Mesophases draw-induced

Finally we mention that draw-induced mesophases have also been observed in some other semi-rigid chain polymers. Examples of such studies are cold drawing of glassy PEN [108,109] and oriented crystallization of PET/PEN copolymers [110, 111]. In all cases a smectic mesophase has been reported. In the absence of crystallization it can attain a degree of stability that resists decay from chain relaxation. This supports the view that the mesophase represents a thermodynamic state [101]. We expect that similar mesophases based on semi-rigid monomers can be observed in other polymers with intrinsic rigid building blocks, such as other aromatic polyesters and polyamides. 

Figure 20 presents mesophase rods produced by extrusion alone and by a light draw after extrusion. As Jenkins and Jenkins observed (34), the strong preferred orientation induced by extrusion was easily disturbed by pyrolysis bubbles or even by small flow irregularities. However, modest draws (e.g., draw ratio = 2) after extrusion produced fibrous morphologies with good uniformity. At these draw levels, the nodes and crosses characteristic of wedge disclinations could be resolved on transverse sections. 

Orientational order plays an important role in solid polymers. It is often induced by industrial processing, for example in fibers and injection- or compression-modulated parts. In polymers with liquid-crystalline properties of the melt or solution, the anisotropies generated by the flow pattern are particularly pronounced. In order to improve the mechanical properties of polymer fibers or films, the degree of orientation is intentionally enhanced by drawing. At the same time, anisotropy of mechanical properties can result in low tolerance to unfavourably directed loads. In many liquid-crystalline polymers, in the mesophase near the transition to the isotropic phase, electric or magnetic fields can induce macroscopic orientational order [1]. Natural polymers such as silk protein fibers, which are biosynthesized and spun under biological condition, also have good mechanical properties because of their ordered structure [2]. 

Specially difficult systems are represented by fibers of macromolecules. All the topics treated in the prior sections must be considered under the additional aspect of the presence of crystal deformations caused by the drawing process (see Sect. 5.2.6 and 5.3.6) and possible strain retained in the amorphous areas (see Sect. 6.3.3), as well as the existence of sttain-induced mesophases (see Figs. 5.69-72 and 5.113-115). 

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