A Review on the Impact of Humidity during Electrospinning: From the Nanofiber Structure Engineering to the Applications

Electrospinning is the process of choice for the elaboration of nanofibrous mats. During the process, a thin and continuous charged jet of a polymer solution is traveling from an emitter subjected to a high voltage toward a grounded collector. Although the duration of the jet travel is in the order of few tens of milliseconds, the physical interactions acting between the jet and the air play a key role on the resulting fiber morphology. These interactions mainly rely on the amount of water molecules in air. This review deals with the effect of humidity during electrospinning on solvent evaporation, the solidification rate of nanofibers and finally, on the morphology at length scales ranging from the non-woven mat, the nanofiber itself down to the polymer crystal. Original electrospinning processes operating under specific environmental conditions as well as specificities encountered in needleless and free-surface electrospinning dedicated to industrial-scale mass production are also discussed. Then, it is shown how the control of humidity during electrospinning and the understanding of its influence on the fibrous structure can be exploited to target various applications dedicated to energy, environment, and health. Finally, current challenges and ideas for future research and new developments are presented.     

Effect of applied voltage on jet electric current and flow rate in electrospinning of polyacrylonitrile solutions

BACKGROUND: In the electrospinning process, through subjecting a pendent drop of a polymer solution to a high electric field, a fluid jet is ejected from the drop. To have a stable process, the rate at which the fluid is forced into the drop and the rate at which the fluid is carried away by the jet must be equal. A method is reported to find the point at which the flow into the drop is equal to the flow out of the drop. RESULTS: In the electrospinning of polyacrylonitrile solutions, by applying different voltages at a constant solution feed rate, two jet regimes were observed: stable jet and fluctuating jet regimes. The stable jet regime occurred at low voltages where the jet flow rate was lower than the feed rate, and the fluctuating jet regime occurred at higher voltages where the jet flow rate exceeded the feed rate. The highest voltage in the stable jet regime was the point where the jet flow rate was equal to the feed rate. This point was determined for different feed rates. CONCLUSION: By applying various voltages at different feed rates, and investigating the jet current, a curve showing stable processing points can be obtained. Copyright © 2008 Society of Chemical Industry     

Nanofibers from gas-assisted polymer melt electrospinning

The concept of a gas-assisted polymer melt electrospinning process is presented. This technique allows for reduced quenching of the melt jet in the spinning region, and thus increasing the jet attenuation rate and resulting in production of sub-micron scale fibers. A comprehensive melt electrospinning model was used to analyze the effects of the heated air stream on the polymer jet. It was found that under the investigated conditions in electrospinning of polylactic acid (PLA) melt, air drag produced an additional 10% thinning compared to the un-assisted melt electrospinning process, and the heating provided by the air stream resulted in an additional 20-fold jet thinning.     

The use of Weber number to predict morphology in the electrospinning of poly(ethylene oxide) nanofibers

In the electrospinning of polymer nanofibers, an electrically driven jet of polymer solution travels to a grounded target to be collected. The morphology of the resulting nanofibers can be manipulated through process parameters, though little work has been done to correlate electrospinning parameters with those of the free‐jet flow of pure liquids. This is essential when the nanofibers hold entrained beaded structures indicative of jet breakup. The effects of applied voltage and solution concentration on the fiber morphology of electrospun aqueous solutions of poly(ethylene oxide) were investigated. Solution concentrations of 4–8 wt % were used along with voltages of 4.5–11 kV to produce nanofibers with and without entrained beads. It was determined that the calculated Weber number for each condition correlated well with the resulting morphology. These results may suggest that Weber number may also be used to predict nanofibers morphology in the electrospinning of other polymer systems. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Multi‐jet nozzle electrospinning on textile substrates: observations on process and nanofibre mat deposition

Electrospinning is a simple and versatile process for producing small‐diameter fibres (nanofibres). However, in spite of the many potential applications of electrospun nanofibres, further process developments are still necessary to achieve a decisive productivity breakthrough for electrospinning plants. Increasing knowledge of multi‐jet electrospinning is crucial for developing industrial devices for large‐scale nanofibre production. This paper reports on the effect of a non‐conducting textile substrate placed between a jet‐emitting source (nine‐nozzle arrangement) and collector. Shielding the electric field changes the electrospinning conditions, nanofibre morphology, stability of jets and fibre deposition on the collecting surface. Various perturbation phenomena of the electrically driven jets were recorded and are described. The intensity of the perturbations increases as the weight of the non‐woven substrate increases resulting in defects in the nanofibrous mat (i.e. beaded nanofibres), production of tick fibres or failure to produce fibrous materials (e.g. films, droplets). The paper also reports an objective image‐processing procedure to enhance the evaluation of the collector after nanofibre deposition. Copyright © 2010 Society of Chemical Industry     

