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Properties of Polymers

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Properties of Polymers

Polymers are large molecules built up of repeating units called monomers.

We can form polymers in two different ways:

  • Addition polymers are formed from monomers with a C=C double bond. The double bond breaks and bonds with an adjacent monomer to form a single long polymer chain with a C-C backbone.
  • Condensation polymers are formed from monomers with two different functional groups. When these monomers join together into a polymer, they lose some of their atoms, which combine to form a small molecule called a condensate.

For more information about the different types of polymers, see Polymerisation Reactions.

Examples of addition polymers include high and low-density polythene (HDPE and LDPE respectively), and PVC. Examples of condensation polymers include Terylene, Nylon, and Kevlar. In this article, we’ll explore their structures, properties and uses.

What is HDPE?

The first polymer we’ll look at is HDPE. HDPE, properly known as high-density polythene, is a plastic formed from many hundreds of ethene monomers.

Properties of polymers, ethene monomer, StudySmarterAn ethene monomer. Anna Brewer, StudySmarter Originals

HDPE is strong and dense and is used for products such as washing up bowls, plastic pipes, and milk bottles. In order to understand its properties, we must first look at its formation.

How is HDPE made?

HDPE is formed in an addition polymerisation reaction at low temperatures and pressures of about 60℃ and 2-3 atm respectively. A Ziegler-Natta catalyst is used. This consists of a mixture of titanium and aluminium compounds. These conditions result in long, straight hydrocarbon chains with very little random branching.

The repeating unit in HDPE. Anna Brewer, StudySmarter Originals

Properties of polymers, HDPE chains, StudySmarterHDPE chains. Notice how they are predominantly straight with little branching. Anna Brewer, StudySmarter Originals

What are the properties of HDPE?

Because the hydrocarbon polymer chains of the plastic HDPE are predominantly straight and there are very few branches, the molecules can pack together tightly. This makes HDPE very dense. It also results in greater intermolecular forces, namely van der Waals attraction, between the molecules. (Refresh your memory about this attraction with a look at Intermolecular Forces.) These van der Waals forces mean that HDPE has a high melting point and is very strong.

Did you know? HDPE implants have been used in plastic surgery since 1985 as part of facial augmentation procedures, thanks to their strength and low toxicity.

What is LDPE?

Now let’s take a look at LDPE, HDPE’s close cousin. LDPE, properly known as low-density polythene, is also a plastic polymer made from ethene monomers. However, it has quite different properties to HDPE. It is relatively weak and flexible and so is used for carrier bags and food packaging.

How is LDPE made?

LDPE is formed in an addition reaction at temperatures of around 200℃ and a pressure of 2000 atm. The reaction mechanism uses free radicals and results in a high proportion of random branching along its hydrocarbon chains.

A free radical is an atom, ion or molecule with an unpaired outer shell electron. They are all extremely reactive.

Properties of polymers, LDPE chains, StudySmarterLDPE chains. Note the high proportion of random branching. Anna Brewer, StudySmarter Originals

What are the properties of LDPE?

Because the chains of LDPE are randomly branched, they cannot pack together as tightly as the chains in HDPE. This makes LDPE less dense and significantly less strong than HDPE, as the van der Waals forces between chains are weaker.

Did you know? In 2010, the world’s first bio-based plastic to be produced on an industrial level was created by Braskem, Latin America’s largest petrochemical company. It is a polythene made from monomers derived from sugar cane, which makes it entirely renewable.

What is PVC?

Let’s turn our attention to another addition polymer. Polyvinyl chloride, or PVC, is properly known as poly(chloroethene) and is a polymer made from chloroethene monomers. PVC tends to be hard and rigid, and is used for drainpipes, cable insulation, and shoes. In fact, it is the world’s third-most produced plastic.

How is PVC made?

PVC is formed in an addition polymerisation reaction. It forms long hydrocarbon chains with the chlorine atoms arranged with random orientations.

