Polymers & Serendipity
Case Studies
Marcy Copeland, Yellow Medicine East High School,
Granite Falls, MN
Dan Larson, Anoka High School, Anoka, MN
Dan Morton, Roseville Area High School, Roseville, MN
Many important scientific discoveries have occurred
because researchers pursued 'chance' or accidental events. Nowhere
is this more evident than in the history of polymers and organic
chemistry. Teachers interested in teaching about the process
of science will want to convey something about the role of 'serendipity'.
Yet it can be difficult for students to appreciate through their
own lab experiences how mishaps can be transformed into successes
or how 'mistakes' can lead productively to new knowledge. One
strategy is to use historical case studies that can connect with
easy lab demonstrations. The following familiar molecules each
have a fascinating story behind them that helps convey a lesson
about the nature of science and--when linked to 'hands-on' lab
activities--can also enrich a student's understanding of fundamental
organic chemistry:
- Dyes
- Nylon
- Rayon
- Synthetic Rubber (neoprene and thiokol)
- Others (list)
These stories and related
lab activities also offer an ideal opportunity for
introducing a unit on organic chemistry into
a standard chemistry course (including polymers).
- We also recommend as supplemental reading:
- Serendipity: Accidental Discoveries in Science,
by Royston M. Roberts (New York: John Wiley and Sons, 1989).
- and the VIDEO:
- Lucky Accidents, Great Discoveries and the Prepared Mind. --
1/2-inch video #0342, NSP Film Series, 6518 Walker St., Ste.
20, St. Louis Park MN 55426.
The seeds of great discoveries are constantly
floating around us, but they only take root in the minds
well prepared to receive them. --Joseph Henry
The curriculum material here was developed as part of a project
sponsored by SciMath-MN and The Bakken Library and Museum. Click
to see directory of other
curriculum modules using history and philosophy of science in
this series.
Dyes -- Fortunate Accidents
One of the premier scientists of all time, Louis Pasteur, made
an important statement for students and educators: "In the
field of observations, chance favors only the prepared mind." Many
well trained scientists have made discoveries while seeking other
results or methods that their background learning and experience
called to the attention of their ready minds.
Mauve Dye
One such example resulted in British Patent No. 1984 for the synthesis
of mauve dye in 1856. An eighteen-year-old chemistry assistant,
William Henry Perkin, undertook the project of trying to prepare
artificially the anti-malarial drug, quinine, on his Easter vacation.
He started with a simple waste product, aniline, from coal tar.
He failed at synthesizing quinine but did produce a mysterious
black powder. Given his training and curiosity he tried to discover
what it was. He soon found that the powder dissolved in alcohol
to produce a stunning purple color. Instead of discarding the solution,
Perkin wondered if it might dye fabric. He found that not only
did it color silk and cotton, but the color did not wash out with
soap or fade when exposed to sunlight. Perkin built a factory to
produce his mauve dye and it made him a rich man, allowing him
to continue research on coal tar products. Using his accidental
experimental results, William Henry worked out the synthesis of
the red dye alizarin from anthracene, a component of coal tar.
The value of these dyes is not limited to the textile industry.
Researchers have found that bacteria can be stained and show up
for microscopy when certain dyes are used. Tuberculosis and cholera
bacilli were discovered using this technique.
Indigo
Another important dye was produced by a somewhat careless accident
and an accompanying alert observation. Indigo was an important
dye prepared from the indigo plant. In India in 1897 about two
million acres were under cultivation growing indigo plants. A chemist
named Sapper was heating some organic chemicals together when he
accidentally broke a thermometer into the mixture. (No student
would ever do this!) His prepared mind noticed a difference in
the reaction and, upon further testing, he discovered that a product,
phthalic anhydride, had been produced, which could readily be converted
into indigo. The mercury in the thermometer had produced a catalyst
that helped oxidize the coal tar component napthalene into the
unexpected but desired phthalic anhydride.
Monastral Blue
A final example occurred in 1928 in Scotland. A.G. Dandridge was
operating a chemical plant that produced phthalimide from ammonia
and molten phthalic anhydride. Since the temperatures were quite
high, the reaction was performed in a large iron sealed container.
Dandridge noticed some strange blue crystals on the cover and sides
of the container and was curious enough to collect some for examination.
he discovered that they resulted from a reaction between the iron
container and the contents. Further study found the chemical structure
of the pigments and he named them phthalcyanines. By substituting
copper for iron, he produced an even better pigment called 'monastral
blue'. This family of pigments, which have resulted in over thirty
patents, have become some of the most valuable coloring materials
for paints, lacquers and printing inks.
Click here for extended LAB
ACTIVITIES on dyeing and for more information on the history
of dyes in Colonial and Native America.
PLAYING AROUND PRODUCES WONDER FIBER--NYLON
A team of organic chemists from Du Pont led by Wallace Hume Carothers
had been trying to unravel the composition of natural polymers,
such as cellulose, silk, and rubber. From this knowledge they hoped
to develop synthetic materials that mimicked the properties of
these natural polymers. This remarkable group of chemists had developed
a group of compounds, polyamides, which had no remarkable or useful
properties.
These compounds were shelved in order to concentrate their
work on a more promising series of compounds, polyesters. Polyesters
possessed more desirable properties such as having more soluble
products, easier to handle and simpler to work with in the laboratory.
Julian Hill, working with polyester, noticed that if you gathered
a small amount of this soft polymer on the end of your stirring
rod and drew it out of the beaker, it produced a silky, fine
fiber. One afternoon when their boss, Wallace Carothers, was
not in the lab, the chemists decided to see how long a silky
thread they could produce. Hill and his cohorts took a little
ball on a stirring rod and ran down the hall and stretched them
out into a string. The realization struck them during this horseplay
that by stretching the strand of fiber they were orienting the
polymer molecules and increasing the strength of the product.
