Lesson video

Hello, my name is Mrs. Collins, and I'm going to be taking you through the learning today.

This lesson forms part of the unit materials and is called Structure of Polymers.

During the lesson, we're gonna have a look at different types of polymers, and then look at how we can strengthen them and increase their flexibility.

So let's get started.

By the end of this lesson, you'll be able to give examples of polymers, and explain how the properties of polymers depend upon their structure.

Here are the keywords for today's lesson, polymer, forces of attraction, synthetic, plasticiser, and cross-link.

Some of those words will be familiar to you, others will be new.

We are going to explore them during the lesson, but you might want to pause the video now, have a look at some of the definitions, and maybe make some notes.

Today's lesson is gonna be divided into two parts.

Firstly, we're gonna talk about polymers, and then we're gonna talk about how we can change the properties of those polymers.

Let's get started on part one.

A good place to start is to think about what the word polymer means.

So poly means many, and mer means parts.

So a polymer is something made of many parts, and actually these parts are called monomers.

So a polymer is made of a whole series of monomers chemically bonded together to form the polymer.

Here's a question based on that initial bit of learning.

So how many atoms are there in a typical polymer molecule? Have a look at the four answers, and consider them carefully.

Which one do you think is correct? I'll give you a moment to think about it.

So a polymer has more than a thousand atoms in its structure, and actually, it can have hundreds of thousands of atoms in its structure.

So polymers can be made synthetically, that means manmade, but they also incur naturally.

So we're gonna have a look at some synthetic polymers, and then we're gonna go on and have a look at some natural polymers afterwards.

So polypropene and polystyrene are two good examples of synthetic polymers.

Polypropene is made of the monomer propene, and actually the clue is in the name, polypropene, many propenes.

If you have a look at polystyrene, what's the name of the monomer in polystyrene? So the monomer is styrene.

Polystyrene, many styrene molecules joined together to form the polymer.

DNA is a good example of a naturally-occurring polymer, and you will have come across this in biology.

But other ones include proteins, carbohydrates, like starch and cellulose.

They're all naturally-occurring polymers.

Let's have a look at some examples of natural polymers and the monomers that they're made of.

So first of all, cellulose.

You'll know this from biology.

It's found in the cell walls of plants, and it's a long chain of glucose monomers all joined together.

Starch is another example of a polymer made from the monomer glucose.

Now, that suggests there must be something different in their structure, because these are two different polymers, but made from the same monomer.

And another good example comes in proteins.

So silk is an example of a protein, and it's made from the monomer amino acids.

But wool is also a protein.

It's different to silk.

It's made from the protein keratin, but that's also a chain of amino acids.

So that suggests there must be something slightly different in its structure.

Here's a question based on that learning.

Which of these objects are made from synthetic polymers? So think about what the word synthetic means, first of all.

Then look at the three examples, and decide which ones are synthetic.

I'll give you a moment to think about it.

So hopefully you've recognised that polyethene is an example of a synthetic polymer, and polystyrene is an example of a synthetic polymer.

Remember, the word synthetic means manmade, and wool is an example of a naturally-occurring.

So polymers are molecules that are incredibly long, and because they're long, there are forces of attraction between each of the polymer chains.

And we can show that in a diagram.

So here's an example of polymer molecules all laid across each other, and there are forces of attraction between those polymer molecules.

And the longer the polymer molecules are, the stronger the forces of attraction because these occur across its length, and this forms the structure of the plastic.

So the melting point of the polymers is higher than the simple molecules that they're made of.

When a polymer melts, those forces of attraction are broken.

So, not the chemical bonds between the monomers, but the forces of attraction between the polymer molecules.

And that's important to remember.

So there are lots of different types of polymers, but as we've seen, even if they've got the same monomer, they can form different structures.

So high-density polyethene and low-density polyethene are both made from the monomer ethene.

Let's consider high-density polyethene, first of all.

So you'll be familiar with this because it's used to make things like milk bottles.

And in actual fact, the melting point of high-density polyethene is higher than low-density polyethene.

And this is to do with the forces of attraction.

So the forces of attraction in high-density polyethene are higher than in low-density polyethene, and we need to consider why.

We can show it in the diagram.

Again, remember this diagram from before? So you've got these polymer molecules lying across each other, and forces of attraction between them.

And these forces of attraction need to be broken for the substance to melt.

So high-density polyethene actually melts between 120 and 180 degrees Celsius.

Low-density polyethene has got a lower melting point than high-density polyethene.

So there's an example of a plastic made from low-density polyethene, a plastic bag.

This is because of the polymer chains.

And the polymer chains this time have got branches, which we can show in the diagram.

So rather than just being straight, you can see they've got branches across them.

And what this does is it disrupts those forces of attraction between the polymer molecules, and that means it has a lower melting point.

