POLYMERS & PLASTICS
Polymer - poly = many, mer = parts includes many natural and synthetic materials which
can be further classified as organic or carbon based and inorganic or non-carbon based
and can be regarded as being assembled from a simple repeating units held together by
covalent bonds.
Natural Polymers
Organic: wool, linen, cotton, hemp, jute, wood, coal, spiders’ web, silk, starch, cellulose,
proteins, latex rubber, chicle (chewing gum), etc.
Inorganic: sand, clay, asbestos, volcanic glass (obsidian), etc.
Synthetic Polymers
Organic: polyethene, polypropene, polyvinyl chloride, perspex, polycarbonate, poly vinyl
acetate, teflon, neoprene rubber, etc.,
Inorganic: pottery, cement, concrete, silicones, etc.,
Although all the above are polymeric materials the term polymer usually refers to the
synthetic organic polymers often referred to as plastics or synthetic fibres (or synthetics).
Many are household names, e.g., nylon, terylene, teflon, rayon, acrylic, polyester, perspex,
polystyrene, etc., and most are derived from natural petroleum. All polymers are assembled
from repeating units. The repeating unit may be derived from one or more monomers. There
are many ways of classifying these synthetic organic polymers as indicated below:
hermoplastic and thermosetting - the former soften and melt on heating and can be
moulded, e.g., polyethene, the latter decompose on heating and must be shaped at
manufacture, e.g., bakelite. addition and condensation - the former are made from the simple
addition of monomers, e.g., polyethene from the addition of ethene, the latter are usually
synthesized from two different monomers with the elimination of biproduct, e.g., nylon 6-6
from hexandioyl chloride and 1,6-diaminohexane with elimination of HCl. homopolymer and
copolymer - the former are assembled from one repeating unit, e.g., propene in polypropene,
the latter are based on two different repeat units, e.g., nylon.
atactic and isotactic (also syndiotactic) - the former has chemical groups attached to the
polymer chain randomly on either side of the carbon chain, the latter has the chemical groups
attached to the same side of the carbon chain, e.g., atactic and isotactic polypropene,
syndiotactic polymers have the groups alternately on oppostite sides of the chain.
crystalline and amorphous - the latter are stronger and have the polymers chains stacked
together in a more or less ordered form, e.g., high density polyethene (HDPE), the former are
of lower strength and have the polymer chains randomly arranged because of branching or
bulky side groups, e.g., low density polyethene (LDPE) or atactic polypropene.
linear (or straight chain), branched and cross-linked - the former have long chains of
carbon atoms only, e.g., natural latex rubber, branched chain polymers as the name indicates
are branched, cross-linked polymers have polymer chains joined together by other atoms, e.g.,
vulcanized rubber.
Teacher Tips
Use paper clips to act as monomers and these can be linked together to give chains, branched
chains and cross-linked chains. Coloured paper clips could be used to model homo- and copolymers.
Use uncooked spagetti to model a crystalline polymers -many sticks of spagetti lie side by side
in an ordered fashion; cooked spagetti is a model for an amorphous polymer.
The Structure of Polymers - Polythene
Polythene is a polymer formed by the polymerization of ethene. Its formula can be
variously represented as CnH2n+2 (n = large integer 100 - 200 000) or CH3-(-CH2-)n-CH3 or (-CH2-CH2-)-n etc., all of which amount to the same thing, that is long chains of CH2 groups
though at the ends of the chains there must be CH3 groups to ensure that each carbon atom
always forms four bonds. The value of n in the above formula is variable in polyethene from
hundreds to hundreds of thousands and the average value of n is a measure of the average
chain length - because of this variation polyethene is not, strictly speaking, a pure compound
as the molecules are not all the same it is better to call it a mixture of hydrocarbons. The
simplest hydrocarbon is methane CH4 (n=1) which is familiar as natural gas; bottled heating
and cooking "Calor" gas is propane CH3CH2CH3 (n=3); camping gas and lighter fuel is
mainly butane CH3CH2CH2CH3 (n=4); petrol is mainly CH3CH2CH2CH2CH2CH2CH2CH3
or octane; paraffin wax for candles has on average twenty carbon atoms in a chain, i.e.,
typically C20H42; the waxes found in shoe polish and vaseline have upto 70 carbon atoms in
the chain. The chains are not straight chains but in fact zig-zag chains - the term straight
refers to the fact that they are not branched! The zig-zag nature of the chain arises because
when we look down a carbon-carbon bond the atoms attached to adjacent carbon atoms
should always be "staggered" rather than "eclipsed" - this is known as the staggered
conformation and is more stable. Notice also that as the chain of carbon atoms gets longer
the materials change from gases to volatile liquids to oils to waxes to solid plastics. The only
bonds in these compounds are covalent bonds between the atoms in the molecule (C-C and
C-H bonds which are strong) but there are no bonds between the molecules. So what is
holding the molecules together? Why are all hydrocarbons not gases like methane? There are
very weak intermolecular forces called London or dispersion forces that give rise to van
der Waals interactions between molecules. These forces of attraction between molecules
arise because electrons in the covalent bonds are always in motion and at any instant a pair
of electrons may not be equally shared between the two atoms of a covalent bond, i.e., there
will be instantaneous bond polarization Cd +-Hd -. This bond polarization induces bond
polarization in neighbouring molecules leading to very weak electrostatic attraction
bewteen molecules. In methane however the van der Waals forces are to small to overcome
the thermal motion of the molecules due to their kinetic energy at room temperature and
methane is a gas. As the temperature is lowered and the thermal kinetic energy gets less then
methane eventually liquefies and at very low temperatures freezes. Although molecular size
increases in longer hydrocarbon chains, the van der Waals forces do not increase and the
thermal energy remains the same as for methane at a given temperature. But, because the
molecules are heavier, the speed at which they move becomes less (kinetic energy is 1/2mv 2
m = mass, v = speed) and are thus less likely to be able to escape from the weak van der
Waals attraction of another molecule. Also longer molecules have more van der Waals
contacts as shown below. The lecture diagram showed sections of zig-zag polythene stacked
together in a highly idealized manner. In practice polymer chains are never stacked as
regularly as shown above for several reasons:
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the polymer chains are very flexible and can be twisted into other orientations,
the chains tend to form loose spirals or coils,
the chains may be branched,
not all polymers are ordered or crystalline.
