Polymers have ranges of properties which are not available in traditional materials. The properties of different polymers depend upon their structure and bonding.
A polymer molecule is a long molecule made up from lots of small molecules called monomers. The process of joining small molecules together to form polymers is called polymerisation. If all the monomer molecules are the same, and they are represented by the letter A, an A-A polymer forms:
... A + A + A + A ... ... - A - A - A - A - ...
Polyethene and pvc are examples of A-A polymers.
If two different monomers are used, an A-B polymer is formed, in which A and B monomers alternate along the chain:
... A + B + A + B ... ... - A - B - A - B - ...
Nylon 6,6 and polyesters are examples of this type of A-B polymer.
There is a short-hand way of writing polymers, e.g. for polypropene:
where n is a large number e.g. 10,000.
Polymer properties vary widely. Polymers which are soft and springy, which can be deformed and then go back to their original shape, are called elastomers, e.g. rubber.
Polyethene is not so springy and when it is deformed it tends to stay out of shape, undergoing permanent or plastic deformation. This is an example of a plastic.
Stronger polymers, which do not deform easily, are suitable for making into threads and woven. These are called fibres, e.g. nylon.
The physical properties of a polymer, such as its strength and flexibility depend on:
Thermoplastics are polymers without cross links between the chains. The intermolecular forces between the chains are relatively weak. The chains can slide over each other. When this happens the polymer changes shape and can be moulded. Over 80% of plastics are like this, e.g polyalkenes and polyamides like nylon.
Thermosets have extensive cross-linking which prevent the chains sliding over each other. The polymer stays set in the same shape when heated. It cannot be re-moulded, e.g. bakelite.
In general, the longer the polymer chain, the stronger the polymer. There are two reasons for this:
Areas in a polymer where the chains are closely packed in a regular way are said to be crystalline. The percentage of crystallinity in a polymer is very important in determining its properties. The more crystalline the polymer, the stronger and less flexible it becomes.
When a polymer is stretched (cold-drawn), a neck forms. In the neck the polymer chains line up producing a more crystalline region. Cold-drawing leads to an increase in strength.
The first polyethene which was made contained many chains which were branched. This resulted in a relatively disorganised structure of low strength and density. This was called low density polyethene (ldpe).
Ziegler used organometallic catalysts to produce polythene with little branching along the chain. The chains could pack together more closely resulting in more crystalline regions. The result was high density polyethene (hdpe) which was stronger and more dense.
Natta used Zieger's catalyst to polymerise propene. The reaction mixture contained two forms of polypropene - a crystalline form and an amorphous (non-crystalline) form.
In the crystalline form, the methyl groups all have the same orientation along the chain. This is called the isotactic form. In the amorphous form, the methyl groups are randomly orientated. This is called the atactic form.
Polymers with a regular structure are said to be stereoregular.
Isomers are two molecules with the same molecular formula, but differ in the way the atoms are arranged. Geometrical isomers are one form of stereoisomers which have identical molecular formulae, the atoms are bonded together in the same order but the arrangement of atoms in space are different. Optical isomers are another form of stereoisomers. Geometrical isomerism is also called cis-trans isomerism.
Cis-trans isomers occur in organic compounds which contain a carbon-carbon double or triple bond. An example is in but-2-ene.
The cis form is where the substituent groups are on the same side of the double bond. The trans form is where they on opposite sides.
Alkynes are hydrocarbons which contain a carbon-carbon triple bond, e.g. ethyne (C2H2):
H - C C - H.
When ethyne is polymerised, the product contains alternating single and double bonds. Due to geometrical isomerism, two forms of polyethyne are produced: cis-polyethyne and trans-polyethyne. By using a Ziegler-Natta catalyst it was possible to produce just the cis form which on warming was found to conduct electricity.
Amines are organic compounds which contain the -CO2H functional group. Carboxylic acids contain the -CO2H functional group. An amine group reacts with a carboxyl group to produce an amide group -CONH-. A molecule of water is eliminated. These are called condensation reactions.
Diamines and dicarboxylic acids which contain reactive groups in two places in their molecules can be linked together to form a chain. Polymers made where the monomer units are linked together by amide groups are made by a process called condensation polymerisation.
Nylon is an example of this which can be made from:
These polymers are called polyamides or nylons.
Usually the acid chloride derivative of a carboxylic acid is used rather than the carboxylic acid because it is a faster reaction, e.g.
|n H2N(CH2)6NH2||+||n ClCO(CH2)8COCl||-[NH(CH2)6-NH-CO-(CH2)8CO]-n||+||2nHCl|
[The first digit refers to the number of carbon atoms in the diamine, the second digit to the number of carbon atoms in acid].
Polyesters are condensation polymers made from dicarboxylic acids and diols (they have the OH group at each end of the molecule).
Nylon has found many uses in engineering because of its strength, toughness, rigidity and abrasion resistance. This is due to the strong intermolecular forces between the polymer chains. Due to the presence of the N-H and C=O groups in the chains, hydrogen bonding and permanent dipole - permanent dipole attractions are possible between the chains. In polyethene the much weaker instantaneous dipole-induced dipole attractions are only present, hence polythene is a much weaker polymer.
Kevlar is an aromatic amide -aramid, it contains benzene rings in the chain.
Kevlar is extremely strong because the polymer chains line up parallel to each other held together by hydrogen bonds to form sheets. The sheets then stack together regularly to give an almost perfectly ordered structure.
Solids on heating eventually melt to form a liquid. With polymers it is not so simple. Rubber on cooling (in liquid nitrogen) becomes brittle or glassy. Many polymers have a mixture of ordered (crystalline) regions and random (amorphous) regions. In the glassy state the tangled chains in the amorphous region are frozen so movement of chains is not possible. The polymer is brittle.
If the glassy material is heated, the chains reach a temperature at which they can move. This temperature is called the glass transition temperature Tg. Above this temperature the polymer is flexible. At the melting point, the crystalline regions break down and the polymer becomes a viscous liquid.
The glass transition temperature of a polymer can be changed by two different ways:
Most plastics are not degradable because decomposer organisms do not have the enzymes to break them down. There are two important categories of degradable plastic.
It is possible to recycle some plastics. It is necessary first to separate the plastics into the different types. It is not yet economical to recycle mixed plastic from domestic waste, although some plastic recycling schemes are now in operation.
Part of this site was last updated on 21st January 2009.
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