Molecular structure and crystallinity

Work on poly(ethylene oxide) gels indicates that the presence of solvents such as chloroform and carbon disulfide contributes to the formation of a uniform helical conformation. The gelation behavior and the gel struc-ttue depend on the solvent type which, in turn, is determined by solvent-polymer interaction. In a good solvent, polythiophene molecules exist in coiled conformation. In a poor solvent, the molecules form aggregates through the short substituents.  

Polyvinylchloride, PVC, which has a low crystallinity, gives strong gels. Neutron diffraction and scattering studies show that these strong gels result from the formation of 

The interaction of polymer-solvent affects iheological properties. Studies on a divinyl ether-maleic anhydride copolymer show that molecular structme of the copolymer can be altered by the solvent selected for synthesis. Studies have shown that the thermal and UV stability of PVC is affected by the presence of solvents.  

Temperature is an essential parameter in the crystalhzation process. Rapid cooling of a polycarbonate, PC, solution in benzene resulted in extremely high crystallinity (46.4%) as compared to the typical PC crystallinity of about 30%. 

Polymer-solvent interaction combined with the application of an external force leads to the surface crazing of materials. The process is based on similar principles as discussed in this section formation of fibrilar crystalline structures. 

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Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

Since in general they are poor conductors of heat plastics require large inputs of energy as heat to bring them to the working temperatures the amounts of heat needed differ between materials—essentially because of their molecular structures and crystallinity—as illustrated in Table XI. 

Figure 6.6 Molecular structures and crystalline organization of (a) p6P, (b) PPXTPP and (c) TTPPTT. Arrows crystal lattice directions. Bottom figures-fluorescence micrographs of the different types of nanofibers. Microgranh...

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

Fig. 47. Molecular structure and packing in the crystalline 7 imidazole (1 1) associate with an indication of the H-bonding network136) (H-bonds as broken lines backbone H atoms of the host are shown as sticks only O atoms dotted N atoms hatched)...

FIGURE 5.8 Unit cells (outlined in each diagram) and helix packing in A and B polymorphs of starch. Reprinted from Carbohydrate Research, Vol. 61, Wu and Sarko (1978b), The double helical molecular structure of crystalline A-amylose, Pages 27-40, with permission from Elsevier. 

Wu, H.-C. H. and Sarko, A. (1978a). The double helical molecular structure of crystalline... 

The overall molecular structure and morphology of a semicrystalline polymer is generally admitted to represent a very complex situation. (1) (3 ) Crystallinity is rarely if ever... 

Perchlorates are characterized by a CIO4 fragment/anion in their molecular structures and are crystalline materials used in propellants and explosives.P i] qq g oxy-... 

The forward search starts from the name of a chemical compound, proceeds to finding its molecular structure, and then its physical and chemical properties, such as the boiling point, melting point, density, etcetera, in a handbook. Many databases for single compounds are also organized by classes and families of similar structures. Fluid solutions represent the next level of complexity. For the most important fluids, such as water, air, and some refrigerants, we can find extensive tables for the thermal properties of mixtures. For complex fluids, such as paint and emulsion, which are difficult to characterize and to reproduce, specialized books and journals should be consulted. The properties of some crystalline solids can be found, but usually not for multicrystal composite and amorphous solids. 

Since the properties of a polymer can be noticeably influenced by small variations in the molecular structure, and these in turn depend on the preparation conditions, it is necessary when reporting data to indicate not only the type of measurement (e.g., molecular weight by end group analysis crystallinity by infrared measurement or by X-ray diffraction etc.), but also the type of preparation (e.g., radical polymerization in bulk at 80 °C polymerization with a particular organometallic catalyst at 20 °C). 

In addition to the nature of particulate platelet orientation induced during injection moulding, the associated consequences on molecular orientation and crystalline order of the host thermoplastic matrix have also been reported with particular regard to various flake-filled polypropylenes [174], together with an attempt to interrelate these higher order structural parameters with physical properties of the composites [175]. 

Note that this is only true if the liquid crystalline state is caused by the molecular structure and not its superstructure (see Sect. 5.1)... 

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