Low density polythene LDPE is obtained by polymerization of ethene at high pressure
(1000 atmospheres) and high temperature (200° C) which leads to a waxy solid in which the
polymers chains have irregular branches and cannot pack together in an ordered way shown
above - there are still van der Waals attractions between the chains but these are of random
orientation. The result is that LDPE is an open polymer of low density and little mechanical
strength and rather a tangled mess at the molecular level. Polythene can also be prepared
catalytically by use of Ziegler-Natta catalysts at lower pressures (1-10 atmospheres) and
temperatures (50-100° C). The catalyst is a metal compound and the polymerization occurs at
the metal atom in a conveyor belt fashion which gives rise to a regular non-branched chain
polymer which is highly ordered or crystalline with regular van der Waals attractions between
chains and is known as high density polyethene HDPE. HDPE is tough and strong and the
ordered structure means that it has higher density HDPE and LDPE can be distinguished by
their transparency: LDPE is fairly transparent (sandwich bagsand bread bags) whereas HDPE
is whitish and at best only translucent (supermarket carriers & plastic milk bottles). Even in
HDPE the ordering of the polymer chains is not complete - there are definite regions of the
polymer where the chains are packed in an orderly fashion and these regions are called
crystallites. The number of crystallites determines the crystallinity of the polymer. In
between the crystallites are areas where the polymer chains are more randomly arranged. The
crystallites act rather like the cross-links in a thermosetting polymer and by holding the
polymer chains tightly together give HDPE the strength and toughness that are lacking in
LDPE. Although van der Waals attractions are weak (5-20 kJ mol-1) when concentrated in an
orderly fashion in the crystallites they can give a polymer strength. Polythene is very familiar
as the plastic of plastic carrier bags from supermarkets which are made of thin HDPE. The
way HDPE is manufactured and its structure ensures that most the polymer chains are
arranged so as to give the internal structure a fibrous nature or grain. As with any "grained"
material the strength varies according to whether tension is applied across or with the grain,
this can be demonstrated with a supermarket carrier bag.
Cut out two similar large squares (6" x 6") from a plastic carrier bag noting carefully which
edges correspond to the top and bottom and side of the bag. Make a small cut in one edge of
each square - the top edge of one square and the side edge of the other square.
Grasp each square in turn firmly with both hands and pull gently so as to tear each square
starting at the small cut. Repeat the experiment several times to convince yourself (or not as
the case may be) that the polymer does not tear as easily in both directions. As supermarkets
bags become thinner, for reasons of economy, this experiment becomes less reliable but you
should find that the bag tears more easily from top to bottom than from side to side. This is
because the polymer chains are aligned from top to bottom and tearing in this direction or
with the grain involves breaking mainly weak van der Waals attractions. Tearing from side to
side or against the grain must involve breaking not only van der Waals attractions but some
covalent bonds as well which requires more energy and is thus more difficult. The bags made
in this away because most of the strain is in the vertical direction which parallel to the grain
and would tend to pull the bag apart perpendicular to the grain. Next cut out a vertical strip
from your bag. It should be about 1" or 2.5 cm and 6" or 15 cm long. Grab hold of each end of
the strip and pull gently but steadily - observe and describe what happens and try to monitor
changes in stress in the plastic as you increase the strain. If necessary repeat the experiment
several times to ensure that it is reproduceable. When you start to pull on the strip nothing
happens at first except a little stretching but then as you increase the applied strain one or
more "necks" form in the strip and as you continue to increase the strain the necks get longer
as material appears to "flow" into the necks from the taut regions, either side of the neck,
which get smaller. Eventually the neck grows to the whole length and the polymer becomes
stiffer and ridged - it is difficult to stretch at this stage and eventually, as you increase the
strain beyosd a certain point, the strip snaps.
