Polymers

Polymers are large molecules
composed of smaller molecules called monomers.

Monomers are produced and either grow together
or are assembled to produce a single polymer.

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There are synthetic and natural polymers. Some
examples of natural polymers would be wood,
starches, fingernails, and hair. Synthetic polymers
are usually referred to as plastics. Petroleum, is the
primary monomer used to produce polymers. An
English chemist named Alexander Parkes was the
first scientist to produce the first synthetic polymer
in 1862. John Wesley Hyatt, an American, was
the first person to produce a useable polymer two
years later. He named the product celluloid. The
prime virtue of polymers is a high
strength-to-weight ratio. Industrial-strength
polymers surpass titanium in tensile strength. To
add strength and improve flexibility, polymers are
sometimes fortified with short-fiber additives,
mostly fiberglass. This is known as a polymer
composite. One particular polymer has three times
the strength of tempered steel and is being used in
bullet proof vests. Another composite will be used
to fasten together the sections proposed space
stations. Polymers have also been used in cars,
including the Chevrolet Camaro and the Pontiac
Fiero. New polymers are being created with more
strength and flexibility by combing two chemically
different polymers and producing a block
copolymer. Combinations of block copolymers
and composites and intended for use in booster
rockets and in materials of Earth-orbiting
installations. Most common polymers are usually
solid, but a new class of polymers is being
introduced in a liquid crystal state. Although these
polymers still have the physical characteristics of
liquid, they are structured more like solids. Many
liquid crystals are transparent at one temperature
and colored at another temperature. This makes
them suitable for use in liquid crystal displays, such
as in digital watches, hand-held calculators, and
lap-top computers. A new liquid polymer,
consisting of a mixture of iron and nickel, is being
used to make metal links that can be used in
paper, glass, and on electronic circuit boards.

Despite the development and widespread use of
polymers, scientific understanding is still sketchy.

Polymer development has occurred through trial
and error. Scientific shortcomings are becoming
more apparent in the search for polymers that can
meet the demands for high technology of today.

The new study is on the microstructure of
polymers while still in a liquid state. The purpose is
to learn how the solid-state structure is developed.

The ultimate goal is to be able to predict
properties from a specific material under a
constant set of processing conditions.
Science

Polymers

Polymers Polymer, substance consisting of large molecules that are made of many small, repeating units called monomers, or mers. The number of repeating units in one large molecule is called the degree of polymerization. Materials with a very high degree of polymerization are called high polymers. Polymers consisting of only one kind of repeating unit are called homopolymers. Copolymers are formed from several different repeating units. Most of the organic substances found in living matter, such as protein, wood, chitin, rubber, and resins, are polymers.

Many synthetic materials, such as plastics, fibers, adhesives, glass, and porcelain, are also to a large extent polymeric substances. Structure of Polymers Polymers can be subdivided into three, or possibly four, structural groups. The molecules in linear polymers consist of long chains of monomers joined by bonds that are rigid to a certain degree-the monomers cannot rotate freely with respect to each other. Typical examples are polyethylene, polyvinyl alcohol, and polyvinyl chloride (PVC). Branched polymers have side chains that are attached to the chain molecule itself. Branching can be caused by impurities or by the presence of monomers that have several reactive groups.

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Chain polymers composed of monomers with side groups that are part of the monomers, such as polystyrene or polypropylene, are not considered branched polymers. In cross-linked polymers, two or more chains are joined together by side chains. With a small degree of cross-linking, a loose network is obtained that is essentially two dimensional. High degrees of cross-linking result in a tight three-dimensional structure. Cross-linking is usually caused by chemical reactions.

An example of a two-dimensional cross-linked structure is vulcanized rubber, in which cross-links are formed by sulfur atoms. Thermosetting plastics are examples of highly cross-linked polymers; their structure is so rigid that when heated they decompose or burn rather than melt. Synthesis Two general methods exist for forming large molecules from small monomers: addition polymerization and condensation polymerization. In the chemical process called addition polymerization, monomers join together without the loss of atoms from the molecules. Some examples of addition polymers are polyethylene, polypropylene, polystyrene, polyvinyl acetate, and polytetrafluoroethylene (Teflon).

In condensation polymerization, monomers join together with the simultaneous elimination of atoms or groups of atoms. Typical condensation polymers are polyamides, polyesters, and certain polyurethanes. In 1983 a new method of addition polymerization called group transfer polymerization was announced. An activating group within the molecule initiating the process transfers to the end of the growing polymer chain as individual monomers insert themselves in the group. The method has been used for acrylic plastics; it should prove applicable to other plastics as well.

Bibliography Polymer, Microsoft Encarta 98 Encyclopedia. 1993-1997 Microsoft Corporation. All rights reserved. Science.

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