Paracrystalline materials

Figure 8.21. Features of a ID correlation function, yi (x/L) for perfect and disordered topologies. L is the number-average distance of the domains from each other (i.e., long period). Dotted Perfect lattice. Dashed and solid lines Paracrystalline stacks with increasing disorder. a = — v/j / (1 — v/j) with 0 < v/j < 0.5 is a measure of the linear volume crystallinity in the material, which is either or 1 —...

Oriented material can be described in terms of a paracrystalline structure. On doping with AsFs, order is retained, with a new spacing developing perpendicular to the chain axis and a regular positioning of dopant along the chains 499>. 

Methods for estimating lattice distortion generally require two or more orders of a particular reflection to be present, and most polymers have only one order available. A method for estimating both crystallinity and lattice disorder, which does not need higher orders of a reflection, and indeed takes into account the whole of the diffraction trace, is that due to Buland (27). This method has been applied to many different fibres by Sotton and his colleagues, who have discussed their results both here (28) and elsewhere (12). The major problem with Ruland s method is that an arbitrary separation of the crystalline scatter from the non-crystalline scatter must be made other restrictions are that the method cannot be used to measure crystallite size and cannot give any indication of the presence of paracrystalline or intermediate-phase material. 

So paracrystalline polymers are nearly identical with liquid-crystalline polymers, filled with amorphous defect domains. And network polymers or copolymers maybe compared with filled materials, cross links playing the same role as sub microscopic filler particles. 

While, properly speaking, all packaging plastics are either amorphous or partially crystalline, in practice the term crystalline polymer is generally used to refer to a polymer that has any substantial degree of crystallinity. The term semicrystalline is sometimes used for polymers that have only a slight amount of crystallinity. The term paracrystalline is used to refer to materials, such as polyacrylonitrile, which have a substantial degree of molecular order, but fall short of true crystallinity. Polymers with only a very limited ability to crystallize are sometimes, incorrectly, referred to as amorphous polymers. Polyvinyl chloride, for example, has an extremely small ability to form crystals since it is primarily atactic but has some syndiotactic tendency. Nonetheless, it is often reported to be an amorphous polymer. 

Characterization of polymer orientation is most often accomplished via X-ray techniques which are suited to crystalline and paracrystalline regions (i-d). However, semicrystalline polymers present a complex system of crystalline, amorphous, and intermediate pluses ( -d) and complete characterization of semicrystalline polymers can only be achieved by application of a variety of techniques sensitive to particular aspects of orientation. As discussed by Desper (4), one must determine the degree of orientation of the individual phases in semicrystalline polymers in order to develop an understanding of structure-property relationships. Although the amorphous regions of oriented and unoriented semicrystalline polymers are primarily responsible for the environmental stress cracking behaviour and transport properties of the polymers, few techniques are available to examine the state of the amorphous material at the submicroscopic level. 

Because of the presence of structural disorder, the X-ray diffraction patterns of mesophases show a large amount of diffuse scattering and need a special care for a quantitative evaluation. Paracrystalline distortions of the lattice [59,60] usually affect the shape and the width of the diffraction peaks to a large extent. The analysis of disorder necessarily implies a multidisciplinary approach, in order to unravel the complicated nature of disorder in disordered crystalline materials [61]. 

Figure 7 Part of a large paracrystalline aggregate of Xenopus lamin-A, expressed in E. coli. Note the characteristic 25 nm periodicity along the filamentous aggregate. Bar=0.5nm. (The sample material was kindly provided by Gieffers C and Krohne G and the specimen was produced from a solution of protein in 2 mol I ammonium acetate by the negative staining-carbon film procedure.)...

Interference function scattering was modelled by using a paracrystalline lattice model as a basis, with the adoption of the equations for X-ray scattering from low molecular weight crystalline materials. Thus the Bragg peaks essentially result from a powder diffraction pattern from the block copolymers. Reasonable agreement... 

From our viewpoint, there are three most interesting aspect of those materials to study. First, the particular features of crystallization of these compounds, generally random copolymers, are of quite general interest. Two recent and essentially different models for crystallization of such irregular polymeric materials, that is, the model of nonperiodic layers (1) and the model of the paracrystalline lattice (2), are still under discussion. 

If the monomeric units have different lengths, as it is the case for another copolyester under study CPE-2, aperiodic reflections in the meridian and several Bragg spots on the equator and in quadrants are observed in X-ray patterns of oriented fibers. That means that the crystalline phase of the material may be described by one of two models proposed by Windle (1) or Blackwell (2) for such unusual structures, namely, the model of non-periodic layer crystals and paracrystalline model, respectively. 

Figure 6.37 The paracrystalline model of Hosemann (134). Amorphous structures are illustrated in terms of defects. A radius of gyration approaching amorphous materials might be expected.

The concept of paracrystallinity introduced by Hosemann et al. in 1966 was used earlier to describe the phenomenon of ammonia iron. They developed the theory of paracrystallinity from XRD data which seemed to explain the special properties of the activated iron catalyst. A three-dimensional, endotactic incorporation of hercynite (FeAl204) motives into the a-iron lattice was thought to create substitutional point defects in the crystal lattice leading to a modified bulk and surface structure of the activated catalyst material. The interplanar spacings change... 

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