We can explain what is observed by considering the changes on a molecular scale. The
individual polymer chains are held together by van der Waals forces so that the chains are
parallel within crystallites. However, although the chains are parallel, they are not straight but
are randomly coiled. When tension is applied there is a tendency for the uncoiling to occur
which results in a highly aligned molecular structure -this occurs as the "necks" are formed in
the strip of plastic. The uncoiling and alignment continues until all the molecules are highly
aligned and at this stage the strip has become noticeably more rigid and ridged reflecting the
highly organized linear structure. The uncoiling and alignment can occur because van der
Waals attractions are weak and easily broken and reformed. Eventually when enough strain is
applied the highly aligned molecules slide past each other, overcoming the van der Waals
attractions, and the polymer breaks. The high level of strain required to break the polymer
explains why it is so difficult to tear open a bag of frozen peas - you have to tug and pull very
hard and then suddenly it rips open and you’ve got peas all over the kitchen floor! It is much
easier to open the bag if the manufacturer provides a weakness or flaw into the packaging
which allows you to concentrate your applied tension in one area.
Change of State
A convenient way of identifying a pure solid organic compound is by the determination of its
melting point, e.g., pure benzoic acid crystals C6H5COOH have a melting point of 122° C.
HDPE is an organic compound but differs from other organic compounds in two ways:
i.
ii.
it is only partly crystalline or ordered unlike benzoic acid crystals which are almost
perfectly ordered,
it is not a single compound but is a mixture of compounds of formula CnH2n+2 where n
is large and variable, i.e., it is not a pure substance.
HDPE thus does not have a sharp melting point. When we heat it up slowly nothing happens
for a while then there comes a temperature at which it starts to gradually change from a hard
solid to a treacle like substance. The temperature at which this starts is known as the glass
transition temperature, so called because glass behaves in a similar way - it starts to soften
and flow but does not melt to a runny liquid.Chewing gum, either based on natural chicle
from the sapodilla tree or synthetic based on poly(vinyl acetate), is a relatively brittle
material at room temperature - a stick of gum straight from the packet is not chewable.
However most chewing gum has a glass transition temperature of around 25-30° C (pva gum
28° C) and in the mouth, temperature 37° C, it becomes the soft chewy material we are
familiar with.
The glass transition temperature varies from polymer to polymer and depends on:
(i) the average chain length,
(ii) the rigidity of the polymer chains,
(iii) the strength and nature of interactions between chains,
(iv) the degree of crystallinity.
When polymers soften (we should not use the word melt) they do not become runny liquids why not? Ice melts to become water! The reason is that when polymers soften each molecule
cannot behave independently, like a small molecule, because they are all entangled and there
are still van der Waals attractions, although these are random rather than ordered. To use the
cooked spagetti analogy again, the softened polymer chains are like a tangled mass of cooked
spagetti - try to move one string of spagetti on its own, it cannot be done! In a true liquid (a
molten solid) each individual molecule can behave independently - this is not possible in a
softened polymer. The glass transition temperature marks a boundary between little or no
molecular movement (solid) and restricted molecular movement (treacle-like).Cotton has a
glass transition temperature of 225° C so cotton fabrics keep their shape because the cotton
molecules cannot move at room temperature. Cotton is a carbohydrate polymer of glucose
which contains lots of hydroxy groups, -OH, and these can hydrogen bond (another story for
another lecture) to -OH groups in another polymer chain. The inter-chain hydrogen bonding is
much stronger than van der Waals attractions in polyethene so cotton is a much tougher
material than polyethene and can form strong fibres which can be formed into threads, which
cannot be done for polyethene. The high glass transition temperature reflects the strength of
the forces between the polymer chains. However cotton absorbs water which, because it
contains -OH groups, will also hydrogen bond to the polymer - one reason why
undergarments are cotton rich is because they are absorbent. Water acts as a plasticizer or
lubricant between the chains allowing them to move more freely and lowers the glass
transition temperature to 20° C. Cotton shirts and blouses thus crease most where they absorb
most moisture and are under most pressure - inside the elbows, under the arm pits, where they
are tucked into trousers, etc. Where they are not subject moisture or pressure they remain
quite flat and uncreased. The lubricating power of water is used when we use a steam iron to
press our cotton shirts or blouses - the high temperature speeds up molecular motion and
make the pressing process more rapid, it could be done at 20° C but it would be a very slow
job! PVC (polyvinyl chloride) is a rigid polymer that is difficult to process for most
applications but we are all familiar with pvc in clingfilm, shower curtains, waterproof
clothing, pvc wellies, synthetic upholstery and all these applications rely on substantial
amounts (upto 50%) of plasticizers. The plasticizers in clingfilm had a bad press a few years
ago when people worried about migration from the film to the food. A typical plasticizer is
dibutyl phthalate - it is responsible for the oily film that condenses on the inside of the
windscreen of a new car as it is released from the vinyl upholstery and dashboard and is also
responsible for the smell of a new car. Of course when you have your new double-glazing
installed you want only the best - UPVC frames - which should be strong and rigid and so are
made from Unplasticized PVC.
