Not to be brushed off: pushing molecular limits with neutron reflectometry
Thanks to a recent upgrade of ILL’s neutron reflectometer D17, molecular motions can be tracked with unprecedented precision
Polymer brushes are macromolecular structures consisting of a dense array of polymer chains tethered to a microscopic surface. Recently, they have received much attention in academic research due to their impressively versatile application potential, ranging from lubrication, the reduction of biofouling or providing antibacterial properties to surfaces. In addition, polymer brushes can be used in biocompatible molecular detection devices (sensors), e.g. to detect blood glucose levels. For this purpose, polymers which can change their shape in response to changes in their environment are of particular interest. An example of such a polymer is Poly(N-isopropylacrylamide) (PNIPAM for short), which is known to collapse from an extended to globular shape at temperatures above 30-32°C.
The characterisation of such fast, small-scale molecular transitions with a high spatial and temporal resolution requires complex experimental setups. While great results have been achieved with techniques such as optical spectroscopy, these have multiple limitations, for example the need to label polymers with special fluorescent dyes. These may affect some properties of the polymer molecules, such as their mass, and thereby influence experiments. To advance the understanding of polymers in solution, non-invasive techniques, such as neutron-based methods, are urgently sought after.
Neutron reflectometry is a highly specialised method for investigating the behaviour of polymers, including polymer brushes, on flat surfaces. One of ILL’s reflectometry instruments, D17, has recently been extensively upgraded. “Amongst other improvements, the instrument’s neutron flux is now greatly increased and allows for studies of molecular transitions with even better resolution than prior to the upgrade”, explains D17’s co-responsible Ben Humphreys, who led a study investigating the solution response of PNIPAM chains attached to a silicon surface as a function of temperature and glucose concentration.
His team’s experiments showed that increasing concentrations of glucose destabilised PNIPAM, thereby making it collapse at progressively lower temperatures. In samples without glucose, the collapse occurred around 32°C as expected. Interestingly, a so-called vertical phase separation was observed for both glucose- and temperature-induced collapse. This means that the polymer first collapses next to the silicon wafer and then in areas towards the solvent. Furthermore, the fast, time-resolved reflectometry measurements revealed a significant difference between the glucose and thermal-driven transitions of the PNIPAM brush.
“We are very happy with these studies. Not only did our findings allow for detailed insights into stimulus-responsive transitions of PNIPAM which are of strong interest for the development of smart materials, but they also highlight the unmatched strength of the next generation of ILL’s reflectometers for high-resolution studies of molecular kinetics in an aqueous environment”, says Ben Humphreys.
Reference: Edwin C. Johnson, Hayden Robertson, Erica J. Wanless, Grant B. Webber, Ben A. Humphreys. Neutron reflectometry can capture the rapid collapse and swelling of a polymer brush. Journal of Colloid and Interface Science, Volume 699, Part 2, 2025. https://doi.org/10.1016/j.jcis.2025.138248
ILL instrument: D17
ILL contact person: Ben Humphreys