CHARACTERISTICS OF ELASTOMERIC NANOFIBER MEMBRANES PRODUCED BY ELECTROSPINNING

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1 CHARACTERISTICS OF ELASTOMERIC NANOFIBER MEMBRANES PRODUCED BY ELECTROSPINNING Yoshihiro Yamashita*, Akira Tanaka* and Frank Ko** *Department of Materials Science, The University of Shiga Prefecture 5 Hassaka, Hikone, Shiga , Japan **Department of Materials Science and Engineering, Drexel University 3nd Chestnut Street, Philadelphia, PA 94, USA ABSTRACT This study examines the feasibility of co-electrospinning of vapor grown carbon nano fibers (VGNF) within thermoplastic elastomer. The reinforcement effect was studied. The diameter of the thermoplastic elastomer was found to be the ranging from.5 to 3 µm. The mechanical properties of the electrospun fibrous structure were found to be similar to that of the cast elastomeric films. However, the electrospun fibrous membranes have superior tactile properties. The addition of VGNF was significantly increases the bulkiness of the fibrous elastomeric membranes KEY WORDS: Materials-Elastomer, Polymeric Resin, Fiber Reinforcement-Other. INTRODUCTION The electrospinning process has been rediscovered in recent years as an attractive and convenient technique for the fabrication of submicron level fibers. Most of the polymers that have been successfully electrospun are crystalline polymer having a molecular weight over,(). Only a few studies have been reported on elastomers(-4). Crystalline polymer becomes a fibrous easily because solvent evaporates at the spinning and it is crystallized though the molecular chain is orientation. But it is difficult to spin nanoscale fibers from non-crystalline polymers, because relaxation of the molecular chain is given to priority. In addition, it is difficult to obtain dissolution of cross-linking agents such as sulfur and peroxide within the solvent. Accordingly, it is our objective to examine the feasibility of producing nanofibers by the electrospinning of a thermoplastic elastomer by polymer blending. By using a sulfur free thermoplastic elastomer, this material system may be applicable to electronic components and devices, neogenesis cure material and separation membrane. We will specifically investigate the feasibility of improving the conductivity and mechanical properties by adding various carbon nanotubes and nanofibers to this elastomer.. EXPERIMENTAL. Materials Styrene butadiene styrene polymer blend thermoplastic elastomer (SBS -TR, JSR Co. Ltd) was selected for this study. This SBS blend consisted of 4% styrene and 6% butadiene by weight. The weight average molecular weight of the polymer is,. For comparison purposes, butadiene elastomer (BR83, JSR Co. Ltd.) with 3% crystal component by weight and polystyrene having a molecular weight of 6, were also used for this study.

