Polymer Background
Science Connections: Natural Polymers
Natural polymers are those long-chain molecules of repeating smaller units found
in nature. These include proteins, lipids (fats), latex, nucleic acids (DNA and
RNA), and the carbohydrates starch and cellulose, among many others. Natural
polymers perform a huge variety of functions. For example, the polymer lignin
gives rigidity to wood, fats provide some animals with insulation, and waxes are
used by bees to construct honeycombs.
Natural polymers can be artificially altered to change or enhance their properties.
One of the first synthetic polymers made was celluloid, from cellulose as a raw
material. Natural rubber (from latex) was made harder when heated with sulfur
as a cross-linking agent. This cross-linking process, invented by Charles
Goodyear (1800-1860, American inventor), is termed vulcanization. It made the
rubber much more useful for tires and other products.
Natural polymers are also used in combination with synthetic polymers to yield a
material that has the most useful properties of both substances. Polyester fibers,
for example, are frequently interwoven with cotton fibers to make a blended
fabric. The fabric has the “breathability” of cotton and the lightness and wrinkleresistance of polyester. By reading fabric labels, it is easy to identify a large
number of synthetic/natural polymer blends that are used in modern clothing.
Definition
Polymers are a class of chemicals that occur naturally or can be made in a
laboratory. Polymers are long-chain molecules made up of repeating smaller
chemical units (monomers).
Natural Polymers
Natural polymers are long-chain molecules made by plants and animals. You
are probably familiar with materials made from many of the natural polymers, but
you may not know the names of the polymers themselves. One common natural
polymer is cellulose. This is the substance that gives plants much of their
structure. Cellulose is made up of a long chain of linked glucose molecules.
(Glucose is a simple sugar made by plants.) Other natural polymers are
starches, proteins, latex, and fats.
Synthetic Polymers
Synthetic polymers are long-chain molecules made by chemists in the
laboratory. Synthetic polymers developed in the early 1900s included BakeliteTM,
rayon, and cellophane. One of the earliest of the synthetic polymers made from
petroleum products was nylon, invented in 1939. Nylon could be spun into
fibers and was very strong. It could be woven into fabrics that were quite sheer,
with a smooth texture and high gloss. Because nylon had many of the same
properties as silk, during World War II nylon became a substitute for the silk used
for parachutes. The success of nylon products led to the invention of many other
synthetic polymers, such as rayon, DacronTM, OrlonTM, and polyester.
Once synthetic polymers appeared on the scene, their uses seemed endless.
They were used for clothing, packaging materials, containers, coatings, toys, car
parts, appliance parts, wall coverings, floor coverings, and many other products.
The properties of synthetic polymers often enhanced or complemented the
properties of natural polymers in products. Shirts made of polyester/cotton
blends, for example, have the breathability and body of cotton and the wrinkle
resistance of polyester.
As more types of synthetic polymers were developed, products became available
that were not possible before. Plastics revolutionized the packing industry. The
invention of TeflonTM made nonstick cookware a reality. Levels of stretch in
fabrics reached new heights with the introduction of LycraTM and SpandexTM
fibers. Fabrics strong enough to stop bullets became possible with Kevlar TM.
Structure and Composition
Polymers are useful and versatile because polymer chemists can modify their
structure in a number of ways:
 by changing the length of the molecular chain or its branching structure
 by changing the chemical composition of the monomer units
 by changing the way monomer units are arranged within the chain
The length of polymer chains and the presence or absence of cross-links
between chains determines the physical and chemical properties of polymers.
For example, in polyethylene (a thermoplastic with many commercial uses), as
the number of ethylene (CH2) units grows in the polymer chain, the properties of
the polymer are changed. One thousand to three thousand ethylene units linked
together yield a tough material used to make bottles and other containers.
Testing
Synthetic and natural polymers are used in many commercial products.
Consumers expect these products to meet certain standards. Products should
not facture, shatter, or melt during normal use. Therefore, raw materials are
used in the manufacturing process. The tests that technicians run in the
laboratory play a key role in determining how a specific polymer can be used, or
how it may have contributed to the failure of a product.
Polymers are tested in the laboratory for:
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behavior at different temperatures
permeability
resistance to flow (viscosity)
resistance to pull (tensile strength)
resistance to sudden impact
resistance to a pushing force (stress/strain)
They are also analyzed for appearance, texture, odor, opacity, shear, and many
other physical properties. The properties of the polymers determine the products
for which the polymers would be appropriate.
