Polymers are a large class of materials consisting of many small molecules (called ) that can be linked together to form long chains, thus they are known as .
A polymer can have any of three basic molecular shapes. The shape is determined by the functionality of the monomers which make up the polymer. The three configurations are:
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Linear |
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Branched |
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Network |
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For example:
many methylene groups -CH2- tied together creates polyethylene.
CH3 - CH2 - CH2 - CH2 - ... - CH2 - CH2 - CH3
or
CH3 [ - CH2 - ]n - CH3 (a shorthand method)
or finally
[ - CH2 - ] n or -CH2CH2-
n is a subscript to count the number of repeat units. This last shorthand method
requires that you remember to add a hydrogen atom to each end group (CH3 on the
end rather than CH2.)
Other shapes of the polymers include:
Star polymers (i.e. dendrimers); Comb polymers; Ladder polymers.
Summary of polymers:
Inorganic
Organic
Natural
Synthetic
Polymers are useful because they are lightweight, are corrosion resistant, are easy to process at low temperatures, and are generally inexpensive.
Some important characteristics of polymers include their size (or molecular weight), softening and melting points, crystallinity, and structure. The mechanical properties of polymers generally include low strength and high toughness. Their strength is often improved using reinforced composite structures.
3. Industrially Important Polymers
Polymers are a very important industry. Four specific polymers dominate the market because of their low cost and ability to be produced in high volume. About 85% of the world plastics consumption is from just four polymers. These polymers are produced in high volume at very low cost. They are all thermoplastics.
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Polymer |
Repeat Unit |
Applications |
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electrical wire insulation, flexible tubing, squeeze bottles |
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carpet fibers, ropes, liquid containers (cups, buckets, tanks), pipes
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packaging foams, egg cartons, lighting panels, electrical appliance components |
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bottles, hoses, pipes, valves, electrical wire insulation, toys, raincoats |
Exceptions:
Some metal complexes attached on the polymer side chain.
Example:
4. Polymer types:
Homopolymer- a polymer containing a single repeat
unit.
Copolymer- A mixture of two polymers. It may be
composed of two bifunctional units and may alternate to give a well-defined
recurring unit or the two different monomers may be joined in a random fashion
in which no recurring unit can be defined. A copolymer contrasts with a
.
A
copolymerization that results in A-B-A-B-A- etc., is called an
.
A copolymerization where the sequence of A's and B's is random,
A-A-B-A-B-B-A-B-A-B-B-B-A etc is an example, is called a ____________________.
what if it isn't alternating, but it isn't completely random either?
A block copolymer is built from first one polymer, and then another, as
in A-A-A-A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B-B-B-B.
A graft copolymer is shown below, where a polymer of 'B' was grafted onto
a polymer of 'A'.
-A-A-A-A-A-A-A-A-A-A-
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B
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B
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B
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Crosslinkage- The formation of crosslinks. Long polymer chains form because each bifunctional monomer unit has two "bonding sites" so it can link to two other monomers. You should be able to see how C=C gives two bonding sites in addition polymerization. Now, if you include monomers which possess three bonding sites (see A* below), then when one of these trifunctional monomers is incorporated into a polymer chain, it has a third site that monomers can attach to:
- A - A - A* - A - A - A - A -
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A
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A
Degree of Polymerization (n)- the number of monomer units that have polymerized together.
Polymer Processing Fundamentals, By Tim A. Osswald, Hanser Gardner Publications, 1998.
Polymer Principles, 1999.
Polymer Chemistry, American Chemical Society (1986).
Smith, William F., Foundations of Materials Science and Engineering, McGraw Hill, NY (1993).
Billmeyer, F, Textbook of Polymer Science, 3nd ed., John Wiley and Sons, Inc., NY (1984).
Young, R. J.; Lovell, P. A. Introduction to Polymers, 2nd ed. Chapman & Hall: London, 1991
Munk,
P. Introduction to Macromolecular Science; Wiley: New York, 1989