COTTON and LINEN
The nature of cotton was briefly discussed in the last lecture. Cotton and linen are both
natural cellulose polymers based on glucose as the repeating unit. They both form strong
fibres that can be woven into threads. Polyethene does not form strong fibres and cannot be
woven into threads. This difference in behaviour can be explained by the different nature of
the forces of intermolecular attraction between the polymer chains. In polyethene the only
forces between the polymer chains are van der Waals attractions. In polymers based on
cellulose there are, in addition to van der Waals attractions (these are always present),
hydrogen bonding interactions. Hydrogen bonds are about five times stronger than van der
Waals attractions but only about one tenth of the strength of a covalent bond.
van der waals attraction 5-10 kJ mol-1
hydrogen bond 25-50 kJ mol-1
covalent bond 250-500 kJ mol-1
The hydrogen bond occurs between hydrogen atoms that are themselves attached to oxygen or
nitrogen and oxygen or nitrogen atoms in another molecule. -OH groups are known as
hydroxyl groups and N-H groups as amino groups or, in some cases, amido
groups.Hydrogen bonding arises because oxygen and nitrogen are very electronegative
elements, i.e., they strongly attract electrons towards themselves in compounds, and as a
result O-H and N-H bonds are very polar. The electrostatic force of attraction of the
positive hydrogen end of an O-H or N-H bond for another electronegative oxygen or nitrogen
atom in another molecule results in the hydrogen bond. Hydrogen bonds are about twice as
long as a normal covalent bond, e.g., O-H or N-H typically 90 to 100 pm and O----H and N---H 180-200 pm [pm = picometre or 10-12 m]. Don’t try to remember the structure of cellulose!
Just remember that it is a polymer of glucose, which is a carbohydrate, and has lots of
hydroxyl groups -OH that can hydrogen bond to oxygen atoms in other polymer chains
giving rise to strong intermolecular attractions leading to a strong polymer that can be
woven into threads.
WOOL, SILK and HAIR
Other natural fibres that contain hydrogen bonded polymer chains are wool and hair,
examples of animal fibres. These are however a different sort of polymer as the repeating
groups are amino acids and this sort of polymer is a protein which is a polyamide known as
a polypeptide. The term polypeptide is reserved for polymers containing the amide link
derived from amino acids. As the lecture diagram showed, there is scope for hydrogen
bonding between amido groups. There are, however, many different amino acids so the
protein chains in animal fibres can have a variety of different repeating units made up of
different combinations of amino acids. The different repeating units leads to different
possibilities in the secondary structure of the animal fibre because of different hydrogen
bonding possibilities not only between polymer chains but within the polymer chain. a Keratin the protein of wool, human hair, nails and feathers has the individual polymer chains
coiled into a right handed helix, or a -helix, held together by hydrogen bonds. The helical
chains of wool and hair are then coiled about one another, held together by further
hydrogen bonds, to form a superhelix which accounts for the thread-like nature of these
proteins.
Another protein known as silk fibroin has different protein chains hydrogen-bonded together
to give a puckered layer structure known as a b -pleated sheet. (The hydrogen bonding
pattern is very similar to the pattern of van der Waals forces between zig-zag chains of
polyethene.) However in the case of wool or hair this is not the complete story. One of the
amino acids that make up the protein of hair and wool is cysteine HSCH2C(NH2)CHCOOH.
When two cysteine molecules in adjacent protein chains are close together they react
together, or are oxidized, to give cystine which contains an S-S bond that holds the protein
chains by covalent bonds, the protein or polymer chain is crosslinked. The crosslinks that
hold the chains together give a very durable fibre that retains its shape. The crosslinks
survive stretching and wool is springy and when when pulled or stretched returns
immediately to shape when released. The curliness in hair is due to the S-S bonds in cystine
which hold the hair protein chains together. The S-S bonds can be removed by reducing
agents. In the reduced form the hair can be brushed, combed, rollered and held into a new
style. The hair can then be re-oxidized to form new S-S bonds which then retain the new style.
This is the basis of the permanent wave or perm of the ladies’ hairdressers - and the
process is responsible for some of the stranger odours found in those establishments! A
sulphur crosslink is also found in vulcanized rubber (see below).
SYNTHETIC POLYAMIDES - NYLON and KEVLAR
Synthetic polymers that contain the amide link are known as polyamides and have the same
repeating units throughout unlike the natural proteins. Nylon and Kevlar are examples of
polyamides. Like proteins the polymer chains are held together by hydrogen bonds but they
do not have the complex hydrogen bonding within the chains that give rise to the complex
spiral structure of some proteins. Nylon was discovered in 1935 by an American Wallace
Carothers (1996-1937) who worked in the Research Division of the chemical company Du
Pont - however despite his discovery he was obsessed with self-failure and committed suicide
before the large scale manufacture of nylon (after New York and London) began in the U.S.
during WW2.Nylon 6-6 or nylon 66 is a condensation copolymer made from hexandioyl
(adipyl) chlorideand 1,6-diaminohexane. In the laboratory last week you saw the preparation
of nylon 68 in the nylon rope trick - this used octandioyl (sebacoyl) chloride as one of the
monomers (two more carbon atoms in one of the monomers). In the reaction hydrogen
chloride (hydrochloric acid gas) is formed or "condensed" from the reactants. Another type of
nylon called nylon 6 is a homopolymer made from only one monomer called caprolactam.
