Core-sheath composite fibers

Figure 5.15 Schematic mechanism for the formation of core-sheath composite fibers during emulsion electrospinning [67]...

McCann et al. [50] described the use of electrospinning in the fabrication of polymeric, ceramic, and composite nanofibers with core-sheath, hollow-fiber, or porous structure, as well as the efforts made to improve their morphological homogeneity, functionality, and device performance. Figure 12.9a shows the schematic diagram for the fabrication of core-sheath nanofibers with a coaxial spinneret, which was used by McCann et al. [50]... 

A nylon fiber whose surface was coated with carbon particles in the finishing stage was reported to have antistatic properties[93]. The substrate fiber had a composite structure of core-sheath, whose core and sheath were composed of copolymer of nylon-6 and nylon-6,6 and low temperature melting nylon-6,6,... 

Composite fibers n. Fibers composed of two or more polymer types in a sheath-core or side-by-side (bilateral) relation. 

Encapsulation of fluorescein isothiocyanate - conjugated BSA (FitcBSA) with PEG in core, wrapped in PCL sheath, has also been reported. A comparison was made between the protein release profiles from the two composite fibers, one made by the co-axial process, and the other as a mixture using single-jet electrospinning. The results showed a more sustained... 

To solve the problem of poor adhesion between fiber mats and the substrate, Kim et al. (2006) introduced an additional hot-pressing step after titania fiber deposition. Besides improving adhesion, this treatment was found to have an impact on the microstracture of the fibers as shown in Fig. 2.11. The as-spun metal oxide-polymer composite fibers exhibit a range of diameters from 200 to 500 nm (Fig. 2.11a). When calcined without hot-pressing to remove the organic vehicle, a bundle structure composed of sheaths of 200-500 mn diameters was obtained. In some cases, the outer sheaths were broken, revealing cores filled with 10-nm-thick fibrils as shown in Fig. 2.11c. By introducing the... 

Core-sheath nanofibers of PEG-PLA and PEG can be prepared by electrospinning a water-in-oil emulsion in which the aqueous phase consists of a PEO solution in water and the oily phase is a chloroform solution of an amphiphilic PEG-PLLA diblock copolymer. The fibers obtained are composed of a PEO core and a PEG-PLA sheath with a sharp boundary in between. By adjusting the emulsion composition and the emulsification parameters the overall fiber size and the relative diameters of the core and the sheath can be changed. As shown in Fig. 5.15, a mechanism is proposed to explain the process of transformation fi-om the emulsion to the core-sheath fibers, i.e., the stretching and evaporation induced de-emulsification. In principle, this process can be applied to other systems to... 

Table 1 lists some key production and compositional details for a variety of SiC-based fiber types of current interest and availability as CMC reinforcement. The polymer-derived types range from first-generation fibers with very high percentages of oxygen and excess carbon, such as Nicalon and Tyranno Lox M, to the more recent near-stoichiometric (atomic C/Si 1) fibers, such as Tyraimo SA and Sylramic. For the CVD-derived types, such as the SCS family with carbon cores, the only compositional variables in the SiC sheaths are slight excesses of free sihcon or free carbon. Table 2 lists some of the key physical and mechanical properties of the SiC fiber types in their as-produced condition, as well as estimated commercial cost per kilogram, all properties important to fiber application as CMC reinforcement. These SiC fiber properties in Tables 1 and 2 are in most part those published by the indicated commercial vendors. It should be noted that the Sylramic fiber... 

Among carbon fillers, carbon black is most commonly used due to good conduction performance, and metallic oxides are often used to make fiber white. Du Pont produced a composite nylon fiber made up of nylon sheath and conductive polymer core formed by dispersing about 30% carbon Hack in LDPE matrix. When the conductive core content was ca. 4%, the was around 10 cm[96,97]. Toray[98] developed a composite nylon fiber made up of nylon-6 sheath and conductive polymer core formed by dispersing about 30% carbon black in nylon-6 matrix. When the conductive core content was ca. 5%, the was 10 to 10 cm. Other conductive nylon fiber was reported by Unitika[99,100], in which 25% acetylene black was dispersed in nylon-6, which was combined with the same nylon 6 base polymer at a ratio of20/80. The conductive polymer was exposed onto the fiber surface to increase efficiency. A white-colored conductive nylon fiber was also obtained by using titanium dioxide particles with diameters of 2 pm or less coated with tin oxide. A heat resistant conductive nylon fiber was obtained by dispersing carbon black in an aromatic polyamide[101]. 

In certain instances it may be of interest to examine the etched surface of a fiber or determine the proportion of a particular cross-section in a fiber mixture. This is done by scanning electron microscopy (SEM), where magnifications of up to 100 000 x can be achieved with greater depth of focus and resolution. Additionally, the cross-sectional appearance of bicomponent and polyblend fibers may be examined by SEM, revealing side-by-side (Figure 4), sheath-core , or island-in-the-sea composites. 

Melt spinning from supercooled fluoride melts affords optical single and bicomponent glass fibers [7]. The latter have a concentric fluoride core and a fluoride sheath or clad with a slightly different composition. 

Continuous sapphire fibers (Chapter 4) and continuous sheath/core bicomponent silicon carbide/carbon fibers (Chapter 3) offer impressive performance as reinforcing fibers and in ceramic and metal matrix composites. Here are some noteworthy commonalties and differences. 

A glass composition which was found particuiarly suitable for these applications [67] contained 52% SiOs, 30% Na20,15% CaO and <3% P2O5 (in mole %). Fiber bundles or tow of up to 5000 filaments were drawn from a melt of this composition and were interwoven with carbon fiber tow into a cylindrically braided sheath/core textile preform. The carbon fibers formed the core of the braided structure and provided the required stiffness and load support, and the bone bioactive fibers formed the sheath and functionality. 

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