polymer liquid getting really stressed. Predicted stress patterns for a liquid polymer made of branched molecules flowing through a constriction – the flow determines the final properties of the plastic
Plastic drinks bottles may seem uninspiring but they combine the strength needed to hold a drink, with the flexibility required not to burst when stuffed into your bag. Behind the unique materials used for such everyday objects lies the latest research from the very small scale of molecular physics right up to the large scale of industrial processes.
'It is the behaviour of the individual molecules in a material that determines how it behaves as it is processed and moulded,' explains Tom McLeish, a physicist at the University of Leeds. This in turn determines the final properties of the material'. Tom co-ordinates the unique Microscale Polymer Processing (MuPP) team, funded by the Engineering and Physical Sciences Research Council (EPSRC), who are unravelling the complexities of molecular behaviour in liquid polymers and leading the way to the development of new and advanced materials, for everything from drinks packaging to high performance sports equipment and the new generation of polymer-based electronics.
'Most plastics are formed in their molten form flowing into moulds and fibres as liquids. The giant string-like molecules or polymers that make up these materials interact to form entangled networks as they are processed, making the behaviour of these molten polymers very complex and hard to predict. Observing and mapping how these polymers behave is time consuming and expensive,' says Tom. 'A key challenge is therefore to be able to accurately predict the final properties of a new material from the basic physical properties of the molecules used to create it'.
To test theories of how polymers might behave as they flow in liquid form, the MuPP team has combined expertise from chemists, mathematicians, physicists, computer scientists and engineers, from Academia and Industry. Physicists use their expertise in the fundamental properties of molecules to identify the ideal molecule to test a particular theory. Chemists then build these molecules developing what the team calls a 'Model Polymer System'. In these model systems every molecule is pretty much identical, leaving no room for hiding failures of the theory,' explains Tom. After some refinement, chemical engineers then watch as molten samples flow through complex sets of channels, mapping the direction and speed of flow at every point. They use lasers to measure the direction of the molecules and a technique known as neutron scattering to pick out the exact shape of the whole molecule within the flow. X-ray scattering is also used to determine the structure of polymer crystals that form during processing. Finally, the mathematicians and computer scientists take the physicists' theories of molecular behaviour and, using a technique known as multiscale modelling, scale these up to develop predictive computer models for the complex flow of hot plastics.
This approach obviously yields results, as MuPP were the first team worldwide to relate results obtained from observing a polymer flow to current theories of molecular movement. 'The significance is that now we can place molecules where we want and how we want in designed materials,' says Tom. 'Already this work has led to the next generation plastic for such diverse applications as crack resistant bottles and all-weather protection covers for lighting'.