Conductive chitosan/multi walled carbon nanotubes electrospun nanofiber feasibility

The current study focuses on the electrospinning of chitosan (CHT)/multi walled carbon nanotubes (MWNTs) composite nanofiber using a highly stable dispersion. The acetic acid (1–100%) and trifluoroacetic acid/dichloromethane (TFA/DCM 70: 30) was tested as solvent, and the TFA/DCM (70 : 30) is most preferred for fiber formation process with acceptable electrospinnability. Moreover, a new protocol was used to establish proper technique for preparation of electrospinning solution. FT-IR spectroscopy utilized to infer the extent of interaction between CHT polymer chain and MWNT filaments. A quite simple technique was employed to show the stability of electrospinning solution before nanofiber formation process. Scanning electronic microscope (SEM) was employed to show the influence of spinning parameters on surface morphology of electrospun fiber. Under optimized condition, homogeneous and beadfree CHT/MWNTs nanofibers and known physical characteristics were prepared. The formation of conducting nanofibers based on CHT nanocomposites can be considered as a significant improvement in electrospinning of CHT/CNT dispersion. The direct outcome of the current study includes the homogeneous CHT/MWNTs nanofibers with an average diameter of 275 nm and a conductivity of 9×10⁻⁵ S/cm. These results are extremely important for further investigation regarding biomedical applications.     

Modeling of melt electrospinning for semi-crystalline polymers

A comprehensive model for the stable jet region in electrospinning of crystallizing polymer melts has been presented. First, the conventional flow-induced crystallization (FIC) model by Ziabicki was coupled with the non-isothermal melt electrospinning model. The modeled initial jet profiles were compared to digitized experimental images of the stable Nylon-6 melt jet near the spinneret. The final jet diameters were also compared to the average thickness of collected fibers. The results were in good agreement with the flow visualization experiments for various melt temperatures and flow rates. The modeled crystallinity predictions were also in agreement with experimental data from collected fiber mats. Then, a new FIC model that can provide microstructure information, such as crystallite number density and average size, has been proposed and validated under isothermal and non-isothermal conditions in the bulk as well as in the confined geometry of the polymer melt jet in electrospinning. Nylon-6,6 was used as the model polymer in this crystallization study, and the results are in good agreement with the widely-used Ziabicki FIC model.     

Theory and kinematic measurements of the mechanics of stable electrospun polymer jets

We present a simplified approach to understanding the mechanics of stable electrospinning jets based on electrohydrodynamic theory that explicitly incorporates the extensional rheology of polymeric fluids. Flow regimes of electrospun jets are identified by analogy to uniaxial extension of a fluid jet. These flow regimes predict the limiting kinematics of electrospinning jets and identify dimensionless parameters important to the control and operation of electrospinning processes. In situ kinematic measurements validate model assumptions and scaling predictions, and allow the reduction of entire jet radius and velocity profiles to several key parameters. The model predictions are shown to hold both above and below the entanglement concentration, as well as for solutions with added electrolyte and increased conductivity. The analysis also enables direct measurement of the apparent extensional viscosity of solutions at the high extension rates experienced during electrospinning. Finally, dimensional analysis of the model yields a correlation for electrospun fiber diameter in terms of measurable fluid properties, controlled process parameters, and measured jet variables, demonstrating the influence of mechanics in the straight portion of the jet on ultimate fiber morphology.     

