Crystallite perfection imperfection

The theoretical melting point of a perfectly S5mdiotactic PVC (a configuration which supports crystalline structure formation) is estimated to be around 400°C, but such a polymer has not yet been synthesized. The melting point of PVC synthesized by radical pol5mierization is in the range of 102-230"C because of small size crystallites and imperfections in its ordered stmctuie. 

The diffraction peaks obtained with a perfect crystal are in theory expected to be infinitely sharp. The finite widths of the observed diffraction peaks as seen in Figure 3.2 reflect the fact that crystallites in semicrystalline polymers are not perfect, and the analysis of the line widths can tell us about the nature and degree of imperfection in the polymer crystal lattices and the size of the polymer crystallites if they are small. 

Melting/recrvstallisation temperature determinations Semi-crystalline polymers generally melt over a wide temperature range. This behaviour is related to imperfections in the crystallites and non-uniformity in their size the smaller and/or less perfectly formed crystallites will melt at lower temperatures. The endothermic fusion effect as measured by the DSC is in many cases indicated by the temperature of the maximum heat flow (the Tm-value) and by the total heat involved in the fusion process (the Hf-value). Often reported is also the Te-value i.e. the temperature at which the last crystallite has fused. 

In practice, all crystals have imperfections. If a substance crystallizes rapidly, it is likely to have many more imperfections, because crystal growth starts at many sites almost simultaneously. Each small crystallite grows until it runs into its neighbors the boundaries between these small crystallites are called grain boundaries, which can be seen on microscopic examination of a polished surface. Slow crystal growth reduces the number of grain boundaries, because crystal growth starts from a smaller number of sites. However, even if a crystal appears to be perfect, it will likely have imperfections on an atomic level caused by impurities in the material or by dislocations within the lattice. 

However, uncertainties due to the imperfect orientation of the crystallites and their defects limit the number of spots and can make their identification very difficult. In such a case, the various atoms are first assumed to occupy certain positions after considering the steric hindrance of the molecular groups and the interatomic distances then, diffracted intensities are calculated and the values obtained are compared with experimental results. If there is no agreement, the assumed structure is false if the agreement is satisfactory, the assumption made is close to reality. The structure first assumed is then modified until obtaining a perfect agreement (trial-and-error method). 

Basically, this means that imperfect crystallites melt before the perfect crystalline phase and finally amorphous material is formed. 

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