The change of bead morphology formed on electrospun polystyrene fibers

Polystyrene (PS) dissolved in the mixture of tetrahydrofuran (THF) and N,N-dimethyl formamide (DMF) was electrospun to prepare fibers of sub-micron in diameters. Electropinning parameters such as polymer concentration, applied voltage and tip-to-collector distance were controlled. From these parameters it was determined that while the surface tension of polymer solution had linear correlation with the critical voltage, throughput was dependent on electric conductivity. The electrospun PS fibers produced contained irregular beads and electrospinning certainly was enhanced with increasing DMF content. The bead concentration was also controlled by DMF content. The aspect ratio of the formed beads and the diameter of fibers were increased with increasing solution concentration. When PS was dissolved in only THF, an unexpected half hollow spheres (HHS) structure appeared. Also, different shape forms of PS non-woven mats have been prepared by controlling electrospinning parameters.     

Investigation of pore formation for polystyrene electrospun fiber: Effect of relative humidity

Electrospinning method uses electrical force to produce a polymer nanofiber from a polymer solution. The surface morphology and the pore formation of e-spun fiber have been studied by many variables that are involved in different polymer concentrations and solvent mixing ratios. Another major factor affecting fiber morphology and size distribution is the relative humidity. The interaction between the relative humidity and the solvent evaporation affects the distribution of electric charge on the surface of the e-spun fiber. The higher the electric density, the thinner the fiber that can be produced in low humidity conditions. The relative humidity and solvent evaporation can create pores on the fiber surface. The pores can be formed under the condition of 30% relative humidity using 100% of THF solvent. The boundary of the pores has expanded and becomes formless due to the agglomeration of each pore, which can decrease the evaporating capacity.     

Latent orientation in the skin layer of electrospun isotactic polystyrene ultrafine fibers

Isotactic polystyrene was electrospun into ultrafine fibers from the chloroform or tetrahydrofuran solutions. Tubular or ribbon-like fibers with many small pores were obtained from the chloroform solution. Densely stacked lamellae were formed by annealing them. On the other hand, fibers with many small dents on the skin layer were obtained from the tetrahydrofuran solution. The skin layer changed into the densely stacked lamellae by annealing, while the interior was not crystallized under the utilized annealing condition. These results suggested that the skin layer of the as-spun fiber should contain a finite amount of highly-oriented molecular chains, while the interior may be unoriented. Despite the annealing, the orientation of the chains in the skin layer has been preserved and worked as the nuclei for the densely stacked lamellae.     

Electrospinning of uniform polystyrene fibers: The effect of solvent conductivity

By means of the electrospinning technique, micron- and nanofibers can be obtained from polymer solutions under a very high electrical field. A special challenge is to produce bead-free uniform fibers since any minor changes in the electrospinning parameters such as slight variations in the polymer solutions and/or electrospinning experimental parameters may result in significant variations in the final nanofiber morphology. Furthermore, it is often not trivial at all to obtain reproducible uniform electrospun nanofibers for the optimized electrospinning conditions. Here we report that the conductivity of the solvent is the key factor for the reproducible electrospinning of uniform polystyrene (PS) fibers from dimethylformamide (DMF) solutions. It is shown that even slight changes in the conductivity of the DMF solutions can greatly affect the morphology of the resulting electrospun PS fibers. Here, we have carried out a thorough and systematic study on the effect of solution conductivity on the electrospinning of bead-free polystyrene (PS) fibers when dimethylformamide (DMF) was used as the solvent. Interestingly, we found out that different grades of solvent as-received (DMF) from various suppliers have slightly different solution conductivities. Consequently, the polymer solutions prepared with the same PS concentration have different conductivities, which are shown to have significant changes on the morphology of the PS fibers resulting in beaded or bead-free uniform fibers when electrospun under the identical electrospinning conditions. Such as, bead-free PS fibers were obtained from PS solutions in the range of 20% (w/v) through 30% (w/v) depending on the DMF grade used. In brief, it was observed that solutions with a higher conductivity yielded bead-free fibers from lower polymer concentrations, which confirms that the solution conductivity plays a very significant role in producing bead-free uniform PS fibers.     