In Situ Viscosity‐Controlled Electrospinning with a Low Threshold Voltage

In the present study, a novel electrospinning method is proposed,where jet formation is aided by shearing the solution in situ. With a generalpolymer solution, viscosity decreases by shearing, that is, the solution isshear‐thinning. Poly(ethylene‐oxide) is used as a model polymer andthe effects of rotation speed, solution concentration, and gap size (the widthof the annular orifice) on the process and the morphology of the obtainedfibers are investigated. It is found that the threshold voltage for generatingmultiple jets decreased from 35 to 12 kV when rotation speed is higher than60 rpm (or shear rate more than 310 s^?1). Additionally, the results show thatfiber diameter increases as the concentration of the solution increases. Thechi‐square two‐sample test is used to compare the distribution of fibersproduced by the capillary method and the novel electrospinning process. Inthe authors' method, the viscosity of the solution can be changed by applyingmechanical forces on it during the electrospinning process, which results inthe initiation of the electrospinning jet at a low threshold voltage. It is alsofound that gap size has a similar effect on fiber diameter as needle diameter in classical electrospinning.     

Control of structure and morphology of highly aligned PLLA ultrafine fibers via linear-jet electrospinning

In this work, poly(l-lactic acid) (PLLA) ultrafine fibers with different morphology and structure were fabricated by a novel linear-jet electrospinning method which relies on a conventional electrospinning set-up with continuous rotating drum. To control the morphology and structure of PLLA electrospun fibers, different solution systems and electrospinning conditions were investigated. Two PLLA solution systems (PLLA/DMF/CH₂Cl₂ and PLLA/CH₂Cl₂) with different concentration and conductivity were used for the electrospinning and their influences on the formation of the linear electrospinning jet were discussed. Two types of collecting patterns with aligned buckling and linear structure were achieved under the linear electrospinning jet. Highly aligned PLLA electrospun fibers with porous surface could be formed by using the highly volatile solvent CH₂Cl₂. Here, it should be emphasized that the diameter and surface porosity of such highly aligned PLLA electrospun fibers can be fine tuned by varying the winding velocity. The results of SEM images and polarized FTIR investigations verified that the as-spun PLLA porous surface fibers were highly aligned and molecularly oriented, leading to the enhanced mechanical performance as compared to the non-woven PLLA electrospun fibers.     

Polymeric nanofibers via flat spinneret electrospinning

Electrospun nanofibers are most often produced by needle electrospinning process, which has inherent disadvantages like clogging and low efficiency. In this study, an alternative needleless electrospinning process is reported for the fabrication of nanofibers based on a novel spinneret. Firstly, a spinneret with a 0.5‐mm diameter hole in the middle of a flat plastic cap was custom‐made that may be readily scaled up for mass production. Then, polyethylene oxide (PEO) aqueous solution with 6.0 wt% concentration was used to demonstrate the needleless electrospinning process. The processing window for the jet formation in the flat spinneret electrospinning process was determined. The relationships between various processing parameters (applied voltage, working distance, and flow rate) and the resultant PEO nanofibers were also investigated. It was found that stable fluid jet launched from the tip of the coned droplet anchored at the rim of the hole and formed fibers. The morphology and diameter of electrospun fibers were examined using scanning electron microscopy. The results show that PEO nanofibers produced by this needleless electrospinning have similar structure and morphology to those from the single needle source. Finally, the hole number of spinneret was increased to four holes, which was still able to produce smooth nanofibers with a higher production rate. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Electrospinning from a convex needle with multiple jet toward better controlling and enhanced production rate

Multiple jet was successfully generated from a convex needle spinneret based on a conventional electrospinning setup. A convex channel was created in the front part of the needle to generate a limited free surface in the electrospinning process. A high flow rate was implemented with more than one polymer jet being produced, resulting in the production rate 2–3 times higher than conventional electrospinning. Finer nanofibers were produced from the convex needle when the applied voltage was 19 kV. The electric field intensity distribution of this spinneret was analyzed and compared with conventional needle spinneret by Comsol Multiphysics modeling. The research work has demonstrated that scaling up the production rate of nanofibers from needle-based free surface electrospinning is possible. It will benefit further development of electrospinning with enhanced throughput and more precise controlling. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48014.     

