Polymeric nanofibres by electrospinning

Electrospinning process is the only established method for producing continuous polymeric fibres with diameters in the nanometer range. Thandavamoorthy Subbiah & S S Ramkumar analyse the process

Materials with dimensions below 100 nm are generally characterized as nanomaterials. Some of the latest attractions in this cutting edge technology are nanofibres, single wall nanotubes (SWNT), nanoparticles, etc. Polymeric fibres of submicron diameter can be produced by different methods like electrospinning, meltblowing and spunbonding technologies. Of the different viable techniques, electrospinning is the only well-established process for producing fibres consistently in the submicron range.

Electrospinning is known for decades and it uses high voltage electric field to produce fibres in the submicron to nanometer range. When a threshold voltage is applied to the polymer solution, electrostatic forces overcome the surface tension of pendant droplet to form nanofibres. Formhals in the 1930s patented his first invention of creating synthetic fibres using electrostatic forces. Taylor et al studied the fundamentals of fibre forming process giving importance to the shape of the fibre initiating surface, later known as ‘Taylor Cone’. Though electrospinning has been experimented for decades, it has received greater attraction in the last five years due to the growing importance on nanomaterials and its properties.

Doshi and Reneker were successful in electrospinning different polymers below submicron range. They studied the effects of different process parameters like solution concentration, applied voltage, needle tip-collector distance, solution conductivity and spinning environment. The high surface area polymeric nanofibres find applications in protective membranes, filtration, tissue scaffolds, controlled drug delivery systems, fuel cells, catalytic substrates and high performance textiles.

Electrospinning process

Electrospinning is a unique approach using electrostatic forces to produce fine fibres. Electrostatic precipitators and pesticide sprayers are some of the well known applications that work similar to the electrospinning technique. fibre production using electrostatic forces has invoked glare and attention due to its potential of forming fine fibres. Electrospun fibres have small pore size and high surface area. There is also evidence of sizable static charges in the electrospun fibres that could be effectively handled to produce three-dimensional structures.

The apparatus used for electrospinning is simple in construction which consists of a high voltage electric source with positive or negative polarity, a syringe pump with capillaries or tubes to carry the solution from the syringe or pipette to the spinnerette and a conducting collector. The collector can be made of any shape according to the requirements like flat plate, rotating drum, etc.

Many researchers have used the apparatus with modifications depending on process conditions to spin a wide variety of fine fibres. Polymer solution or the melt that has to be spun is forced through a syringe pump to form a pendant drop of polymer at the tip of the capillary or syringe needle. High voltage is applied to the polymer solution inside the syringe through a connected electrode thereby inducing free charges into the polymer solution. These charged ions move in response to the applied electric field towards the electrode of opposite polarity thereby transferring tensile forces to the polymer liquid. At the tip of the capillary, the pendant hemispherical polymer drop takes a cone like projection in the presence of an electric field. And, when the applied potential reaches a critical value that is required to overcome the surface tension of the liquid, a jet of liquid is ejected from the cone tip. Most charge carriers in organic solvents and polymers have lower mobilities and hence the charge is expected to move through the liquid for larger distances only if given enough time. After the initiation from the cone, the jet undergoes a chaotic motion or bending instability and is field directed towards the oppositively charged collector, which collects the charged fibres.

As the jet travels through the atmosphere, the solvent evaporates leaving behind a dry fibre on the collecting device. For low viscosity solutions the jet breaks up into droplets while for high viscosity solutions it travels to the collector as fibres. A detailed account on the theory of electrospinning and different polymers electrospun has been elaborated by Subbiah et al.

Experimental details Materials and method

Electrospinning setup consists of a high voltage power source (gamma high voltage research, ES30P-5W, 30KV, 166 A), ultra precision syringe pump (Harvard Apparatus, PHD 2000 infuse/withdraw, 0.0001 l/min-220.82ml/min), positive and negative electrodes, grounded conductive collector screen, and syringes with syringe needles (18G and 22G). Syringes from 0.5l to 140ml can be used with this syringe pump. Aluminum foil was used as the collector screen. Polyethylene oxide (PEO) (Molecular weight:400,000) was obtained from Sigma Aldrich in powder form. This material was dissolved in the HPLC grade water purchased from Aldrich to form a homogeneous polymer solution required for electrospinning. Pellethane, thermoplastic polyurethane (TPU) sample received from Dow Chemicals was used in the study. Tetrahydrofuran (THF) 99.7% and Dimethylformamide (DMF) 99.8% A.C.S reagent were the solvents selected for dissolving polyurethane and were purchased from Sigma Aldrich. Chemicals were used as received without further purification.

Homogeneously dissolved polymer solution was taken in the glass syringe and mounted on the syringe pump. 18 gauge and 22 gauge stainless steel needle tips from Harward apparatus were used in the experiment. The positive electrode from the high voltage source was connected to the syringe needle tip by means of an alligator clip. The negative terminal of the power source and the collector screen were grounded. The experimental set up for the studies on process parameter variation were similar. Nanofibres were collected on a smooth aluminum foil. The structure and the morphology of nanofibres spun were characterized using Scanning Electron Microscopy (SEM) (Hitachi S570) and Atomic Force Microscopy (AFM). Sample preparation for the SEM characterization requires the usage of sputter coater (Technics Hummer V Sputter Coater). The samples to be analyzed were placed over a carbon tape on an aluminum pedestal and sputter coated with gold-platinum alloy to a thickness of 200Ao-300Ao.