Nanoparticle filtration by electrospun polymer fibers

Polyacrylonitrile (PAN) fibers with mean diameters in 270-400 nm range were prepared by electrospinning for use as a filter media. Compared to commercial filters made of polyolefin and glass, the fibers of electrospun filters were more uniform in diameter. The performance of electrospun filters was evaluated by measuring the penetration of monodisperse NaCl nanoparticles (below 80 nm in size) through the filters. It was found that electrospun filters could be made which had nanoparticle penetration values comparable to commercial filters but with substantially less filter mass. The penetration of nanoparticles through the electrospun filter media could be reduced by increasing the filter thickness, which is controlled by the collection time during the electrospinning process. Nanoparticle collection by electrostatic forces was found to be negligible for electrospun filters. Filter quality factors and single fiber collection efficiencies were found to be independent of filter thickness for electrospun filters, and the penetration of nanoparticles through electrospun filters was in better agreement with theoretical predictions than was the measured penetration through a commercial filter. This study shows that electrospinning is a promising technology for the production of high performance nanoparticle filters.     

Studies on the controlled morphology and wettability of polystyrene surfaces by electrospinning or electrospraying

Polystyrene (PS) surfaces with various morphologies have been produced by electrospinning or electrospraying, such as beads with different sizes and shapes, bead-on-string structures with different aspect ratios of the beads and fibers with different diameters and shapes. Both the solution properties and the electrospinning conditions affected the PS surface morphology obtained. The results of water contact angle (CA) measurement indicated that the surface morphology could affect the wettability distinctively. It was found that CA values of PS surfaces comprised merely fibers were in the range of 140°-150°. The CA values of PS surfaces comprised bead-on-string structures were usually about 150°. However, the CA values of PS surfaces consisted of particles could reach up to 160°, which shows a superhydrophobic property. A bilayer fibers-on-beads surface was verified to be stable and superhydrophobic.     

Spontaneous core-sheath formation in electrospun nanofibers

In recent research, electrospun nanofibers (NFs) from polymer solutions containing metal salts were used to produce high-temperature superconducting ceramic (HTSC) NFs through pyrolysis. In this research, the production of phase separated nanostructures inside NFs spun from polymer solutions containing metal salts was investigated. The metal salts were expected to be preferentially driven into one of the phases (the amorphous phase in a semi-crystalline polymer and the more hydrophilic phase in a triblock copolymer) and yield nanostructured ceramics upon pyrolysis. Surprisingly, the electrospun NFs exhibited the spontaneous formation of a core-sheath structure. The metal-atom-rich core exhibited a 10 nm scale structure while no such structure was observed in the metal-atom-poor sheath. Both the repulsion of metal atoms by the positive surface charge and the exclusion of the metal atoms from the crystallizing front that moves inward from the surface were shown to contribute to the spontaneous formation of the core-sheath structure.     

Ion-assisted collection of Nylon-4,6 electrospun nanofibers

In electrospinning, electrostatic interaction between charged fibers and the collection substrate can result in poor and non-uniform coverage, particularly when electrically insulating substrates are used, because they are prone to surface charge accumulation. Charged electrospun Nylon-4,6 nanofiber coatings were deposited onto substrates of varying size, conductivity and morphology. The density and uniformity of the nanofiber coatings were significantly enhanced, both on insulating and on conducting substrates, by a new method based on rapid sequential deposition of charged nanofibers and oppositely charged ions onto substrates that were mounted onto a rotating collecting electrode (mandrel) located between an electrospinning source and a focused ion source. Sequential fiber/ion deposition presumably led to surface charge neutralization or reversed charging, and minimization of electrostatic fiber/substrate interactions. An electrostatics model was developed to interpret the experimental results. It was also theoretically argued that any degree of ion charging will induce continuous fiber accumulation.     

