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Chapter 29 Synthetic Polymers. 29.1 Some Background. Natural and Synthetic Polymers. A polymer is a substance composed of macromolecules, molecules with a large number of atoms and high molecular weight. Starch, cellulose, silk, and DNA are examples of naturally occurring polymers.
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Natural and Synthetic Polymers • A polymer is a substance composed of macromolecules, molecules with a large number of atoms and high molecular weight. • Starch, cellulose, silk, and DNA are examples of naturally occurring polymers. • Nylon, polyethylene, and Bakelite are synthetic polymers.
Synthetic Polymers • In 1839, Charles Goodyear modified natural rubber by vulcanization. Vulcanization caused the rubber to maintain elasticity over a wide temperature range. • In 1910, Leo Baekland patented the first totally synthetic polymer, Bakelite. • Wallace Carothers discovered nylon in 1938 while working for DuPont. Unfortunately, Carothers died before he could be awarded a Nobel prize for his work.
Nomenclature • Although there are IUPAC rules for naming polymers by structure, an alternative IUPAC source-based system is used. • The source-based system names polymers according to the monomers they are made from.
Addition Polymers • Addition polymers are formed by reactions of the type: The product A-B contains all the atoms of the reactants, A + B. When A = B, the resulting polymer is a homopolymer. When A ≠ B, the resultingpolymer is a copolymer.
Condensation Polymers Condensation polymers are prepared by the formation of a covalent bond, followed by loss of a small molecule.The small molecule can be water, an alcohol, or a halogen halide.
Chain-Growth • In chain-growth polymerization, monomers add one by one to the same end of a growing chain. • Each chain has only one growth point. • The amount of monomer decreases until it is depleted.
Structure • Linear polymers have a continuous chain of repeating units. • Because of the conformations of the repeating units along the chain, chains can be random or ordered. • Random polymers are said to be amorphous. • Ordered polymers are said to be crystalline. • Branched polymers have branches which extend from the main chain. • In general, branching reduces crystallinity.
HDPE vs. LDPE Polymers which can pack more efficiently have higher density. Branching reduces density and strength. Low density polyethylene is more highly branched, and has a lower melting point. High density polyethylene is more densely packed and has fewer branch points. LDPE is used for grocery bags, while HDPE is stronger, and is used for milk jugs and gasoline tanks. (Table 29.1)
Thermoplastic Polymers Thermoplastic polymers are those that soften when heated. At the glass transition temperature, Tg, thermoplastic polymers go from a glass to a flexible state. Crystalline polymers go through a second transition which is the melting temperature, Tm. HDPE tends to have a high Tm (137oC) while LDPE has a lower Tm (115oC).
Thermosetting Polymers Thermosetting polymers pass through a liquid state upon heating and then solidify or cure. The solidified material is called a thermoset. Thermosets are formed by irreversible chemical reactions which create crosslinking. Bakelite and Melamine are both thermosets.
Elastomers Elastomers are flexible polymers. These polymers can stretch, but return to their original state. Most amorphorous polymers become rubbery past their Tg. Not all rubbery polymers are elastomers.