Surface morphology of the nanofibre samples were characterized using the AFM images. The fibre diameter was measured from the electron micrograph using ImageJ (Freeware version 1.31) software.

Results and discussions

The indigenously assembled electrospinning apparatus was initially tested for its ability to produce nanofibres with Polyethylene Oxide (PEO) solution. PEO could be electrospun easily and plenty of literature evidences are available in the public domain to establish the easy spinnability of nanofibres. 10 wt% PEO in water was a highly viscous solution and it was electrospun at a flowrate of 50 l/min. The critical voltage required to overcome the surface tension was slightly above 5 KV and continuous stable jets were formed only above 7 KV. Doshi and Renekker and Deitzel et al have worked extensively to characterize the morphology of PEO fibres for relative changes in process parameters.

By increasing the applied voltage the deposition rate increased with an increase in the velocity of the fibre jets. The fibre cloud caused by multiple splitting and whipping at a small distance from the needle tip was not easily visible in the electrospinning of PEO solution. This may be due to faster occurrence of the splaying and whipping process. The highly conductive nature of PEO and water was an important reason for electrospinning PEO fibres without many defects. PEO fibres with diameter in the range of 100-200 nm were produced.

Polyurethane nanofibres

Polyurethane (PU) dissolves equally well in tetrahydrofuran (THF) and dimethyformamide (DMF) solvents. THF is a highly volatile solvent (Bp: 66øC; Vp: 129 mm of Hg @20øC) with low dielectric constant (7.6 @ 25øC) and dipole moment (1.7D). In comparison, dimethylformamide has lower vapor pressure (2.6mm of Hg @20øC, Bp: 153øC) and high dielectric constant (36.7 @25øC). Electrospinning of polyurethane with THF gave highly unstable fibre jets which moved in all directions. At lower flowrates, due to the high volatility of the solvent, the polymer solution got dried and clogged the needle tip. With increased flow rate and voltage, fibre jets started emanating from the stainless needle tip at around 6.0 KV. Due to the unstability caused by the high evaporation rate of the solvent, the fibres formed had larger diameter and beaded structural defects. However, polyurethane electrospun with DMF produced droplets with intermediate fibre formation at 10wt% concentration.

The solvent effect plays a major role in the formation of defect free fibres in the nanometer range. In order to utilize the higher vapor pressure of THF and the polyelectrolyte behavior of DMF, mixed solvents were prepared and its effect on the morphology of the nanofibre were studied.

Process parameter tuning of electrospinning process

Solvent effect

Structure and morphology of nanofibres produced by the electrospinning process depends on the process parameters like the applied voltage, needle tip-collector distance, solution concentration and conductivity and solvent volatility. Other parameters like needle tip or capillary diameter, surrounding gas stream, conductivity of the collector screen, etc. could also influence the fibre morphology and orientation.

The solvent effect on the morphology of polyurethane nanofibres was characterized by preparing PU solutions in a mixed solvent of THF and DMF at percentage levels: 20/80, 40/60, 60/40, 80/20 THF/DMF (v/v). The volatility of the mixed solvent increases in this given order. Though 20/80 solvent produced a narrow distribution of fibres with diameter in the 100-300 nm range, it also produced droplets and wet fibres. Similarly, the 80/20 solvent system produced fibres with more beads. Surface tension and the viscosity of the polymer solution plays an important role in the fibre formation. Lower surface tension of the spinning solution facilitates defect free fibre formation. Lee et al have reported the surface tension profile of THF/DMF mixed solvent system. 60/40 THF/DMF mixture has the lowest surface tension and the relative decrease in bad defects is evident.

Voltage and distance effect

Applied electric field is the foremost important parameter in the electrospinning process due to its direct impact on the dynamics of the fluid flow. The changes in the applied voltage will be reflected on the shape of the pendant droplet at the needle tip, its surface charge, dripping rate, velocity of the flowing fluid and hence on the fibre structure and morphology. Similarly the needle tip to collector distance is also a major factor in determining the time available for fibre drying and the space available for splaying and whipping of fibres to take place. The effects of varying voltage and needle collector distance were studied individually by keeping other parameters constant.

Conclusion

Results on the electrospinning of polyethylene oxide and polyurethane fibres have been presented in this paper. Limited experimental trails show that the electrospinning technique is a viable method to produce fibres in the nanometer range. The influence of process parameters such as the solvent volatility, applied voltage and the needle to collector distance on the structure and morphology of nanofibres produced has been elaborated. Solvent volatility greatly influences the fibre structure and diameter of the nanofibres. A detailed investigation is necessary to understand the process-structure-property relationship on a variety of polymers.

(Mr Subbiah is with the Department of Chemical Engineering, Texas Tech University, Box 43121, Lubbock, TX 79409-3121, USA. Mr Ramkumar is with the Institute of Environmental and Human Health Texas Tech