Skeleton-reinforced electrospun nanofiber membrane with enhanced mechanical strength

Electrospun membranes are made of thousands of nanofibers and are widely used in many fields. However, their poor mechanical properties seriously restrict their industrialization. In this paper, a simple post fabrication strategy is proposed to enhance the strength of electrospun nanofiber membranes. The polymer skeleton was melted electrohydrodynamic direct-writing into the electrospun nanofiber membrane and penetrated the nanofiber membrane to enhance its mechanical properties. As a verification, composite membrane with polyimide nanofiber and polylactide skeleton was generated, and tensile strength increased from 14.9 MPa to 34.2 MPa without significant loss of fiber morphology. In addition, finite element analysis was conducted. Our method makes it possible to predict the mechanical properties of nanofiber membranes in advance.


Introduction
Electrospinning is a simple and versatile technique to fabricate nanofibers from many different polymeric materials [1].The electrospun nanofiber membranes have shown great advantages in many fields due to the small diameter, high porosity, and large specific surface area.However, it has been observed that the electrospun nanofiber membranes have little inter-fiber cohesion and structural integrity [2].The nanofibers were deposited and collected in the form of webs with randomly or directionally orientation, lack the structural coherence and strength required for effective use in various applications.Therefore, post fabrication strength enhancement strategies are usually required for practical applications of electrospun nanofiber membranes.
In the recent years, many progresses have been made to enhance the mechanical strength of electrospun nanofibrous membranes [3].Chemical and physical techniques including chemical crosslinking, solvent dissolution, thermal annealing, and hot pressing/stretching are usually adopted.One of the challenges for the strength enhancement techniques is to maintain the structural morphologies of nanofiber membranes such as porosity, specific surface area and other inherent advantages as much as possible while improving the mechanical properties.
In this paper, skeleton-reinforced electrospun nanofiber membrane was produced to improve the mechanical performance, of which the polymer skeletons with higher Young's modulus were direct-writing on the electrospun nanofiber membrane.The tensile strength increased without significant loss of fiber morphology.Our method provides a simple way to balance the porosity and mechanical properties of nanofiber membrane.

Experimental details
Polyimide (PI) solution with mass fraction of 20% was purchased from Hangzhou Surmount Science and Technology Co. Ltd.Polylactide (PLA, Mw = 80000 g/mol) was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd.All the materials were used directly without any post-processing.
The production process of the skeleton-reinforced electrospun nanofiber membrane is shown in figure 1.The electrospinning and melt electrohydrodynamic direct-writing processes were carried out on a commercial apparatus (NL-E-ES-2, Narai) at an ambient temperature of 25 ℃ and a humidity of 45%RH.The nozzle used was of an inner diameter of 0.2 mm.Firstly, the PI solution was electrospun to form nanofiber membrane.The applied voltage, nozzle-to-collector distance, and solution supply rate were 10 kV, 12 cm, and 10 μL/min, respectively.Melt electrohydrodynamic direct-writing technology was then taken to manufacture the skeleton.The PLA fibers were deposited on the PI nanofiber membrane at an applied voltage, nozzle-to-collector distance, nozzle temperature, and collector temperature of 2 kV, 3 mm, 230 ℃, and 120 ℃, respectively.After the preparation, the nanofiber membrane was heated on a heating plate at a temperature of 230℃ for 1 min and then cooled at normal temperature.Due to the highly thermal stability and porosity of PI nanofiber membrane, the molten PLA would completely penetrate the nanofiber membrane and served as skeletons.The morphology of nanofiber membrane was observed by scanning electron microscope (SEM, Zeiss Sigma 500) and optical microscope (VHX2000, Keyence).The mechanical properties of the membrane were examined using a tensile tester (CTM8010, Xie Qiang Instrument Manufacturing) with simple size of 1 × 2 cm.The holding distance was set as 2 cm and the moving speed was 5 mm/min.The related data such as stress and strain were recorded and averaged from five repeated experiments on each sample.The crack strength was calculated according to following equation: where σ (MPa) was the fracture strength, F (N) was the maximum load, B (mm) was the pattern width, and D (mm) was the sample thickness.

