Highly oriented polymers

A third observation obtained from Figure 27 is the disappearance of these fissures as soon as the film is locally separated (Fig. 27 a, arrow B). It must be assumed, therefore, that the deformation lines close upon stress relieve just as in hard-elastic materials. A characteristic feature of some of those highly crystalline, highly oriented polymers is the fact that they can be extended by 50-100%, this extension being practically completely and inmiediately reversible... 

Brittle fracture in glassy polymers occurs by the prior formation of crazes which then fracture. In the craze, fibrils of highly oriented polymer are produced and it is the fracture of these fibrils that almost certainly produces the observed radicals. Even in this case, then, the signal derives ultimately from a fibre in which... 

It has been observed that in these particular samples the crack is followed by the craze. The microhardness increases rapidly in this region (see Fig. 3.15). Table 3.3 shows the bulk microhardness values of the polymers investigated. In Table 3.4 the H values of the craze fibrils are listed. The data of Table 3.4 show that the microhardness of the fibrils is at least twice that of the bulk polymer. This is consistent with the concept of highly oriented polymer chains within craze fibrils (Bin Ahmad Ashby, 1988). It is noteworthy that the microhardness of craze fibrils in amorphous polymers is of the same order of magnitude as the microhardness of... 

It will be shown that the level of uniaxial orientation and the variation of local mechanical properties generated by controlling the injection temperature can be conveniently characterized by microhardness measurement in combination with the measurement of optical birefringence An and DSC. In Section 2.7 we saw that microhardness is a very useful mechanical property, which can provide direct information about the anisotropy developed within highly oriented polymers. 

Figure 2.29 A WAXD pattern of highly oriented polymer crystals with their reciprocal lattices and the Ewald sphere.

Fig. 3.5 Structural hierarchy in liquid-crystalline fibers. The mechanical performance of highly oriented polymers can approach the ultimate theoretical properties at high degrees of elongation. Anisotropic, rod-like macromolecules, like aromatic copolyesters composed of 2,6-naphthyl and 1,4 phenyl units, often form oriented structures, which can exhibit liquid crystallinity. Extensive structural studies of fibers of these oriented copolyesters showed a hierarchical structure like the one depicted in this Figure. In aramids (Kevlar or Twaron) similar structures may exist. Adopted with permission from [17]...

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