The Oxford Science Lecture Series
Professor Dame Julia Higgins FRS
Imperial College
"Seeing in Believing"
Dorothy Hodgkin Memorial Lecture 2021
Online, Oxford, 4th March 2021
Dame Julia Higgins is Professor of Polymer Science at Imperial College, a Fellow of the Royal Society and a Chevalier de la Legion d’Honneur. She studied physics at Somerville for her undergraduate degree, moving to the Department of Physical Chemistry for her DPhil. She began her lecture with the crest of the Royal Society which was founded in 1660 to focus on experimental work and the need to ‘see results’. Its motto can be translated into ‘Seeing is Believing’, the title of Dame Julia’s lecture.
Dame Julia began by describing how her inspirational physics teacher, also a Somerville graduate, inspired her to apply to Oxford and Somerville in 1961. She was one of only 10 female physics students, 6 of whom were at Somerville, in a class of 200. For her DPhil she was based at Harwell and studied neutron scattering. On graduating, she went into teaching for a couple of years but missed research and so took up a Research Fellowship in chemistry at the University of Manchester. She worked on neutron scattering in Strasburg, Paris and Grenoble before taking a lectureship in the Department of Chemical Engineering at Imperial College.
Polymers are very useful but at the moment they are getting a bad name as many are not biodegradable. Dame Julia explained that they turn up in multiple disciplines from medicine to aeroplanes. They are formed from multiple units of chemical building blocks, most of which are simple but not symmetrical and so the same units can be arranged in different ways to create materials with a range of properties. She gave a brief overview of the chemistry of polymers, from the discovery of rubber in 1770 and polystyrene in 1839 to the development of industrial synthesis in the 1930s and of catalysis in the 1960s. 2020 was the centenary of the publication of a paper by the Nobel Laureate Hermann Staudinger which showed that polymers were long covalent molecules which are tangled up like a ball of string. Another Nobel Laureate, Paul Flory, showed in 1953 that polymers adopted a random structure akin to a drunkard’s walk and increased in size by the square root of the number of units. Dame Julia commented that she normally demonstrated the random nature of a drunkard’s walk but that sadly that wasn’t possible in an online presentation.
Dame Julia’s early research investigated methods to measure the length of a polymer molecule which is not straightforward as each molecule is within a tangle of identical polymers. She wanted to see one particular molecule within the tangle and see how it moved when the whole material was bent. To do this she used neutron scattering. She showed a video of glass marbles in water or in toluene. The marbles could be seen in the glass but not in the toluene as the refractive index of glass is different from that of water but not that of toluene. Dame Julie explained that they developed a way to change the neutron beam equivalent of the refractive index of one molecule within the mixture of many so that it can be picked out. They did this by replacing hydrogen with deuterium in the polymer as this doesn’t change the chemical properties of the polymer but does change the way it scatters neutrons.
To work with neutrons, one needs a reactor and Dame Julia described that she worked at the centre in Grenoble where neutrons are generated as a beam and can be scattered in the same way as light can be scattered, using the formation of a halo around the moon as an example of light scattering. The scattering pattern gives information about the structure of the molecule, as the structure gets bigger the scattering angle gets smaller. In 1974, Dame Julia published a paper with J. des Cloizeaux describing the technique to determine the conformation of a polymer chain in a melt or solvent. In Grenoble, she lived in a tower block facing the French alps but, in 1976, she moved from there to central London to work at Imperial College. She said that some questioned why she would make such a move, but she explained that hitherto she had been working on method development in neutron scattering and felt it was now time to switch to using the technique to answer her scientific questions.
The first question she set out to answer was what happens to the shape of a polymer molecule when you stretch the material? Small molecules can stretch with the bulk structure without having to change shape. Longer molecules have a more tangled conformation and so stretching needs energy to alter the arrangement of the molecule. They used deuterated polystyrene which they heated above the glass transition temperature, so that it was pliable, and then quenched it rapidly. Using neutron scattering, they could see that the molecule had stretched with the material. If they held it at temperature, they could see the polymer gradually change shape as it relaxed back to a lowest energy conformation.
The second question she asked was how does one molecule move around within the tangled web of molecules? Her colleague, Sam Edwards developed a mathematical model by considering that one molecule in the tangle reacted in the same way as if it was in a tube formed by the surrounding molecules. He postulated that the polymer would move through the tangle like a snake moves through a tube. Eventually the molecule might escape from the original ‘tube’ into a new set of constraining molecules. Dame Julia looked at molecules at the boundary between two thin layers. Neutron beams can be reflected at a surface in the same way as light. By looking at the interference pattern between the layers she could see how the molecules were moving as the layers were merging and confirm that they moved as had been predicted.
Dame Julia concluded her talk by saying that the work she had described was some of the simplest uses of neutron scattering to look at polymer molecules. As the field has progressed, more and more complex questions are being answered using this technique. She related it to the field of X-ray crystallography which Dorothy Hodgkin used to study proteins which are natural polymer molecules. Deuterium labelling and neutron scattering has been used by Dorothy’s students and many others to study protein structure and Dame Julia met many of them during her time in Grenoble and at the European Molecular Biology Lab there. The online session finished with a vote of thanks and questions from the audience.
Prof. Carolyn Carr
Associate Professor of Biomedical Science
University of Oxford