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high Tg . Decreased mobility of polymer chains, increased chain rigidity, and high Tg are found where the chains are substituted with several substituents as in poly(methyl methacrylate) and polytetra uoroethylene or with bulky substituents as in polystyrene. The Tm values of crystalline polymers produced from such rigid chains would also be high. The effects of substituents are not always easy to understand. A comparison of polypropene, poly(vinyl chloride), and poly(vinyl uoride) with polyisobutylene, poly(vinylidene chloride), and poly(vinylidene uoride), respectively, shows the polymers from 1,1-disubstituted ethylenes have lower Tg and Tm values than do those from the monosubstituted ethylenes. One might have predicted the opposite result because of the greater polarity and molecular symmetry of the polymers from 1,1-disubstituted ethylenes. Apparently, the presence of two side groups instead of one separates polymer chains from each other and results in more exible polymer chains. Thus, the effects of substituents on Tg and Tm depend on their number and identity. The rigidity of polymer chains is especially high when there are cyclic structures in the main polymer chains. Polymers such as cellulose have high Tg and Tm values. On the other hand, the highly exible polysiloxane chain (a consequence of the large size of Si) results in very low values of Tg and Tm . Although Tg and Tm depend similarly on molecular structure, the variations in the two transition temperature do not always quantitative parallel each other. Table 1-3 shows the various polymers listed in order of increasing Tg values. The Tm values are seen to generally increase in the same order, but there are many polymers whose Tm values do not follow in the same exact order. Molecular symmetry, chain rigidity, and secondary forces do not affect Tg and Tm in the same quantitative manner. Thus polyethylene and polyoxymethylene have low Tg values because of their highly exible chains; however, their simple and regular structures yield tightly packed crystal structures with high Tm values. An empirical consideration of ratio Tg =Tm (Kelvin temperatures) for various polymers aids this discussion. The Tg =Tm ratio is approximately 1/2 for symmetric polymers [e.g., poly(vinylidene chloride)], but the ratio is closer to 3/4 for unsymmetric polymers (e.g., poly[vinyl chloride]). This result indicates that Tm is more dependent on molecular symmetry, while Tg is more dependent on secondary forces and chain exibility. It should be evident that some of the factors that decrease the crystallization tendecy of a polymer also lead to increased values of Tm (and also Tg ). The reason for this is that the extent of crystallinity developed in a polymer is both kinetically and thermodynamically controlled, while the melting temperature is only thermodynamically controlled. Polymers with rigid chains are dif cult or slow to crystallize, but the portion that does crystallize will have a high melting temperature. (The extent of crystallinity can be signi cantly increased in such polymers by mechanical stretching to align and crystallize the polymer chains.) Thus compare the differences between polyethylene and poly(hexamethylene adipamide). Polyethylene tends to crystallize easier and faster than the polyamide because of its simple and highly regular structure and is usually obtained with greater degrees of crystallinity. On the other hand, the Tm of the polyamide is much higher (by $130 C) than that of polyethylene because of the much greater secondary forces. 1-6 APPLICATIONS OF POLYMERS 1-6a Mechanical Properties
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Many polymer properties such as solvent, chemical, and electrical resistance and gas permeability are important in determining the use of a speci c polymer in a speci c application. However, the prime consideration in determining the general utility of a polymer is its
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