Properties of Polymers

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.

Fig. 1 - An ethene monomer

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.

Fig. 2 - The repeating unit in HDPE

Fig. 3 - HDPE chains. Notice how they are predominantly straight with little branching

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.

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.

Fig. 4 - LDPE chains. Note the high proportion of random branching

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.

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.

Fig. 5 - The polymer of PVC

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.

Fig. 6 - The structure of Terylene

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.

Fig. 7 - The structure of Nylon

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.

Fig. 8 - The amide functional group

Fig. 9 - Hydrogen bonding between two amide groups

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.

Fig. 10 - The structure of Kevlar

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.

Fig. 11 - A table comparing the different polymers discussed in this article

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.

Fig. 12 - Hydrolysis of polyamides

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.