Caprolactam, a ring amide, is heated with water to give a chain compound 6aminohexanoicacid which on strong heating forms the condensation polymer (releasing
water) nylon 6. In caprolactam the amide groups are formed intramolecularly to form rings
whereas in nylon 6 the amide links are formed intermolecularly to give long chains. In
addition to van der Waals forces and hydrogen bonding in nylon there are dipole-dipole
attractions between chains. These are electrostatic attractions between polar bonds that are
similar to, but weaker than, hydrogen bonds. In addition to side to side bonding between
chains by hydrogen bonding the additional attractions can give rise to bonds to other polymer
chains above and below a polymer chain. As a result of all three inter-chain attractions nylon
is extremely strong and tough. Thin extruded nylon wires, like fishing line, can withstand
surprisingly high tensional stress. Nylon can be used to prepare cogs for motors and gears
because it is very hard wearing and abrasion resistant, it is lighter and more durable than
metal cogs and requires less lubrication. Nylon, because it is so strong, can be spun into very
fine threads suitable for weaving into cloth. In the 1960s there was a vogue for nylon shirts
(BriNylon was the British product) -they were drip-dry and required little or no ironing but
they did not keep their shape like cotton and were non-absorbent and "sweaty" to wear. Nylon
shirts were however very hard wearing and did not shrink or stretch - one snag was that the
cotton stitching wore out long before nylon material. What gave you street cred in 1960s
would definitely make you a nerd today! Nylon curtains were a failure too, as nylon is
degraded by ultra-violet light in sunlight.
Kevlar is another type of polyamide called an aramid (from aromatic amide) prepared from
the reaction of 1,4-diaminobenzene and 1,4-benzenedicarboxylic acid. Any compound
containing the ring of carbon atoms found in benzeneis said to be aromatic - the name arises
because many simple benzene compounds have very characterisitic odours. The benzene rings
are planar and ensure that the polymer chain is almost flat. The flat nature of the Kevlar
chains means that they hydrogen bond together to give sheets and these sheets are stacked one
upon another and are held together by dipole-dipole attractions. The highly organized or
crystalline nature of Kevlar has been demonstrated by a special form of X-ray microscopy
called XANES and contributes to its enormous strength
Kevlar cables are many times stronger than steel cables and much lighter and corrosion
resistant and find application in marine engineering. Bullet proof vests, windsurfing sails that
can stand 60 mph gales without tearing, skis, golf clubs, aircraft parts, gaskets and brake
linings are all manufactured from Kevlar. The comparative strengths of some polymers is
given below.
Kevlar 20-30 gram/denier
Rayon 1-2
Nylon 66 3-10
Nomex 4-5.5
Nomex is very similar to Kevlar but the amide groups are attached to the benzene rings in a
different ori
RUBBERS or ELASTOMERS
Natural rubber is a unique polymer and was exploited by South American Indians to make
waterproof clothing and footwear long ago. Columbus in 1493 recorded that Indians played
with a ball made from a tree gum ". . . which tho’ heavy would fly and bound better than
those fill’d with Wind in Spain." [J. Chemical Education, 1990]. Rubber arrived in Europe in
1735 as "caoutchouc" from the Indian word "caa" (wood) and "o-chu" (to flow or weep).
Joseph Priestley coined the word India Rubber in 1770 when he found that it erased pencil
marks on paper better than using breadcrumbs. Charles de la Condamine observed that
Amazon people poured rubber tree sap on their feet to make waterproof boots - instant
wellies! He did not record whether they were also practised at instant birth control! In 1823
Charles Mackintosh made a rubber solution that enabled him to make a sandwich of rubber
between two layers of cloth and made the first mackintoshes.
Rubber is derived from the sap or latex from the rubber tree Hevea braziliensis. The sap is a
suspension of a natural polymer in water which, on on acidification, coagulates and is made
into sheets of crêpe rubber, a tacky elastic waterproof material. Michael Faraday found that
rubber was made up of C5H8 units or cis-isoprene units . Gutta percha is another natural
polymer of trans-isoprene which differs in the arrangement of atoms about the double bond it is hard and horn-like and was used to make golf balls.
The polymer chains in rubber, because of the shape of the repeat unit and its double bond, are
strongly coiled and held as coils by van der Waals attractions. When pulled the coils
straighten out to become extended chains and the rubber stretches. When the tension is
removed the rubber chains return to their orginal coiled arrangement. The stretched coils do
not form new van der Waals attractions, and hence retain their stretched shape, because the
rubber polymer chains are not packed tightly together because of the side chain (CH3)
attached to the double bond. Rubber is an example of an elastomer. Natural rubber is a
thermoplastic and in hot weather becomes soft and sticky and in cold weather hard and
brittle. In 1839 the American Charles Goodyear made the accidental discovery that the
addition of sulphur to natural made a much more useful polymer that was stable to heat and
cold and no longer soluble in organic solvents but still rubbery. He had invented the process
of vulcanization. When natural rubber is heated with sulphur a reaction occurs between
some of the double bonds in neighbouring rubber chains and the sulphur to give sulphur
cross-links between the chains. This gives rise to changes Goodyear noticed. The more
sulphur is added the greater degree of cross-linking occurs - elastic bands are soft and very
stretchy and have only small amount of cross-linking. Tyres are made of extensively crosslinked rubber which is quite hard and bouncy rather than elastic. 1% cross-linking actually
improves the elasticity of rubber as it prevents molecular slippage. The situation is very
similar to that in wool described earlier. Much rubber is now synthetic including neoprene,
butyl rubber (pond and landfill linings), polybutadiene (modern golf balls) and SBR
(Styrene Butadiene Rubber) for modern tyres, chloroprene (like ordinary rubber except CH3
group replaced by Cl) which has a greater resistance to oils, heat and oxygen, BUNA S or
GRS which is a copolymer rubber of butadiene and styrene.
