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Polymer

Polymer
SCIENCE • Form 4 • Chapter 9: Chemicals in Industry

9.3 Polymer

Polymers are large molecules constructed from repeated links of smaller units known as monomers. They are widely used in daily life, ranging from everyday plastic containers and packaging to vehicle tyres and latex gloves.

Learning Objectives

  • Define polymers, monomers, polymerisation, and depolymerisation.
  • Classify polymers into natural polymers and synthetic polymers with their respective monomers and uses.
  • Explain the process of addition polymerisation using ethene as an example.
  • Describe the characteristics of natural rubber and the actions of acids, alkalis, and bacteria on latex coagulation.
  • Explain the process of vulcanisation of rubber and compare the properties of natural rubber with vulcanised rubber.
  • Identify latest rubber-based technologies in industry.

The Coagulation and Prevention of Coagulation in Latex

How rubber particles repel each other, how acid causes coagulation, and how alkali keeps latex liquid.

Coagulation and prevention of coagulation in latex Three panels showing rubber particles with negative charges on their protein membranes repelling each other; acid neutralising the charges so particles collide and coagulate; and alkali neutralising bacterial acid so particles continue repelling. Latex particles repel − − − − Negative charges on protein membranes keep particles apart. Acid causes coagulation H⁺ Hydrogen ions neutralise the charges; particles collide and entangle. Alkali prevents coagulation − − − − Hydroxide ions neutralise bacterial acid; particles keep repelling.

Short Explanation

Definition

A polymer is a large molecule formed by chains of smaller molecular units. A monomer is the smaller molecular unit that makes up a polymer.

Polymers are broadly divided into natural polymers that occur in living things and synthetic polymers manufactured through chemical processes.

Natural Polymers

Starch has the monomer glucose and is used to produce energy. Protein has the monomer amino acid and is used to build cells and body tissues. Natural rubber has the monomer isoprene and is used to make rubber-based products.

Synthetic Polymers

Polythene has the monomer ethene and is used to make plastic bottles, plastic bags, and pails. Polystyrene has the monomer styrene and is used to make packing containers for electrical equipment. Perspex has the monomer methyl methacrylate and is used to make aeroplane mirrors and windows for vehicles. Synthetic rubber has the monomer neoprene and is used to make gloves, tyres, and shoe soles.

Polymerisation and Depolymerisation

Polymerisation is the process of joining monomers through the formation of chemical bonds to produce a long-chain polymer. Depolymerisation is the process of splitting a long-chain polymer into its monomers through chemical reactions.

Addition Polymerisation

Addition polymerisation involves monomers of the same type by breaking a double bond into a single bond. For example, ethene molecules join together to form polythene.

Natural Rubber and Its Characteristics

Latex is the milky fluid obtained from tapped rubber trees, which is processed into natural rubber. Natural rubber is elastic, soft, cannot withstand heat, is a good electrical insulator, and is not permeable to air.

Action of Acid and Alkali on Latex

Rubber molecules in chains are surrounded by a protective layer of protein membrane carrying negative charges on its outer surface. These like charges cause the rubber particles to repel one another, keeping latex in liquid form.

When acid is added, positively-charged hydrogen ions, \(\text{H}^+\), from the acid neutralise the negative charges on the protein membrane. The neutralised rubber particles collide with one another. Collisions break the protein membrane, allowing rubber polymer chains to entangle and coagulate into solid lumps.

When latex is left exposed to air, bacteria in the air produce acid that neutralises the negative charges on the protein membrane, causing latex to coagulate naturally over time.

When alkali is added, hydroxide ions, \(\text{OH}^-\), from the alkali neutralise the hydrogen ions produced by bacterial action. Negative charges remain on the protein membrane. The rubber particles continue to repel one another and do not collide, keeping latex in liquid form.

Vulcanisation of Rubber

Natural rubber is soft and melts easily on hot Malaysian road surfaces, making it unsuitable for vehicle tyres. Vulcanisation is a process of heating natural rubber with sulphur. It was discovered by Charles Goodyear in 1839 by heating rubber with \(1\% - 3\%\) sulphur by mass. During vulcanisation, sulphur atoms form cross-links between the chains of rubber polymer molecules.

Sulphur cross-links prevent the polymer chains from sliding easily over each other when a force is applied. The cross-linked structure resists breaking when heated, making vulcanised rubber harder and highly resistant to heat.

