Polymers

Polymers
1. Polymer – many small units (monomers) joining together to formed large molecule.
2. Polymer can be classified into two groups:

• synthetic polymers / man-made polymers (polythene; PVC – polyvinyl chloride; artificial silk; and polypropene)
• natural polymers (natural rubber; starch; cellulose; and proteins)

3. Natural polymer: Carbohydrates (polysaccharides) (starch, glycogen and cellulose)

• General formula: C_x(H₂O)_y with the ratio of H:O = 2:1
• Carbohydrates have cyclic structure.
• Monomer: glucose (C₆H₁₂H₆)
• Reaction to form polymer: condensation reaction (- H₂O)

4. Natural polymer: Protein (polypeptide)

• Protein consists of carbon, hydrogen, oxygen and nitrogen (some have sulphur, phosphorus and other elements)
• Monomer: amino acids
• Amino acids have two functional group which are carboxyl group (-COOH) and amino group (-NH₂)
• Reaction to form polymer: condensation reaction (- H₂O)

5. Natural polymer: Natural rubber

• Extracted from the latex of rubber tree (Hevea brasiliensis) which the tree originates from Brazil.
• A molecule of rubber contains 5000 isoprene units.
• Monomer: isoprene, C₃H₈ or 2-methylbuta-1,3-diene.
• Reaction to form polymer: additional polymerisation (one of the double bond in isoprene becomes single bond)

6. Structure of rubber molecule

• Latex is colloid (35% rubber particles and 65% water).
• Rubber particle contains rubber molecules which are wrapped by a layer of negatively-charged protein membrane. Same charge of rubber molecules repels each other. This prevent rubber from coagulate.

7. Coagulation process of latex
The process for the coagulation of latex is summarised as:

1. Acid (H⁺) can neutralise the negatively-charged protein membrane. Example of acid: formic acid, methanoic acid, suphuric acid and hydrochloric acid.
2. The rubber molecules will collide after the protein membrane is broken.
3. Rubber molecules (polymers) are set free
4. Rubber molecules combine with one another (coagulation).

8. Natural coagulation process of latex
For the natural coagulation of latex:

1. Latex is exposed to air without adding acid (duration – overnight).
2. Coagulation process occurs in slower pace due to the bacteria (microorganism) action which produce acid)

9. Prevent coagulation process of latex
The following are latex coagulation prevention method:

1. Alkaline / Basic solution is added to the latex. Example: ammonia (NH₃).
2. Positively-charged hydrogen ion / H⁺ produced by bacteria can be neutralised by negatively-charged hydroxide ion / OH^- from ammonia solution.

10. Properties of natural rubber

• elastic
• cannot withstand heat (become sticky and soft – above 50°C; decompose – above 200°C; hard and brittle – cooled)
• easily oxidised (present of C=C)
• insoluble in water (due to the long hydrocarbon chains)
• soluble in organic solvent (propanone, benzene, petrol etc.)

11. Vulcanisation of rubber
Vulcanisation – process of hardening rubber and increases rubber elasticity by heating it with sulphur or sulphur compounds.
Methods:

• heating natural rubber with sulphur at 140°C using zinc oxide as catalyst or
• dipping natural rubber in a solution of disulphur dichloride (S₂Cl₂) in methylbenzene.

12. Properties of vulcanisation of rubber

• The sulphur atoms are added to double bonds in the natural rubber molecules to form disulphide linkages (-C-S-S-C-) / sulphur cross-links between the long polymer chains. Therefore, vulcanised rubber is more elastics and stronger.
• This increases the molecular size and the intermolecular forces of attraction between rubber molecules. Therefore, vulcanised rubber is more resistant to heat (does not become soft and sticky when hot).
• This also reduces the number of carbon-carbon double bonds in rubber molecules. Therefore, vulcanised rubber is more resistant to oxygen, ozone, sunlight and other chemicals.

13. Comparison between the properties of vulcanised rubber and unvulcanised rubber

Properties	Vulcanised rubber	Unvulcanised rubber
Double bonds	Decreases (formation of sulphur cross-links)	More number of double bonds
Melting point	High (presence of sulphur)	Low
Elasticity	More elastic (sulphur cross-links prevents the polymer chain or rubber from slipping past.	Less elastics
Strength and hardness	Strong and hard (depends on degree of vulcanisation)	Weak and soft (polymer chain of rubber will break when rubber is over stretched.
Resistant to heat	Resistant to heat	Poor resistant to heat
Oxidation	Resistant to oxidation (reduction of number of double bonds per rubber molecule)	Easily oxidised by oxygen, UV light (presence of many double bonds per rubber molecules)

14. R & D of rubber

• RRIM – Rubber Research Institute of Malaysia
• MRB – Malaysian Rubber Board
• Rubber Technology Centre
• Various local higher institutions of learning