CONTACT LENSES
For many years contact lenses were made of glass which had to be individually ground, were
"hard" on the cornea, and inflexible. About 50 years ago "perspex" contact lenses were
introduced which was more easily machined than glass, was less prone to chip or break if
accidentally dropped, and were physiologically harmless to the eye. However it was stil
"hard" on the eye - what was wanted was a flexible contact lens.Perspex is an addition
polymer of methyl methacrylate or methyl2-methylpropenoate and is known as
poly(methyl methacrylate) or PMMA. By replacing one of the methyl groups with an
hydroxyethyl group to give 2-hydroxyethylmethacrylate a new polymer can be prepared
which has all the desireable properties of PMMA but has some additional useful features. The
presence of the hydroxyl group allows the polymer to absorb water by hydrogen bonding
which lowers the glass transition temperature and it becomes a clear flexible gel - the
absorbed water is a plasticizer. Water also effectively lubricates the contact surface between
the cornea, the eyelid and the contact lens and the contact lenses are much less irritating.This
new polymer is known as a hydrogel and finds application not only as a contact lens but also
in lens replacement after cataract removal.
POLY(ETHYLENE TEREPHTHALATE) (PET) or TERYLENE (POLYESTERS)
This polymer is the most notable example of a polyester which contains the ester linkage and
is a condensation polymer made by elimination of water between molecules of 1,2-ethanediol
(ethylene glycol - antifreeze) and terephthalic acid (benzene-1,4-dicarboxylic acid).
PET can be manufactured into a wide variety of different forms - as thin films it is the base of
magnetic tape, as thicker film it is used for "boil-in-bag" food packaging, a thicker layer is
used for soft-drink bottles and oven ready trays for convenience foods, and in the fibrous form
it can be woven into polyester materials, e.g., TeryleneTM, CrimpleneTM or DacronTM.
A look at the structure of PET shows that there are no hydrogen atoms attached to the oxygen
atoms so there is no possibility of hydrogen bonding between polymer chains. The inter-chain
attractions in polyesters are of van der Waals and dipole-dipole nature between the C=O or
carbonyl groups. Polyester is stronger than polyethene (van der Waals only) but less strong
than nylon (van der Waals + hydrogen bonds + dipole-dipole).
BIOPOL
Biopol (ICI 1980s) is a polyester copolymer of 3-hydroxypentanoic acid and
hydroxybutanoic acid that is slowly and harmlessly dissolved in the body so it is used in
sutures (stitches). The interesting feature of this polymer is that it is produced by
fermentation. A bacterium, Alcaligenes eutrophus, is cultured in a medium containing
glucose, pentanoic acid etc. As the bacteria grow they manufacture the polymer to store as a
food reserve (much as we store fat) and by the end of fermentation the dried bacteria may
contain 70% or more by mass of the polymer. The polymer can be made from entirely natural
sustainable materials (plant sugars) and is also biodegradeable as the ester bonds are slowly
attacked by water (hydrolysed) and the products are gradually oxidized in the environment to
carbon dioxide and water. In May 1997 the Cooperative Bank announced that its bank cards
were to be made of Biopol to emphasise its credentials as the the U.K.’s "green" bank.
GLASS
Pliny the Elder recorded that Phoenician sailors came ashore and, not finding any local
materials suitable for supporting their cooking pots above the sandy shore, used lumps of of
trona (natural soda or sodium carbonate) that formed part of their ship’s cargo. As the
trona was heated in the fire it combined with the sand to give a material that flowed. This
story may not be entirely accurate but points to the fact that glass as a man-made material has
a long history and some of the oldest dated Middle Eastern pieces are over 4000 years old; a
deep blue charm is estimated to date from 7000 BC. Egyptians and Romans fashioned a wide
range of intricate and coloured glass objects. A highly developed glass industry in Syria in
1500 BC. In medieval times the centre of the glass blowing expertise was Venice (1200-1600
AD).
The serendipitous preparation of glass by the Phoenician sailors is basically the same as that
used today. A mixture of purified sand is heated with sodium and calcium carbonate together
with some sodium sulphate. The gases evolved help to stir the mixture. The addition of
calcium is necessary to make the glass insoluble in water - simple sodium glass is water
soluble to give a very viscous liquid known as water-glass (used as an egg preservative in
WW2). For those who like the chemistry:
Na2CO3 + SiO2 -> Na2SiO3 + CO2
CaCO3 + SiO2 -> CaSiO3 + CO2
Na2SO4 + SiO2 -> Na2SiO3 + SO3
Glass made as above is known as soda-glass. Replacement of sodium with some pstassium
give a harder glass that is familiar as window and bottle glass. The molten glass is made by a
continuous process and is floated on a bath of pure molten tin, with which it does not mix, and
cooled to give flat smooth sheets. The scale of production is enormous - in 1939/40 a glass
melting plant poured out a 51" wide sheet of glass without interruption for 600 days - each
day 3.25 miles of glass poured from the plant which is 46,000 tons or 42,100,000 square
feet!!! Common glass is sodium (+ potassium) calcium metasilicate but by replacing the metal
ions (Na,K,Ca) by other metal ions (Pb,Ba,Fe,Co) and by replacing the silicate (SiO3) with
borate (BO3) or phosphate (PO3) a wide variety of different glasses can be made.
