Crystallinity molecular structure

The ability of a material to crystallise is determined by the regularity of its molecular structure. A regular structure is potentially capable of crystallinity whilst an irregular structure will tend to give amorphous polymers. Structural irregularities can occur in the following ways ... 

Isotactic Type of polymeric molecular structure that contains sequences of regularly spaced asymmetric atoms that are arranged in similar configuration in the primary polymer chain. Materials having isotactic molecules are generally in a highly crystalline form. 

Figure 17.7 Molecular structures of (a) CIF3 and (b) BrFa as determined by microwave spectroscopy. An X-ray study of crystalline CIF3 gave slightly longer distances (171.6 and 162.1 pm) and a slightly smaller angle (87.0 "). (c) Structure of laCl showing planar molecules of approximate Da symmetry.

The molecular structure of yellow crystalline Xe(OTep5)4 has been determined by X-ray analysis (see Pig. 18.3b) / the Xe atom is surrounded by a square-planar array of four O atoms, with the adjacent TeFj groups pointing, curiously, pair-wise up and down from this plane (Xe-O 203.9(5) and 202.6(5) pm, Te-O 188.5 pm). 

While cooling the melt to solidify in a processing operation, alignment of molecules controlling the crystallinity and overall morphology depends on the molecular structural parameters of PEs. 

The infoiination obtained from an application of IR spectroscopy to a surface investigation includes the molecular structure, orientation, chemical reaction, conformation, crystallinity, and so on. 

Mechanical properties of plastics are invariably time-dependent. Rheology of plastics involves plastics in all possible states from the molten state to the glassy or crystalline state (Chapter 6). The rheology of solid plastics within a range of small strains, within the range of linear viscoelasticity, has shown that mechanical behavior has often been successfully related to molecular structure. Studies in this area can have two objectives (1) mechanical characterization of... 

In this article some literature studies together with studies conducted recently in our laboratories on the crystalline and molecular structure of polymorphic polymers are reviewed, also with the aim of showing possible influences of the polymorphism on the properties and, as a consequence, on the applications of polymeric materials. 

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. 

G. Herzberg, Molecular Spectra and Molecular Structure, Vol. II - Infrared and Raman Spectra of Polyatomic Molecules, Van Nostrand Reinhold, New York, 1945 In the crystalline state, it is more convenient to speak about multi-phonon processes since the modes from the whole dispersion range of the first Brillouin zone are allowed to contribute according to the conservation of energy and momentum of the phonons involved in the process... 

With regard to the molecular structure of CCHn, both cyclohexyl rings having a chair conformation are substituted in the equatorial positions and the alkyl chain is completely extended in the all-trans conformation. The cyclohexyl rings are nearly coplanar. The crystal structures of the investigated CCHn show that various types of molecular overlapping are present in the crystal. The molecular packing in the crystalline state is quite different in all three compounds. 

Cement formation requires a continuous structure to be formed in situ from a large number of nuclei. Moreover, this structure must be maintained despite changes in the character of the bonds. These criteria are, obviously, more easily satisfied by a flexible random structure than by one which is highly-ordered and rigid. Crystallinity implies well-satisfied and rigidly-directed chemical bonds, exact stoichiometry and a highly ordered structure. So unless crystal growth is very slow a continuous molecular structure cannot be formed. 

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