Properties of natural rubber compared with vulcanised rubber.
Property Natural Rubber Vulcanised Rubber
Hardness Soft Hard and more elastic
Heat resistance Cannot withstand heat Resistant to heat
Electrical insulation Good electrical insulator Good electrical insulator
Permeability Not permeable to air Not permeable to liquid and air
Oxidation Easily oxidised in air Difficult to be oxidised in air
Reaction with acid and alkali Reacts with acid and alkali Does not react with acid and alkali

Latest Rubber-based Technology

Cuplump Modified Asphalt (CMA) is used as concrete asphalt for road pavements to last longer, resist heat, reduce noise, and reduce road cracks. Getah Colour is a paint produced from latex used for visual arts. Rubber Rail Pad is placed between the railway track and train engine to dampen vibration and reduce noise.

Try to Answer First

Answer in your mind, then press “Check Answer”.

1

What is the difference between polymerisation and depolymerisation?

Answer: Polymerisation is the process of joining monomers through chemical bonds to form a long-chain polymer, whereas depolymerisation is the process of breaking down a long-chain polymer into its constituent monomers through chemical reactions.
2

Why does fresh latex turn solid when left exposed to the air for a few days without any added chemicals?

Answer: Bacteria present in the air produce acid that releases hydrogen ions. These ions neutralise the negative charges on the protein membranes of rubber particles, allowing them to collide, break their membranes, and coagulate into solid lumps.
3

How do sulphur cross-links improve the elasticity and heat resistance of vulcanised rubber?

Answer: Sulphur cross-links form chemical bonds between adjacent rubber polymer chains, preventing the chains from easily sliding over one another when pulled and preventing the molecular structure from breaking down when exposed to heat.

Common Mistakes

  • !Confusing the monomers of natural and synthetic polymers. For example, stating ethene is the monomer of natural rubber instead of isoprene.
  • !Thinking that adding alkali causes latex to coagulate. In fact, alkali prevents coagulation by neutralising acids.
  • !Assuming that vulcanised rubber is less elastic than natural rubber. Vulcanised rubber is harder and more elastic because it returns to its original shape effectively without permanent chain slippage.
  • !Believing that natural rubber is permeable to air. Natural rubber is not permeable to air.

Concept Misunderstandings

Misconception

Natural rubber coagulates on its own because the rubber molecules naturally stick together over time.

Correct Concept

Rubber particles in latex naturally repel each other due to negative surface charges on their protein membranes. Coagulation only occurs over time because environmental bacteria produce acid that neutralises these charges.

Misconception

Vulcanisation changes the polymer chains into completely new types of atoms.

Correct Concept

Vulcanisation does not change the isoprene rubber chain itself; it introduces sulphur atoms that create cross-links between existing rubber polymer chains.

Summary

  • Polymers are large molecules made of repeating monomers.
  • Polymers are grouped into natural polymers (starch, protein, natural rubber) and synthetic polymers (polythene, polystyrene, Perspex, synthetic rubber).
  • Addition polymerisation forms polymers by breaking double bonds into single bonds without forming by-products (e.g., ethene to polythene).
  • Latex contains negatively charged protein membranes that keep particles dispersed; acids neutralise the charges causing coagulation, while alkalis preserve the liquid state.
  • Vulcanisation heats natural rubber with sulphur to form sulphur cross-links, producing rubber that is harder, more elastic, heat resistant, and chemically resistant.
  • Advanced rubber applications include Cuplump Modified Asphalt (CMA), Getah Colour, and rail pads.

Short Activity

A. Objective Quiz

1 Which substance can be added to fresh latex to prevent it from coagulating?

2 What structural feature gives vulcanised rubber higher resistance to heat compared to natural rubber?

B. Fill in the Blanks

3 The process of joining monomers together through chemical bonds to produce a long-chain polymer is known as .

 

4 The monomer of natural rubber is .

 

C. Drag and Drop: Match Each Polymer to Its Monomer

Drag each polymer to its correct monomer. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.

Polymers
Starch
Protein
Polythene
Synthetic rubber
Monomers
1 Glucose
2 Amino acid
3 Ethene
4 Neoprene

C. Match Each Polymer to Its Monomer

Write the correct monomer next to each polymer.

Polymer Monomer
1. Starch 
2. Protein 
3. Polythene 
4. Synthetic rubber 

Answer choices: Glucose • Amino acid • Ethene • Neoprene

Keywords

Polymer Monomer Polymerisation Depolymerisation Addition Polymerisation Latex Coagulation Vulcanisation Sulphur cross-links
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

Polymer

Polymer
SCIENCE • Form 4 • Chapter 9: Chemicals in Industry

9.3 Polymer

Polymers are large molecules constructed from repeated links of smaller units known as monomers. They are widely used in daily life, ranging from everyday plastic containers and packaging to vehicle tyres and latex gloves.