Write down what you consider to be the most important advantages and disadvantages
of glass.
Let us look at some of the features you have listed. Hopefully you have placed transparency
as one of the most important advantages of glass.
Transparency - the ability to transmit (conduct) visible light without distortion. What
other common materials are transparent? most liquids and some plastics (perspex,
polycarbonate). Note that coloured glass is transparent but only to selective wavelengths of
light. What have many of these materials in common? They have non-ordered structures they all have the "liquid" structure, i.e., molecules in close contact but free to move. Glass
can be considered as a very viscous liquid. Does glass flow like a liquid? Yes it does! When
stained glass windows at Chartres Cathedral were restored it was found that many pieces of
glass were much thicker at the bottom than at the top because the 500 or 600 year old glass
has flowed under the influence of gravity. Glass, like many polymers, does not melt - it
softens into a treacly liquid above the glass transition temperature (see polymer notes). Any
of you who use a dishwasher may have noticed that glass items that are regularly washed in a
dishwasher become cloudy or, in extremes case, opaque. This devitrification occurs because
certain components of dishwasher detergents chemically scour the glass surface and induce
the growth of tiny crystals which continue to grow (much like clear honey eventually
crystallizes) and render the glass opaque. The devitrification extends into the glass and cannot
be removed by polishing.
Structure and Bonding
The structure of glass can be regarded as a mixture of an ionic compound (containing cations
and anions) and a covalent polymer(containing long branched chain molecules). The basic
structure comprises long branched polysilicate chains that are sometimes known as network
formers. The chains are tangled and branched and even when molten the chains are not free
to move and molten glass is viscous. The silicon atoms can be, partly or wholly replaced by,
e.g., boron in borosilicate glass or PyrexTM or phosphorus in crown glass (optical glass). The
lecture diagram showed a tiny fragment of a polysilicate chain. Each silicon is covalently
bonded to four oxygen atoms and each oxygen can either form two covalent bonds to
silicon OR forms one covalent bond and carries a negative charge. In order to balance the
accumulated negative charge on the silicate chains there are metal cations that randomly
occupy suitably sized cavities in the silicate network. The bonding between the cations and
the chain anions is ionic.
When simple glass is subject to stress - dropped on to a hard surface or hit with a hammer - it
shatters readily because it is brittle. This brittle nature arises, in part, from the ionic nature of
glass - ionic compounds are usually brittle, e.g., a large crystal of rock salt will shatter is
struck with a hammer. However the way in which glass breaks is different to that of ionic
compounds. Ionic compounds contain highly regular arrays of ions and split or cleave cleanly
between layers or rows of ions - ionic compounds display good cleavage. Glass does not
cleave - it fractures - or breaks randomly to give curved (conchoidal) surfaces with sharp
edges. The random fracture arises, in part, because of the lack of order on the molecular
scale.
Why does glass sometimes shatter when heated suddenly? There are three properties of sodaglass that lead it to shatter:
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expands significantly when heated,
is a poor conductor of heat,
the structure contains voids.
When hot water is poured into a cold glass vessel the inner wall of the glass in contact with
the hot water expands. However because glass is a poor conductor of heat the outer walls
remain cool and do not expand but remain in contact with the expanded inner surface. The
temperature difference thus sets up strain in the glass which may lead to cracking or
shattering. Thin walled tumblers are much less likely to crack as a result of thermal stress
than are thick walled tumblers. The expansion of glass is concentrated around voids in the
structure of glass. An article manufactured out of thick glass needs careful annealing (heat
treatment) and slow cooling to remove strain - the 20 ton glass mirror used to manufacture the
200" reflecting mirror of the Mount Palomar telescope required one year’s cooling!
PyrexTM or borosilicate glass contains about 12% boric oxide replacing some silicate and
contains much less sodium and more aluminium. The resultant glass contains more branching
and cross-links and fewer voids, expands very little when heated, is not subject to the thermal
stresses of common glass and is widely used for oven-to-table ware. More sophisticated
glasses can be used to manufacture pans that can be placed directly onto a gas flame or red
hot electric cooker ring.
Ultra low expansion glass VycorTM is made by moulding a vessel from glass and then
leaching out the metal oxide with strong acid to leave only the silica network. The vessel is
then baked at high temperature wherupon it fuses and shrinks to give a translucent glass
resembling quartz that can be plunged from red-heat into ice-cold water without shattering.