The Polymer Industry
Polymer manufacturing is a very big business. There is great, worldwide
demand for synthetic polymers, as well as natural polymers that have been
altered to make them more useful. Because polymer manufacturing uses mostly
petroleum products for its raw materials, it is classified as a petrochemical
industry. Polymer chemists working in industry laboratories make a huge variety
of new polymers every year by altering the molecular structures of existing raw
materials. Not all of the new polymers made by chemists are commercially
useful, however. It is the job of the technicians who work in the polymer research
and development to measure the properties of the new polymers and record
these data. Their test results help the chemists to recommend practical uses for
the new polymers.
Science Connections: Synthesizing Polymers
Most synthetic polymers start from monomers derived from petroleum, natural
gas, or coal, such as ethylene (C2H4) and propylene (C3H6). Chemists who
develop new polymers set up polymerization reactions to link these small
molecules into much longer chains. The process happens in one of two ways,
either by addition or by condensation.
In addition polymerization, many monomer molecules join to form long chains
through covalent bonding. An initiator molecule starts the reaction by reacting
with a monomer molecule. More and more monomers join the growing chain-a
process called propagation. (A chemical called a catalyst can be used to
increase the rate of the reaction.) Finally, a chemical called an inhibitor stops the
reaction (termination).
In condensation polymerization, a small molecule (usually water, H2O) is
released from the reaction site as monomer molecules become covalently linked.
Polymer chains can be made of several thousands to millions of monomers. The
number of monomers can affect the properties of a polymeric material. For
example, long-chain polyethylene, the polymer made from ethylene monomers,
yields a harder, stronger plastic than does short-chain polyethylene.
Science Connections: The Relationship of Polymer Properties to Structure
and Composition
The types of monomers in a polymer, as well as the type of bonding between
those monomers, the number of monomers in a chain, and the relationship
between the chains, all determine what the physical and chemical properties of a
polymeric material will be. For example, polyvinyl chloride (PVC), which has a
chlorine atom in each of its monomers (in addition to carbon and hydrogen), has
very different properties from polyethylene (PE), which has monomers made of
only carbon and hydrogen. If you are familiar with PVC pipe or house siding (an
opaque, tough solid), you can easily see that its properties are very different from
those of polyethylene, the plastic used to make food wraps and trash bags.
Differences in branching, as well as arrangements of the same type of polymer
chain, can also yield materials with different properties. Close-packing chains of
polyethylene yield high density polyethylene (HDPE), a sturdy material that can
be used for milk and detergent containers. Packing the very same chains more
loosely yields low density polyethylene (LDPE), a weaker material usually used
for food wraps and storage bags.
Science Connections: Cross-linking of Polymer Chains
Cross-linking occurs when an added substance (cross-linking agent) forms
bridges between polymer chains.
Typically, polymer chains that are not cross-linked are fairly free-moving and can
slide over each other. White glue, a mixture of the polymer polyvinyl acetate and
water, is free-flowing in its non-cross-linked form. When a solution of sodium
borate, however, is added to the glue, it cross-links the polyvinyl acetate chains
and produces a much more rigid material that can be formed into a ball that
bounces, or be pulled apart into a long strand. Cross-linking adds rigidity and
strength to polymeric materials, making them useful for a wide variety of
products.
Name: _________________
Block: ______
Date: ________________
Introduction to Polymer Questions
1. Name 3 examples of natural polymers:
2. Why are natural polymers sometimes combined with synthetic polymers?
3. What are polymers?
4. What is the most basic unit of a polymer?
5. What are synthetic polymers?
6. When were synthetic polymers first developed?
7. Identify 3 examples of synthetic polymers:
8. Describe the three ways in which the structure of polymers can be
modified:
9. What is the importance of the structure and composition of polymers?
10. In your opinion, what are the three most important polymer laboratory
tests? Explain why you believe each is important.
11. What is the main raw material used in the development of polymers?
12. Describe the two methods in which polymers are synthesized.
13. How are polymer properties related to the structure and composition of
polymers? Explain using two examples.
14. What is the benefit of cross-linking polymer chains?
15. In your opinion, what is the most important concept about polymers?