Learning Objectives

  • Define polymers, monomers, polymerisation, and depolymerisation.
  • Classify polymers into natural polymers and synthetic polymers with their respective monomers and uses.
  • Explain the process of addition polymerisation using ethene as an example.
  • Describe the characteristics of natural rubber and the actions of acids, alkalis, and bacteria on latex coagulation.
  • Explain the process of vulcanisation of rubber and compare the properties of natural rubber with vulcanised rubber.
  • Identify latest rubber-based technologies in industry.

The Coagulation and Prevention of Coagulation in Latex

How rubber particles repel each other, how acid causes coagulation, and how alkali keeps latex liquid.

Coagulation and prevention of coagulation in latex Three panels showing rubber particles with negative charges on their protein membranes repelling each other; acid neutralising the charges so particles collide and coagulate; and alkali neutralising bacterial acid so particles continue repelling. Latex particles repel − − − − Negative charges on protein membranes keep particles apart. Acid causes coagulation H⁺ Hydrogen ions neutralise the charges; particles collide and entangle. Alkali prevents coagulation − − − − Hydroxide ions neutralise bacterial acid; particles keep repelling.

Short Explanation

Definition

A polymer is a large molecule formed by chains of smaller molecular units. A monomer is the smaller molecular unit that makes up a polymer.

Polymers are broadly divided into natural polymers that occur in living things and synthetic polymers manufactured through chemical processes.

Natural Polymers

Starch has the monomer glucose and is used to produce energy. Protein has the monomer amino acid and is used to build cells and body tissues. Natural rubber has the monomer isoprene and is used to make rubber-based products.

Synthetic Polymers

Polythene has the monomer ethene and is used to make plastic bottles, plastic bags, and pails. Polystyrene has the monomer styrene and is used to make packing containers for electrical equipment. Perspex has the monomer methyl methacrylate and is used to make aeroplane mirrors and windows for vehicles. Synthetic rubber has the monomer neoprene and is used to make gloves, tyres, and shoe soles.

Polymerisation and Depolymerisation

Polymerisation is the process of joining monomers through the formation of chemical bonds to produce a long-chain polymer. Depolymerisation is the process of splitting a long-chain polymer into its monomers through chemical reactions.

Addition Polymerisation

Addition polymerisation involves monomers of the same type by breaking a double bond into a single bond. For example, ethene molecules join together to form polythene.

Natural Rubber and Its Characteristics

Latex is the milky fluid obtained from tapped rubber trees, which is processed into natural rubber. Natural rubber is elastic, soft, cannot withstand heat, is a good electrical insulator, and is not permeable to air.

Action of Acid and Alkali on Latex

Rubber molecules in chains are surrounded by a protective layer of protein membrane carrying negative charges on its outer surface. These like charges cause the rubber particles to repel one another, keeping latex in liquid form.

When acid is added, positively-charged hydrogen ions, \(\text{H}^+\), from the acid neutralise the negative charges on the protein membrane. The neutralised rubber particles collide with one another. Collisions break the protein membrane, allowing rubber polymer chains to entangle and coagulate into solid lumps.

When latex is left exposed to air, bacteria in the air produce acid that neutralises the negative charges on the protein membrane, causing latex to coagulate naturally over time.

When alkali is added, hydroxide ions, \(\text{OH}^-\), from the alkali neutralise the hydrogen ions produced by bacterial action. Negative charges remain on the protein membrane. The rubber particles continue to repel one another and do not collide, keeping latex in liquid form.

Vulcanisation of Rubber

Natural rubber is soft and melts easily on hot Malaysian road surfaces, making it unsuitable for vehicle tyres. Vulcanisation is a process of heating natural rubber with sulphur. It was discovered by Charles Goodyear in 1839 by heating rubber with \(1\% - 3\%\) sulphur by mass. During vulcanisation, sulphur atoms form cross-links between the chains of rubber polymer molecules.

Sulphur cross-links prevent the polymer chains from sliding easily over each other when a force is applied. The cross-linked structure resists breaking when heated, making vulcanised rubber harder and highly resistant to heat.

