Orientation fibrillar

Crazing in anisotropic polymers has seldom been studied by means of optical interferometry. A block copolymer (Isoprene-Styrene) with a highly oriented fibrillar structure, and a PMMA oriented by means of drawing above Tg, have been studied so far. These results are reported in and The same subject has often been studied, however, using other experimental techniques... 

Dry spinning generates a fiber that initially appears different from typical wet-spun fibers as there is no opportunity for the spin bath to diffuse into the fiber. However, when the unoriented dry-spun fiber is stretched, an oriented fibrillar structure develops, indistinguishable from a stretched wet-spun fiber. Hence, all acrylic fibers, whether dry- or wet-spun are fundamentally similar. 

Hierarchical fibrillar model [80] This model was proposed for drawn TLCP fibers. They are composed of bundles of macro fibrils (5 p,m), fibrils (0.5 (Jim), and micro fibrils (0.05 p.m) in a hierarchical order (see Figure 8.17). This type of highly oriented fibrillar morphology is also found in the inner skin region of the injection-molded parts, as illustrated in Figure 8.18. 

Hughes, K.E., Fink, D.J., Hutson, T.B. and Veis,. . (1984) Oriented fibrillar coUageii and its application to biomedical decices./. American Leather Chemists Association, LXXDC, 146-158. 

The liquid crystal polymerization method was modified to prepare a vertically aligned poly acetylene film [58]. As a result, a film with a very curious morphology was formed that was composed of two layers. The layer on the solvent side had a vertically oriented fibrillar structure, whereas the layer on the acetylene gas side had a randomly oriented one. The modified method is very promising for visualizing the polymer growth process and clearly demonstrates that PA chains grow in a liquid crystal solvent. 

During the drawing step, the blend components are oriented, and nano- or microfibrils are created fibrillation step). In the subsequent thermal treatment stage, when melting of the lower-melting component occurs isotropization step), the oriented fibrillar structure... 

Ziabicki and Jarecki have studied the effect of thermodynamic and hydro-dynamic parameters on the orientation distribution of the nuclei, to explain oriented fibrillar growth. Wu has also taken a thermodynamic approach to stress induced crystallization of cross-linked rubbers and its effect on morphology. He considers that extended chain crystals are only formed above the zero-strained melting point, Tva, and folded crystals are formed below Tm if the molecules are stretched less than some critical amount. 

These phenomena are modeled in Figure 13.37. A crack is an open fissure in the material. However, a craze constitutes a relatively stable structure, with highly oriented fibrillar and microfibrillar structures running from the roof to the floor of the craze. The rubber domains may cavitate, forming internal voids, thus relieving triaxial stresses inside the plastic. 

Pi om a mesoscopic point of view, when a semicrystalline polymer is submitted to a large deformation, the spherulitic structure is gradually broken down, while increasing moleculai orientation. In a tensile deformation, the spherulitic could be reformed ultimately in a new oriented microstructure the oriented fibrillar texture [190]. Such cold processed polymer fibers may have a Young s modulus, which could nearly reach 150 GPa for polyaramid fibers (Kevlar ). The strong link between microstructme and mechanical properties can be easily predicted. 

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