Results and discussion
The images of skeleton-reinforced electrospun nanofiber membrane were shown in figure 2. The dimension of the nanofiber membrane was 1 × 2 cm, and 6 PLA skeletons were arranged along the length direction.The SEM image of PI nanofiber was shown in figure 2(a).The averaged diameter of nanofiber was about 0.88 μm.The nanofibers were randomly stacked and overlap with each other to form many through-holes, of which allowed the molten PLA to be penetrated the membrane and formed skeletons of specific shape after solidification.The average width of molten PLA patterns just deposited on nanofiber membrane was about 103 μm, as depicted in figure 2(b).After deposition, the molten PLA would diffuse and infiltrate through the holes of nanofiber membrane.The optical images of the front and back of the nanofiber membrane were shown in figure 2(c) and 2(d), respectively.The skeletons were observed in both sides, demonstrating that the PLA had fully penetrated the PI nanofiber membrane.In addition, the width of skeleton increased with the infiltration of molten PLA.The average width of solidified PLA skeleton was about 275 μm, and the porosity of the whole nanofiber membrane was reduced by 16.2%.The structural loss of the nanofiber membrane would be proportional to the area occupied by the skeleton.In the area outside the skeletons, the structure of the nanofibers had not changed.The stress-strain curves of PI nanofiber membrane and PLA skeleton-reinforced nanofiber membrane were shown in figure 3. The stress of each membrane increased with strain until the membrane fractured.The overall strength of the nanofiber membrane was significantly improved after the implantation of PLA skeletons.The fracture strength of PI nanofiber membrane was 14.9MPa.After implanting the PLA skeletons, the fracture strength was increased to 34.2MPa, which was 130% higher than that of the original membrane.Meanwhile, the skeleton-reinforced membrane had smaller strain changes while bearing larger loads and could reduce the deformation during stress.Moreover, finite element analysis was conducted to further verify the effectiveness of the proposed strategy to enhance the strength of nanofiber membrane.The simulation model was illustrated in figure 4, as the flat plate represented the nanofiber membrane with low Young's modulus and density, the long bar array represented the polymer backbone with high Young's modulus and density.The dimension of the nanofiber membrane was 1 × 2 × 0.25 cm (width × length × thickness), the width of skeleton was 0.5 mm, and the tension applied was 2 N. Figure 5 illustrates the simulated stress diagram on the membrane and the distribution curve on the membrane midline.Due to the difference in Young's modulus between the nanofiber membrane and the implanted polymer skeletons, the tensile strength of the membrane was mainly provided by the part which has the higher Young's modulus.The skeletons borne most of the external stresses and protected the membrane from breaking.Moreover, the stresses on each skeleton would decrease with the increasing in the number of skeletons and would effectively improve the mechanical properties of the membrane.

Conclusions
In this paper, a simple post fabrication strategy is proposed to enhance the strength of electrospun nanofiber membranes.PLA skeletons were added to the electrospun PI nanofiber membrane through melt electrohydrodynamic direct-writing processes.The tensile strength increased from 14.9 MPa to 34.2 MPa, and the porosity of the whole nanofiber membrane was reduced by 16.2% without significant loss of fiber morphology.Finite element analysis was also conducted to further verify the effectiveness of the proposed strategy.Our method makes it possible to predict the mechanical properties and porosity of nanofiber membranes in advance.

Figure 2 .
Figure 2. (a) SEM image of the nanofiber.(b) Optical image of a PLA pattern just deposited on the membrane.(c) Photograph of the front of membrane.(d) Photograph of the back of membrane.

Figure 4 .
Figure 4. Simulation model for finite element analysis.

Figure 5 .
Figure 5. Stress simulation results on the membrane.