Branched polymers molecular configuration

There is no such thing as a pure polymer. All polymers comprise molecules that exhibit chemical and physical distributions of many variables these include molecular weight, branching, steric defects, molecular configuration, preferential chain orientation, and crystallite size and shape. The properties and characteristics that we exploit in polymers are controlled by the overall balance of these distributions. 

HDPE is a semi crystalline plastic, whose crystallinity varies from 40 to 80%. depending on the degree of branching and molecular weight. Polymer chains in crystalline HDPE have a flat zigzag configuration. The principal crystalline form of HDPE is orthorhombic, with a density of 1.00 g/cm3 and the cell parameters a =0.740, b = 0.493. and c ... 

Type polymers. If the linear and branched configurations of the fractions can be positively established, then these materials should serve as type substances for physical and colloidal research. They are unique in that they are composed of the same structural unit thus such phenomena as adsorption, coacervation, gelation, etc. can be related to molecular configuration. 

The advance of synthetic techniques in the last decade made possible the synthesis of model branched polymers of known and controlled structure and, equally important, relatively homogeneous in molecular weight and structure (204, 221, 228). Investigations (101, 136, 141, 196) (Fig. 9) of a variety of branched polymers of different branched chain configurations but of the same chemical stmctiure and total molecular weight permits direct determination of the effect of coil dimensions and configurations on rj. 

The unique architecture of dendritic polymers affords distinct physical properties to these molecules as compared to their linear polymer analogues. For example, as shown in Figure 30.2, branched molecules have a much lower intrinsic viscosity than their linear analogues. Their compact and dense molecular configuration also leads to enhanced solubility at high molecular weights. 

The configuration of a molecule refers to the fixed arrangement of the atoms in the molecule, which is determined by the chemical bonds that have been formed. The configuration of a polymer chain cannot be altered unless chemical bonds are broken and reformed. The linear, branched, or cross-linked architecture of polymer molecules, and the different types of copolymers discussed in Section 3.3 are all examples of different molecular configurations. Even within a linear homopolymer, there can be different configurations of molecules, as will be explained in Sections 3.6 and 3.7. 

Precisely branched polymers include hyperbranched polymers, dendrimers and den-drons. Dendrimers and dendrons are characterized by perfectly controlled structures in three dimensions such as tree branch architecture, and they have attractive features such as a well-ordered chemical structure, molecular mass, size and configuration of polymers [5], Although the precise order of shape of hyperbranched polymers is less than that of dendrimers and dendrons, hyperbranched polymers have unique properties such as low viscosity attributed to the lack of entanglanent of polymer segments, and the possibility of chemical modification in terminal functional groups such as in dendrimers [1-3]. 

The third kind of heterogeneity mentioned above refers to differences in the physical configuration of the macromolecules, such as those between linear and branched polymers, and also to differences in the tacticity of the several molecular species present in the mixture, which is usually reflected in varying amounts of amorphous and crystalline materials in the substance. 

Polymer products synthesized in laboratories and in industry represent a set of individual chemical compounds whose number is practically infinite. Macro-molecules of such products can differ in their degree of polymerization, tactici-ty, number of branchings and the lengths that connect their polymer chains, as well as in other characteristics which describe the configuration of the macromolecule. In the case of copolymers their macromolecules are known to also vary in composition and the character of the alternation of monomeric units of different types. As a rule, it is impossible to provide an exhaustive quantitative description of such a polymer system, i.e. to indicate concentrations of all individual compounds with a particular chemical (primary) structure. However, for many practical purposes it is often enough to define a polymer specimen only in terms of partial distributions of molecules for some of their main characteristics (such as, for instance, molecular weight or composition) avoiding completely a... 

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