The polyesters had very low melting points, too low for textile
uses, so they retrieved the polyamides from the shelf and began
to experiment with this need 'cold-drawing process.' They found
that the strand of polyamide produced by this cold-drawing technique
produced a strong , excellent fiber. The patent for the composition
of nylon was never applied for by Du Pont, rather they chose
to patent the production process -- cold-drawing -- developed
by unsupervised adults playing around in the lab.
In January-February 1939, this consumer product hit the US
market. It is without equal in its impact before or since. Nylon
stockings were exhibited at the Golden Gate International Exposition
in San Francisco and were sold first to employees of the inventor
company Du Pont de Nemours. On May 15, 1940, nylon stockings
went on sale throughout the US, and in New York City alone four
million pairs were sold in a matter of hours.
Naming this new polymer too many twists and turns. Initially
the name norun was proposed for this new product because it was
more resistant to laddering than silk. But there were problems
and the name was then reversed to read nuron. However, it was
pointed out that this was too close to the word neuron which
may be construed to be a nerve tonic. Hence, nuron was changed
nulon. However this ran into trade mark problems and the name
was again changed to nilon. English speakers differed in their
pronunciation of this, so, to remove ambiguity the name finally
became nylon.
Two years before the basic patent on nylon had been filed,
the discoverer of nylon, Wallace Hume Carothers, suffering from
one of his increasingly frequent attacks of depression, caused
by his conviction that he was a scientific failure, drank juice
containing potassium cyanide. He would be pleased to know that
half of all the chemists in the US work on the preparation, characterization,
or application of polymers.
THE COLD-DRAWING PROCESS:
The most common nylon is nylon-6,6. The sixes refer to the number
of carbon atoms in the two monomer units (see below), one of which
is a six-carbon diacid and the other a six-carbon diamine. When
these two monomers combine they do so by eliminating a water molecule
(condensation reaction) producing a chain of alternating monomers.
In the cold-drawing process, the long polymer molecules line up
with one another so that the oxygen atom can hydrogen bond with
a nitrogen atom on an adjacent chain. The individual polymer molecules
are bound together just as strands in a rope. When twisted together,
they hydrogen bond forming fibers with great strength. This appears
to mimic the natural silk which is also extruded from the silk
worm in a cold-drawing process.
Accidental Discovery of Rayon, Artificial Silk
In the 1870s, Louis Pasteur was involved in an effort to save the
French silk industry from an epidemic affecting silkworms. His
assistant, a young chemist, Hilaire de Chardonnet, spilled a bottle
of collodion while working in the dark room. Like many of us, he
left the clean-up of the spill for another time. When he returned
to clean-up his mess, he found that the collodion had become a
tacky, viscous liquid due to partial evaporation of the solvent.
As he wiped it away, he noticed long, thin strands of fiber which
resembled silk. His observation of this fiber-like material and
the strong desire to find a silk substitute, was enough to encourage
Chardonnet to experiment further with the collodion.
Within six years after the accidental spill, a material resembling
silk had been produced. His starting material was mulberry leaves,
the natural food of silkworms, dissolved in ether and alco hol.
He drew the fibers out and coagulated them in warm air. The unveiling
of this artificial silk took place at the Paris Exposition of
1891, where the enthusiasm for this product quickly resulted
in financial backing to begin commercial production. This new
fiber was called 'artificial silk' until 1924 when the name rayon
was first used.
This 'artificial silk' was not only used for clothing, but
also to produce movie film. There were some significant problems
with this fiber, cellulose nitrate, as it was highly flammable.
This material's flammability resulted in several disastrous fires
in movie theaters when the projector jammed and the film stayed
in the path of the intense light for only a few seconds. Because
of this, if was replaced with a 'safety film' produced from cellulose
acetate.
Newer rayons have been developed. The two most common are xanthate
rayon and acetate rayon. The xanthate rayon, regenerated cellulose,
it prepared by converting cellulose into a soluble form, cellulose
xanthate. This is then extruded through fine holes into a chemical
bath that converts the cellulose xanthate back into cellulose.
This process gives the regenerated cellulose a smooth, silky
finish unlike the fuzzy appearance of cotton, a natural cellulose.
Xanthate rayon is found on labels simply as rayon.
The acetate rayon, found on labels as acetate, is prepared
in a similar fashion to Chardonnet's early rayon. Cellulose is
converted to an acetate ester, rather than a nitrate, which is
soluble and can be extruded into smooth fibers. This cellulose
acetate is not flammable, but is somewhat soluble in organic
solvents, such as acetone. The xanthate rayon is impervious to
organic solvents.
- Click to see a LAB ACTIVITY for
students to prepare cuppammonium rayon.
INTRODUCING ORGANIC CHEMISTRY INTO A CHEMISTRY COURSE
Organic molecules are among the most important
to our lives and society--plastics, fossil fuels and other
petroleum products, biological molecules (DNA, proteins, etc.),
drugs, dyes, artificial sweeteners (saccharin and aspartame)
and new fat 'substitutes' (olestra). Yet many chemistry classes
do not include them. We hope the exercises presented here and
the guidelines for a unit in organic chemistry will inspire
many teachers to include them, to expand their treatment, or
to enrich what they already teach with perspectives in the
history and philosophy of science.
A good warm-up activity is to explore a
list of'Polymers in Everyday
Life'.
- Background information on polymers can be obtained
via FTP:
- ftp://rtfm.mit.edu/pub/usenet/news.answers/polymers-faq
- or by sending e-mail message to
- mail-server@rtfm.mit.edu with
the single-line message ("Subject" line blank):
send/pub/usenet/news.answers/polymers-faq
|
