiber membranes. For 0^∘ and 0^∘/90^∘ fiber structures, the curve shows obvious linear elastic feature and exhibits brittle characteristics. This is due to the fact that there is no crimp for the fibers inside the material, and 0^∘ fibers along loading direction act as the main load-bearing object which can bear load up to fracture. However, the curves for 0^∘/90^∘/+45^∘ and 0^∘/90^∘/+45^∘/−45^∘ fiber structures show significant nonlinear transition and exhibit an obvious plastic platform which attributes to different failure mechanisms, the interfaces between the +45^∘ and −45^∘ fibers are the main load-bearing objects, the degradation of the matrix between the fibers increases gradually which exhibits progressive loss of stiffness. At the same time, it can be found that the curves go down gradually with the increase of the fiber orientation angle. The peak stress and the slope of the curves decreases gradually, moreover, the materials showed an extended strain response with increased fiber orientation.

Figure 9

Tensile stress-strain curves with different fiber structures

Table 1 shows the comparison of the tensile properties for four fiber structures. It can be found that for the 0^∘ fiber structure, the tensile strength and modulus are the highest, followed by the 0^∘/90^∘ structure, then the 0^∘/90^∘/+45^∘structure, and finally the 0^∘/90^∘/+45^∘/−45^∘. Tensile strength for 0^∘, 0^∘/90^∘, 0^∘/90^∘/+45^∘ and 0^∘/90^∘/+45^∘/−45^∘ fiber structure are 36.50±4.11, 23.44±2.29, 13.83±1.66 and 11.21±1.41MPa respectively and the modulus are 1.146, 0.856, 0.273 and 0.201GPa. Compared with the 0^∘ structure, the strength decreased by 35.78%, 62.11% and 69.29%, respectively, and the modulus decreased by 25.31%, 76.18% and 82.46%. The reason is that the fibers with no crimp are the main load-bearing objects for nanofibers membrane and the number of fibers along the loading direction determines the properties of the material. However, with increasing the fiber orientation angle, the number and the effective carrying–loading capacity of fibers decreases significantly. Therefore, the fiber architecture is an important factor affecting the tensile properties of aligned nanofibers membranes which increase significantly with decreasing fiber orientation angle along the loading direction.

3.3.1 Fracture morphology and failure mechanism with different concentrations

Figure 10 showed the fracture morphology of nanofiber membranes with different concentrations. From Figure 10A, at 8wt%, it can be seen that the fracture surface is L-shaped, which is affected by lots of beads of different sizes in nanofibers. Beads of different sizes cause different defects in nanofibers, and the membrane breaks at a larger defect formed L-shaped fracture under the tensile and shear stress. At 10wt%, zigzag fracture was shown in Figure 10B. Because beads and defects gradually become decrease and uniform in 10wt% nanofiber membrane, the fracture was caused at uniform beads and defects under tensile normal stress, and the nanofibers are pulled out and broken at different levels. From Figure 10C and 10D, the fracture morphology of nanofiber membranes with concentration of 12wt% and 14wt% are ductile fracture with V-shaped under the tensile and shear stress coupling due to the fact that the directions of each fiber in the membrane are different and randomly distributed. So, the main failure for non-woven nanofibers structure is shear fracture feature. The nanofibers that are parallel to loading direction are fractured under tensile normal stress, and that the nanofibers which are not parallel to the loading direction are fractured under tensile and shear stress.

Figure 10

Fracture morphology of PAN nanofiber membranes with different concentrations: A) 8wt%; B) 10wt%; C) 12wt%; D) 14wt%.

3.3.2 Fracture morphology and failure mechanism with different rotary speeds

Figure 11 shows the fracture morphology of nanofiber membranes obtained at different rotary speeds. From Figure 11A, 11B and 11C, the fracture morphology of nanofiber membranes with rotary speeds 1000rpm, 1500rpm and 2000rpm are shear fracture with V-shaped. It is because each fiber in the membrane is also randomly distributed and the stress state of each fiber is different. The nanofibers that are parallel to loading direction are fractured under tensile normal stress, and that the nanofibers which are not parallel to the loading direction are fractured under tensile and shear stress, and the energy travels along the easiest path causing shear fracture. However, from Figure 11D, at 2500rpm, it can be found that the fracture becomes flush. This is mainly because the fabricated nanofibers have better orientation in 0^∘ direction. The 0^∘ nanofibers act as the main load-bearing object and the main failure mode is the nanofibers fracture along 0^∘ direction under tensile normal stress. These fracture features also prove that the orientation of nanofibers increases significantly with increasing rotary speed and fabricated nanofibers membrane has best orientation at 2500rpm.

Figure 11

Fracture morphology of PAN nanofiber members with different rotary speeds: A) 1000rpm; B) 1500rpm; C) 2000rpm; D)2500rpm.

3.3.3 Fracture morphology and failure mechanism with different structures

Figure 12 shows the fracture morphology of nanofiber membranes possess different fiber structure with 0^∘, 0^∘/90^∘, 0^∘/90^∘/+45^∘ and 0^∘/90^∘/+45^∘/−45^∘. From Figure 12A, for 0^∘ structure, it can be seen that the fracture is flush, the 0^∘ nanofibers become loose, and there is obvious pull out under tensile normal stress, because the 0^∘ nanofibers act as the load-bearing object. From Figure 7B, for 0^∘/90^∘ structure, the fracture for nanofibers are also flush due to the tensile normal stress, which proved that the 0^∘ nanofibers still act as the main load-bearing, which is similar to the 0^∘ structure. Due to the introduction of 90^∘ nanofibers layer, the interlayer between the 0^∘ and 90^∘ nanofibers layer are bearing the coupling of tensile and shear stress which lead to the delaminating. For the 90^∘ fiber layer, the nanofibers hardly bear loading and are broken, firstly the interface is damaged under the tensile stress, then the fibers are torn from the cracked matrix. From Figure 7C, for 0^∘/90^∘/+45^∘ structure, the nanofibers appear clustering, zigzag delaminating and shear fracture can be observed. It is due to the discontinuity of stress state for different nanofiber layers and the fibers breakage is not simultaneous. On the other hand, the nanofibers in different layers have different failure mechanism. The main failure mode is the nanofibers torn along 90^∘ and +45^∘ direction, 0^∘ nanofibers tensile fracture, as well as the fibers shear breakage. From Figure 7D, for 0^∘/90^∘/+45^∘/−45^∘ structure, the material is delaminated from the surface and the fracture are along +45^∘ and −45^∘ direction. The nanofibers in 0^∘ layers are broken under tensile normal stress. The nanofibers in 90^∘ layers are torn along fiber interface. While for nanofibers in +45^∘/−45^∘ layers, the main failure mode is that the +45^∘/−45^∘ nanofibers are torn along +45^∘/−45^∘ fiber interface. The main damage is in the form of interlayer delaminating, interface debonding, fibers tearing and breakage of the nanofibers.

Figure 12

Fracture morphology of PAN nanofiber members with different structures: A) 0^∘ B) 0^∘/90^∘ C) 0^∘/90^∘/+45^∘ D) 0^∘/90^∘/+45^∘/−45^∘.