Darrell H. Reneker

Nanometre diameter fibres of polymer, produced by electrospinning

Electrospinning uses electrical forces to produce polymer fibres with nanometre-scale diameters. Electrospinning occurs when the electrical forces at the surface of a polymer solution or melt overcome the surface tension and cause an electrically charged jet to be ejected. When the jet dries or solidifies, an electrically charged fibre remains. This charged fibre can be directed or accelerated by electrical forces and then collected in sheets or other useful geometrical forms. More than 20 polymers, including polyethylene oxide, nylon, polyimide, DNA, polyaramid, and polyaniline, have been electrospun in our laboratory. Most were spun from solution, although spinning from the melt in vacuum and air was also demonstrated. Electrospinning from polymer melts in a vacuum is advantageous because higher fields and higher temperatures can be used than in air.

Bending instability of electrically charged liquid jets of polymer solutions in electrospinning

Nanofibers of polymers were electrospun by creating an electrically charged jet of polymer solution at a pendent droplet. After the jet flowed away from the droplet in a nearly straight line, it bent into a complex path and other changes in shape occurred, during which electrical forces stretched and thinned it by very large ratios. After the solvent evaporated, birefringent nanofibers were left. In this article the reasons for the instability are analyzed and explained using a mathematical model. The rheological complexity of the polymer solution is included, which allows consideration of viscoelastic jets. It is shown that the longitudinal stress caused by the external electric field acting on the charge carried by the jet stabilized the straight jet for some distance. Then a lateral perturbation grew in response to the repulsive forces between adjacent elements of charge carried by the jet. The motion of segments of the jet grew rapidly into an electrically driven bending instability. The three-dimensional paths of continuous jets were calculated, both in the nearly straight region where the instability grew slowly and in the region where the bending dominated the path of the jet. The mathematical model provides a reasonable representation of the experimental data, particularly of the jet paths determined from high speed videographic observations.

BEADED NANOFIBERS FORMED DURING ELECTROSPINNING

Electrospinning is a straightforward method to produce polymer fibers from polymer solutions, with diameters in the range of 100 nm. Electrospun fibers often have beads in regular arrays. The viscoelasticity of the solution, charge density carried by the jet, and the surface tension of the solution are the key factors that influence the formation of the beaded fibers.

Bending instability in electrospinning of nanofibers

A localized approximation was developed to calculate the bending electric force acting on an electrified polymer jet, which is a key element of the electrospinning process for manufacturing of nanofibers. Using this force, a far reaching analogy between the electrically driven bending instability and the aerodynamically driven instability was established. Continuous, quasi-one-dimensional, partial differential equations were derived and used to predict the growth rate of small electrically driven bending perturbations of a liquid column. A discretized form of these equations, that accounts for solvent evaporation and polymer solidification, was used to calculate the jet paths during the course of nonlinear bending instability leading to formation of large loops and resulting in nanofibers. The results of the calculations are compared to the experimental data acquired in the present work. Agreement of theory and experiment is discussed.

Taylor cone and jetting from liquid droplets in electrospinning of nanofibers

Sessile and pendant droplets of polymer solutions acquire stable shapes when they are electrically charged by applying an electrical potential difference between the droplet and a flat plate, if the potential is not too large. These stable shapes result only from equilibrium of the electric forces and surface tension in the cases of inviscid, Newtonian, and viscoelastic liquids. In liquids with a nonrelaxing elastic force, that force also affects the shapes. It is widely assumed that when the critical potential φ0* has been reached and any further increase will destroy the equilibrium, the liquid body acquires a conical shape referred to as the Taylor cone, having a half angle of 49.3°. In the present work we show that the Taylor cone corresponds essentially to a specific self-similar solution, whereas there exist nonself-similar solutions which do not tend toward a Taylor cone. Thus, the Taylor cone does not represent a unique critical shape: there exists another shape, which is not self-similar. The exp...

Enzyme-carrying polymeric nanofibers prepared via electrospinning for use as unique biocatalysts.

Improvement of catalytic efficiency of immobilized enzymes via materials engineering was demonstrated through the preparation of bioactive nanofibers. Bioactive polystyrene (PS) nanofibers with a typical diameter of 120 nm were prepared and examined for catalytic efficiency for biotransformations. The nanofibers were produced by electrospinning functionalized PS, followed by the chemical attachment of a model enzyme, α‐chymotrypsin. The observed enzyme loading as determined by active site titration was up to 1.4% (wt/wt), corresponding to over 27.4% monolayer coverage of the external surface of nanofibers. The apparent hydrolytic activity of the nanofibrous enzyme in aqueous solutions was over 65% of that of the native enzyme, indicating a high catalytic efficiency as compared to other forms of immobilized enzymes. Furthermore, nanofibrous α‐chymotrypsin exhibited a much‐improved nonaqueous activity that was over 3 orders of magnitude higher than that of its native counterpart suspended in organic solvents including hexane and isooctane. It appeared that the covalent binding also improved the enzymes stability against structural denaturation, such that the half‐life of the nanofibrous enzyme in methanol was 18‐fold longer than that of the native enzyme.

