Elastomers fiber orientation

Acrylonitrile (AN), C H N, first became an important polymeric building block in the 1940s. Although it had been discovered in 1893 (1), its unique properties were not realized until the development of nitrile mbbers during World War II (see Elastomers, synthetic, nitrile rubber) and the discovery of solvents for the homopolymer with resultant fiber appHcations (see Fibers, acrylic) for textiles and carbon fibers. As a comonomer, acrylonitrile (qv) contributes hardness, rigidity, solvent and light resistance, gas impermeabiUty, and the abiUty to orient. These properties have led to many copolymer apphcation developments since 1950. 

The properties of elastomeric materials are also greatly iafluenced by the presence of strong interchain, ie, iatermolecular, forces which can result ia the formation of crystalline domains. Thus the elastomeric properties are those of an amorphous material having weak interchain iateractions and hence no crystallisation. At the other extreme of polymer properties are fiber-forming polymers, such as nylon, which when properly oriented lead to the formation of permanent, crystalline fibers. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonates, etc. 

Uniaxial deformations give prolate (needle-shaped) ellipsoids, and biaxial deformations give oblate (disc-shaped) ellipsoids [220,221], Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and the long fibers frequently used for this purpose in thermoplastics and thermosets. Similarly, oblate particles can be considered as analogues of the much-studied clay platelets used to reinforce a variety of materials [70-73], but with dimensions that are controllable. In the case of non-spherical particles, their orientations are also of considerable importance. One interest here is the anisotropic reinforcements such particles provide, and there have been simulations to better understand the mechanical properties of such composites [86,222],... 

Both thermoplastics and thermosets can be used in four of the five major application areas plastics, elastomers, coatings, and adhesives. But, only thermoplastics can be used in making fibers. During the spinning and drawing process of fiber processing, it s necessary to orient the molecules. Only unbranched, linear polymers (not thermosets) are capable of orientation. 

Polymers have been valued since antiquity for their solid state properties. By this is meant their ability to undergo chain entanglement or co-linear orientation and microcrystalll-zatlon in the solid state. This underlies their use as structural materials, films, fibers, and elastomers. Such properties still constitute the driving force for most pol)nner-orlented research, especially with respect to the synthesis of heat-stable, radiation-stable, or highly flexible materials. 

It has been shown using X-ray fiber patterns, that a weakly crosslinked sidechain LCE can be oriented by an external mechanical stress with the polymer chains oriented parallel to the stress axis and the director associated with the mesogenic units perpendicular to the stress axis [38]. X-ray investigations also have been done for nematic phases of combined elastomers [33]. 

Fibers are used only rarely as reinforcing materials in elastomer moldings, so that this type of orientation is of no relevance for the particle types used. 

When the polymeric component forms the continuous phase, spheres, cylinders, or platelets may be added, as illustrated under reinforced polymers. The fiber composites are the most highly researched, as far as different modes of mixing are considered. The filaments may be continuous or discontinuous, or oriented or random in the matrix, with many subclasses of partial orientation possible (not shown). The tape composites are interesting since in some quarters these may be considered a two-dimensional analog of the highly oriented, continuous fibers embedded in a plastic matrix. The reinforced elastomers differ from the reinforced plastics in two ways the mechanical properties of the polymeric substrate, and the size of the reinforcing particles with respect to polymer chain dimensions. Because of the poor properties often obtained, it is rare to see a research paper on large particles dispersed in an elastomer. 

The X-ray pattern taken at room temperature from a stretched sample is reported in Figure 11. A broadened wide angle reflection at the equator is evident. The smectic layer reflections can be observed at the meridian, indicating a perpendicular orientation of the smectic layers to the stress direction. The orientation is therefore dominated by the polymer chains, as it was reported also in the case of highly stretched fibers of a smectic main chain elastomer (35). 

Breuer O, Uttandaraman S (2004) Big returns from small fibers a review of polymer/carbon nanotube composites. Polym Compos 25(6) 630-645 Brigitte V, Alain P, Claude C, Cedric S, Rene P, Catherine J, Patrick B, Philippe P (20(X)) Macroscopic fibers and ribbons of oriented carbon nanoUibes. Science 290(5495) 1331-1334 Cadambi RM, Ghassemieh E (2012) Optimized process for the inclusion of carbon nanotubes in elastomers with improved thermal and mechanical properties. J Appl Polym Sci 124(6) 4993-5001... 

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