Glass can also easily shatter as a result of impact and in many situtations this is highly
undesireable. Car windscreens are regularly hit by flying grit and insects - a cockchafer beetle
hitting a windscreen at 40 mph packs quite a punch! It is essential that a windscreen such be
able to withstand such impacts and, if it does fail, that it does so safely. The glass windscreen
is carefully annealed or tempered during manufacture to strengthen it and also incorporates a
polymer interleaving which prevents the glass shards from dispersing if the windscreen
shatters. The dis-benefit is that if the windscreen shatters whilst the car is in motion the driver
cannot see a thing through the crazed glass until he/she is able to break a hole in the
windscreen. Specially toughened glass AmourplateTM and HerculiteTM are used in the
construction industry (glass doors etc.) and are manufacture to shape, then heated to softening
point and chilled suddenly to leave the surface of the glass under a good deal of compressive
tension - such glass is four or more times stronger than ordinary glass but cannnot be
machined further. Glass can also be reinforced with steel mesh to give security glass (burglar
proof). What colour is ordinary glass? Look through a section of the base of a milk bottle and
you’ll see that it is perceptibly green. The old ink-bottle, shown to you in the lecture, is more
distinctly green. The green colour is due to ferrous Fe2+ ions in the glass which come from
impurities in the sand - mainly iron oxide (Fe3+)(O2-)3 (rust). The Fe3+ ions are reduced to Fe2+
during the manufacture of the glass. However by using purified sand the green colour is
usually not a problem. An alternative is to add oxidizing manganic ions Mn4+ (MnO2 or
pyrolusite) during manufacture which oxidize green Fe2+ to almost colourless Fe3+ (the black
MnO2 is reduced to almost colourless Mn2+). However for certain applications, e.g., optical
fibres, the glass must be absolutely transparent (100% transmittance or light or zero
absorbance). Fibre optic cables transmit information as light pulses rather than electic pulses.
Optical fibres are usually double glass with an outer sheath of lower refractive index which
prevents light escaping out of the fibre. The high purity optical glass is drawn out into fine
fibres (5-100 µm diameter) which are then packed into bundles of several thousand. The
bundles are strong but retain the flexibility of individual fibres. Such a bundle of fibres can
transmit 30 000 times more information than an equivalent diameter copper communications
cable as well as being cheaper and lighter. Fibre optic technology has contributed to the
communications revolution and a fibre optic cable link Europe with the States and three
cables link Japan with the States. An undersea fibre optic cable will (or may already) connect
the U.K. with Japan. Fibre optics are also used in endoscopes for internal bodily examination
and, in conjunction with lasers, in controversial key-hole surgery.
For other purposes many other different sorts of glass may be required. These are just a few
other types of glass:
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lead crytstal (or flint glass) - by the addition of lead oxide gives a brilliant glass of
high refractive index that can be easily cut and polished is obtained,
crown glass - 0% silicon 5% boron 57% phosphorus used in lenses and other optical
applications, barium flint glass is used in bifocal lenses,
amber glass - for protection of light sensitive liquids (drugs, chemicals) contains Fe3+
and S2- which make the glass dark brown, also green glass for wine (Fe2+ rich glass),
cobalt blue glass - contains Co2+ and was formerly used for poisons and by chemists
in flame tests to mask the yellow colour of sodium ion which is a common
contaminant,
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white opaque glass - is made by the addition of calcium fluoride (CaF2 fluorspar) or
stannic oxide (SnO2) to the glass and is used in the food and drink packaging industry,
nuclear storage glass - molten glass is an excellent solvent for metal oxides which are
accommodated in the voids in the structure; metallic nuclear waste (caesium, cobalt,
strontium etc.) is converted into oxides and dissolved in molten glass (vitrification) to
give a relatively inert shiny black glass which is stored in sealed steel containers
underground in geologically sound areas (nimby!),
one-way glass, heat/light reflecting glasses, low transmission, and insulating
glasses are designer glasses manufactured to meet a wide range of security and
construction purposes,
optically variable or photochromic glass is used in spectacles and car windows in
sunny climates and is borosilicate glass containing silver and copper compounds - the
silver ions are photochemically reduced by copper ions to silver metal in strong
sunlight causing the glass to darken, when the light fades the reverse reaction occurs
and the glasses becomes light again.
low light Cu+ + Ag+ -> Cu2+ + Ag strong light
Fibre glass is coarsely spun conventional glass and is widely used in insulation and the
manufacture of fireproof cloth (replacing dangerous asbestos) in safety curtains and fire
blankets. Mixed with resins, glass fibre forms light rigid materials for construction of boats,
skis, canoes etc. Glass fibre filter papers are used by chemists to filter corrosive solutions.
http://www.lbl.gov/MicroWorlds/Kevlar/KevlarIntro.html - an excellent MicroWorlds
resource with many simple ideas, experiments, analogies, and demonstrations.
http://www.lexmark.com/ptc/book.html - an introduction to plastic by the Plastics
Technology Center.
http://www.umr.edu/~wlf/ - polymer chemistry hypertext, an educational resource compiled
by students, a bit heavy going.
http://www2.ncsu.edu/ncsu/pams/science_house/ctchem.html - not exactly polymers but
some recipes for some gooey colloids that can be made in the kitchen including slime, silly
putty, clear slime, glurch and cat’s meow??? Have fun!