9.3 - Polymer

Polymer and Monomer

A polymer is a large chain-shaped molecule formed by the combination of smaller molecular units.
A monomer is the small molecular unit that forms a polymer.
Polymers are classified as natural polymers or synthetic polymers.
A natural polymer occurs naturally; a synthetic polymer is made by humans using chemicals.

Polymer	Monomer	Use
Starch	Glucose	Provides energy
Protein	Amino acid	Builds cells and body tissues
Natural rubber	Isoprene	Makes rubber-based products
Polythene	Ethene	Plastic bottles, plastic bags and pails
Polystyrene	Styrene	Packaging containers for electrical equipment
Perspex	Methyl methacrylate	Aeroplane mirrors and vehicle windows
Synthetic rubber	Neoprene	Gloves, vehicle tyres and shoe soles

Polymerisation: monomers join through the formation of chemical bonds to produce a large, chain-shaped polymer molecule.
Depolymerisation: a chain-shaped polymer molecule is split into its monomers through a chemical reaction.

Monomers polymerisation depolymerisation Polymer

Addition polymerisation involves the same type of monomer; double bonds break and become single bonds.
Example: ethene monomers form polythene.

n C H_{2} = C H_{2} ⟶ [- C H_{2} - C H_{2} -]_{n}

The milky fluid obtained by tapping a rubber tree is called latex.
Latex is processed into natural rubber, a natural polymer that contributes to Malaysia’s economy.
Natural rubber is elastic, soft, unable to withstand heat, a good electrical insulator and not permeable to air.

Each chain-shaped rubber molecule is covered by a protein membrane.
Negative charges surround the outer surface of the membrane.
Like charges make the rubber molecules repel one another; they do not collide, so latex remains liquid.

Acid supplies positively charged hydrogen ions.
Hydrogen ions neutralise the negative charges on the protein membrane.
Rubber molecules collide; the protein membranes break.
Rubber polymer chains are released, become entangled and form solid lumps.
Latex left exposed also coagulates because bacteria produce acid that neutralises the protein membranes.

Alkali supplies hydroxide ions that neutralise hydrogen ions from the acid produced by bacteria.
Negative charges remain on the protein membranes.
Rubber molecules continue to repel one another; latex remains liquid.

Observation: The strip returns to its original shape; natural rubber is elastic.

Observation: The rubber becomes less elastic; natural rubber cannot withstand heat.

Label three beakers P, Q and R; add 20 ml latex to each.
Add 10 drops ethanoic acid to P and stir → latex coagulates; a white solid forms.
Add 10 drops ammonia solution to Q and stir → latex remains liquid and does not coagulate.
Add nothing to R → latex eventually coagulates because bacterial action produces acid.

Natural rubber is too soft and not sufficiently heat-resistant for products such as vehicle tyres.
Vulcanisation is the process of heating rubber with sulphur.
Charles Goodyear’s method heated natural rubber with 1%–3% sulphur by mass.
During vulcanisation, sulphur atoms form sulphur cross-links between the chains of natural-rubber polymer molecules.
The product is vulcanised rubber.
When force is applied, cross-linked chains are difficult to slide over one another.
When heated, its molecular structure is difficult to break.
Therefore, vulcanised rubber is harder and more heat-resistant than natural rubber.

Natural rubber	Vulcanised rubber
No sulphur cross-links between polymer chains	Sulphur cross-links join polymer chains
Soft and less elastic	Hard and more elastic
Cannot withstand heat	Resistant to heat
Polymer chains slide more easily	Polymer chains are difficult to slide

Cuplump Modified Asphalt (CMA): used as asphalt concrete for road surfacing so that roads last longer, resist heat, produce less noise and develop fewer cracks.
Getah Colour: painting paint made from latex for visual art.
Railway pads: rubber pads placed between the rails and the train engine reduce vibration and noise.