- Bakelite:
The world's first commercially produced purely synthetic plastic --
a combination of phenol with formaldehyde and suitable fillers --
has been discovered by
Leo Hendrik Baekeland in 1909.
Within two years Bakelite was being produced in both Europe and America.
Bakelite is a thermoset,
a material that holds its shape permanently
once it is formed and cannot be remolded. A smash hit, Bakelite transformed
the look of the everyday world and was used to make everything from
hairbrushes to telephone housings until it was eventually replaced by
more modern plastics.
- chain connectivity:
Monomers of polymer chains are connected by bonds along the
chains
(mainly by covalent bonds).
Normally, chemistry does not allow bonds to cross.
- conductive polymers:
Most polymers are insulators, which is a good thing, because they are
often used to insulate electrical wires. But in the late 1970s it was
discovered that polymers can also conduct electricity. This can be
achieved either through the addition of carbon or metal flakes or by
adding chemicals like iodine and sulfur trifluoride to the polymer chain.
Conductive polymers are promising because they are much lighter than metals
and can potentially be used to make lightweight batteries for electric cars,
for example. Conductive polymers also have potential use in electronics and
solar collectors.
- fiber:
polymeric fiber is a polymer whose chains are stretched out straight
(or close to straight) and lined up next to each other,
all along the same axis:
Polymers arranged in fibers like this can be spun into threads and used as
textiles. The clothes you're wearing are made out of polymeric fibers.
So is carpet. So is rope. Here are some of the polymers which can be drawn
into fibers:
polyethylene,
Kevlar,
Nylon,
polyester,
cellulose,
polyurethanes,
polyacrylonitrile,...
It's important to point out that fibers are always made of polymers
which are arranged into crystals.
They have to be able to pack into a
regular arrangement in order to line up as fibers.
(In fact, fibers are really a kind of crystal, a really long kind of crystal.)
We can show this by taking a closer look at the way nylon 6,6 packs into
a crystalline fiber:
Those hydrogen bonds
and other secondary interactions between the individual chains hold the
chains together very tightly. So tightly that they don't particularly
like to slide past one another.
This means that when you pull on the nylon fiber it won't stretch very much,
if at all. This is why fibers are good for using as rope and thread.
While fibers have good tensile strength, that is,
they're strong when you pull or stretch them, they usually have bad
compressional strength, that is, they're weak when you try to
squash or compress them. Also, fibers tend to be strong only in one direction,
the direction in which they're oriented. If you pull in them in the direction
at right angles to their orientation, they tend to be weak:
Because of this weird combination of strengths and weaknesses,
it is often a good idea to use fibers with another material,
such as a thermoset.
Often fibers are used to reinforce thermosets.
The fiber compensates for the thermoset's weaknesses,
but the thermoset's strengths make up for the fiber's weaknesses as well.
When a thermoset or any other polymer is reinforced with a fiber like this,
it's called a
composite material.
- Kevlar:
A special type of polyamide, the aramid Kevlar,
entered commercial production in the early 1970s.
Kevlar, a very rigid rod polymer, forms an opalescent solution
when dissolved, the first observed polymer liquid crystal.
(Paul Flory had predicted the existence of polymer liquid crystals
some years earlier.)
Aramid fibers are woven into
bullet-proof vests, tire cords and are even integrated into
structural components for aircrafts.
Figure:
A fragmentation-resistant vest containing Kevlar.
Kevlar vests have saved the lives of more than 2000 police officers.
The bullet embedded in this vest was fired to demonstrate the vest's
effectiveness. The bullet flattened on impact.
- macromolecule:
a single molecule containing many thousands of atoms
linked together by
covalent bonds.
Macromolecules are also called
polymers.
- molecular weight (molar mass):
the total weight of all the atoms in a molecule. The weight of each atom is
simply its total number of protons and neutrons in the atom. Thus, the molecular
weight of a molecule is the total number of protons and neutrons in all the
atoms in that molecule. Obviously, the more atoms there are in a molecule,
the higher its molecular weight will be.
A water molecule has a molecular weight of 18. A sugar (sucrose) molecule has
a molecular weight of 342. Macromolecules have molecular weights in the
thousands or even millions.
A good sample of
nylon
normally has an average molecular weight of around 18,000.
- monomer:
molecule being a building block and repeat unit
of a polymer chain
- molecule:
group of atoms being the repeat unit of a polymer chain.
- neoprene:
one of the most widely used synthetic polymers.
Neoprene has somewhat poorer rubbery properties than
natural rubber but exhibits much greater oil resistance.
The chemical name for this elastomer is polychloroprene,
neoprene being DuPont's trade name.
- nylon:
synthetic fiber obtained by polymerization of
adipic acid (C6) and hexamethylene diamine (C6).
Nylon was designed in the DuPont research group
of Carothers (1896-1937)
to replace natural silk.
In May 1940 nylon stockings hit the hosiery stores in the US.
At just over one dollar a pair, five million pairs were sold on
the first day. When the States entered the Second World War and
arrived in the UK, a few pairs of nylons could buy anything!
Figure left:
With the rising hemlines of the 1920s, stockings became an important part
of women's fashion, but silk, the fiber of choice, was expensive and had to
be imported from East Asia. Nylon stockings first became available in 1939,
when the Second World War had begun to disrupt trade routes, and demand for
them quickly outstripped production capacity. After the United States entered
the war, nylon became almost unavailable for civilians, and supply did not
catch up with demand until well after the war's end in 1945.
