Studying polymers - neutron scattering and ISIS

Materials World magazine
,
1 Jan 2009

Neutron scattering underpins advances in polymer science. Professor Dame Julia Higgins, former Principal of the Faculty of Engineering at Imperial College London, UK, explains the process and why it is important.

Polymers are complex yet common materials that play a vital role in many areas of our lives. For scientists, understanding the molecular structure of polymers is important as it provides the building blocks to design and build new polymeric materials or modify existing ones. Natural and synthetic polymers are composed of many repeating structural units of relatively simple molecules linked together, generally end-to-end, to form long chains.

Neutron scattering is a research technique that has significantly contributed to the understanding of polymers. It was first used to study the structure of polymers in the 1970s. At that time it was a little-known technique, but it has served materials science well over the years.

This scattering provides critical insight into the conformation of macromolecules in melts and solutions. It has also helped to demonstrate some of the consequences of the polymer snake-like motion in melt samples (called reptation), reveal polymer mixture interactions and clarify interfacial phenomena.

Probing atoms

Before neutron scattering there was limited information on the shape and organisation of polymer molecules in solid samples. The available information was based on theoretical models but not experimental evidence. The scattering provides a non-destructive tool that enables materials to be examined at the atomic level. By scattering neutrons off sample materials, it is possible to visualise the positions and motions of atoms and make further discoveries.

Neutrons are excellent probes for polymeric systems because of their well-matched wavelength and energy characteristics. Also, isotopic substitution by deuterium in place of the hydrogen atoms in polymer molecules allows whole and parts of molecules to be highlighted within a sample, providing evidence of their behaviour.

Neutron scattering enables us to see how molecules move and interact with each other and view individual molecules, even if surrounded by others. It is also possible to examine the effects of stretch or flow on polymeric materials. This has provided key evidence for understanding behaviour and developing new materials.

ISIS

The UK has its own neutron scattering facility. ISIS is a world-leading centre for research in the physical and life sciences and is operated by the Science and Technology Facilities Council at the Rutherford Appleton Laboratory in Oxfordshire.

The facility has attracted users to neutron scattering, and recently doubled in size to satisfy demand from research groups for neutrons. Its £200m addition includes a second target station to complement the existing neutron source, it will be optimised for the production of long-wavelength neutrons.

I’ve had a long association with ISIS. In the 1980s she used its first reflectometer – Crisp. It is now one of five neutron reflectometers at ISIS. The machines are routinely used by a large number of scientists from across the UK and further afield to study the behaviour of complex and new types of polymers.

In the summer of 2008 ISIS produced its first neutrons at the second target station. New reflectometers built at this station will be extremely useful in probing polymers. When combined with existing instruments and other neutron scattering techniques, ISIS will significantly enhance scientists’ capabilities in tackling complexity, kinetics and in-plane ordering issues.

Looking to the future

Research in the UK will continue to be at the forefront of polymer science. Its interests are increasingly motivated by the commercial significance of products. Many of these problems involve complex mixtures of components and understanding the flow and processing of so-called soft matter (which includes polymers but also emulsions and gels) in industrial processes. Neutron scattering will continue to provide unique answers in this area.

The second target station will enable scientists to make further breakthroughs in materials research for the next generation of applications. The efforts of a growing neutron community, and continued advances in instrumentation at ISIS, will ensure polymers are better understood and that new discoveries continue to be made. Investment in facilities like ISIS is what UK science needs to stay on top.

Further information: ISIS Pulsed Neutron and Muon Source