CH09_EOC_Questions - A NSW E R S T O END-OF-CHAPTER...

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9 - 2 ANSWERS TO END-OF-CHAPTER QUESTIONS CHAPTER 9: THE WORLD OF PLASTICS AND POLYMERS Emphasizing Essentials 1. Give two examples each of natural and of synthetic polymers. Answer: Examples of natural polymers: Cotton, silk, natural rubber, cellulose, wool, and DNA. Examples of synthetic polymers: Kevlar, vinyl, nylon, Dacron, polyethylene, polypropylene, and synthetic rubber. 2. Polymers sometimes are referred to as macromolecules. Explain. Answer: The size and mass of a polymer makes the name macromolecule seem reasonable, because the prefix macro – means large. Individual polymers may involve thousands of atoms, and molecular masses can reach over a million grams per mole. 3. Equation 9.1 contains an n on both sides of the equation. The one on the left is a coefficient; the one on the right is a subscript. Explain. Answer: The n on the left side of the equation gives the number of monomers that react to form the polymer. Thus, it is a coefficient. The n on the right side is a subscript; it represents the number of repeating units in the polymer. 4. In equation 9.1, explain the function of the over the arrow. Answer: The represents a free radical that initiates the polymerization. 5. Describe how each of these strategies would be expected to affect the properties of polyethylene. Also provide an explanation at the molecular level for each effect. a . increasing the length of the polymer chain b. aligning the polymer chains with one another c. increasing the degree of branching in the polymer chain Answer: a. At the molecular level, increasing the length of the polymer chain would increase its molar mass and the extent of its interactions with neighboring chains. This would be expected to somewhat increase the polymer’s rigidity, strength, and melting point. b. At the molecular level, aligning polyethylene chains with one another means that the structure is more crystalline and highly ordered. This would be expected to give the polymer
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9 - 3 slightly more density, more rigidity, and more strength. The melting point would also increase. c. At the molecular level, this would be just the opposite of the previous question. The structure would be less crystalline, less ordered, and possibly somewhat tangled. This would be expected to make the polymer slightly less dense, less rigid, and not as strong. The melting point would decrease. 6. Figure 9.3a shows two bottles made from polyethylene. How do the two bottles differ at the molecular level? Answer: The bottle on the left appears to be made of low-density polyethylene; the one on the right of high-density polyethylene. The molecular structures of LDPE and HDPE can help explain this difference in properties. LDPE is somewhat more branched, lessening molecular attractions between the chains and causing the plastic to be softer and more easily deformed, as shown in the photo. HDPE molecules, with fewer branches, can approach each other more closely and form regions that are more crystalline. The result is an increase in rigidity, again
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CH09_EOC_Questions - A NSW E R S T O END-OF-CHAPTER...

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