Properties of polymers, PVC polymer, StudySmarterThe polymer of PVC. Anna Brewer, StudySmarter Originals

What are the properties of PVC?

Because of the large size of their chlorine atoms and the randomness of their orientation, PVC’s hydrocarbon polymer chains cannot pack together tightly. You would expect this to make the plastic weak and flimsy, like LDPE, but instead PVC is hard and strong. This is because the C-Cl bond is polar and there are permanent dipole-dipole forces between molecules, holding the chains tightly together.

To make the plastic more flexible, plasticisers can be added. These are small molecules that fit between the polymer chains, forcing them apart and enabling them to slip over each other. This reduces the intermolecular forces between the chains and reduces the plastic’s strength. PVC made in this way can then be used to make softer products such as imitation leather.

What is Terylene?

The first condensation polymer we’ll look at is Terylene. Terylene, also known as PET, has the proper name of poly(ethylene terephthalate) and is a polyester polymer. It is used for a variety of purposes, such as clothes and drinks bottles, and accounts for 18 percent of the world’s polymer production.

How is Terylene made?

Terylene is formed in a condensation polymerisation reaction between benzene-1,4-dicarboxylic acid and ethane-1,2-diol. It consists of hydrocarbon chains based around the ester functional group, -COO-. A small molecule is released in the process. In this case, that small molecule is water. Its structure is shown below.

Properties of polymers, Terylene structure, StudySmarterThe structure of Terylene. Anna Brewer, StudySmarter Originals

What are the properties of Terylene?

Terylene has polar bonds and so experiences permanent dipole-dipole forces between chains. Its properties can vary, from flexible to rigid, which is reflected in its variety of uses. For example, you’ll find it in clothing under the generic term polyester, in microfibre towels and cleaning cloths, and in plastic drinks bottles.

What is Nylon?

Nylon is a polyamide polymer that was first synthesised in 1935. It is used in products such as clothes, toothbrushes, food packaging and electrical equipment, and was the first example of a thermoplastic polymer. These are polymers that melt at high temperatures and resolidify once cooled, instead of becoming brittle and snapping.

How is Nylon made?

Nylon is commonly formed in a condensation polymerisation reaction between an amine and a carboxylic acid. Water is released in the reaction. For example, Nylon-6,6 is made industrially by the reaction between 1,6-diaminohexane and hexane-1,6-dicarboxylic acid, as shown below.

Properties of polymers, Nylon structure, StudySmarterThe structure of Nylon. Anna Brewer, StudySmarter Originals

The two reactants first form a salt, which is heated at 350℃ under suitable pressure.

Nylon can also be formed in the reaction between an amine and an acyl chloride. This uses hexanediol dichloride, takes place at room temperature, and is a much faster reaction. In this case, the small molecule released is .

What are the properties of Nylon?

Because Nylon contains the amide linkage group, -CONH-, it experiences hydrogen bonding between polymer chains. This occurs between the hydrogen atoms bonded to nitrogen, and the nitrogen on an adjacent chain. This increases Nylon’s strength dramatically.

Properties of polymers, amide functional group, StudySmarterThe amide functional group. Anna Brewer, StudySmarter Originals

Properties of polymers, amide hydrogen bonding, StudySmarterHydrogen bonding between two amide groups. Anna Brewer, StudySmarter Originals

Did you know? Nylon’s first commercial use was as toothbrush bristles in 1938. Its popularity rose during the Second World War. After the war was over, nylon parachutes were commonly recycled into women’s dresses.

What is Kevlar?

The final polymer we’ll investigate is Kevlar. Kevlar is also a polyamide. It is extremely strong and light and can withstand high temperatures, making it suitable for use in bulletproof vests, lightweight mountaineering ropes, and oven gloves.

How is Kevlar made?

Kevlar is made from the condensation polymerisation reaction between benzene-1,4-diamine and benzene-1,4-dicarboxylic acid. Due to its benzene rings, it forms long chains that are predominantly planar.

