Voices of Science

Why do you actually need to know what the rotation of these molecules is within the polymer?

This is to do with the wriggling motion, which isn’t really a pure rotation. People were trying to understand the properties of polymeric materials. What they’re particularly interested in is the so called rheology, how do they flow, okay, viscosity. So they don’t flow in the same way as small liquids. They have – have a memory for a start, so that if you – if you take some liquids and pour them from the bottle and then pull it back, sometimes they go [slurps] back into the bottle. Some cough mixtures will do that. Some viscous polymers will do that. So you – an ordinary liquid wouldn’t do that, but it’s flowing slowly, it’s viscous and it gets pulled back into the bottle rather than going down. It’s quite an interesting experiment to try. I believe cough mixture is quite a good way of doing it [laughs]. So people wanted to understand – I mean, all of this – don’t forget, I’ve told you the molecules are doing this, but nobody’s seen it. You know, you can’t put a microscope on it. You don’t know where it is. That was the theoretical view of what was going on. What people wanted to try and do was have a molecular picture of how the molecules were moving and then be able to describe theoretically the flow behaviour, ‘cause the flow behaviour is extremely important. It determines how you’re going to be able to process the polymers, okay? So you’ve got a vat full of polyethylene. You want to make it into sheets. You want to make it into pipes. You have to know how to deal with it. If you try and push it through the pipe too fast it won’t flow. I’ve seen some processing where you push things through pipes too fast and you get what’s called shark shin. It goes in judders. And when it comes out the other end you can actually see, it looks like shark skin. If you go too slowly you waste time, you know. Processes – you want to do things fast. If you go too fast you might leave so called residual stress. The molecules might be not quite where they’d like to be and they have to relax back to where they want to be. To understand the rheology means understanding how to make things from polymers. People had a lot of information about the rheology but the molecular scientists were attempting – the physicists were attempting to start to try and relate the molecules to the – to what was going on.