Episode 172 29 min 05 sec Natural and synthetic: What we're still discovering about polymers
Emeritus Professor of Polymer Science in the Department of Chemical Engineering in Imperial College, London, her research career has focussed on the application of scattering techniques, notably neutron scattering, to the understanding of polymer behaviour. She was Dean and then Principal of the Faculty of Engineering in Imperial College.
She is a Fellow of both the Royal Society and the Royal Academy of Engineering and a Foreign Member of the US National Academy of Engineering. She is a Dame of the British Empire and a Chevalier of the Legion d’Honneur.
Chair of EPSRC from 2003 to 2007, Vice President and Foreign secretary of the Royal Society 2001 to 2006. She currently chairs the Advisory Committee on Mathematics Education (ACME), and is a council member of the Royal Academy of Engineering.
Host: Dr Shane Huntington
Producers: Kelvin Param, Eric van Bemmel
Audio Engineer: Gavin Nebauer
Episode Research: Dr Dyani Lewis
Voiceover: Nerissa Hannink
Series Creators: Eric van Bemmel and Kelvin Param
Welcome to Up Close, the research talk show from the University of Melbourne, Australia.
I’m Shane Huntington. Thanks for joining us. When we hear the word polymer we often think of plastics but polymers - large molecules of repeating structures - are much more than that. They’re found everywhere and they’re an essential feature of life on this planet. From nature we find many examples of polymeric materials such as nucleic acids, proteins, amber and rubber to name a few.
Over the last two centuries we have learned to synthesise many more polymeric materials. Bakelite, nylon, PVC, polystyrene, silicone and many others have become household names and play a central role in contemporary human life and lifestyles. Despite their widespread use polymers are not completely understood. And until recently theory and observation have not converged. As we push the applications of polymers through new functionality, careful modelling and examination becomes crucial.
Today in Up Close we speak to Professor Dame Julia Higgins about polymers and how we image their structure. Professor Dame Julia Higgins is a polymer scientist in the Department of Chemical Engineering and Chemical Technology at Imperial College, London. Welcome to Up Close, Julia.
Very nice to be here.
Firstly, let’s start with an explanation of what a polymer is and how it differs from other chemical structures.
The basic thing is in the two words poly and mer. The poly means many. The mer means a unit and essentially, as you’ve just already mentioned, it’s a large molecule. Usually the synthetic polymers I’m talking about are composed of quite simple building blocks but repeated many, many times and linked together with proper chemical bonds, not clusters of molecules as you might get in a colloid. They are actually single molecules.
The absolute simplest one of all is polythene. Polythene is essentially a carbon with two hydrogens on it but you might get millions of them joined together so the carbons are hand in hand, the hydrogens stick out each side and you finish up with a molecule which when you make it into a material could be a plastic bag, the casing of a telephone, a bottle to put your milk in or many other things.
Probably people who are specialist sportsmen are aware that almost anything they use, wear or put on, ride on or swim in or anything else is made of a plastic material. We might not immediately think that modern medicine would be impossible without plastics. For example, almost any of the inserts you put into the body have plastic materials. If you have a replacement hip, one of the surfaces is a polymer. If you have a stent inserted in your arteries some of that will be plastic. If you have a heart-lung operation then while you’re busy being operated on your blood is being circulated through yards of plastic pipe. None of that would be possible without plastics.
Why is it that we use polymers and plastics specifically over other materials? What are the advantages?
Well, first of all, because you can make many different sorts so you can control what you’ve got. So, for example, if you wanted to do the heart-lung machine with glass you’ve got glass and it’s not very flexible. You might use natural polymer in rubber but it’s not easy to sterilise or anything else. So actually what would you use, I think is my answer back to you. It’s quite difficult to think what you would use.
If you’re thinking of the supermarket, you imagine going to the supermarket and everything you buy in a container is in glass your shopping bag would be extraordinarily heavy coming home. Everything you buy, if it’s not liquid, will be packed in wood and paper, equally difficult. So actually what the plastics have done is make possible a lot of things that we couldn’t do without them.
Julia, we’ve had a number of guests on Up Close over recent years talking about particle physics and one of the things we find there is that the experiments take quite a while to catch up to the theoretical predictions. How does that work in the case of polymers? Is there a similar lag as we invent the technologies required?
Yes. I mean, let me go back, if I can, a little bit into history. As you mentioned, people were making polymeric materials way back, in fact, in the 19th Century. Bakelite was late 19th or early 20th Century so they could make things that we would recognise as plastic materials but they actually had no idea what they had made. It was a purely empirical way of making them.
