Polymer chain, regularity

The diamine part has six methylene units and the diacid part four methylene units and two acid carbon atoms. Thus, it has an even number of methylene units in both die diamine and the diacid part, which gives the polymer chain regularity so it is able to crystallize easily. 

These resulting polymers are solid, linear, internally cyclized, thermoplastic structures containing unreacted allylic groups spaced at regular intervals along the polymer chain. 

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. 

If a rubbery polymer of regular structure (e.g. natural rubber) is stretched, the chain segments will be aligned and crystallisation is induced by orientation. This crystallisation causes a pronounced stiffening in natural rubber on extension. The crystalline structures are metastable and on retraction of the sample they disappear. 

In spite of possessing a flexible backbone and low interchain attraction polyethylene is not a rubber. This is because its chain regularity enables a measure of crystallinity which does not disappear until temperatures of the order of 100°C are reached. It therefore follows that if crystallinity can be substantially reduced it should be possible to obtain an ethylene-based polymer which is rubbery. The means by which this objective has been achieved on a commercial scale may be classified into three categories ... 

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. 

Introduction of bulky lateral substituents on monomer units to increase interchain distance and prevent close packing in polymer crystal. The use of unsymmetrically substituted monomers, resulting in a random distribution of head-to-head and head-to-tail structures in polymer chains, further helps in disrupting regularity. Some examples of substituent effects are given in Table 2.16. 

The synthesis of comb-like polymers with regular branching (in contrast to random branching) has been performed in the following way 91) A linear polystyrene precursor fitted with carbanionic sites at both ends is reacted first with 1,1-diphenylethylene (to decrease the nucleophilicity of the sites) and then with a calculated amount of triallyloxytriazine to get chain extension. Each triazine residue still carries one allyloxy... 

Omstein [276] developed a model for a rigidly organized gel as a cubic lattice, where the lattice elements consist of the polyacrylamide chains and the intersections of the lattice elements represent the cross-links. Figure 7 shows the polymer chains arranged in a cubic lattice as in Omstein s model and several other uniform pore models for comparison. This model predicted r, the pore size, to be proportional to I/Vt, where T is the concentration of total monomer in the gel, and he found that for a 7.5% T gel the pore size was 5 nm. Although this may be more appropriate for regular media, such as zeolites, this model gives the same functional dependence on T as some other, more complex models. 

Stereodefects reduce the overall regularity of an isotactic polymer chain and hinder its ability to crystallize. As the concentration of defects increases, the degree of crystallinity falls, resulting in reduced density, reduced melting temperatures, lower heat distortion temperatures, reduced modulus, and reduced yield stress. 

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