This customer couldn't even wait to get home before trying hers on.
Figure right: Before World War II, parachutes were made of Japanese silk.
When supplies were cut off by the war, DuPont persuaded the army to
try nylon as a substitute.
- plastic:
The word plastic comes from the Greek "plastikos," meaning "moldable."
Most polymer plastics are formed through the application of heat and pressure.
Thermoplastic polymers can be reheated and reshaped after once
being molded.
Thermosetting
polymers retain their shapes even at relatively high
temperatures and cannot be reformed. An example for the latter class
of polymers is Bakelite,
the world's first commercially produced synthetic polymer.
- polyamide:
a macromolecule
that contains many amide groups in its chemical structure.
Nylon,
proteins,
and
Kevlar,
are all polyamides.
- polyacrylonitrile:
- polyester:
- polyethylene:
Polyethylene is the most popular plastic
in the world. This is the polymer that makes grocery bags, shampoo bottles,
children's toys, and even bullet proof vests. For such a versatile material,
it has a very simple structure, the simplest of all commercial polymers.
A molecule of polyethylene is nothing more than a long chain of carbon atoms,
with two hydrogen atoms attached to each carbon atom:
Sometimes some of the carbons,
instead of having hydrogens attached to them, will
have long chains of polyethylene attached to them.
Linear polyethylene is much stronger than branched polyethylene,
but branched polyethylene is cheaper and easier to make.
Linear polyethylene is normally produced with molecular weights
in the range of 200,000 to 500,000, but it can be made even higher.
Polyethylene with molecular weights of three to six million is referred to
as ultra-high molecular weight polyethylene, or UHMWPE.
UHMWPE can be used to make
fibers which are so strong they
replaced Kevlar for use in bullet proof
vests. Large sheets of it can be used instead of ice for skating rinks.
Lightweight yet durable, polyethylene is ideal for food manufacturing,
food handling, light industrial, and similar industries where splash
protection is necessary. This thin, polymer material is an economical
way to keep clothing clean and protected from water based liquids and
certain chemicals.
The figure on the right shows a polyethylene shower cap.
- polymer:
from the Greek "polymeres" meaning "having many parts";
giant molecule in which N monomeric repeat
units are connected to form long chains.
[G79,
DE86,
RC03]
(More precisely, this definition refers to linear homopolymers,
i.e., linear chain molecules consisting of one monomer species only.
By contrast, polymer chemistry can nowadays synthesize various other
topologies, such as cyclic, star- or H-polymers.)
Most polymers are formed by
monomers
held together by
covalent bonds.
The typical chain size exceeds that of a monomer by several orders of magnitude.
However, contrary to granular materials, the chain is not so large
that thermal energy would be unimportant. Not at all!
Thermal energy is the important energy scale for polymers.
It provokes conformational transitions so that the polymer
can assume a multitude of different configurations at ambient conditions.
Polymer materials fall in the more general classes of
soft matter
and
complex fluids.
Some polymers occur naturally, such as starch,
protein,
and cellulose.
Starch, as you know, is found in many foods (who can resist the beloved
starchy potato?). Cotton and wood are made of
cellulose.
And your silk dress or tie? Protein.
A real cultural and commercial revolution came when chemists began to
synthesize polymers in the lab. These polymers, appropriately called
synthetic polymers, can be found in everything from plastic bags
to rubber tires,
from water-proof fabrics to cell phones.
If you've heard of
nylon,
rayon,
Kevlar,
Teflon, or Tupperware, then you've heard of polymers.
- polymerization:
the creation of a long chain through the joining of smaller groups.
It ceases whenever the length of the molecule becomes so
great that no additional groups are available or by a
specific termination step creating an intert chain end.
- polyurethane:
- rubber:
Natural rubber latex (caoutchouc)
is useful in warm climates,
but it becomes stiff and brittle in the cold. What's more,
it gets runny and sticky when it gets too warm. That
all changed in 1839, when Charles Goodyear (1800-1860) accidentally spilled a
mixture of rubber latex, sulfur, and white lead on a hot stovetop.
The result was a rubber that was more resistant to temperature extremes.
Goodyear named the process vulcanization.
Vulcanized rubber became the wonder material of the 1800s due to the ease
to control its toughness and elasticity by varying the amount of added sulfur.
In 1888 John Dunlop (1840-1921) spurred the craze for bicycling and stimulated
the demand for caoutchouc (natural rubber) by placing an air-filled rubber tube
on his son's bicycle. Seven years later, the name Dunlop identified a
twenty-five-million-dollar pneumatic tire business.
The birth and boom of the automobile in America, led by the Model T,
brought the center of world rubber consumption to the United States.
The great rise in demand for rubber made the search for a commercially
viable synthetic product attractive long before the crisis of the 1940s.
The Germans had experimented with making rubber from butadiene, a petroleum
byproduct, using the metal sodium to initiate the polymerization of
butadiene molecules.
By 1930 two variants of these Bu-na (butadiene + Na or sodium)
rubbers had been developed, in which butadiene linked in a chain
with either styrene (Buna-S) or with acrylonitrile (Buna-N).
- vinyl polymers:
Vinyl polymers are polymers made from vinyl monomers; that is, small
molecules containing carbon-carbon double bonds. They make up
largest family of polymers.
An example is polyethylene.
Polyethylene is made from the monomer ethylene, which is also called
ethene. When polymerized, the ethylene molecules are joined
along the axes of their double bonds, to form a long chain of many
thousands of carbon atoms containing only single bonds between atoms.