Tailoring the grooved texture of electrospun polystyrene nanofibers by controlling the solvent system and relative humidity

In this study, we have successfully fabricated electrospun polystyrene (PS) nanofibers having a diameter of 326 ± 50 nm with a parallel grooved texture using a mixed solvent of tetrahydrofuran (THF) and N,N-dimethylformamide (DMF). We discovered that solvent system, solution concentration, and relative humidity were the three key factors to the formation of grooved texture and the diameter of nanofibers. We demonstrated that grooved nanofibers with desired properties (e.g., different numbers of grooves, widths between two adjacent grooves, and depths of grooves) could be electrospun under certain conditions. When THF/DMF ratio was higher than 2:1, the formation mechanism of single grooved texture should be attributed to the formation of voids on the jet surface at the early stage of electrospinning and subsequent elongation and solidification of the voids into a line surface structure. When THF/DMF ratio was 1:1, the formation mechanism of grooved texture should be ascribed to the formation of wrinkled surface on the jet surface at the early stage of electrospinning and subsequent elongation into a grooved texture. Such findings can serve as guidelines for the preparation of grooved nanofibers with desired secondary morphology.     

Electrospinning of polyurethane fibers

A segmented polyurethaneurea based on poly(tetramethylene oxide)glycol, a cycloaliphatic diisocyanate and an unsymmetrical diamine were prepared. Urea content of the copolymer was 35 wt%. Electrospinning behavior of this elastomeric polyurethaneurea copolymer in solution was studied. The effects of electrical field, temperature, conductivity and viscosity of the solution on the electrospinning process and morphology and property of the fibers obtained were investigated. Results of observations made by optical microscope, atomic force microscope and scanning electron microscope were interpreted and compared with literature data available on the electrospinning behavior of other polymeric systems.     

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.     

Relationship between surface concentration of amphiphilic quaternary ammonium biocides in electrospun polymer fibers and biocidal activity

Electrospinning was utilized to generate antimicrobial Nylon and polycarbonate fibers for potential applications including self-decontaminating fabrics, wound dressings, and filtration media. The effects of quaternary ammonium salt concentration on fiber morphology, diameter, and antimicrobial activity of the resulting fiber mats were investigated. Fibers were characterized utilizing scanning electron microscopy and X-ray photoelectron spectroscopy, while antimicrobial activity was evaluated against Staphylococcus aureus. The co-electrospinning of soluble quaternary ammonium biocides within polymeric solutions generated uniform fibers with diameters ranging from 91 to 278 nm for Nylon and 0.55–2.34 μm for polycarbonate. Fiber morphology and diameter of the resulting fibers were shown to be dependent on polymer type and biocide concentration. A positive correlation between surface concentration of quaternary ammonium salts and antimicrobial activity was observed as fibers loaded with biocides exhibited up to a 7 log reduction of viable bacteria.     

Processing and characterization of electrospun poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) nanofibrous membranes

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) nanofibrous membranes were first fabricated via electrospinning from chloroform (CHCl₃) or CHCl₃/dimethylformamide (DMF) polymer solutions. The electrospinning conditions such as the polymer concentration, the solvent composition, and the applied voltage were optimized in order to get smooth and nano-sized fibers. The crystalline structure, the melting behaviors and the mechanical properties of the obtained nanofibrous membranes were characterized. With pure CHCl₃ as the solvent in the electrospinning process, the finest smooth PHBHHx fibers were about 1 μm in diameter. When DMF is added to CHCl₃ as a co-solvent, the conductivity and volatility of the solution increased and reduced, respectively, and the electrospinnability of the polymer solution increased as a result. The averaged diameters of PHBHHx fibers could be reduced down to 300-500 nm when the polymer concentration was kept at 3 wt%, the ratio of DMF/CHCl₃ was maintained at 20/80 (wt%), and the applied voltage was fixed at 15 kV during electrospinning. WAXD and DSC results indicated that the crystallization of the PHBHHx nanofibers was restricted to specific crystalline planes due to the molecular orientation along the axial direction of the fibers. The crystallization behaviors of the electrospun nanofibers were significantly different from that of the cast membranes because of the rapid solidification and the one-dimensional fiber size effect in the electrospinning process. Mechanically, the electrospun PHBHHx nanofibrous membranes were soft but tough, and their elongation at break averaged 240-300% and could be up to 450% in some cases. This study demonstrated how the size of electrospun PHBHHx fibers could be reduced by adding DMF in the solvent and gave a clue of the presence of oriented molecular chain packing in the crystalline phase of the electrospun PHBHHx fibers.     