There was a German scientist in the 1920s called Hermann Staudinger who was the first to really suggest that these might be made of long molecules and, to begin with, he was laughed out of court by the chemists. They said it’s not possible to make something with bonds hundreds and hundreds and hundreds of units long.
Up until the Second World War, although people were learning how to make these large molecules, they didn’t have a huge amount of control of what they made. Synthetic rubber was made early in the century. The first polythene was just before the Second World War but controlling what they got was quite tricky.
We ought to make a digression into why you want to control what you’ve got in order to control the properties. If you think of this as a long molecule - I described the simplest possible one, polyethylene which is a line of carbons with hydrogens on the side. All the other polymers are more complicated so you can imagine that the building block isn’t just a simple bead. It’s got a shape to it.
If you imagine making a long, long line of Lego pieces and the pieces are not symmetrical, that is to say it’s a rectangle with a nob on one side, you’ve got a number of options. You could join them together in a line with the nobs all on one side. You could join them together with the nobs alternating. In terms of polymers; those two I described as isotactic, meaning on the same side; syndiotactic, meaning on opposites sides. The third way is you could join them together randomly. So you have a little run of nobs on one side and then a little run with the nobs on the other side. They’re called atactic and they have very different physical properties.
First of all, only the regular ones, the isotactic and the syndiotactic could make crystals. If you want to make a crystal structure you’ve got to make a regular pattern. If you imagine a line of Lego with the nobs alternate - randomly on each side you simply can’t pack that together into a nice structure. So the atactic polymers are what we call amorphous. They never make crystals and a lot of the materials we work with are amorphous.
People knew that polymers had different properties. For example, the scientific name is polymethyl methacrylate; Perspex in Australia. It’s called Plexiglass in the USA. Anyway, we’re fairly familiar with it. It’s a synthetic plastic glass. If you make the isotactic form then it will melt at about 40 or 50 degrees centigrade; quite a low temperature. If you make the syndiotactic form it doesn’t melt until about 140. If you make the atactic form, which is what most of what you will see around you when you buy Perspex, it melts at about 105 degrees centigrade.
So just one simple change in the arrangements makes a big change in what’s going on. To come back to the history of polymer science, once people realised this local structure was important they wanted to learn to control it and that was a real breakthrough in about the ‘60s. A chemist called Ziegler managed to invent a catalyst which is a help to growing the polymers which caused a polymer - the particular polymer was polypropylene but it was applied to many others subsequently. He designed a catalyst that made the polypropylene grow in a regular structure.
There was a scientist called Natta. So Ziegler was German, Natta was Italian and Natta was the physical scientist. He understood the crystal structure so he proved that these new molecules were indeed regular structures and could make crystals. Between them, they won a Nobel prize and they are forever linked as Ziegler-Natta catalysts. The interesting twist to the story, as I’m told, they actually hated each other so they’ve been linked together for the rest of their lives in the name of what they invented. But the breakthrough was that chemists had now begun to learn how to really control what’s called the microstructure of the polymer and therefore to control the properties in a way that they simply couldn’t before.
To come back to the history, until these control samples were available, the physicist didn’t have a great deal of chance from their experiments understanding what had happened because they got a mixed mess and they couldn’t relate the properties back to the structure.
This is Up Close coming to you from the University of Melbourne, Australia. I’m Shane Huntington and we’re delving into polymer science today with physicist, Julia Higgins. Julia, you use a technique called neutron scattering to examine some of these polymers. What is neutrons gathering?
Well, the first question we ought to address is what is the neutron. So the neutron is a neutral particle that’s found in the nuclei of every atom around us except the hydrogen atoms. Hydrogen atoms have one proton. All the other atoms have more protons and some neutrons. So the nuclei contain the neutrons but the chemical behaviour of the atom is determined by the protons and the number of matching electrons in the atom.
Why is the neutron interesting? Because it’s a neutral particle, its properties as a particle for probing materials mean that it looks at some of the same regions as x-rays would. Its wavelength, which determines whether you can make patterns from the structure of materials matches the distance between atoms and materials. So it’s good just exactly as x-rays are for looking at how materials are structured but its energy is very much less than the energy of an x-ray which means you can also look at small energy changes.
