**7. Electro-spinning of polymers**

Electro-spinning or electrostatic deposition is one of the easiest and most inexpensive methods of generating nanoscopic polymer / composite fibers of macroscopic lengths. In other words, the ease of processing, the low learning curve and inexpensive instrumentation render this technique extremely popular and versatile. One electro-spin viscous liquids, so having the right resin / solvent ratio, or in the case of composites, the right filler / binder / solvent proportions (and perhaps access to a good rheometer in order to characterize the polymer solution) is of some importance.

precursor fluids or fluctuations in the deposition parameters. The mats permeability is a function of the pores dimensions distribution. These dimensions are statistically distributed and they tend to correlate inversely with the mat thickness or electro-spinning deposition time. The dimensions of most pores for reasonable deposition times (a few hours or less) lie in the

Materials and Processes for Ion Permeable Separating Membranes by Electro-Spinning

http://dx.doi.org/10.5772/57094

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In our studies, solutions of PLLA and PLLA- multi walled carbon nano-tubes (MWCNTS) composites were used. It is know that the addition of carbon nano-tubes to a polymeric solution can improved the mechanical properties of the resulting fibers and/or mat [16]. The PLLA was dissolved in 1:3 acetone/chloroform to prepare solutions of 13, 15 and 17 % w/v concentrations. The viscosity of the solutions were measured in a Bolin Rheometer with the following results, namely (1.1 +/\_ 0.1, 2.8 +/\_ 0.4 and 3.2 +/\_0.6) Pa.s, respectively. As expected, viscosity increased as the content of polymer in solution is increased. These initial solutions were spun using the standard electro-spinning set-up, where the resulting fibers were collected in silicon wafers for subsequent structural analysis. The applied electric field during deposition was 2.3 kV/ cm, with a collection time of 10 s and a pump rate of 0.5 mL/hr. Usually for large anode-cathode distance, in excess of let's say 5 cm and a homogeneous solution, fibers diameters are randomly

One problem when working with very small fibers, with diameters comparable to the wavelength of the visible light, is you inability to see or clearly image structural details such as diameters, asperities, and general morphological features. It is in such cases that scanning electron microscopy (SEM) can be a formidable characterization tool. Even when one ignores the differences in magnification between optical and SEM instruments (500,000 x, that is 250 times better magnification than a good optical microscope), the depth of field is what makes SEM such a versatile instrument. In optical microscopy, you can see the top of a micrometric diameter fiber, but not its sides or bottom, unless you move the focus. In SEM one sees around the complete fiber in focus, that is the top, sides and bottom. Of course, SEM requires a vacuum for the electron optics, and liquid samples, or samples involving liquids present serious imaging challenges. In our experimental work, SEM has been the instrument of choice for

Analysis of SEM data for the fiber diameter reveals that as the solution viscosity increased, the diameter of the obtained fibers was smaller. One can see the effect by examining the distribu‐ tion with a higher mean value of the fiber diameter (4 µm) for the 13 % w/v solution, and compare it to those with smaller mean diameters, i.e. (1 µm), for the 17 % w/v solution. Figure 5, show the SEM images of deposited fibers from different solutions and a histogram with the

These experimental results on fibers diameter distribution suggested continuing further experimentation using the 15 % w/v solution. This particular solution produced fiber with an average diameter of (2.2±0.5) µm. It also, have no beading and more uniform morphology than

range between micro and nano-meters, as evidenced by SEM micrographs.

**8. Composites and characterization**

distributed usually following a log-normal distribution.

imaging and measure.

diameters frequency.

**Figure 4.** Details of a simple "home brew" electro-spinning apparatus.

The "home brew" configuration utilized in electro-spinning usually consists of a small (low power), high voltage power supply capable of voltage outputs in the range to several tens of kilovolts at very low current levels (below µA is desirable, see figure 4). A syringe pump, (to controlled the rate of fluid outcome from the syringe), syringe (containing the polymer solution), and an electrically grounded collector, complete the "home brew" set-up as shown above.

The morphology of the out coming fibers or mats will depend of several experimental conditions including: viscosity of the solution, applied electric field (applied voltage/needle to collector distance), the pump rate, temperature and pressure, solvent vapor pressure (evapo‐ ration rate), and collection time. Another method employed to control the fibers morphology (alignment) during electro-spinning is using a rotating collector or an oscillating needle [15]. These make electro-spinning a good tool to produce mats with a better-controlled morphology. A common morphological feature in electro-statically deposited fiber and mats is the formation of beads or beading. This particular feature involve the formation of random beads, preferen‐ tially where fibers cross each other, often due to the visco-elastic nature of the electro-spinned precursor fluids or fluctuations in the deposition parameters. The mats permeability is a function of the pores dimensions distribution. These dimensions are statistically distributed and they tend to correlate inversely with the mat thickness or electro-spinning deposition time. The dimensions of most pores for reasonable deposition times (a few hours or less) lie in the range between micro and nano-meters, as evidenced by SEM micrographs.