Elastomeric Nanofibers of Styrene-Butadiene-Styrene Triblock Copolymer

Nanofibers of a commercial styrene-butadiene-styrene triblock copolymer were electrospun from solution, and collected either as a nonwoven elastomeric fabric, or on a layer of graphite that was evaporated onto a glass microscope slide. The resulting nanofibers were elastic, birefringent, and most had diameters around 100 nm. A few thin, beaded fibers were found among the smooth nanofibers. The diameter of the fibers between the beads was as small as 3 nm. After staining with osmium tetroxide, the nanofibers were examined using transmission electron microscopy. Separated phases of styrene and butadiene blocks were observed. The single-phase domains were irregular in shape, but elongated along the axis of the fiber. Wide-angle X-ray diffraction patterns showed a weak indication of molecular orientation along the fiber axis, and the birefringence confirmed that such orientation was present. The single-phase domains grew larger in nanofibers that were held at room temperature (∼ 25 °C) for several days. Annealing at a temperature 70 °C greatly accelerated the growth of the single-phase domains. The nanofibers softened and flattened on the evaporated graphite during annealing.

DNA fibers by electrospinning

Thin fibers of calf thymus Na-DNA were electrospun from aqueous solutions with concentrations from 0.3% to 1.5%. In electrospinning, a high voltage is used to create an electrically charged jet of liquid solution, which dries to leave a polymer fiber. The electrospun DNA fibers have diameters around 50 to 80 nm. The diameter of the electrospun fibers is an order of magnitude or more smaller than that of previously reported fibers. The DNA fibers were observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Bead-like structures were observed on many of the fibers. During electrospinning a process called splaying causes the jet to split longitudinally into two smaller jets, which split again, repeatedly, until the very small diameter fibers are formed. The small-diameter fibers are transparent in ordinary 100 kV electron microscopes. Fibers can be spun from samples of DNA as small as 1 mg.

Nanofiber garlands of polycaprolactone by electrospinning

Abstract Over a period of time, the typical path of a single jet of polymer solution, in the electrospinning process follows the nearly straight electric field lines for a certain distance away from the tip, and then develops a series of electrically driven bending instabilities that cause the path of the jet to explore a cone shaped envelope as the jet elongates and dries into a nanofiber. The multitudes of open loops that are formed are rarely observed to come into contact with each other until the dry nanofiber is collected at the end of the process. A new phenomenon is reported in this paper. Electrospinning a solution of polycaprolactone in acetone caused the dramatic appearance of a fluffy, columnar network of fibers that moved slowly in large loops and long curves. The name ‘garland’ was given to the columnar network. Open loops of the single jet came into contact just after the onset of the bending instability and then merged into a cross-linked network that created and maintained the garland. Contacts between loops occurred when the plane of some of the leading loops of the jet rotated around a radius of the loop. Then a small following loop, expanding in a different plane, intersected a leading loop that was as many as several turns ahead. Mechanical forces overcame the repulsive forces from the charge carried by the jet, the open loops in flight made contact and merged at the contact point, to form closed loops. The closed loops constrained the motion to form a fluffy network that stretched and became a long roughly cylindrical column a few millimeters in diameter. This garland, which was electrically charged, developed a path of large open loops that are characteristic of a large-scale electrically driven bending instability. Over a long period of time, the fluffy garland never traveled outside a conical envelope similar to, but larger than the conical envelope associated with the bending instability of a single jet.

Branching in Electrospinning of Nanofibers

Electrospinning of polymer nanofibers often begins with a single, straight, elongating, and electrified fluid jet that emanates from a droplet tip when the electric field at the surface is high enough. After some distance an electrically driven bending instability of the elongating jet occurs. For a polymer solution suitable for electrospinning, capillary instability does not cause the jet to become a spray of droplets. Under some conditions, a sequence of secondary jet branches emanates from the primary jet. This paper describes an experiment in which many closely spaced branches along the jet were observed during the electrospinning of a polycaprolactone solution. A theoretical description of the branching phenomenon is proposed.