What polymers are: A world of plastic spaghetti

by Ignacio Martín-Fabiani*

Department of Physics, University of Surrey, GU2 7 XH Guildford, United Kingdom.

*I am part of the Soft Matter Group, where we are particularly interested in all sorts of applications of soft materials. The project I am working on aims to develop new paints and coatings which are more environmentally friendly than the ones in the market. So I could say I am living proof that you can get a salary for watching paint dry!

Think of the plastic bags you use when you go shopping. Now, think of the bottles of water you buy at the supermarket. And of the gloves that you put on before doing the dishes. What do these things have in common? One could say: ‘They are all plastics’. And he or she would be right, for these things share certain characteristics such as flexibility or low weight that make our brain classify them in the same category. But what if I asked you what do these things have in common with silk, wool or the material your eyeglasses are made of? Then you would probably have not a clue. But they do share something that is true for all the materials I mentioned since the beginning of the text. If you were able to have a look at them under a very powerful microscope, you would see they are made of very long chains. And these chains are completely entangled, looking like a plate of spaghetti (see Fig. 1a). 

 Figure 1. (a) Polymer chains are entangled like a spaghetti plate! (b) Different daily life items made of polymers. All images via Pixabay, Public Domain Dedication (CC).

Everyone of these very long chains is built by repeating a small molecule again and again.  That is the reason why they are called polymers. The word comes from greek, ‘poli’ meaning many and ‘meros’ part. 

These kind of materials are present in nature in many forms: silk, cotton, natural rubber...and at a smaller lengthscale, proteins and sugars. And there are also synthetic polymers, fabricated by humans for certain applications: the nylon used for tights (called this way because it was discovered at the same time by independent people in New York and London), the poly(ethylene terephthalate) (PET) used in plastic bottles, or the polycarbonate eyeglasses are made of. In Figure 1b you can find some examples of things made of polymers, and probably you are using at least one of them in your daily life right?

There is a common feature for all polymers that gives them unique properties. Such long chains are not easy to rearrange and organize, and therefore the arrangement they present is not optimized (as you can see in Fig. 1a it is a mess!). Instead of being tightly packed a lot of free volume (air) is present. In fact, if you think of the plastics you know they are normally light even if they are quite large, especially if you compare them to metallic items. This is because metals pack much better than polymers and they have almost no free volume.

One of the implications of having that free space is that if the chains have enough energy they will move and rearrange themselves. It may have happened to yourself: If you feel plenty of energy and have space to move around you will probably end up wandering around instead of just standing still! And at the molecular level, energy is proportional to temperature. In other words, temperature is a measurement of how fast the molecules are moving. Thus above a certain temperature, which is different for each material, polymer chains have enough energy and develop sufficient mobility to move around. This temperature is known as glass transition temperature or, in short, T_g. 

Why is this temperature T_g  so important? On the one hand,  above their T_g polymeric materials become rubbery. As an example, the T_g for natural rubber is about -70°C, much lower than the temperature you have in your room/office (about 25 °C). The fact that they are way above their glass transition temperature is what makes elastic bands so elastic (Figure 2a). On the other hand, below their T_g polymeric chains become completely frozen and the material becomes rigid. This is what happens to Lego-like bricks (Figure 2b). Their glass transition temperature is much higher than ambient temperature, thus they are rigid. So, as you can see, the glass transition temperature of the material influences strongly its potential applications. 

Figure 2. Two  polymeric materials with glass transition temperature, Tg, (a) below ambient temperature (rubbery) and  (b) above ambient temperature (glassy). All images via Pixabay, Public Domain Dedication (CC).

Hopefully I convinced you of at least two things. First, polymers are present in our daily life in many forms. Nowadays it is not possible to live without them: clothes, toothbrushes, eyeglassess, TV’s, cellphones, laptops, cars...the list would go on forever. Second, their unique internal structure determines a temperature above which they are rubbery, but below they are rigid. This influences the possible application areas and service temperature range for each individual polymer. 

In following posts I wil try to explain some other relevant properties and applications of these materials. In the meantime, you can start identifying all polymeric materials that surround you. After that, try to stretch them and ask yourself: is its glass transition temperature above or below room temperature? Or just chill and wait for the next post, I do not want to be responsible for breaking anything!


References

Angell, C. A. 1995. Formation of Glasses from Liquids and Biopolymers. Science, 5206, pp. 1924-1935.

Strobl, G. R. 2007. The Physics of Polymers, 3rd ed. Springer-Verlag Berlin Heidelberg.