4.1 Polymer Basics
A polymer is a long-chain macromolecule formed by joining together many small repeating units called monomers through a process known as polymerisation.
Classification of Polymers
Polymers are classified according to three main criteria:
1. Source of Polymer
- Natural Polymers: Polymers obtained naturally from plants or animals.
- Natural rubber (Polyisoprene) — Monomer: Isoprene (2-methylbuta-1,3-diene)
- Starch — Monomer: Glucose
- Cellulose — Monomer: Glucose
- Protein — Monomer: Amino acid
- DNA / RNA — Monomer: Nucleotide
- Synthetic Polymers: Man-made polymers synthesized through chemical reactions in laboratories or factories.
- Polyethene (PE), Polypropene (PP), Polyvinyl chloride (PVC), Polystyrene (PS)
- Synthetic rubber (Neoprene, SBR), Terylene, Nylon
2. Physical Properties (Thermal Behavior)
- Thermoplastics: Polymers that melt upon heating and solidify upon cooling. They can be repeatedly melted, remoulded, and recycled because they have weak intermolecular forces between linear chains (e.g., Polyethene, PVC, Polystyrene).
- Thermosetting Plastics: Polymers that decompose or char upon heating and cannot be remoulded or recycled. Cross-links between polymer chains prevent melting (e.g., Bakelite, Melamine).
- Elastomers: Polymers that possess high elasticity; they stretch easily when force is applied and return to their original shape when released (e.g., Natural rubber, Vulcanised rubber, Neoprene).
3. Polymerisation Reactions
- Addition Polymerisation: Monomers containing carbon-carbon double bonds ($\text{C}=\text{C}$) join together without the loss of any small molecules or atoms.
- Example: Ethene $\rightarrow$ Polyethene ($\text{n CH}_2=\text{CH}_2 \rightarrow \text{[-CH}_2-\text{CH}_2\text{-]}_n$)
- Condensation Polymerisation: Monomers containing at least two functional groups react to form a polymer accompanied by the elimination of a small molecule such as water ($\text{H}_2\text{O}$) or hydrogen chloride ($\text{HCl}$).
- Examples: Terylene (Polyester), Nylon (Polyamide).
4.2 Natural Rubber
Natural rubber is a polymer of isoprene ($\text{C}_5\text{H}_8$), systematically named 2-methylbuta-1,3-diene.
Properties of Natural Rubber
- Soft, white, and elastic at room temperature.
- Low resistance to heat (becomes soft and sticky when heated).
- Easily oxidized by atmospheric oxygen or ozone because of double bonds ($\text{C}=\text{C}$) in its structure.
- Insoluble in water, but soluble in organic solvents (e.g., propanone, benzene).
- Insoluble in dilute acids or alkalis.
Coagulation of Latex
- Latex consists of rubber particles suspended in water. Each rubber particle is surrounded by a negatively charged protein membrane.
- Mutual repulsion between negatively charged membranes prevents rubber particles from clumping together.
- Coagulation Process (Adding Acid): Adding an acid (e.g., ethanoic acid, $\text{CH}_3\text{COOH}$) supplies hydrogen ions ($\text{H}^+$). The $\text{H}^+$ ions neutralise the negative charges on the protein membrane. The neutralised particles collide, breaking the protein membrane, causing rubber polymers to entangle and coagulate.
- Natural Coagulation: Bacteria in latex produce lactic acid, which supplies $\text{H}^+$ ions and slowly causes coagulation over time.
- Preventing Coagulation (Adding Alkali): Adding an alkali (e.g., aqueous ammonia, $\text{NH}_3\text{(aq)}$) supplies hydroxide ions ($\text{OH}^-$). $\text{OH}^-$ ions neutralise any $\text{H}^+$ ions present, maintaining the negative charge on the membranes and keeping the latex liquid.
Vulcanisation of Rubber
Vulcanisation is the process of improving the physical properties of natural rubber by heating it with sulfur (or sulfur compounds).
- Mechanism: Sulfur atoms react with double bonds ($\text{C}=\text{C}$) in rubber polymers to form sulfur cross-links between adjacent polymer chains.
- Properties of Vulcanised Rubber:
- More elastic and returns to original shape faster after stretching.
- Harder, stronger, and more resistant to heat.
- More resistant to oxidation by air/ozone due to fewer remaining double bonds.
- Alternative Vulcanisation Methods: Using metal oxides (e.g., zinc oxide), peroxide compounds, or irradiation without using sulfur.
4.3 Synthetic Polymers and Environmental Impact
Synthetic polymers are widely used due to their light weight, low cost, durability, chemical inertness, and versatile physical properties.
Synthetic Rubber vs. Natural Rubber
- Examples of synthetic rubber: Neoprene, Styrene-Butadiene Rubber (SBR), Silicon rubber.
- Advantages: High resistance to heat, chemicals, oil, and atmospheric oxidation.
- Disadvantages: Less elastic than natural rubber and non-biodegradable.
Environmental Impact and Synthetic Polymer Waste Management
- Most synthetic plastics are non-biodegradable and persist in the environment for hundreds of years.
- Environmental Issues: Blocked drainage systems causing flash floods, accumulation in landfills, marine life ingestion, toxic gas emissions upon incineration (e.g., $\text{HCl}$ gas from PVC burning).
- Sustainable Solutions:
- 3R Practice: Reduce, Reuse, Recycle.
- Biodegradable Plastics: Developing polymers that decompose naturally via microorganisms (e.g., polylactic acid, PLA).
- Pyrolysis: Thermal degradation of waste plastics in the absence of oxygen to produce fuel oil.