And that takes it down to 105 to 115 degrees Celsius.

And remember, the lowest melting point for high-density polyethene was 120.

So just having those branches in the structure has an impact on the melting point, because it disrupts the forces of attraction between the polymer molecules.

So here's a question based on that learning, and this time we've got true or false.

So a low-density polymer has a higher melting point than a high-density polymer.

Firstly, you need to decide if that's true or false, and then you need to read the justification statements and decide which one of those you're gonna use.

What I suggest you do is pause the video at this stage, consider the question, and then come back when you're ready.

Welcome back.

So hopefully you've identified that this statement is false.

Actually, it's the wrong way round.

So a low-density polymer has actually got a lower melting point than a high-density polymer.

We need to consider why from the statements, and the correct statement this time is A.

So a high-density polymer has more forces of attraction between the molecules.

Remember, this is because a low-density polymer actually has these branches that affect those forces of attraction.

Well done if you've got that right.

Now we're gonna have a go at task A, which has got three questions.

So we're gonna look at question one, first of all.

So why do polymers have higher melting points than monomers? We've got a series of statements there, and you need to tick a column for each statement.

So pause the video here and have a go at that question.

Okay, let's go through those answers then.

So atoms in a polymer are more strongly held together.

That's actually incorrect.

It's not about the atoms, remember.

The atoms in the polymer are chemically bonded together with each other.

It's all about the forces of attraction between the polymer units.

So be really careful.

So there are greater forces of attraction between the molecules is correct.

And then more atoms in the molecules need to be separated is incorrect.

It's all about those forces of attraction between all the different polymers.

Remember the diagram that we looked at earlier.

Okay, let's consider question two then.

Some students are making a model of a polymer.

They have decided to use a long chain of beads as a model, and what we're going to do is critique that model.

So for part A, you need to say why it's a good representation, why is it a good model? And then for B, you're gonna give three ways where it's not an accurate representation.

So why is it not a good model? And then for question three, starch is a natural polymer and nylon is a synthetic polymer.

What is the difference? So pause the video here, answer the questions, and then I'll see you when we return.

Welcome back.

So let's have a look at question two to start off with.

So part A says state one way in which the beads are a good representation of a polymer molecule.

Now, each of those blue beads in that chain actually can represent monomers.

So that's what makes it a good example.

So it shows those monomers all joined together in one long chain.

Now for part B, there are lots of potential correct answers, and you may have some of the ones I'm gonna show you, but you also might have your own ones as well.

So the beads are joined together on one string.

Remember when we looked at the structure, a polymer is that one chain, and then it forms a plastic by lots of different layers of polymers on top of each other.

Atoms in real life aren't blue.

It only represents a small portion of the polymer, as the polymers are made from thousands of atoms. And obviously it's not to scale, because monomers are not that large.

So we need to think about this when we are thinking about models.

We need to think about what makes them good models and what makes them poor models so that we can recognise that.

Let's have a look at question three.

So starch is a natural polymer and nylon is a synthetic polymer.

What is the difference? So natural polymers come from living things, and synthetic products are manufactured, or manmade, and that's the main difference between the two.

Well done if you got that right.

That's the end of the first part of today's lesson.

So we're gonna move on to the second part now, and we're gonna look at how we can change the properties of polymers.

So polymers are incredibly versatile.

We can use them for lots of different things, and we can actually change the properties of the polymers quite easily as well.

So PVC can either be flexible or rigid, and this means we can use it for different things.

So we use it for light switches, and that's where we've got rigid PVC.

And we use it for Wellington boots, where it's flexible PVC.

And you can see two completely different uses there of the same polymer.

So if we want to increase the flexibility, we need to add a plasticiser.

And this diagram shows how a plasticiser works.

So you can see there it's found between the polymer molecule layers.

And the plasticiser actually allows these polymer molecules to move over each other more, pushing the long molecules apart so the forces holding them together are weaker.

So it weakens the forces of attraction between the polymer chains.

Now we're just gonna talk a little bit about rubber.

Rubber's a natural polymer.

It's obtained from the sap of rubber trees.

So we can extract the sap of rubber trees and turn it into rubber.

And it's got lots of different uses.

So we use it for elastic bands, we use it for balls, and we use it actually for tyres.

And it's got elastic properties, and it can be stretched and bounced back.

So that's important for us.

It means we can use it for lots of different things.

And we're gonna talk about rubber in a little bit more detail later.

But before we do that, you've just got a question based on this part of the learning.

So plasticisers are added to polymers to make them what? So I'll just give you a moment to think about your answer to that question, and then we'll go through it.

So plasticisers make polymers more flexible.

We've talked about how to make plastics or polymers more flexible.