A fundamental study of chitosan/PEO electrospinning

A highly deacetylated (97.5%) chitosan in 50% acetic acid was electrospun at moderate temperatures (25-70 °C) in the presence of a low content of polyethylene oxide (10 wt% PEO) to beadless nanofibers of 60-80 nm in diameter. A systematic quantitative analysis of the solution properties such as surface tension, conductivity, viscosity and acid concentration was conducted in order to shed light on the electrospinnability of this polysaccharide. Rheological properties of chitosan and PEO solutions were studied in order to explain how PEO improves the electrospinnability of chitosan. Positive charges on the chitosan molecule and its chain stiffness were considered as the main limiting factors for electrospinability of neat chitosan as compared to PEO, since surface tension and viscosity of the respective solutions were similar. Various blends of chitosan and PEO solutions with different component ratios were prepared (for 4 wt% total polymer content). A significant positive deviation from the additivity rule in the zero shear viscosity of chitosan/PEO blends was observed and believed to be a proof for strong hydrogen bonding between chitosan and PEO chains, making their blends electrospinnable. The impact of temperature and blend composition on the morphology and diameter of electrospun fibers was also investigated. Electrospinning at moderate temperatures (40-70 °C) helped to obtain beadless nanofibers with higher chitosan content. Additionally, it was found that higher chitosan content in the precursor blends led to thinner nanofibers. Increasing chitosan/PEO ratio from 50/50 to 90/10 led to a diameter reduction from 123 to 63 nm. Producing defect free nanofibrous mats from the electrospinning process and with high chitosan content is particularly promising for antibacterial film packaging and filtration applications.     

Behavior of melt electrospinning/blowing for polypropylene fiber fabrication

The fabrication process of polymer fibers has been analyzed in various ways, and several studies have been conducted to develop new processes and optimize existing ones. Several studies have been conducted on the electrospinning process, which can easily fabricate nanofibers, and the development of materials manufactured through electrospinning has also been investigated. However, research on the nanofiber fabrication and processing of thermoplastic polymers, such as polypropylene (PP), polyethylene and polyethylene terephthalate, is relatively lacking. Therefore, research on nanofiber fabrication is essential. In this study, PP fibers were successfully manufactured through a melt electrospinning/blowing process, which combined melt blowing and electrospinning. To analyze the melt electrospinning/blowing process, the dynamic behavior of the spinning process was observed using a charge-coupled device camera in real time, and the effects of the different spinning conditions were compared and analyzed. As the hot air or high voltage was increased, the spinning jet area tended to increase. In addition, the average diameter of the fabricated fibers tended to decrease as a high voltage was applied at a hot air pressure of 0.01 MPa; conversely, the average diameter tended to increase at a hot air pressure of 0.03 MPa. A similar trend was observed for the tensile stresses in the PP web fabrics. The polymer fibers produced by this melt electrospinning/blowing process can be applied as a production process for nanomembranes, filters and battery separators. © 2022 Society of Industrial Chemistry.     

The thermal effects on electrospinning of polylactic acid melts

We demonstrate that melt electrospinning can be a feasible way to produce sub-micron scale polylactic acid (PLA) fibers in this paper. This solvent-free approach to produce sub-micron scale fibers is more environmentally benign than common solution electrospinning processes, and has a potential to increase the production rate significantly. Our experimental results show that temperatures at the spinneret and in the spinning region are critical to produce sub-micron sized fibers: a high-speed photographic investigation reveals that when spinning temperature is below glass transition temperature, whipping of the jet is suppressed by fast solidification in the spinning region, leading to a larger jet diameter. Both thermal and mechanical degradations of PLA in melt electrospinning can be significant but no change in chemical composition is found. Due to rapid solidification, melt electrospun PLA fibers are mostly amorphous, and the small presence of β crystals is noted in the sub-micron scale PLA fibers by XRD studies. The highly oriented structure of PLA fibers gives rise to cold crystallization at around 95 °C, and the degree of crystallinity of fibers increases with increasing the degree of annealing. Finally, PLA nanofibers have directly been electrospun onto cellulose filter media, and a drastic enhancement in collection efficiency of sub-micron sized dust particles is presented. Melt electrospun PLA nanofiber mats with no residual solvent may serve as better filter media and tissue scaffolds than those obtained from solution electrospinning processes.     

Blow-assisted multi-jet electrospinning of poly-L-lactic acid nanofibers

Regardless the low production rate, electrospinning remains the attractive technique for the nanofibers production in various fields. Thus, the development of a multi-jet technologies for electrospinning gives an opportunity to scale up and increase throughput of the fibers production. However, the multi-jet electrospinning technologies exhibit one major drawback– electrostatic mutual jet repulsion issue. In present research, we propose air blow-assisted multi-jet electrospinning system allowing production of nanofibers with yield, at least, tenfold higher than single jet electrospinning. The system produces nanofibers in two modes: multi-jet electrospinning and blow-assisted multi-jet electrospinning. In case of the latter, the application of sheath air stream allows the system to overcome the electrostatic mutual repulsion issue. These lead to the reduction of deviation of the polymer solution jets, the reduction of instabilities of the jets and the improvement of the control of the nanofibers deposition. Nanofibers morphology and size were investigated based on the scanning electron microscope micrographs. The comparison of the two modes shows changes in nanofibers morphology from beaded structure to fine nanofibers, and the slight increase in fiber mean size when the blowing assistance was applied to the process.     

