Polymer chain defects

During the polymeriza tion process the normal head-to-tad free-radical reaction of vinyl chloride deviates from the normal path and results in sites of lower chemical stabiUty or defect sites along some of the polymer chains. These defect sites are small in number and are formed by autoxidation, chain termination, or chain-branching reactions. Heat stabilizer technology has grown from efforts to either chemically prevent or repair these defect sites. Partial stmctures (3—6) are typical of the defect sites found in PVC homopolymers (2—5). 

Thermogravimetric analysis and other studies made on low-molecular weight model compounds such as 1,3, 5,-trichlorohexane [7,8] corresponding to the idealized head-to-tail structure of PVC show these structures to be considerably more stable than the polymer. This abnormal instability of the polymer is attributed to structural irregularities or defects in the polymer chain, which serve as initiation sites for degradation. 

Tacticity or stereochemical arrangement of atoms in three-dimensional space in relation to each other along the polymer chain cannot really be termed a structural defect. But researchers have shown that tacticity has an important bearing on the reactivity and thermal stability of PVC. For this reason tacticity is being discussed under this section. 

Regiodefects are less readily incorporated into crystallites than defect-free chain sequences. In semicrystalline polymers, increasing levels of misinsertion result in reduced crystallinity. This can affect numerous physical properties, resulting in reduced modulus, lower heat distortion temperature, and decreased tensile strength. 

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. 

In many polymers, degradation begins at defects in the polymer chain. Why is this true ... 

The deformation of polymer chains in stretched and swollen networks can be investigated by SANS, A few such studies have been carried out, and some theoretical results based on Gaussian models of networks have been presented. The possible defects in network formation may invalidate an otherwise well planned experiment, and because of this uncertainty, conclusions based on current experiments must be viewed as tentative. It is also true that theoretical calculations have been restricted thus far to only a few simple models of an elastomeric network. An appropriate method of calculation for trapped entanglements has not been constructed, nor has any calculation of the SANS pattern of a network which is constrained according to the reptation models of de Gennes (24) or Doi-Edwards (25,26) appeared. 

In the simpler cases, the discrimination between the two faces of the prochi-ral monomer may be dictated by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit (chain-end. stereocontrol) or by the chirality of the catalytic site (chiral site stereocontrol). The distribution of steric defects along the polymer chain may be indicative of which kind... 

In the via precursor method, however, it is difficult to prepare the ji-conjugated polymers with ideally developed -conjugation system the -conjugated polymer chains contain many conformational defects because the jc-conjugated chains are caused to develop from disordered precursor polymer, which form random coil conformation, in solid state. For the preparation of polymers with well-developed jc-conjugation system by the via precursor method, accordingly, it is necessary to introduce orientational and conformational orderliness of the precursor polymers in the films. 

More recently, it was shown by List et al. [293-296] and later by Moses et al. [246] that the green emission of the PFs is due to fluoren-9-one defects in the polymer chain. This was confirmed by comparison of PL films annealed in an inert atmosphere and in air a progressive additional band in the green region was observed on annealing in air (Figure 2.12) [246],... 

In chlorinated plastics, such as PVC, decomposition takes place by dehydrochlorination, which is accelerated by oxidation but depends principally on the presence of defect structures within the polymer chain. 

Figure 5.26. Running along the centre of the tube is a primitive chain. This is the shortest path down the tube. The deviations of the polymer chain from this path can be considered as defects. The motion of these kinks or defects in the chain away from the primitive path allows the chain to move within the tube. The polymer creeps through the tube, losing its original constraints and gradually creating a new portion of tube. This reptilian-like motion of the chain was named by de Gennes from the Latin reptare, to creep, hence reptation.

Polymer molecular properties. Making a polymer of high quality is much more complicated than making butanal, for example, because the material properties of a polymer depend heavily on a number of molecular properties. For example, 1% of mistakes in a propene polymer chain can spoil the properties of a polymer completely (crystallinity for instance), while 10% of a by-product in a butanal synthesis can be removed easily by distillation. PVC contains only 0.1% defects as allylic and tertiary chlorides and this necessitates the use of a large package of stabilisers ... 

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