Properties of polymers, Kevlar structure, StudySmarterThe structure of Kevlar. Anna Brewer, StudySmarter Originals

What are the properties of Kevlar?

Like Nylon, Kevlar contains the amide linkage group and so experiences hydrogen bonding between chains. Because its chains are rigid and mostly planar, they can pack together tightly, increasing the strength of the intermolecular forces.

Comparing polymers

Yes, we know - that was a lot of information! To help you consolidate your understanding, here is a table that compares all of the polymers we’ve discussed and their formation, structure, and properties.

Properties of polymers, comparing polymers table, StudySmarterA table comparing the different polymers discussed in this article. Anna Brewer, StudySmarter Originals

How are polymers disposed of?

As explored above, polymers play many roles in modern life, and are part of many of the products we use on a daily basis. As we produce more and more plastics and polymers, they accumulate in the environment, and their disposal becomes an increasingly larger issue.

Disposal of plastics made from alkenes

Polymers such as polyethene are long-chain hydrocarbons, containing only nonpolar bonds. This makes them unreactive and resistant to attack. Their C-H and C-C bonds are very strong and cannot be broken down by hydrolysis or other common reactions. Although they can be burnt, this releases carbon dioxide and other harmful pollutants like carbon particles, carbon monoxide, and styrene vapours. Therefore, there is no simple way to dispose of these plastics.

Disposal of polyesters and polyamides

Unlike plastics such as polythene, polyesters and polyamides can be broken down by hydrolysis into carboxylic acids and either alcohols or amines respectively. Although this is a slow process, it can be sped up by adding an acid or base catalyst. A general equation for the hydrolysis of polyamides is shown below.

Properties of polymers, hydrolysis of polyamides, StudySmarterHydrolysis of polyamides. Anna Brewer, StudySmarter Originals

What are alternatives to the disposal of polymers?

Because many types of plastics like PVC and polythene cannot be hydrolysed or easily disposed of, their levels in the environment have been increasing since their introduction. They can pollute ecosystems and may even end up in the food chain, in the form of microplastics that are eaten by small animals or fish. A solution to the problem is recycling.

  • In mechanical recycling, plastics are sorted, ground up into tiny pellets, then melted and remoulded.
  • In feedstock recycling, plastics are heated to high temperatures to break them down into their monomers. These can then be reformed into new polymers.

However, recycling plastics has its limitations. For example, the polymer chains may be damaged by each subsequent heating and so become shorter and shorter. This means plastic can only be recycled a finite number of times before it becomes unusable. The most sustainable solution is to turn away from plastics derived from crude oil and instead develop alternatives from renewable resources that can be broken down easily, once you are finished with them. One such alternative is packaging made from cellulose. Cellulose is a major component of plant cell walls and can be broken down into compost when disposed of.

Properties of Polymers - Key takeaways

  • Different types of polymers have different properties depending on their structure and bonding.
  • LDPE is branched and relatively flexible whereas HDPE is denser, stronger and formed predominantly of straight chains.
  • Kevlar and Nylon are both polyamides and used for items such as clothing and ropes.
  • Plastics made from alkenes cannot be broken down easily due to their nonpolar C-C and C-H bonds.
  • Polyamides and polyesters can be hydrolysed using an acid or base catalyst.
  • Recycling is an alternative solution to the disposal of polymers but has its limitations.

Frequently Asked Questions about Properties of Polymers

Cross-linking makes polymers much harder and stronger.

The properties of synthetic polymers depend on their structures. For example, HDPE consists of straight chains packed together densely and is hard and strong. On the other hand, LDPE consists of branched chains, and is much softer and more flexible. 

The structure of a polymer determines which intermolecular forces are present between polymer chains. This affects the polymer's properties. For example, Nylon contains the amide linkage group. This means that it can form hydrogen bonds between chains, making it hard and strong.

Final Properties of Polymers Quiz

Question

What is a polymer?

