Macromolecular crystals crystal lattices

Another important sub-case, of disorder in macromolecular crystals, corresponds to the statistical occurrence of two specific orientations only, at well defined positions in a 3-D lattice, of a group of macromolecules or of each single macromolecule. 

Macromolecular crystallinity differs in three important ways from low-molecular weight molecular crystallinity polymers never crystallize completely, the unit cells are always smaller than the macromolecule, i.e. the crystal lattice is formed by the subunits rather than by the whole molecule and a long polymer chain can thread through different crystallites. Complete crystallization is impeded because of the structural polydispersity and the topological constraints resulting from the fact that... 

In the polymerization scheme of reaction (9-59), insertion of a monomer results in an interchange of the polymer substituent and the lattice vacancy. These are not equivalent positions in the crystal lattice of the catalyst. Under normal polymerization conditions the macromolecular alkyl appears to shift back to its original position before the next monomer is added. Isotactic polymers are produced from olefins and catalysts of this type because the monomer always inserts by cis addition with the unsubstituted carbon of the olefin attached to the transition metal which always has the same chirality. If the polymer chain and vacant orbital were to exchange initial positions, however, then the placements of successive monomers would alternate stereochemically, providing syndiotacticity. 

Polarized analysis There is useful spectral information arising from the analysis of polarization of Raman scattered light. This, typically called as polarized analysis, provides an insight into molecular orientation, molecular shape, and vibrational symmetry. One can also calculate the depolarization ratio. Overall, this technique enables correlation between group theory, symmetry, Raman activity, and peaks in the corresponding Raman spectra. It has been applied successful for solving problems in synthetic chemistry understanding macromolecular orientation in crystal lattices, liquid crystals or polymer samples and in polymorph analysis. 

The liquid channels and solvent cavities that characterize macromolecular crystals are primarily responsible for the limited resolution of the diffraction patterns. Because of the relatively large spaces between adjacent molecules and the consequent weak lattice forces, every molecule in the crystal may not occupy exactly equivalent orientations and positions in the crystal but may very slightly from lattice point to lattice point. Furthermore, because of their structural complexity and their potential for conformational dynamics, protein molecules in a crystal may exhibit slight variations in the course of their polypeptide chains or the dispositions of side groups. 

Crystal lattices can possess a number of different defects. Some of these are characteristic of all nonmetallic solids others are specific to crystalline macromolecular substances. Phonons, electrons, holes, excitons, site defects, interstitial defects, and displacements are commonly occurring lattice defects. 

Typical crystalline macromolecular substance lattice defects result from end groups, kinks, jogs, Reneker defects, and chain displacements. Distortion of the whole crystal lattice can be conceived in terms of the paracrystal. The defects can be classified in terms of point, line, and network defects. 

Special Macromolecular Point Defects. End groups have a different chemical structure than that of the main-chain monomeric unit. They consequently produce a defect in the crystal lattice (see also Figure 5-21). 

Twin crystals based on the lattice of the repeating unit are also common in the growth of macromolecular crystals, as shown, for example, in the poly(oxyethylene) crystal of Fig. 5.55. It grew out of a small droplet of a melt. The multiple twin has its chain axis tilted to the a-axis by 126°, so that the axis shown in the figure is the... 

Let us simplify the macromolecular crystal lattice by a onedimensional row of atoms, a so-called linear chain (Fig. 10). The... 

Molecular orientation measurements of polymers are an important way of determining how a product will perform in service. The overall bulk orientation of Kevlar has been directly related to its stiffness and indirectly related to its strength. As far as structure is concerned, the orientation is a measure of the degree of order and how the macromolecular chains are placed in the crystal lattice. 

The polarization of Raman scattered light also contains useful structural information. This property can be measured by using plane polarized light and a polarization analyzer. Spectra recorded with the analyzer set both parallel and perpendicular to the excitation plane can be used to calculate the depolarization ratio of each vibrational mode. This provides insight into molecular orientation and the symmetry of the vibrational modes, as well as information about molecular shape. It is often used to determine macromolecular orientation in crystal lattices, liquid crystals, or polymer samples. 

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