2 The carbon nanotubes used in this research were vapor-grown carbon fibers (VGCF, fiber diameter 5nm, length µm, Showa Denko Co. Ltd.). VGCF is cheap and it is excellent in the dispersibility to the elastomer. In order to overcome the dispersibility issues of carbon nanotube, the nanotubes were stirred in a ball mill for 4 hours prior to being introduced into the solution. Dispersion was further promoted by sonication for 4 hours. VGCF were added to the elastomer respectively.. Processing The elastomer was dissolved in chloroform and the solution placed in a glass syringe. Figure shows the photograph of the electro spinning device. A 6--gauge stainless steel needle on the syringe provided the electrical contact. An alternative method, also employed, was to charge the solution by placing the wire directly in the solution in a glass pipette. The voltage used was 5-5kV, and the distance from the point of the syringe to the negative electrode target was 5-cm. The negative electrode used was a sheet of copper 5 square centimeters in size..3 Mechanical testing and SEM observation Tensile measurements of the elastomer nonwoven fabric produced by the electrospinning process were carried out using a KES-G (Kato-tech, Japan) tensile tester. The diameter of the fiber was measured using a scanning electron microscope (XL-3, Philips)..4 Conductivity of membrane Electrical conductivity of the electrospun membrane and cast film were measured by MCP-HT45 and MCP-T6 (Dia-instruments Co. Ltd.). The size of the sample used for measurement is five-centimeter square, and.mm in thickness. Conductivity was measured by using four terminal probings and the double ring probe techniques. 3. RESULT AND DISCUSSION 3. Effect of Polymer Concentration As shown in Ko s group (), nanofibers can be produced by electrospinning making the fiber of the nano order through electrospinning. Polylactic acid, poly acrylic nitrile, and the polyethylene oxide that are crystalline can form nanofibers by electrospinning relatively easily. On the other hand, it is not easy to make nanofibers from an elastomer whose polymer chain is flexible in the amorphous structure. In this study, we examined the condition of the electrospinning using the syndiotactic - polybutadiene (RB83) that contained the crystal structure as a block in the elastomer. Figure shows the relationship between the solution concentration of RB83 and the average diameter of the fiber obtained by electrospinning. The conditions for electrospinning are shown as follows. The polymer solution is retained in a syringe (3ml). It is then spun from the tip of the needle (6G) of the syringe. The angle of the syringe has been kept at 45 degrees. A high positive voltage is applied to the needle. Applied voltage and current are 5kV and ma, respectively. The elastomer spun from the tip of the syringe is sprayed in the form of the very thin fiber on the copper plate that located 5 cm away. The experiment temperature is 5C. In a solution concentration of 6wt% or less, a nonwoven membrane that consists of elastomer fiber of the nano order is obtained. However, the samples contain many beads. On the other hand, the diameter of the fiber increases rapidly in solution concentrations above 6wt%. In a solution concentration of 7.5wt%, no bead formation was found. Also the deviation of the diameter of the fiber is smaller. Therefore, it is a problem to suppress the generation of beads with a solution of 6wt% or less while still obtaining nanofibers. Molecular weight of SBS and Polybutadine are same. Only beads are obtained for dilute solution. When the concentration increases, nano fibers connect between beads. In addition, the bead becomes a spindle when the concentration condenses, and it finally becomes the fiber of a uniform diameter. The diameter of the fiber is proportional to the solution concentration. 3. Production of a droplet free nanofiber nonwoven membrane by using a low concentration polymer solution The answer is in the distance from the tip of the needle to the target. Figure 3 shows the relationship between distance to the target and average fiber diameter.

3 A significant correlation is not observed between distance and fiber diameter. However, it was found that droplets tend to form more easily with a longer jet distance. When the distance to the target is very short, there is no droplet formation. When the solution concentration is low, the liquid tends to form droplets easily in order to lower the surface tension. However, if a large shearing force, which overcomes the surface tension, acts on the solution, a nanofiber nonwoven membrane without droplet can be obtained. Very high voltages and small nozzle diameters are therefore needed. It is necessary to satisfy these requirements so that a non-crystalline polymer may become fibrous. () Entanglement of polymer chains is necessary. () Large charge necessary to draw polymer chains. (3) Use the solvent with the best vapor rate. The fiber diameter increases when the vapor rate of the solvent is too fast. If solvent evaporation rate is slow, it becomes fibrous with the solvent, and a lot of holes found on the fiber surface. However, this is not observed when the fiber diameter is one micrometer or less. Polybutadiene was able to form nanofibers without beads under the conditions of a distance 5cm, solution concentration of 5.5wt% and a voltage 5KV. It was even possible to bring the distance as close as 3cm. However, since a discharge took place between a needle and a target, electrospinning was not possible. 3.3 The formation of nanofibers using a multi-cone system Another method for making nanofiber without the droplets is to form a multi-cone by the tip of the needle. When electrospinning from a non-crystalline polymer solution, the nanofiber is formed as the spinning solution branches from a single cone. However, droplets can still be formed because the drawing force that was once concentrated on the cone-jet is now denied amongst several branches. It was previously described that this problem could be solved by decreasing the nozzle diameter and shortening distance. However, from a practical side of view, a long time is needed for making a lot of nanofibers by this technique. A thick nozzle diameter and spinning to a wide area will be efficient, however, the fiber diameter also increases in the single cone. In addition, droplets are formed quite easily. Then, the electrospinning is carried out having the branch in the tip of the nozzle. This cone is shown in Figure 4. By the tip of the nozzle, the polymer solution that was positively charged forms the multi-cone shaped like a crown. The following two conditions are necessary in order to form this multi cone: The voltage is high and the area of the target is wide. No droplets were observed on fibers made with the multi-cone. This technique has not been reported until now. The formation mechanism of the multi-cone will be examined in detail in the future. 3.4 Effect of VGCF to elastomer Figure 5 shows the relationship between the solution concentration and the fiber diameter for polybutadiene. The quantity of VGCF that was added is wt% for the polybutadiene. In spite of the higher concentration of polymer solution, the fiber diameter is thin and droplets are observed. The viscosity increases when VGCF is added but the fluidity of polymer improves. It is thought that the VGCF flows easily as a result of being fibrous, and an improvement of electric conductivity of the polymer. Figure 6 shows the stress-strain curve of the nonwoven membrane made by electrospinning of the polybutadiene elastomer. Table shows the tensile modulus obtained from these curves. The elastic modulus of the membrane increased 3 times when 5wt% VGCF was added to the elastomer. However wt% of VGCF was added, the improvement in the modulus was not observed. The change in modulus with the addition of VGCF is calculated by using the simplest mixing rule, it becomes the following. φ φ 3 () = ( + ) E E 8 E c E ( VGCF ) = 5GPa, φ ( VGCF ) =.5, φ ( PB) =.95 E ( PB) =.GPa

