Electrospinning bead formation

Figure 4.4 Example of bead formation during electrospinning SEM micrographs of poly (propyl carbonate) (PPC) beads prepared by electrospinning a PPC solution in dichloro-methane. (Reprinted with permission from Biomaterials, Electrospun aliphatic polycarbonates as tailores tissue scaffold materials by A. Welle, M. Kroger et al., 28, 2211-2219. Copyright...

Several approaches are available to control bead formation in electrospinning of polymer nanofibers ... 

The bead formation during electrospinning of polymer solutions studied. 

PEO is frequently used to faeilitate eleetrospinning of diffieult to proeess polymers, since it has good electro-spinnability, bioeompatibility and solubility in aqueous solutions. Duan et al. [168] reported that ultrafine fibers eould not be eleetrospim from chitosan solution in aqueous acetic acid, but formed when a eertain amoimt of PEO was added. Phase separation might have occurred which resulted in chitosan and PEO rich fibers of different diameters. Lou et al. [169] foimd that the chitosan/PEO ratio is an important factor in fiber formation, and a ratio of 60 40 resulted in the optimum fiber mat morphology with fiber diameters below 100 nm. They also confirmed the non-toxicity and the enhanced cell proliferation of the ehitosan/PEO fiber mats. By heating the polymer solution with hot air before it was ejected, Desai et al. [166] was able to electrospin blend solution with PEO concentration as low as 5% without bead formation. 

The surface tension seems more likely to be a function of solvent compositions, but is negligibly dependent on the solution concentration. Different solvents may contribute different surface tensions. However, not necessarily a lower surface tension of a solvent will always be more suitable for electrospinning. Generally, surface tension determines the upper and lower boundaries of electrospinning window if all other variables are held constant. The formation of droplets, bead and fibers can be driven by the surface tension of solution and lower surface tension of the spinning solution helps electrospinning to occur at lower electric field [57],... 

Despite the fact that the electrospinning technique is relatively easy to use, there are a number of process parameters that can greatly affect fiber formation and structure. Listed in order of relative impact to the electrospinning process, the most important parameters are applied voltage, polymer flow rate, and capillary-collector distance. All three parameters can influence the formation of nanofibers with bead-like defects. 

In the electrospinning technique, the ejected charged jet was affected by electrical forces, so it is needed to have high electrical properties, i.e., a good dielectric constant, to enhance the density of charges at the surface of the jet for better stretching and uniform formation of fibers with bead-free morphology, ... 

The morphological structure can be slightly changed by changing the solution flow rate, as shown in Figure 4.11. At the flow rate of 0.3 mL/h, a few big beads were observed on the fibres. The flow rate could affect electrospinning. When the flow rate exceeded a critical value, the delivery rate of the solution jet to the capillary tip exceeded the rate at which the solution was removed from the tip by the electric forces. This shift in the mass balance resulted in a sustained but unstable jet and the formation of fibres with big beads. 

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