Study of Mechanical Properties of Electrospun Polyacrylonitrile Nanofibrous Membrane

Bibhuti Bhusan Sahoo a), Ipsita Priyadarshini , Bibekananda Sundaray b)

Department of Physics, Ravenshaw University,Cuttack,-753003,India.

a ) sahoobibhuti93@gmil.comb) bnsundaray@gmail.com

Abstract The mechanical properties of electrospun nanofiber membranes are critical for their applications. Nanofibrous Polyacrylonitrile (PAN) membranes are prepared by electrospinning method with different degrees of alignment of nanofibers in the membrane. The mechanical properties of PAN membranes with different degree f alignments re studied by universal testing machine (UTM). The surface morphology and mechanical properties of membranes are characterized by scanning electron microscopy (SEM). The results indicate that with the increase of alignment of nanofibers in the, membranes exhibit superior mechanical properties than membranes with lower degree of oriented fibrous membranes .Nearly 315% enhancement in the tensile modulus is observed for the highest degree of aligned and randomly oriented fibrous membranes.

INTRODUCTION

Recently nanofibers are receiving more attention due to their high aspect ratio, excellent physical and chemical properties. These fibers find uses in heat management materials, composite reinforcements, high temperature catalysis etc. [1]. Different methods has been reported to fabricate nanofibers such as chemical vapor deposition [2] template method [3], the emulsion method [4], and electrospinning [5] etc. Electrospinning is an interesting, versatile and efficient process by which nanofibers can be fabricated from the precursor polymer solution under the influence of high electrostatic field [5-7]. Nanofibrous membranes prepared by eletrospinning method find uses as electrodes of supercapacitors [8], biomedical and tissue engineering [9, 10], electrochemical sensing [11] etc. The mechanical properties of these membranes plays an important role for different applications. The fibers obtained generally are randomly oriented, however can be oriented using different methods such as rotating drum collector etc. The alignment of nanofibers in the membrane plays an important role in determining the mechanical properties of the membrane. The current work is an attempt to study the effect of alignment on the mechanical properties of the membrane. Here we present the tailoring of mechanical properties of membranes with the degree of alignment of nanofibers in the membrane. We report nearly 315% enhancement in the tensile modulus for the highest degree of alignment.

EXPERIMENTAL DETAILS

PAN homopolymer (M.wt 1,50,000), N,N dimethylformamide (DMF) were purchased from Sigma Aldrich and used as received. 1g PAN was dissolved in 10ml DMF to form 10 wt% solution under vigorous stirring for about 12 hours. The above solution was loaded into a syringe, with a flow rate 2 ml/h, at 18 kV potential difference by

keeping 20cm distance between the tip and collector. The polymer nanofibrous membranes were collected on the aluminium foil wrap on fixed collector as well as on the rotating drum collector with different speed of rotation. The samples collected are named as PNF-00, PNF-05, PNF-10, PNF-15, PNF-20 respectively for rotation speed at 00RPM, 500RPM, 1000RPM, 1500RPM and 2000RPM. The membranes were dried in a vacuum oven at 40oC prior to further characterizations. The details of electrospinning parameters are given in TABLE 1. Different specimens of fibers are prepared by using ASTM:D 3822 – 07 method.

TABLE 1

Sample code Dc voltage

(kv)

Solution Feed rate

(mlh-1 )

Tip to collector

distance (cm)

Rotation Speed

(rpm)

Temperature

(0 C)

Relative Humidity

(%)

PNF-00 15 2 15 00 32.5 42.4

PNF-05 15 2 15 500 34.2 55.7

PNF-10 15 2 15 1000 34.8 58.2

PNF-15 15 2 15 1500 32.1 57.3

PNF-20 15 2 15 2000 35.3 52.9

Surface morphology of fibers were analysed by Scanning Electron Microscopy (SEM, Zesis Supra, 55). The Image J software was used for analysis of diameter of the individual fibers in the mebrane. The tensile strength of the polymer nanofiberous membranes were characterized using Universal Testing Machine 3382,instron,UK, according to ASTM D 3822 – 07. As per this method prior to testing the mechanical properties, two pieces of double stick tape were placed on a cardboard frame having dimension 4cm*4cm at outer side and 2cm*2cm at inner side. The nanofibrous membrane was attached on the tape followed by a paste of another double stick tape for the strong bonding. During testing process the cardboard containing the fiber mat was cut at the two sides without tape, leaving the fiber membrane along the direction of applied force. The test was repeated five times to give an average value.

RESULTS AND DISCUSSIONS

FIGURE 1 : FESEM image of polymer nanofiber at rotation speed (a)0 RPM, (b) 500 RPM ,(c)1000 RPM (d) 1500 RPM,(e) 2000 RPM

Figure 1 shows the SEM image of polymer nanofibrous membrane at different rotation speed of drum collector. From the SEM image the surface of the fibers are smooth and free from beads. With increase in the speed of the rotary drum collector, the nanofibers were aligned gradually in a particular direction. The degree of alignment of nanofibers in the membrane is highest when the speed reaches to 2000rpm (FIGURE 1(e)).This is further confirmed from the analyses of diameter of fibers and deviation angle from the direction of rotation of drum.

FIGURE 2 (a) Diameter (b) Deviation angle of fibers with rotation speed of collector

FIGURE 2(a) shows the effect of rotation speed on the fiber diameter. From the figure it is seen that the average diameter of polymer nanofibers decreases with increase of rotation speed of drum collector. It may be due to the increasing pulling strength between fibers and surface of the collector due to high rotation speed. On analyzing the tensile modulus with the fiber diameter, it is seen that the tensile modulus of the polymer nanofiber having highest diameter is least. The tensile modulus increases with decrease of the fiber diameter. It may be due to increasing molecular level orientation within the fibers having small diameter [12].FIGURE 2(b) shows the average deviation angle of different fibers from a particular direction of orientation with rotation speed of collector. From the figure it is seen that deviation angle of the fibers is less and the fibers tending to orient in a particular direction when rotation speed of collector increases.

FIGURE 3 (a) tensile modulus (b) % breaking strain of fibers with rotation speed of collector

FIGURE 3(a) shows the tensile modulus of nanofibrous membranes at different rotation speed of collector. The tensile modulus of membranes were increased from 3.43003 MPa to 14.2606 MPa which is about 315%. The enhancement of the mechanical properties is due to the alignment of more number of fibers in a particular direction.

FIGURE 3 (b) is a plot of breaking strain (%) against the rotation speed of collector. From the figure it is seen that breaking strain of the fibers decreases with increasing rotation speed. Hence it may be concluded that fibers collected at higher speed of drum collector are already stretched at the breaking point and these fibers break faster in comparison to the fibers collected at lower speed of rotation.

CONCLUSIONS

Polyacryonitrile nanofibrous membranes with different degree of alignment of nanfibers in the direction of rotation were fabricated by electroinning method using a rotating drum set up. SEM images of the nanofibrous membranes confirmed that the diameter of the fibers decreased with the increase of roation speed. Further the nanofibers in the membrane got aligned in the direction of rotation of drum and the degree of alignment increases with the increase of rotation speed. The mechanical properties of electrospun polyacrylonitrile (PAN) nanofibrous membrane were enhanced with the degree of alignment of nanofibers in the membrane The membranes showed 315% enhancement in the tensile modulus.

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