Properties of natural rubber compared with vulcanised rubber.
Property Natural Rubber Vulcanised Rubber
Hardness Soft Hard and more elastic
Heat resistance Cannot withstand heat Resistant to heat
Electrical insulation Good electrical insulator Good electrical insulator
Permeability Not permeable to air Not permeable to liquid and air
Oxidation Easily oxidised in air Difficult to be oxidised in air
Reaction with acid and alkali Reacts with acid and alkali Does not react with acid and alkali

Latest Rubber-based Technology

Cuplump Modified Asphalt (CMA) is used as concrete asphalt for road pavements to last longer, resist heat, reduce noise, and reduce road cracks. Getah Colour is a paint produced from latex used for visual arts. Rubber Rail Pad is placed between the railway track and train engine to dampen vibration and reduce noise.

Try to Answer First

Answer in your mind, then press “Check Answer”.

1

What is the difference between polymerisation and depolymerisation?

Answer: Polymerisation is the process of joining monomers through chemical bonds to form a long-chain polymer, whereas depolymerisation is the process of breaking down a long-chain polymer into its constituent monomers through chemical reactions.
2

Why does fresh latex turn solid when left exposed to the air for a few days without any added chemicals?

Answer: Bacteria present in the air produce acid that releases hydrogen ions. These ions neutralise the negative charges on the protein membranes of rubber particles, allowing them to collide, break their membranes, and coagulate into solid lumps.
3

How do sulphur cross-links improve the elasticity and heat resistance of vulcanised rubber?

Answer: Sulphur cross-links form chemical bonds between adjacent rubber polymer chains, preventing the chains from easily sliding over one another when pulled and preventing the molecular structure from breaking down when exposed to heat.

Common Mistakes

  • !Confusing the monomers of natural and synthetic polymers. For example, stating ethene is the monomer of natural rubber instead of isoprene.
  • !Thinking that adding alkali causes latex to coagulate. In fact, alkali prevents coagulation by neutralising acids.
  • !Assuming that vulcanised rubber is less elastic than natural rubber. Vulcanised rubber is harder and more elastic because it returns to its original shape effectively without permanent chain slippage.
  • !Believing that natural rubber is permeable to air. Natural rubber is not permeable to air.

Concept Misunderstandings

Misconception

Natural rubber coagulates on its own because the rubber molecules naturally stick together over time.

Correct Concept

Rubber particles in latex naturally repel each other due to negative surface charges on their protein membranes. Coagulation only occurs over time because environmental bacteria produce acid that neutralises these charges.

Misconception

Vulcanisation changes the polymer chains into completely new types of atoms.

Correct Concept

Vulcanisation does not change the isoprene rubber chain itself; it introduces sulphur atoms that create cross-links between existing rubber polymer chains.

Summary

  • Polymers are large molecules made of repeating monomers.
  • Polymers are grouped into natural polymers (starch, protein, natural rubber) and synthetic polymers (polythene, polystyrene, Perspex, synthetic rubber).
  • Addition polymerisation forms polymers by breaking double bonds into single bonds without forming by-products (e.g., ethene to polythene).
  • Latex contains negatively charged protein membranes that keep particles dispersed; acids neutralise the charges causing coagulation, while alkalis preserve the liquid state.
  • Vulcanisation heats natural rubber with sulphur to form sulphur cross-links, producing rubber that is harder, more elastic, heat resistant, and chemically resistant.
  • Advanced rubber applications include Cuplump Modified Asphalt (CMA), Getah Colour, and rail pads.

Short Activity

A. Objective Quiz

1 Which substance can be added to fresh latex to prevent it from coagulating?

2 What structural feature gives vulcanised rubber higher resistance to heat compared to natural rubber?

B. Fill in the Blanks

3 The process of joining monomers together through chemical bonds to produce a long-chain polymer is known as .

 

4 The monomer of natural rubber is .

 

C. Drag and Drop: Match Each Polymer to Its Monomer

Drag each polymer to its correct monomer. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.

Polymers
Starch
Protein
Polythene
Synthetic rubber
Monomers
1 Glucose
2 Amino acid
3 Ethene
4 Neoprene

C. Match Each Polymer to Its Monomer

Write the correct monomer next to each polymer.

Polymer Monomer
1. Starch 
2. Protein 
3. Polythene 
4. Synthetic rubber 

Answer choices: Glucose • Amino acid • Ethene • Neoprene

Keywords

Polymer Monomer Polymerisation Depolymerisation Addition Polymerisation Latex Coagulation Vulcanisation Sulphur cross-links
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Chemicals in Industry
Topic : Polymer
Form 4 Science
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