Regularity of chain

Random E/P copolymers consist of PP chains in which small amounts of ethylene (C2) are more or less randomly distributed. The C2 units act as defects for the regularity of chain configuration and reduce the overall crystallinity of the polymer. 

Although these column fractionation methods are of necessity very slow in order to ensure thermodynamic equilibrium conditions pertain during the process, they can be very useful for the fractionation of polymers which are only soluble in solvents at relatively high temperatures, such as polyolefins. However, care has to be taken to ensure that separation is by molecular weight and not by the ability of polymers to crystallize from solution because of regularity of chain structure. 

When comonomers are introduced, this destroys chemical regularity of chain units. It thereby reduces or even eliminates crystallinity, depending on the comonomer content. An illustration which is also important in applications is provided by ethylene copolymers. If propene is introduced into polyethylene, the level of crystallinity is reduced (therefore also diminishing its modulus and density). This is because it limits the possibilities for a crystalline structure to those sequences made up of (CH2)i units. A whole family of new compounds, called Low Density Linear PolyEthylenes (LDLPE), was developed during the 1980s. Ethylene/propene (50/50) copolymers form amorphous compounds which are used as elastomers (EPDM). 

Figure 5.22 illustrates the regularization of chain aggregation state of aliphatic nylon 1010. We note that some portions of the amide groups are hydrogen-... 

Short chains of amino acid residues are known as di-, tri-, tetrapeptide, and so on, but as the number of residues increases the general names oligopeptide and polypeptide are used. When the number of chains grow to hundreds, the name protein is used. There is no definite point at which the name polypeptide is dropped for protein. Twenty common amino acids appear regularly in peptides and proteins of all species. Each has a distinctive side chain (R in Figure 45.3) varying in size, charge, and chemical reactivity. 

However, significant exceptions to all these attributes occur. The regularity of the amide linkages along the polymer chain defines two classes of polyamides AB and AABB. 

The disruption of chain regularity by the introduction of lateral substituents or kinks on repeating units is a supplementary means to decrease the melting temperature of aromatic polyesters.72 This is illustrated in Table 2.9, where the melting temperatures of unsubstituted and methyl-substituted aromatic-aliphatic and aliphatic acids are reported. Regularity disruptions often cause significant... 

Several approaches have been used, all aiming at disruption of the structural regularity of aromatic polyester chains ... 

Aromatic polyamides, 5 Aromatic polyester chains, disruption of structural regularity of, 50-52 Aromatic polyesters... 

The most commonly used siloxane modifiers are those having phenyl, trifluoro-propyl and cyanopropyl substituents. Introduction of phenyl units into the polydimethylsiloxane backbone either in the form of methylphenylsiloxane or diphenyl-siloxane increases the thermal and oxidative stability, glass transition temperature and the organic solubility characteristics of the resulting copolymers. At low levels (5-10 percent by weight) of incorporation, bulky phenyl groups also break up the regularity of polydimethylsiloxane chains and inhibit the crystallization (Tc... 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

The presence in these copolymers of hetero-substituted monomeric units randomly dispersed along the phosphazene skeleton brings about the extreme difficulty of the polymeric chains to be packed in regular structures. They lose, therefore, the original stereo-regularity of the parent phosphazene homopolymers (microcrystalline materials), and show only amorphous structures, with sharp decrease in the values of the Tg (collapsed up to about -90 °C) and with the onset of remarkable elastomeric properties [399,409,457]. 

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