4 This is a prediction equation for the modulus of a composite material in which continuous fiber is perpendicularly aligned. In addition, the random orientation of the fiber was assumed and a (3/8) knockdown factor applied. This resulted in a modulus of 6MPa, and the measured value was /3 of this amount. It is necessary to improve the interfacial property between the resin and the fiber and dispersibility, because the elastic modulus over 5GPa can be expected if VGCF has been orientated in the fiber direction. In the observation made by SEM, a change of the fiber diameter with the addition of VGCF was not observed. The breaking elongation was decreased greatly when the added amount of VGCF became 4wt% or more. The breaking strain was % or less, when 5wt% VGCF was added. The membrane obtained from electrospinning changes from a light gray into a dark gray when the amount of VGCF increases. Therefore, it seems that the added VGCF was proportional to the quantity is the final membrane. 3.5 In the case of SBS (Polystyrene-Polybutadiene alloy elasotmer, TR) For the SBS alloy elasotomer, the geometrical arrangement of the fibers in the nonwoven membrane and the diameter of the electrospun fibers were observed by SEM. The diameter of the electrospun SBS fibers ranged from to µm. Droplet formation was observed for fabrics spun with an SBS concentration of 5% or less. Spinning was not possible with a SBS percentage over % as a result of the solution high viscosity. Figure 7 shows the photograph of electrospun SBS elastomer fiber from different concentration solutions. The fiber diameter of the SBS elastomer is larger than that of polybutadiene. The SBS originates from the perfect amorphous structure, while the polybutadiene can crystallize. Figure 8 shows the photographs of electrospun polystyrene fiber. The fiber diameter of polystyrene is larger than that of the SBS elastomer. Electrospinning greatly depends on the viscosity of the polymer. Ko found that the multiplication of the intrinsic viscosity with the concentration of polymer solution (Berry number[6]) is an important factor for electrospinning. Figure 9 shows the relationship between the Berry number and the diameter of the fiber obtained by electrospinning. This result shows that the fiber diameter of an amorphous or slightly crystallized polymer greatly depends on the Berry number. The slope of these three kinds of polymers was found to be almost equal. From what do these coefficients originate? It is probably related to the molecular weight, radius of gyration of molecular chains and entanglement of the molecular chains. The molecular weight of each polymer is different. The polystyrene is 48,, polybutadiene is 6, and SBS is,. It is thought that the alpha is related to molecular weight. It is very useful that the gradient is almost the same in order to guess the conditions for producing nanofibers from electrospinning of an amorphous polymer. That is, the polymer concentration necessary to obtain the nanofiber can be predicted using this relationship. Also, electrospinning was possible from a solution that consisted at polystyrene mixed with polybutadiene. Figure is the SEM photograph of the membrane made from the solution of mixed polybutadiene and polystyrene in 4:6 weight ratios. The concentration of the polymer solution is 6wt%. The mean diameter of the fiber is.55 µm, and it is thinner than that spun from SBS. Though diameter of the fiber is dominated by polybutadiene, the obtained membrane is brittle and the polystyrene will dominate its mechanical properties. 3.6 Tensile property of electrospun fiber Figure shows the results of tensile measurements on the SBS elastomer. The stress-strain curve of the nonwoven mat that has.34 µm fiber diameters is almost the same as a cast film. When the fibers are oriented along the loaded direction, a higher level of strength is expected for the nonwoven mats. The characteristic of this nonwoven membrane is that the initial modulus of elasticity is very low. It gives a very soft hand. 3.7 Conductivity of electrospun fiber The conductivity of the VGCF reinforced nano fiber nonwoven indicated a value that was higher than the cast film (Fig. ). Because, VGCF is oriented in the nonwoven in dimensions. however, it is random in the cast film.