You can imagine the neutron comes in and essentially gets batted off by a moving piece of the molecule and comes off with more energy. If you can measure that energy you can tell something about how the molecule was moving. That’s where the neutron is. Why it’s so interesting to me and to people in chemistry in general but in particular in polymer science is the neutron has the property of seeing a hydrogen atom completely differently from a deuterium atom. You might sort of imagine that. Deuterium is what we find in heavy water. It’s an isotope of hydrogen. Isotope means that it has still one proton, as hydrogen does, but it’s got a neutron in the nucleus. The atom is heavier but still has the same chemistry as the hydrogen and the neutron sees the deuterium nucleus completely differently than it sees a hydrogen nucleus. Most polymer materials are made of carbon and hydrogen - a lot of hydrogen - and what the chemists very early on realised was that if they made the same molecule where all the hydrogen had been replaced by deuterium, which is chemically quite feasible, then as far as a neutron was concerned they might as well have painted the molecule red. They’ve made it stand out from the other molecules.
How exactly do we do the imaging work with neutrons? I understand it’s a form of crystallography but what’s required with the polymer and the neutrons in the way they interact?
Can I just interject? It’s not just crystallography. If we’re looking for the shape of the molecule then, if you like, it’s a bit like crystallography but if we were looking for changes in energy then it’s nearer to what’s called spectroscopy - infrared spectroscopy and so forth - but roughly speaking what you’ve got to do is have a source of neutrons. So somebody has to have a reactor so you have a controlled chain reaction and the neutrons get thrown out as the nuclei break up. The other way is to build something much nearer to a particle physics lab, an atom smasher throwing protons at each other at enormous speeds.
If you imagine a different set up but you still accelerate the protons to a very high speed and simply throw them, all them to hit a lump of metal - a heavy metal because you want lots of neutrons in the nuclei - it could be uranium but it doesn’t have to be uranium, then the protons hit those nuclei. They chip off spare neutrons and spallation is actually a term in using stonework and it means chipping off. So the neutrons have chipped off some of the nuclei and you get a burst of neutrons.
Julia, once you’ve actually produced the neutrons what’s the next step in this process?
The neutrons come out of the source and they come out with a very big spread of energy or wavelengths which is analogous and generally speaking, if we want to do an experiment we don’t want a big spread. We want a narrow spread so the first thing you have to do is select out of this huge broad spread a narrow spread and you can do it in numbers of ways. You could do it by shining the broad spread beam onto a crystal. The crystal would diffract the beam, send of a diffraction pattern but the different wavelengths will go in different directions. So you could pick a direction and pick a wavelength.
That’s actually a rather wasteful way of selecting the beam because it’s actually extremely narrow. It’s too narrow for a lot of experiments and one of the problems with neutrons is however big the reactor you build or the spallation source you never have enough neutrons. We’re always short of neutrons for the experiments. So what you might do is use what are called a chopper and literally that chops the beam. If you imagine two gates separated by maybe a metre and the neutron beam comes along and the first gate opens and then closes so it lets a burst of neutrons through. Those neutrons still have the broad spread of energy but by the time they get to the second gate the fast ones will get there a lot of quicker than the slow ones. So you can choose when to open the second gate and select out of that spread a particular speed that you want and by essentially deciding how long you open the gate for you can decide whether you want a broadish spread or a narrow-ish spread. That’s the simplest way of selecting.
Now that you’ve got your neutron beam, what’s the next step?
So now we’ve selected a wavelength, the next thing you do is throw it on to a sample which could just be a piece of material at room temperature. It doesn’t have to be particularly prepared. I mean, obviously from the physics point of view you want to know what you’ve got there but it’s not difficult to make a sample. The only thing is, again because we’re short of neutrons, they tend to be big and it could be cooled down to low temperatures if that’s where you want the information.
One of the things about the neutron is it’s a neutral particle so it will go through a piece of metal quite happily so you can make a cold sample inside a cooling system and still let the neutrons in to do the experiment much more easily than you can the x-rays. So the neutrons have hit the sample, they’re scattered. They may be scattered at different angles. If we were doing a diffraction experiment they’d be scattered at specified angles and we’d have a diffraction pattern. If we were looking at an amorphous material they’d probably be scattered into a uniform halo around the instant beam. That halo is really the small angle scattering I’m interested in and it’s coming from the vapour particles in the air. The size of the halo is essentially related to the size of the vapour particles. And if I were interested in energy changes then the neutrons might come off faster or slower than they went in and I would need some means of detecting how fast they’ve travelled.
