Controlling fiber orientation

As stated in the previous section, achieving control over the orientation of electrospun fibers is an important step towards many of their potential applications. However, if one considers the fact that fiber formation occurs at very high rates (several hundreds of meters of fiber per second) and that the fiber formation process coincides with a very complicated three-dimensional whipping of the polymer jet (caused by electrostatic bending instability), it becomes clear that controlling the orientation of fibers formed by electrospinning is no simple task. 

Various mechanical and electrostatic approaches have been taken in efforts to control fiber alignment. The two most successful methods are the following  

7 Converging electrostatic field on sharp-edged wheel electrode. Reprinted from reference 1. Copyright (2003), with permission from Elsevier. 

Both spinning onto a rapidly rotating collector and the gap alignment effect have been used to obtain short yams for experimental purposes. 

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As a general rule, however, textile fibers do not wet out readily, are difficult to disperse, and tend to tangle with one another. Consequendy, large amounts of water are necessary to keep the fibers suspended. Further, if the slurry is not handled propedy, the fibers tangle and cause poor sheet formation. Two approaches to resolving these difficulties are increasing slurry—dilution ratio and controlling fiber orientation. 

In an experiment, ferrite powders were dispersed in linear polyacrylamide, the gel was crosslinked, and the swollen gel compressed. This process resulted in particle orientation. Industrial processes, such as extrusion, injection, compression, and blow molding, fiber spinning, and thermoforming induce orientation due to flow. Typical parameters which control fiber orientation in these processes include ... 

For short fiber systems, the fiber orientations obtained are less controllable, but techniques are being developed to control fiber orientation during injection molding to some degree. 

See Chapter 8 for details on the coining process. With reinforced plastic this process (also called injection-compression molding) provides a means of controlling fiber orientation, and so on. 

Quench spacer The quiet zone below the spinneret in which there is no quench airflow. Quench spacer distance is important in controlling fiber orientation and birefringence. 

Murugan, R., Ramakrishna, S., 2007. Design strategies of tissue engineering scaffolds with controlled fiber orientation. Tissue Engineering 13 (8), 1845-1866. 

Murugan, R. and S. Ramakrishna, Design Strategies of Tissue Engineering Scaffolds with Controlled Fiber Orientation. Tissue Engineering, 2007,13(8), 1845-1866. 

We control fiber properties by changing the relative speeds of different stages of the process. Orientation is increased and fiber thickness decreased by increasing the final take-up speed relative to the rate at which the molten polymer strands leave the spinneret. To produce high modulus fibers we generally adopt conditions that maximize orientation. Fiber diameters... 

One key requirement in the commercial production of fibers is to control fiber diameters within narrow ranges of the target. Another is to control the internal structure of the fiber, particularly the orientation of the polymer molecules. It is this orientation along the fiber axis that controls the morphology, and hence the fiber properties, such as dye uptake, shrinkage and tensile strength. 

A winding machine is typically equipped with computer control, and its programming is done offline. Winding pattern programs are often provided that interface with CAD programs and finite element analysis code. Such tools greatly enhance the process of concurrent engineering. Moreover, computer control allows the precise placement of the fiber on the part such that gap/overlap is minimized and the fiber orientation is controlled precisely. The... 

Each process provides capabilities such as meeting production quantity (small to large quantities and/or shapes), performance requirements, proper ratio of reinforcement to matrix, fiber orientation, reliability/ quality control, surface finish, materials used, quantity, tolerance, time schedule, and so forth versus cost (equipment, labor, utilities, etc.). There are products when only one process can be used but there can be applications where different processes can be used. 

As mentioned earlier, suspensions of particulate rods or fibers are almost always non-Brownian. Such fiber suspensions are important precursors to composite materials that use fiber inclusions as mechanical reinforcement agents or as modifiers of thermal, electrical, or dielectrical properties. A common example is that of glass-fiber-reinforced composites, in which the matrix is a thermoplastic or a thermosetting polymer (Darlington et al. 1977). Fiber suspensions are also important in the pulp and paper industry. These materials are often molded, cast, or coated in the liquid suspension state, and the flow properties of the suspension are therefore relevant to the final composite properties. Especially important is the distribution of fiber orientations, which controls transport properties in the composite. There have been many experimental and theoretical studies of the flow properties of fibrous suspensions, which have been reviewed by Ganani and Powell (1985) and by Zimsak et al. (1994). 

The sound velocity in a fiber, and the sonic modulus calculated therefrom, are related to molecular orientation (De Vries ). As shown by Moseley ), the sonic modulus is independent of the crystallinity at temperatures well below the T (which means that the inter- and intramolecular force constants controlling fiber stiffness are not measurably different for crystalline and amorphous regions at these temperatures). An orientation parameter a, calculated from the sonic modulus, is therefore taken as a measure for the average orientation of all molecules in the sample, regardless of the degree of crystallinity. The parameter is called the total orientation , as contrasted to crystalline and amorphous orientation, from X-ray data. 

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