Molecular orientation in individual electrospun nanofibers measured via polarized Raman spectroscopy

Using polarized Raman spectroscopy, we have recorded Raman spectra from individual electrospun Nylon-6 nanofibers. Analysis of these single-fiber spectra, compared to those of unoriented and oriented Nylon-6 films, indicates significant molecular orientation. Because electrospinning produces fibers in a jet with a large strain rate, this molecular orientation is expected. We present quantitative measurements of molecular orientation in a single nanofiber and compare these to those of film samples. Such measurements could yield information about the uniformity of the electrospinning process and resulting fibers, and may also allow comparison between spectrally measured orientation functions and single-fiber mechanical properties.     

Effect of evaporation and solidification of the charged jet in electrospinning of poly(ethylene oxide) aqueous solution

The electrospinning process uses electrical force to produce nanofibers. A charged droplet acquires a conical shape known as the Taylor cone and then becomes unstable. A charged jet emerges from the vertex and develops a spiral path due to the electrically driven bending instability, which makes it possible, in a small space, for the jet to elongate by a large amount and produce nanofibers. Evaporation and the associated solidification were identified as important factors that affect the diameter of electrospun nanofibers. In this study, the evaporation rate and solidification of the charged jet were controlled by varying the relative humidity during electrospinning of poly(ethylene oxide) from aqueous solution. As the relative humidity increased, the solidification process became slower, allowing elongation of the charged jet to continue longer and thereby to form thinner fibers. As the relative humidity increased from 5.1% to 48.7%, the diameter of the solidified fiber decreased from 253 nm to 144 nm. As the relative humidity increased above 50%, beads formed on the thinner fibers, indicating that the capillary instability occurred before the jet solidified. The vapor concentration of solvent is an effective electrospinning process control parameter of fiber diameter that also produces a systematic change in the development of beads on the fibers.     

Chitin and chitosan: Transformations due to the electrospinning process

Electrospinning is a complex process that requires numerous interacting physical instabilities. Assuming that a chosen polymer and solvent system can be spun, the chosen polymer exists in various states, which have variable crystallinities starting with the highest degree of crystallinity (when in bulk form) and ultimately being transformed into a non‐woven mat. In an effort to better understand the effects that the electrospinning process has on the biopolymers chitin [practical grade (PG)] and chitosan [PG and medium molecular weight (MMW)], including post‐production neutralization and cross‐linking steps, field emission scanning electron microscopy (FESEM) and solubility studies were performed. An evaluation of diffraction peaks of the bulk, solution, and fibrous forms of chitin and chitosan were evaluated by X‐ray diffraction (XRD) and determined that the formation of chitosan chains is influenced by the addition of solvent and cross‐linking agent. This study is of importance since the crystallinity of chitin and chitosan directly relate to the ability of the biopolymers to chelate metals, and the chemical stability of non‐woven mats aid in the creation of functional filtration membranes. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Mass Production of Functional Amine–Conjugated PAN Nanofiber Mat via Syringeless Electrospinning and CVD

Polymeric nanofiber webs have attained much attention because they can provide high surface area with various functional groups. To obtain the polymeric nanofiber webs, electrospinning is the most attractive method because this can provide the versatility of material selection. However, it is relatively difficult to obtain the nanofiber webs, which have highly reactive functional groups and high mechanical strength with high production rate. Here, the helically probed rotating cylinder (HPRC) system based on syringeless electrospinning and chemical vapor deposition (CVD) is introduced to prepare the polyacrylonitrile (PAN) nanofiber webs, having high functional groups and high mechanical strength in fast production rate. The HPRC system can provide the PAN nanofiber webs in high production rate, and the CVD process can provide high reactive functional groups on the PAN nanofiber. In addition, the nanofiber webs can be applied to diverse potential application fields, which require a high number of functional moieties.