The simplest way in the first experiments I did when I did my doctorate were so-called time of light. You literally time how long they take to get to the detector. The one other thing, I should say, is how do you detect a neutron? The quick among our audience will have noticed if it’s a neutral particle and goes through things how do I know when it’s got there? What happens is you don’t detect the neutron, you put gas filled detectors filled with a gas where the nuclei would interact with a neutron so the neutron comes in, hits the nucleus. It might be Helium-3, it might be Boron trifluoride. They’re different atoms which the neutron interacts with the nucleus and off comes some radioactive particles and what you do is then detect those radioactive particles. And then we’ve got an experiment.
All we’ve got to do is count how many neutrons arrive where, look at the pattern and then decide if it’s telling us something about the sample.
You’ve mentioned that the neutrons, of course, are low energy, they don’t interact that much. Can I assume then this is not a destructive examination technique when you’re looking at polymers?
Absolutely. It’s completely non-destructive. The energy that the neutron carries is typically the sort of energy that an air molecule will have in it around you in everyday life; what’s called thermal energy, so they’re very low energy.
What other sorts of materials do we find neutrons getting used for in examination of small samples?
Well, polymers clearly but one big area is what’s called soft matter so colloidal materials, particles floating around in liquids, certainly a lot of crystal structure.
This is Up Close coming to you from the University of Melbourne, Australia. I’m Shane Huntington and we’re delving into polymer science today with physicist, Julia Higgins. Julia, specifically what aspects of polymers are you studying with these neutron-based techniques?
Well, in my career, I’ve been looking at a lot of different aspects. In particular the very first questions we addressed with what was the shape of a polymer molecule, this was a question that had been postulated theoretically by a guy called Paul Flory who got a Nobel prize in the ‘50s or ‘60s but nobody’s seen a polymer molecule. You can’t take a lump of rubber and shine a laser on to it and say, well, I can see this molecule and not that molecule. Very early on, the group I was working with, a French group, we simply labelled some of the molecules with deuterium. The material we were using was polystyrene because it’s a nice simple material.
So the chemist - they made polystyrene where all the hydrogen had been replaced with deuterium, mixed it with ordinary polystyrene. We could see the molecules and we confirmed that the predicted shape of the molecule was as Flory had said. Now, that was a big deal at the time but it was only the beginning. Subsequently people have looked at the shape of molecules in many different configurations.
They went on and we, to some extent, to look at stretched materials. So, as you will know, plastic materials is one of the properties, you can deform them, you can stretch them, you can bounce them. The question is what are the molecules doing? That was an area that was studied and I was involved with and then, in particular, in the ‘80s one of the big questions was how you could get the people who move polymers around, rheologists who push them through pipes and squash them into shapes and know that they have very special what are called viscoelastic properties. That’s to say they don’t just flow, they bounce back. The question was how they could relate what they measured to what we, the molecular scientists were telling them about the shape and size and movement of the molecules. There were experiments very - to me they were exciting at the time where there was new theory coming from particularly a UK scientist, Sam Edwards who’s still in Cambridge and a French scientist, Pierre-Gilles de Gennes, Nobel prize winner who died two or three years ago. They, in different ways, developed a model which allowed the rheologists, the people who handle the polymers, to talk to the molecular scientists who knew what shape and size they were. That was every exciting.
What I’ve been doing a lot for the last decade or two is looking at mixtures of polymers. Neutron scattering is not the only way of doing that but it has some interesting possibilities. Just a remark, polymers don’t like mixing with each other generally speaking which is one reason why recycling is so difficult. If you’ve mixed together polystyrene, polymethyl methacrylate and some polyethylene you get a nasty crumbly mess. You do not get a nice plastic material.
We were interested both in the ones that mix and the ones that don’t mix and we’ve done experiments on the thermodynamics of mixing on the process of separating out because sometimes you can mix them at some temperatures and then at higher temperatures they fall apart and on additions of materials that help them to mix, so-called copolymers. So a whole range of things but essentially related to mixing of polymers.
From a theoretical point of view, how well do we now understand the relationship between structure and function for polymers?
I would say pretty well. You alluded to it at the beginning, a marching hand-in-hand, a theory of computer modelling which has been absolutely crucial, of chemistry and of physical experiments and it’s really needed all those four things together. The chemists had to make the precise material that would test the theoretical model. The theorists had to come up with these ideas that allowed simple pictures to be translated between properties and molecular behaviour. You needed computer modelling really to handle then those simple scientific ideas and apply them to a big material and finally you needed the engineers to make the material. So you needed all those together and I think we’ve actually come a long way to the extent that most people are now arguing more about the details than the fundamental picture.
