The Crystalline State and Partially Ordered Structures

The occurrence of significant crystallinity in a polymer sample is of considraable consequence to a materials scientist. The properties of the sample — the density, optical clarity, modulus, and general mechanical response — all change dramatically when crystallites are present, and the polymer is no longer subject to the rules of linear viscoelasticity, which apply to amorphous polymers as outlined in Chapter 13. Howcvct, a polymer sample is rarely completely crystalline, and the properties also depend on the amonnt of crystalline order. 

It is important then to examine crystallinity in polymers and determine the factors that control the extent of crystallinity. 

Polymers forming partially ordered phases (polymer liquid crystals) between the solid and the hqnid phase have become increasingly important, and these are also reviewed in this chapter, along with the self-ordered structures formed by many block copolymers. 

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One may consider a series of physical states ranging from the crystalline, where molecular aggregation and orientation are large, to the dilute gaseous state, where there are no significant orientational limits. States of intermediate order are represented by micelles, liquid crystals, monolayers, ion pairs, and dipole-dipole complexes. In the crystalline state, the differences between pure enantiomers, racemic modifications, and diastereomeric complexes are clearly defined both structurally and energetically (32,33). At the other extreme, stereospecific interactions between diastereomerically related solvents and solutes, ion pairs, and other partially oriented systems are much less clearly resolved. 

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. 

The intensity of the electron beam used to examine thin crystalline polymer films by electron microscopy is usually of sufficient intensity to induce cross-linking. It is not surprising, therefore, after initial examination in the crystalline state, that thin films of poly(amides) and polyethylene display ordered structures when subsequently examined in the molten state by this technique. These observations are to be expected. They cannot be construed as evidence that, in general, the liquid state in polymers is an ordered one.(33) The partially ordered liquid represents an interesting, unique situation that results from the nature of the chain arrangement at the time of network formation. 

Polymers in the solid state can be completely amorphous, partially crystalline, or almost completely crystaUine. Polymer crystals have the requiranent that they must accommodate the covalent axis within an ordered structure. For some time the interpretation of X-ray diffraction patterns was that individual polymer molecules were partly crystalline and partly amorphous. The longest dimension of the crystallites in polycrystalline materials is usually about 5-50 nm, which is a small fraction of the length of a fully extended polymer molecule. A graphical representation of this once popular model is shown in Figure 3.12. Here a long polymer chain wanders... 

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