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Polymers - Topic Overview
History is often described in terms of the
materials employed in making useful items - the Stone Age, the Bronze Ag, and
the Iron Age. The 20th century is appropriately known as the
Polymer Age, since most items we use daily are made from polymers.
Natural polymers have existed since the
beginning of life itself - polymers such as DNA, RNA, proteins, cellulose, and
starch. Over two thousand years ago, the Chinese developed paper making;
Central American natives learned how to make rubber balls from the juice of
certain trees. In the 1800s scientists and inventors began to convert
natural polymers into products with useful products.
Charles Goodyear learned how to vulcanize rubber;
Christian
Schonbein changed cellulose into nitrocellulose; John Hyatt made celluloid
from nitrocellulose and camphor; Louis Chardonnet produced artificial silk from
nitrocellulose.
Large-scale production of purely synthetic
polymers began in 1909 when
Leo
Baekeland developed the phenolic resin he called Bakelite. The polymer
industry grew exponentially, and today it is almost impossible to list all the
various polymers available. In the 1920s, the term "plastics" was used to
describe these new materials. It is estimated that 75% of all chemists
working today are in some field of polymers and plastics.
Simply described, polymers are giant
molecules made up of long chains of repeating units. Poly means
"many" and mer means "part." All polymers are made up from
small units, called
monomers, joined in long chains by covalent bonds. For
example, polyethylene is made from many ethylene molecules, and polystyrene is
made from styrene molecules.
The process of polymerization can proceed
in two general ways: addition and condensation, depending upon the types
of monomers used. In
addition polymers (chain-growth polymers), one monomer simply
adds to another, and another, etc. Chains of polyethylene can have
10,000 - 20,000 ethylene units. In
condensation polymers (step-growth polymers), two different
monomers are joined together with the elimination of a small molecule, like
water. Nylon and polyester are examples of condensation polymers.
If the polymer chains are cross-linked, the
product will usually be much more rigid and resistant to heat.
Thermoplastic polymers can be heated and remolded many times, and
thermoset polymers cannot be remolded once they have been
originally shaped because they are cross-linked. Heat does not soften
thermosets; they simply char if exposed to flame. Polyethylene and
nylon are thermoplastics; Formica counter tops and epoxy putty are thermosets.
Toothbrush handles are made of a thermoplastic polymer, while handles of kitchen
utensils are fabricated from thermoset polymers.
Glossary
People
Demonstrations & Labs
Background Webquest
Polymers and People
Timeline of Plastics
From Crude Oil to Plastics
Lessons in Plastics
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