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Unformatted text preview: C h a p t e r 4 / Polymer Structures 76 T ransmission electron mi- crograph showing the spheru- lite structure in a natural rub- ber specimen. Chain-folded lamellar crystallites approxi- mately 10 nm thick extend in radial directions from the cen- ter; they appear as white lines in the micrograph. 30,000 3 . (Photograph supplied by P. J. Phillips. First published in R. Bartnikas and R. M. Eichhorn, Engineering Dielectrics, Vol. IIA, Electrical Properties of Solid Insulating Materials: Molecular Structure and Electrical Behavior. Copy- right ASTM. Reprinted with permission.) Why Study Polymer Structures? A relatively large number of chemical and struc- tural characteristics affect the properties and behav- iors of polymeric materials. Some of these influ- ences are as follows: 1. Degree of crystallinity of semicrystalline poly- merson density, stiffness, strength, and ductility (Sections 4.11 and 8.18). 2. Degree of crosslinkingon the stiffness of rub- ber-like materials (Section 8.19). h 3. Polymer chemistryon melting and glass-transi- tion temperatures (Section 11.17). j L e a r n i n g O b j e c t i v e s After careful study of this chapter you should be able to do the following: 1. Describe a typical polymer molecule in terms of its chain structure, and, in addition, how the mol- ecule may be generated by repeating mer units. 2. Draw mer structures for polyethylene, polyvinyl chloride, polytetrafluoroethylene, polypropylene, and polystyrene. 3. Calculate number-average and weight-average molecular weights, and number-average and weight-average degrees of polymerization for a specified polymer. 4. Name and briefly describe: (a) the four general types of polymer molecular structures; (b) the three types of stereoisomers; (c) the two kinds of geometrical isomers; (d) the four types of copolymers. 5. Cite the differences in behavior and molecular structure for thermoplastic and thermosetting polymers. 6. Briefly describe the crystalline state in polymeric materials. 7. Briefly describe/diagram the spherulitic struc- ture for a semicrystalline polymer. 4.1 I NTRODUCTION Naturally occurring polymersthose derived from plants and animalshave been used for many centuries; these materials include wood, rubber, cotton, wool, leather, and silk. Other natural polymers such as proteins, enzymes, starches, and cellulose are important in biological and physiological processes in plants and animals. Mod- ern scientific research tools have made possible the determination of the molecular structures of this group of materials, and the development of numerous polymers, which are synthesized from small organic molecules. Many of our useful plastics, rubbers, and fiber materials are synthetic polymers. In fact, since the conclusion of World War II, the field of materials has been virtually revolutionized by the advent of synthetic polymers. The synthetics can be produced inexpensively, and their properties may be managed to the degree that many are superior to their natural...
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