Template-assisted assembly of electrospun fibers

Ordinarily, the electrospinning process generates one-dimensional fibers which assemble into non-woven membrane structures due to instabilities in the fluid jet. In this paper, an electrospinning procedure is developed that utilizes patterned collectors to produce aligned membranes with designed topological structures. The template-assisted electrospinning approach is demonstrated using polycaprolactone (PCL) fibers to produce patterns including alphanumeric characters and a printed electronic circuit chip, with feature sizes on the order of several hundred microns. The process has a significant impact on micro-manufacturing, and provides the capability for incorporation of oriented fiber materials in patterned micro-composites and electronic components.     

Temperature dependence of polymorphism in electrospun nanofibres of PA6 and PA6/clay nanocomposite

Polymorphism found in nanofibres of polyamide 6 (PA6) and PA6/clay nanocomposite (PA6-NC), prepared by an electrospinning process, was studied by transmission electron microscopy (TEM) and variable-temperature wide angle X-ray scattering (WAXS), and compared with the polymorphic changes occurring in the pre-electrospun bulk materials. TEM results, concerning morphology and dispersion of the nanoclays, reveal that the produced electrospun nanofibres have an average diameter of 50 nm, and nanoclays are much more uniformly dispersed in the electrospun PA6-NC fibres than in the pristine PA6-NC. According to WAXS measurements, both types of electrospun nanofibres predominantly consist of γ-form crystals of PA6. Upon heating, from room temperature to the melting point, a number of successive transitions are observed for both systems, namely, crystalline γ to α′, α′ to α and α to the “amorphous” δ-form due to breakage of hydrogen bonds. On subsequent cooling, it has been observed, for the first time, that the development of crystalline forms for both systems is quite different from each other. The molten electrospun pure PA6 fibres first crystallize in the high temperature α′-form, and then they show the room temperature α-form. For these nanofibres, during a temperature cycle of heating and cooling, the initial γ-form crystals completely turn into the α-form crystals as in bulk PA6. In contrast, for the electrospun nanofibres of the PA6-NC, the γ-form crystals are preserved after completing a thermal cycle down to room temperature. The present findings on the evolution of polymorphism in the electrospun nanofibres of both systems provide useful information regarding their use as reinforcing elements in polymer composites.     

Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns

The fiber spinning technique of electrospinning was optimized in order to prepare unidirectional aligned, structurally oriented, and mechanically useful carbon precursor fibers with diameters in the nanoscale range. The fiber spinning velocity and fiber draw ratio was measured to be between 140 and 160 m/s and 1:300,000, respectively, for fibers spun from 10 wt% polyacrylonitrile (PAN) solutions with dimethylformamide (DMF). A high-speed, rotating target was used to collect unidirectional tows of PAN fibers. Aligned and (+) birefringent fibers with diameters between 0.27 and 0.29 μm (FESEM) were collected from electrospinning 15 wt% PAN in DMF solutions at 16 kV onto a target rotating with a surface velocity between 3.5 and 12.3 m/s. Dichroism measurements (Polarized FTIR) of the nitrile-stretching vibration show an increase in the molecular orientation with take-up speed. Wide angle X-ray diffraction patterns (WAXD) show equatorial arcs from the reflection at and (1120) reflection at A maximum chain orientation parameter of 0.23 was determined for fibers collected between 8.1 and 9.8 m/s. Twisted yarns of highly aligned PAN nanofibers with twist angles between 1.1 and 16.8° were prepared. The ultimate strength and modulus of the twisted yarns increase with increasing angle of twist to a maximum of 162±8.5 MPa and 5.9±0.3 GPa, respectively, at an angle of 9.3°.     

