Over the past 15 years, electrostatic fiber formation, or “electrospinning”, has evolving into the method of choice for forming submicron diameter fibers of well-controlled size and morphology. For many applications it is desirable to impart a degree of structure to the fiber, either through manipulation of its external surface or through the introduction of internal heterogeneities. Manipulation of the external surface is possible through control of the fiber forming process itself, or by post-processing of the fibers to impart an additional, surface-specific structure. For example, by controlling the viscoelastic nature of the spin solution and the growth of varicose instabilities along the spin line, bead-on-string structures can be introduced to the fibers. Such beaded fibers appear to have particular utility in the field of liquid repellency and liquid-liquid separations, where the highly porous nature and re-entrant geometry of the electospun mat imbue it with exceptional hydropohobic and/or oleophobic properties. Internal heterogeneities can be obtained through controlled dispersion of insoluble nanoparticles during the fiber forming process or through the self-assembly of block copolymers as a post-spinning step. In the latter case, introduction of a second, shell fluid into a coaxial electrospinning geometry is essential to the formation of relatively uniform fibers over a broad range of diameters that can be subsequently annealed to obtain self-assembly. Of particular note, fiber diameters can be realized that approach the characteristic length scale for self-assembly of the block copolymer itself. Under such conditions, the self-assembly process is sensitive to the degree of confinement and the curvature of the confining geometry within the fiber, yielding fundamental insights into the thermodynamics of self-assembly. Of particular interest is the concentric lamellar morphology, in which the number, thickness, and continuity of cylindrical shells of different chemical composition within the fiber can be controlled. Such internal structures create new possibilities for the application of electrospun fibers as sensors, photonic band gap fibers, drug delivery devices, and more.

Superhydrophobic materials and surfaces are being intensively studied in order to provide superior water repellency and self-cleaning behavior. This unique property is potentially very important for many industries, such as textiles, construction, automobiles, microfluidics. etc. Superhydrophobicity results from a combination of low surface energy and high surface roughness. Various methods have been studied to achieve superhydrophobicity. Electrospinning is a relatively straightforward technique to produce micro-/nano-fibers from many kinds of polymers. Advantages of electrospinning are the ability to control: (a) the fiber diameter from micro- to nano-meter dimensions; (b) the various fiber compositions; (c) the spatial alignment of multiple fibers. Electrospinning can produce non-woven fiber mats with high surface roughness, exceptional surface to volume ratios and with high porosity.At Cincinnati, we are investigating the technique of coaxial electrospinning which can produce core-sheath fibers in a single step process. This core-sheath structured fiber is extremely versatile, as it enables us to combine two different properties from core and sheath materials into a single fiber. We have demonstrated the first coaxial fiber of poly(ε-caprolactone) (PCL) core and amorphous fluoropolymer Teflon® AF sheath by using coaxial electrospinning. Electrospinning naturally provides a fiber mat with high surface roughness and the Teflon® AF sheath provides fibers with low surface energy. Secondary structure on fibers, such as striation, enhances the hydrophobicity due to increasing roughness. We have successfully produced superhydrophobic and oleophobic fiber membranes by coaxial electrospinning. Superhydrophobic fiber mats show a very high contact angle > 155°, extremely low rolling angle of ~ 5° and highly elastic (‘bouncing’) behavior for water droplets. Interestingly, the coaxially electrospun fiber mat also exhibits oleophobicity. For example, dodecane oil (~23dynes/cm) placed on PCL-only fiber mat spreads very thoroughly and its contact angle approaches nearly 0°. However, on a coaxial of PCL/Teflon® AF fiber mat, the dodecane droplet did not experience any spreading and actually had quite a high contact angle of ~ 130°.Compared to other methods, coaxial electrospinning has many advantages: (a) it provides a simple one step process for conformal coating of polymer fiber with hydrophobic material: (b) high cost-effectiveness; (c) diversity of available materials for both core and sheath. Treatment by vacuum, high temperature, plasma or sophisticated chemistry are not necessary. We have also demonstrated that the coaxially electrospun fiber can preserve core properties, as demonstrated with mechanical tensile tests. We are confident that coaxial electrospinning will provide a new paradigm for obtaining material combinations very attractive for many fields.

Among the wide variety of materials that can be spun into fibers by electrospinning, block copolymer nanofibers have the interesting feature that the geometry of the fiber imposes a cylindrical confinement on the microphase separation of the block copolymer. In this presentation we show that, for a block copolymer/ PDC precursor nanocomposite that forms a two-dimensional hexagonal morphology in the bulk, helical and stacked toroidal mesostructures were obtained in nanofibers. Continuous fibers were obtained by a two-fluid coaxial electrospinning technique in which an amphiphilic block copolymer/ polymer derived ceramic (PDC) precursor nanocomposite was enveloped as the core component within a rigid polymer shell. The amphiphilic block copolymer is used as a structure directing agent for the PDC precursor. Selective swelling of the hydrophilic microdomains of the block copolymer with the PDC precursor results in cooperative self-assembly of the block copolymer and the PDC precursor into nanostructured morphologies. The structure is permanently set by crosslinking the PDC precursor with a radical initiator. This approach creates continuous ultrathin nanofibers with confinement induced ordered morphologies, which are interesting for their potential towards applications in catalysis, sensing, opto-electronics and separation. The possible chirality of the helical fibers could be particularly interesting for applications like enantioselective separation.

Polymeric nanofibers have been attracting significant attention lately for topical drug delivery applications. The ever growing interest on nanofibers are based on their ability to incorporate a wide range of drugs, the high surface area of the 3D mats for efficient drug release and the ease of fabricating the delivery vehicle in the required architecture. The nanofibers are considered to be useful for wound dressing, artificial organs and multifunctional membranes. Electrospinning system is a useful way to fabricate nanofibers in a continuous process. The dimension of the nanofibers can be controlled from tens of nanometers to a few micrometers. The nanofibers are produce by electrical charge force between niddle and collector. We fabricated core-sheath nanofibers consisting of PCL shell and hydrogel core by using electrospinning method. We have utilized simple method of electrospinning for manufacturing nanofibers and the crosslinked PMMA colloids & PNIPAm hydrogel colloids for drug delivery, release system. Core-shell structured nanofibers have crosslinked PMMA colloids & PNIPAm hydrogel colloids distributed in its core and polymer on its shell. We observed that dye of nanofibers released during experimental period.