Fibers and Thin Films

Single filaments (known as monofilaments) cannot be used directly in fabric products. For fabrics a certain number of monofilaments is first put together, and then twisted in order to increase the friction between the individual monofilaments. These interlocked fibers are called yarns. It is important to always report whether DMA measurements were made on single fibers or yarns the mechanical properties of a yarn are not simply the sum of the mechanical properties of single fibers. 

Films are usually prepared from a polymer melt or solution by slit-die extrusion followed by one- or two-dimensional stretching thus a film may 

Measurements on fibers and films using DMA yield information about the various transitions using the temperature dependence of the storage and loss moduli. In order of decreasing temperatures, these transitions are 

DMA measurements can provide information about the orientation of the fiber or film through the magnitude of the storage modulus or by the glass transition temperature. 

An important topic is the effects of crystallinity and orientation of the fiber on the glass transition temperature. Several research groups have noted that 

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POLYACETYLENE. A linear polymer of acetylene having alternate single and double bonds, developed in 1978. It is electrically conductive, but this property can be varied in either direction by appropriate doping either with electron acceptors (arsenic pentaflnoride or a halogen) or with electron donors (lithium, sodium). Thus, it can be made to have a wide range of conductivity from insulators to n- or >-type semiconductors to strongly conductive forms, Polyacetylene can be made in both cis and trans modifications in the form of fibers and thin films, the conductivity... 

As solute is removed from ihe feed-side membrane-solution interface by dialysis, the layer is depleted and its concentration must he restored from the bulk solution. In laminar flow, which is usual In smallbore hollow-fiber and thin-film plate-and-frame devices, there is no convection and repletion of the interfacial layer is solely by diffusion from the bulk solution. As such diffusion occurs, the concentration gradient from the bulk solution to the imeriacial layer decmases, Tims, the rate of restoration of the inierfacia solute concentration is a fuactlon of the solute size, the transmembrane flux, and the rate of solute supply, that is, the axial feed flow rate. 

Both neat polyolefins and their blends and composites are commonly processed by blow molding, injection molding, extrusion, compression molding, rotational molding, and thermoforming. The final materials can be produced as bulk materials, fibers, and thin films. 

Composites of PPV in films of the polyelectrolyte Nafion have been synthesized by electrostatically binding the dicationic monomer to the film, followed by treatment with base and thermal conversion (48). Ordered nanocomposites of PPV have been synthesized from mixtures of polymerizable lyotropic liquid crystals with PPV precursors (49). Photopolymerization of the host results in a hexagonal architecture, which can be fabricated into fibers and thin-films. A significant enhancement in photoluminescence of the composite relative to PPV was found. 

Fractured bulk polymers and composites require only coating with a conductive layer (Section 4.7.3) before observation in the SEM, although some composite fracture surfaces are so rough as to make deposition of a thin continuous conductive film very difficult. High resolution is rarely required in these materials so the common solution is to use a thick coating, and often carbon is evaporated followed by metal coating. Fibers, particularly textile fibers and thin films, have such a small cross sectional area that the main difficulty is in handling the broken sample. 

Figure 5.53. Illustration demonstrating the necessity of using a pretension in DMA tensile measurements of fibers and thin films sample buckhng will take place below a certain pretension level is the static pretension force, and Fi is the variable dynamic oscillating force imposed during the test (from Grehlinger and Kraft, 1988b, with permission of the Society of Plastics Engineers).

Lin, 2(X)1). However, the sol-gel process presents inherent advantages for the preparation of membrane materials made of nanophase and/or nanoporous ceramic oxides or hybrid materials. In particular the nanostructure of sol-gel derived materials can be controlled together with their porous structmre (e.g., formation ofmesopores or micropores). Other quoted advantages of sol-gel processed ceramics are compositional homogeneity and the ability to prepare shaped-materials such as spherical particles, fibers and thin films (Guizard, 1999). 

Previous sections have demonstrated the use of ChG glasses in a wide variety of fiber and thin-film sensing devices, which requires specific knowledge of the optical, thermal, and mechanical properties of the ChG in use. In general, these material characteristics largely fall into two main categories, optical, and thermal [as well as some application-specific factors such as chemical resistance and radiation hardness). 

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