Continuous yarns from electrospun fibers

A technique for making continuous uniaxial fiber bundle yarns from electrospun fibers is described. The technique consists of spinning onto a water reservoir collector and drawing the resulting non-woven web of fibers across the water before collecting the resulting yarn. Yarns from electrospun fibers of poly(vinyl acetate), poly(vinylidene difluoride) and polyacrylonitrile are used to illustrate the process of yarn formation and fiber alignment within the yarn. A theoretical production rate of 180 m of yarn per hour for a single needle electrospinning setup makes the process suitable for lab-scale production of electrospun yarns.     

Continuous aligned polymer fibers produced by a modified electrospinning method

A novel and simple technique of manufacturing uniaxially aligned electrospun fibers with diameter of sub-micrometers is described. Compared with typical electrospinning setup, two oppositely placed metallic needles are used, and they are connected to positive and negative voltages, respectively. Fibers coming out of the two needles combine in a yarn, which is wound by a cylinder collector rotating at a high speed. Fibers manufactured by this method are continuous, well-aligned, and can be deposited over a large area. Poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) are used to manufacture aligned fibers. An analysis of the possible mechanism of the fibers alignment is given. The influences of the concentration of the solution and the take-up velocity on the alignment of fibers were investigated.     

Evaluation of electrospun nanofiber pore structure parameters

Nanofibers produced by electrospinning method are widely used for drug delivery, as tissue scaffolding materials and filtration purposes where specific pore characteristics are required. For continued growth in these areas, it is critical that the nanofibers be properly designed for these applications to prevent failure. Most of the current methods only provide an indirect way of determining pore structure parameters and contain inherent disadvantages. In this study, we developed a novel image analysis method for measuring pore characteristics of electrospun nanofiber webs. Five electrospun webs with different pore characteristics were analyzed by this method. The method is direct, very fast, and presents valuable and comprehensive information regarding pore structure parameters of the webs. Two sets of simulated images were generated to study the effects of web density, fiber diameter and its variations on pore characteristics. The results indicated that web density and fiber diameter significantly influence the pore characteristics, whereas the effect of fiber diameter variations was insignificant.     

The role of elasticity in the formation of electrospun fibers

The role of fluid elasticity in the formation of fibers from polymer solution by electrospinning is investigated. Model solutions with different degrees of elasticity were prepared by blending small amounts of high molecular weight polyethylene oxide (PEO) with concentrated aqueous solutions of low molecular weight polyethylene glycol (PEG). The elastic properties of these solutions, such as extensional viscosity and the longest relaxation time, were measured using the capillary breakup extensional rheometer (CaBER). The formation of beads-on-string and uniform fiber morphologies during electrospinning was observed for a series of solutions having the same polymer concentration, surface tension, zero shear viscosity, and conductivity but different degrees of elasticity. A high degree of elasticity is observed to arrest the breakup of the jet into droplets by the Rayleigh instability and in some cases to suppress the instability altogether. We examine the susceptibility of the jet to the Rayleigh instability in two ways. First, a Deborah number, defined as the ratio of the fluid relaxation time to the instability growth time, is shown to correlate with the arrest of droplet breakup, giving rise to electrospinning rather than electrospraying. Second, a critical value of elastic stress in the jet, expressed as a function of jet radius and capillary number, is shown to indicate complete suppression of the Rayleigh instability and the transition from ‘beads-on-string’ to uniform fiber morphology.