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Answer

A large molecule made up of repeating units called monomers.

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Question

Compare and contrast addition and condensation polymers.


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Answer

  • Both are large molecules made from monomers. 
  • Addition polymers are made from monomers with one type of functional group whereas condensation polymers are made from monomers with two different functional groups. 
  • There is only one product in addition polymerisation, whereas in condensation polymerisation, a small molecule called a condensate is also produced.


Show question

Question

What monomers make up polyamides?


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Answer

Carboxylic acids

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Question

What monomers make up polyesters?

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Answer

Alcohols

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Question

How is HDPE formed?


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Answer

High temperature and pressure.

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Question

Explain how the structure of HDPE affects its properties.


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Answer

  • HDPE consists predominantly of straight polymer chains with very little branching. 
  • This enables them to pack closely together and means there are strong intermolecular forces between the chains. 
  • Therefore, HDPE is strong and relatively rigid.

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Question

How is LDPE formed?


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Answer

High temperature and pressure.


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Question

Explain how the structure of LDPE affects its properties.


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Answer

  • LDPE has a random, highly branched structure. 
  • This means the polymer chains cannot pack together as closely and the intermolecular forces between them are weaker.
  • This makes LDPE flexible and less strong than HDPE.

Show question

Question

Compare and contrast HDPE and LDPE.


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Answer

  • Both are polymers of ethene formed in an addition reaction. 
  • Both consist of long hydrocarbon chains. 
  • HDPE is strong and rigid whilst LDPE is flexible and weaker. 
  • HDPE has mostly straight chains whilst LDPE has random branching. 
  • HDPE is formed under low temperature and pressure with a Ziegler-Natta catalyst whereas LDPE is formed under high temperature and pressure using a free radical mechanism.

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Question

List a few uses of the plastic PVC.


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Answer

Shoes, drainpipes, cable insulation, vinyl, traffic cones, etc.


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Question

What are plasticisers?


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Answer

Small molecules that fit between the hydrocarbon chains in polymers such as PVC. They force the chains apart, weakening the intermolecular forces between them, and making the plastic softer and more flexible.

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Question

Compare and contrast Nylon and Kevlar.


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Answer

  • Both are polyamides made in a condensation reaction from smaller monomers. 
  • Both are made from an amine and a carboxylic acid and contain the amide linkage group.
  • Nylon can also be made from an amine and an acyl chloride. 
  • Kevlar’s chains contain benzene rings and are planar molecules, whereas Nylon’s polymer chains do not. 
  • Kevlar has a greater strength than Nylon due to stronger hydrogen bonding between chains. 
  • Both plastics are used for multiple products like clothes and ropes.


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Question

Name the strongest type of intermolecular force present in polyesters.


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Answer

Permanent dipole-dipole forces

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Question

Name the strongest type of intermolecular force present in polyamides.


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Answer

Hydrogen bonding

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Question

Explain how Kevlar’s structure makes it suitable for use in bulletproof vests.


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Answer

  • Kevlar is made of long hydrocarbon chains containing benzene rings.
  • These make the molecules planar and relatively rigid. 
  • As a result, they can pack together tightly, which strengthens the intermolecular forces between them. 
  • Because Kevlar is a polyamide with the amide linkage group, these intermolecular forces include hydrogen bonds. 
  • This makes Kevlar dense and very strong.

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Question

Why can’t polymers made from alkenes be broken down easily?


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Answer

They only contain strong, nonpolar C-C and C-H bonds which are resistant to chemical attack.


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Question

What are the products of the breakdown of polyamides?


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Answer

Amines

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Question

Give two positives and two negatives of recycling plastics.


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Answer

(Eg):

  • It means we do not have to create new plastics, which uses crude oil, a finite resource. 
  • It requires less energy than the production of virgin plastics.
  • However, it can only be done a few times before the polymer chains are broken beyond use. 
  • The processes involved in recycling can release toxic chemicals within the plastics into the environment. 

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