We now need to think about how we can make them harder, less flexible, more rigid.

And the way we do that is using cross-links.

So let's look at a diagram to see how that works.

You can see the cross-links actually link the polymer molecules together and change the overall structure of the polymer.

And this is a strong chemical bond between the different chains of atoms. And what that does is it makes the polymer harder, less flexible, and it's harder for those chains to slide past each other.

Remember, adding a plasticiser makes it easier for them to slide past each other.

So let's go back to the naturally-occurring polymer rubber.

Remember, we can use rubber to make things like elastic bands, but we can also use it to make tyres.

And if you think about it, the properties of an elastic band are completely different to the properties of a tyre.

So what we've done here is we've done a process called vulcanisation, and all that does is it adds cross-links between those polymer chains, and that increases the rigidity.

We don't want our tyres to be as bouncy and as elastic as an elastic band.

So it takes more energy to separate the polymer molecules when there are cross-links present, because these are stronger than forces of attraction between those natural rubber molecules.

So the forces of attraction are less strong than those cross-links.

So if we just have the forces of attraction, the melting point of natural rubber is only about 106 degrees Celsius.

But with these cross-links in vulcanised rubber, that increases the melting point to about 120 or 140 degrees Celsius.

So you can see it has a reasonable impact on the melting point of rubber.

Now we've got a question about that piece of work that we've just been covering, and again, it's a true or false question.

So cross-links increase the flexibility of a polymer.

So I want you to consider if that's true or false, and then justify your answer using the statements below.

So pause the video now, and I'll see you when you're finished.

Welcome back.

So hopefully you've recognised that actually that statement is false.

Cross-links do not increase the flexibility of the polymer.

Remember, that is a plasticiser that does that.

Then justify your answer.

It adds strong chemical bonds between the polymer molecules, and that's why it doesn't increase the flexibility.

It actually increases the rigidity of the plastic or the polymer.

Let's consider task B.

Question one says PVA glue is runny and contains a polymer.

Mixing borax solution with PVA glue adds cross-links to the polymer.

It then asks you to predict how the properties will change, and why you think this will happen.

Now remember, you might not know about PVA glue and borax solution, but you do know about cross-links.

So think carefully about your answer.

Question two says, the properties of a polymer can be changed by adding cross-links or plasticisers.

How could you make a polymer more flexible? So we want to increase the flexibility.

Do we add cross-links or plasticisers? I suggest you pause the video now, consider those questions, and I'll see you when you're finished.

Welcome back.

So let's consider those questions then.

So in part A, what we are doing is we're adding cross-links to the polymer.

Now remember, adding cross-links will make it less flexible.

So the PVA should become less runny or less flexible, or more stretchy.

Any of those answers will be correct.

Why do you think this will happen? Cross-links are strong chemical bonds between the long polymer molecules.

It'll be harder for the polymer molecules to slide or move past each other.

Notice the answer explains what cross-links are, first of all, and then explains why they will make a change in the polymer chain.

The properties of a polymer can be changed by adding cross-links or plasticisers.

How would you make it more flexible? So plasticisers make a polymer more flexible.

So you'd add a plasticiser.

So well done if you've got those questions correct.

Let's take a look at question three.

Here we've got two students, Laura and Sam, and they've drawn diagrams to explain how a plasticiser works.

We're gonna consider parts A to E, first of all, and then we're going to look at F.

So what I'd like you to do is pause the video and answer questions A to E.

Welcome back.

So parts A and B related to Laura's diagram, so we'll consider that first of all.

The blue lines are representing polymer molecules, and the pink ovals represent the plasticiser.

In Sam's diagram, the dotted lines represent polymer molecules, the orange ovals represent the plasticiser, and the red lines actually represent the forces of attraction between the polymer molecules.

So, well done if you got that right.

That last part, E, was probably the most difficult.

Let's consider part F now.

Whose diagram helps to explain better why a plasticiser makes PVC polymers flexible? Give a reason for your answer.

So pause the video here, consider the question, and then I'll see you when you're finished.

So actually, Sam's diagram is a better representation.

So both diagrams show the polymer molecules and the plasticisers, but Sam's diagram also shows those weak forces of attraction between the polymer molecules.

So it's a better representation.

It provides more detail as to what is happening.

So, well done if you recognised that Sams was better, and you were able to explain why that was the case.

So here's a summary of today's learning.

Polymers are made from small molecules called monomers that join together to form a long-chained molecule.

Polymers can be made synthetically, but some also exist naturally.

Some polymers have high melting points because their molecules are hard to separate.

A plasticiser added to a polymer gets between polymer molecules and allows them to move over each other more easily, making it more flexible.

And cross-links between polymer molecules can make a polymer harder and less flexible.

So thank you very much for joining me for this lesson today.