Having said that, I wouldn’t say that polymer science is finished because, of course, we’re always trying to use the materials in more complicated situations. There are couple that are quite interesting. One is supposing we add a third material and people are very interested in nanotechnology. The term nanotechnology is everywhere. One part of nanotechnology is to add these tiny what I call soot particles but they’re called fullerenes. The little tiny particles of carbon that have been around for hundreds of years but were only really discovered 10 or 20 years ago. If you add those to polymers, you produce a new set of properties. If you add them to polymer mixtures you change the mixing behaviour. And people are very interested in these because of the possibility - if you add these carbon nanoparticles you produce light strong materials - because the whole of the aviation industry, the car industry, the saving of power and everything.
The other area that there’s a lot of development is electronic use of polymers. Polymers generally are not conducting but you can make them conducting. You can certainly make them conducting in special ways by adding these small particles and there is a whole lot of fundamental work in both the use in electronics and also people are struggling to try and make photovoltaic cells. What we want is an artificial leaf that will collect the sun’s light and turn it into electricity and polymers are going to be important there too.
Do you think there’s a time when these materials will replace things like steel and concrete in construction or will that just neve happen?
People are making quite strong materials. The main place they’re making them is the aircraft industry. A lot of things in the aircraft industry are made from plastics and they’re made strong by essentially mixing them with things like carbon fibres and so forth and, of course, the drive there is you want strength with lightness.
I think it’s extremely unlikely that you could imagine a jet engine running on plastic. The sort of chemical bonds we’ve got simply would not survive at the temperatures you would need to run a jet engine so there’s going to be areas where metals are essential. I think in terms of construction, probably there will be many more uses. Having said that, my guess would be it’s more expensive to make a plastic beam with carbon fibre in it than it is to make a concrete beam.
Julia, just one last question with regards to polymers. Over the years there have been some fairly serious environmental concerns regarding plastics. Where do we stand at this point with regards to those concerns?
Well, I alluded a little bit earlier to the fact that polymers don’t like to mix each other. So the ideal will be to say to melt all the polymer up, press it into a new material and you’d have a new material, is not actually easy to do. They don’t like mixing. People might have noticed that many plastic materials have labels on the bottom, keys. Yoghurt pots have a PS for polystyrene or a PP for polypropylene. That’s to allow people to sort them out so that they can recycle the same polymer with the same polymer, so that’s one option but it’s very labour intensive doing the sorting out.
Another thing is that motor manufacturers, for example, often label the parts, say the bumpers at the front. I know bumpers are called different things in different countries but the protective parts of the car might well be labelled with a key that says what the plastic was so that it can be recycled, not mixed with other polymers. So there is a place for recycling.
Having said that, recycle is a small part in my view. I think we’ve got to train people to be more careful with plastics so that they don’t throw them around all over the place. But my own view, and I think there are quite a lot of people who share it now, is what we should do is burn them. We should use them as a power source. There’s no reason why we can’t do that. You can build power generators that will burn the plastics and burn them safely because what you don’t want is to put fumes out into the air and people are exploring the possibility of recycling at least domestic waste fairly locally. So you might, around the city of Melbourne where we’re sitting, having several recycle centres to which the plastic and probably other materials will be taken, burned in controlled conditions and produce local power. I think a lot of people would argue that’s, in fact, probably the most general way we can do something with the plastics rather than litter the surface of the earth with them.
If you think about it, we’ve taken the oil, made the plastic out of the ground, out of the oil stream - out of, if you like, the energy flow - we’ve put it back into energy later. That’s not a bad thing to do.
Professor Dame Julia Higgins, Professor of Polymer Science in the Department of Chemical Engineering and Chemical Technology, Imperial College London, thank you very much for being our guest on Up Close today and giving us an understanding of the latest work in polymer science.
It’s been great talking to you.
Relevant links, a full transcript and more info on this episode can be found at our website at upclose.unimelb.edu.au. Up Close is a production of the University of Melbourne, Australia. This episode was recorded on 10 November 2011. Our producers for this episode were Kelvin Param and Eric van Bemmel . Audio engineering by Gavin Nebauer, background research by Dyani Lewis. Up Close is created by Eric van Bemmel and Kelvin Param. I’m Shane Huntington. Until next time, good-bye.
You’ve been listening to Up Close. We’re also on Twitter and Facebook. For more info visit upclose.unimelb.edu.au. Copyright 2011, the University of Melbourne.
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