5 4. SUMMARY The preparation of a nanofiber membrane of an elastomer was successful using the electrospinning process. The diameter of the fiber is dependent significantly on solution concentration, voltage, distance and the viscosity of the polymer. Beadless nanofiber membrane that consisted of polybutadiene was obtained under the condition of 5.5wt%, 5KV, and 5cm. The mean diameter of the fiber was.46 µm. In addition, it was found that fibers without beads were obtained by growing the multi-cone in the needle tip. It was possible to co-electrospin VGCF or VGNF with the elasotomer. The viscosity and electrical conductivity of the polymer increased with the addition of VGCF/VGNF. The tensile strength of the elastomeric nanofiber membrane was similar to that of the film. However, the initial elastic modulus was very low. A soft feeling elastomer nonwoven membrane was obtained. 5. REFERENCES () S. Sukigara, M. Gandhi, J. Ayutsede, M. Micklus, F. Ko, Polymer,44, (3) () N. Viriyabanthorn, J.L. Mead, R.G. Stacer, ACS Rubber Div Meet., 6 nd, 9 () (3) N. Viriyabanthorn, J. Shawon, J.L. Mead, R.G. Stacer, The Fiber Society Fall Technical Meeting, October 6-8 () (4) P. Gibson, H. Schreuder-Gibson, ASME MD, 9, 45-6 () (5) Y. Einaga, Y. Myaki, H. Fujita, J. Polym. Sci., Polym. Phys. Ed., 7,, 3-9 (979) (6) B.L. Hager, G.C. Berry, J. Polym. Sci. Part B,, 9-98 (985) Polymer solution Positive wire Ground wire Copper plate T Aluminum GAMMA HIGH VOLTAGE RESEACH ES-3P (3kV,5uA) Figure Photograph of the electro spinning device

6 Fiber diameter (micron meter) Concentration (wt%) single cone(many beads) multi cone (non-beads) single cone (non-beads) Figure Effect of polymer concentration (- polybutadiene, RB83). Electrospinning condition; 5KV, 5cm distance, 6G needle. diameter of fiber, micron m length from target to tip of niddle,c m G 5cm non-beads G m any beads G m any beads 8G m any beads 6G m any beads Figure 3 Effect of distance between needle and target (- polybutadiene, RB83). Electrospinning condition; 5KV, 5.5wt%

7 (a) Single cone spun (b) Multi-cone spun Figure4 Photograph of single cone and multi cone spinning Fiber diameter (micron me Many droplets Polym er concentration (w t%) Figure 5 Effect of the VGCF (VGCF wt% for BR83, 5KV, 5cm)

8 Stress (MPa) VGCF none wt% wt% 3wt% 4wt% 5wt%. 8. Figure 6 Stress-strain curve of tensile test (RB83 concentration is 7.5wt%). 4 Strain. 6 (a) (b) (c) 6% solution,.3 µm 8% solution, 5. µm % solution, µm Figure 7 Electorospinning of SBS elastomer (TR, 5KV, 5cm distance, 6G) (a) (b) (c) 5% solution, 5.9 µm 7% solution, 6.8 µm % solution, 3.8 µm Figure 8 Electorospinning of Polystyrene (5KV, 5cm distance, 6G needle)

9 Figure 9 Effect of B ([η] x c) on fiber diameter; c: concentration (g/dl)[η]:intrinsic viscosity Fiber diameter (micron meter) Dia.=α B 5 9 SBS(TR) PB(RB83) Polystyrene PB/VGCF(phr). B (dl/g), SBS (TR).34,Polybutadiene.534,Polystyrene.84[5] Figure SEM photograph of electropinnning membrane from Polystyrene/Polybutadiene mixed (4:6), 5KV, 6G, 5cm, average fiber diameter is.55µm. Table Tensile modulus for the VGCF reinforced nonwoven membrane VGCF (wt%) Modulus (MPa)

10 8 Electro spun nonwoven (dia..34µm) Stress (MPa) 6 4 film Strain (%) Figure Tensile property of electrospun nanofiber (SBS elastomer) Elctroric conductivity (S/cm) Cast film Electrospun fiber 6 8 VGCF (wt%) Figure Conductivity of the VGCF reinforced electro-spun BR elastomer