Crystalline state, polyamides

In the non-crystalline state polyamide and polyester chains do not contain exclusively anti- or syn- conformers, but a mixture of both placements. Under strain these are easily allowed to interconvert in order to release the stress [412, 663, 664]. In the case of the aromatic polyamides at least, 60° ring flips also contribute to stress release [663]. These isomeric interconversions involve relatively low energy-barrier rotational motions, resulting in substantial reduction in the forces needed to strain such chains and a commensurate drop in modulus. Isolated aromatic polyamide or polyester chains, each free to adopt several conformational isomers, have, then, lower modulus than when present in the crystal, fully extended in a single conformational isomeric state. 

Unusual properties of fully aromatic polyesters are observed if they have at least partially a rigid planar chain structure. In particular, they can form thermotropic liquid crystalline states (see Example 4-5). As already discussed in Sect. 1.2.4 an important structural prerequisit for LCPs of Type A in order to attain the liquid crystalline state of aromatic polyesters (and aromatic polyamides, see Example 4-14), is a rigid main chain according to the following construction principle ... 

Liquid crystalline compounds are remarkable because of their ability to show spontaneous anisotropy and readily induced orientation in the liquid crystalline state. When polymers are processed in the liquid crystalline state, this anisotropy may be maintained in the solid state and can readily lead to the formation of materials of great strength in the direction of orientation. A particularly important example of the use of this property for polymers is in the formation of fibers from aromatic polyamides which are spun from shear oriented liquid crystalline solutions Solutions of poly(benzyl glutamate) also show characteristics of liquid crystalline mesophases, and both of these types of polymers are examples of the lyotropic solution behaviour of rigid rod polymers which was predicted by Flory... 

Whang and Wu [3] have described the liquid crystalline state of polyimide precursors and shown that certain polyamic acids derived from pyromellitic anhydride exhibit lyotropic behaviour. Liquid crystal phases have also been observed by Wenzel et al. [4] in polyimides derived from pyromellitic anhydride and 2,5-di-n-alkoxy-1,4-phenyl ene diisocyanate. Dezern [5] has disclosed a synthesis for linear polyamide-imides derived from benzophenone dianhydride but the occurrence or otherwise of mesophases is not mentioned. 

Linear aliphatic homopolyamides are partially crystalline materials. Therefore they are characterized by both an unordered amorphous state and an ordered crystalline state. The latter may exhibit polymorphism. The extent to which each state or specific modification is represented depends, for a given chemical structure, considerably on processing conditions and treatment operations. It affects the properties of the shaped polyamide product. Thus the corresponding structure parameters are of importance for optimizing fiber processes as well as for assessing the performance of fiber products in particular applications. 

Wu s explanations for this fundamental observation were couched in field theory, such as overlapping of concentrated stress fields around particles or a transition in local stress state from plane strain to plane stress, and could not furnish a specific material dimension, which, as we present below, depends on the type of the polymer and its crystalline state. The required fully consistent explanation for the discovery was provided by the studies of Muratoglu et al. (1995a), who proposed that the material-specific level of A is a consequence of a preferred form of crystallization of polyamide lamellae near particle interfaces, extending to a certain distance I away from the interface. This results in an anisotropic plastic resistance in this layer, which upon percolation through the matrix and in an... 

Let us now consider the structural organization of polyesters and polyamides with alkyl chains [30]. These systems are similar to alkyl-substituted PT and PANi/surfactant. The results obtained for these polymers would be applicable to conducting polymers that also have an aromatic backbone. In the crystalline state, structures of polyesters based on 1,4-dialkyl esters of pyromellitic acid and 4,4 -biphenol (PE-1) were found to depend on the length of the alkyl chains. For n = 12 a layered structure was detected with an interchain distance within layers of 4.55 A. For n = 14, 16, and 18 the interchain distance was 3.45 A. This distance corresponds to the van der Waals radius of the phenyl ring. Such dense packing requires a coplanar arrangement of the aromatic backbone within layers. Coplanar... 

As with solutions of extended-chain polyamides (12), the critical concentration is affected by polymer molecular weight, being higher as molecular weight decreases (Fig. 7). At low inherent viscosities solubility may be insufficient for the attainment of a liquid crystalline state. On the other hand, solvent-polymer interaction may be so strong as to prevent the development of liquid crystalline order. 

This brief description of the appearance of the liquid-crystalline state in solutions concerned rigid-chain polymers, PEG in particular (in coiling solvents), aromatic polyamides with phenyl units in the para position, and some... 

A.2. Infrared Spectrometry. Theoretical studies of the effect of the degree of crystallinity on vibration spectra account only partially for the appearance or the disappearance of certain absorption bands when comparing amorphous polymers with semicrystalline ones. However, these differences are admittedly related both to the appearance (or the disappearance) of intra- or intermolecular interactions and also to stricter selection rules for the crystalline state. In the latter case, absorption bands corresponding to interactions are better defined and thus narrower they are fewer due to stricter rules of selection. For example, in the case of polyamide-6,6, a linear variation of the intensity of two characteristic absorption bands is observed as a function of the density and hence degree of crystallinity (Figure 6.26). One band is due to the crystalline state, and the other one is due to the amorphous state. 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

Both low molecular weight materials [145] and polymers [146,147] can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. One reason such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed (as described for elastomers throughout this chapter), (ii) the deformation produces alignment of the chains, and (iii) alignment of the chains is central to the formation of liquid-crystalline phases. Because of fascinating properties related to their novel structures, liquid-crystalline elastomers have been the subject of numerous studies, as described in several detailed reviews [148-150]. The purpose here will be to mention some typical elastomers exhibiting liquid crystallinity, to describe some of their properties, and to provide interpretations of some of these properties in molecular terms. 

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