Making leaps and jumps in high pressure small-angle neutron scattering techniques

We have been exploring the analytical power of neutrons at the Institut Laue-Langevin (ILL) for many decades, developing some of the world’s most advanced tools for investigating materials. Small-angle neutron scattering (SANS) is one such technique that consists of directing a penetrating beam of neutrons at a sample and utilising the scattering pattern to infer its microscopic structure.

SANS has been shown to be particularly suited to the analysis of soft matter – the term used to describe substances that are easily deformed by thermal fluctuations and external forces. This includes liquids, polymers, gels, and a number of biological materials. Neutron beams are non-destructive as opposed to other experimental techniques such as small-angle x-ray scattering (SAXS), while the structural components SANS can analyse range in size from 1-100 nanometres – these features make it an ideal tool for exploring soft matter without disturbing the often delicate and heterogeneous systems common to biological materials.

Developments in analytical tools for soft matter studies are highly important both for the development of new materials and in enhancing our knowledge about current materials, including those that exist inside human beings such as biological macromolecules. Innovations in SANS experiments are allowing us to gain a deeper and broader understanding of the structure and behaviour of soft matter using neutrons. Two recent studies that have emerged from the ILL demonstrate just how much potential there is for scientific progress through development of the SANS sample environment available, both utilising our D11 instrument.

The researchers at the ILL exploited rapid pressure changes to probe the fast structural response of polymeric materials. To this end, two different strategies were adopted. In the first study, a pressure difference between the cell and the pressure generator is slowly built up and rapidly released, triggering the structural changes. The setup allows to work up to 3 kBar of pressure with no limitation in the amplitude of the pressure jump. However, as only a handful pressure jumps can be performed, the time resolution is limited by the scattering intensity of the samples. In the second study, a specialised stroboscopic high pressure cell has been used. This cell has a pressure range which is limited to 300 bar, but due to its design it allows for fast cycling of pressure jumps of the order of 150 bar. Both the amplitude and the frequency of the pressure cycles can be adjusted and kept for periods up to several hours. Using this cell we could perform 5400 repetitions of pressure jumps in an automatised way, in order to obtain scattering curves with an excellent signal-noise ratio, even down to a time resolution/time slices of 5ms.

A team of scientists from the ILL, Technical University of Munich, Technical University of Berlin, and University of Central Florida, published their work on the use of kinetic SANS using fast pressure jumps in ACS Macro Letters. This ‘high speed’ form of the technique, with a time resolution of 50 milliseconds and almost instantaneous changes in pressure, was used to examine the formation and growth of mesoglobules – capsules composed of a hollow core and polymeric shell. Kinetic SANS utilising a high pressure cell is an ideal way to examine transition kinetics, as unlike using temperature to induce the change, pressure jumps have no gradient when it comes to analysing large sample volumes.

Mesoglobules are of great interest to scientists due to their potential to act as carriers or reservoirs inside the human body, delivering biologically active substances such as therapeutic drugs. Kinetic SANS allowed the researchers to observe how mesoglobules are formed from dissolved polymeric chains, following the process in unprecedented detail across time ranges never seen before -  the potential of the technique to understand these nanoparticles has been so far largely unexplored. As with other soft matter materials, the polymer that forms mesoglobules changes in response to external factors such as pressure or temperature. These factors initiate the so-called phase transition, the point at which mesoglobules are formed.

For the mesoglobules to be exploited for use in medical therapy, it is essential that the mechanism of their formation is thoroughly understood, so that the pathway and behaviour can be effectively controlled. The scientists used a specially designed cell to initiate a sudden change in pressure known as a ‘pressure jump’ to the polymer. This jump triggers the phase transition, which was observed in real-time using SANS, providing detailed information about the size and inner structure of the mesoglobules. One of the advantages of using SANS in these pressure jump experiments is that neutrons have the penetrating power to examine the properties of samples even when encased in solid materials such as pressure cells. Pressure cells are employed in studies involving x-rays also, but neutrons have, as they are electrically neutral, a much higher penetration power and furthermore they do not induce any radiation damage. This study is a leading example of the use of high-pressure SANS towards much improved time resolutions and much faster pressure jumps than has been done so far. These results demonstrate how kinetic SANS, with the added tool of the pressure cell, provide an excellent environment for studying the structure of soft matter, multicomponent systems.

The potential of kinetic SANS was also recently explored in another experiment published in Scientific Reports, by a team of scientists from the ILL, Bielefeld University, University of Cologne, and University of Stuttgart. Using the D11 instrument at the ILL, the research explored another soft matter system of smart colloidal microgels.

Smart microgels, engineered to respond in a certain way to external stimuli, could have many advanced applications including as tuneable catalytic environments, nanoreactors, sensors, or for drug delivery. Crucial to developing microgels with these future applications is a thorough understanding of the response kinetics in response to environmental changes.

In the study, the researchers once again examined the response of the material to pressure by making use of a specialised pressure cell with a very precise temperature control. They were interested in understanding the kinetics of the swelling and collapse that occurs within microgels, a process which is triggered by the sudden increase in pressure. The structure of the microgel had already been investigated with scattering techniques, but the response kinetics was an area in need of further understanding.

Kinetic SANS utilising a high pressure cell is an ideal way to examine transition kinetics with neutrons, as unlike using temperature to induce the change, pressure jumps do not take a long time to affect change in the large sample size needed for neutron analysis. The rapid and homogenous switch in pressure does not allow a period of changeover, and the supreme time resolution means the researchers were able to discover that the swelling of the particles in the gel occurs at least 10 times faster than the collapse of the structures.

The resolution power of SANS also permitted researchers to establish that the cause of the slower particle collapse is likely to be the inhomogeneous collapse of the microgels, resulting from the cross-links throughout the particle being distributed unequally. Subsequently, the collapse occurs in two steps, at a slower rate than the swelling.

The superior techniques for soft matter studies at the ILL have made massive progress in recent years, with these studies using kinetic SANS demonstrating just how much understanding can be gained about these complex systems using neutrons. The pressure jump technique especially opens the doors to a precise analysis of phase transitions. Many competencies came together in the research behind these systems, and the future of advanced SANS techniques relies on the continued progress and refinement of the sample environments. The goal is to make them robust and straightforward enough to use for other research fields and specialties.

Instrument: The Lowest momentum transfer & lowest background small-angle neutron scattering instrument D11

Contacts:

• Ralf Schweins, Instrument Scientist: schweins@remove-this.ill.eu
• Leonardo Chiappisi, Post-doctoral researcher: chiappisil@remove-this.ill.eu

Re.:
Volume phase transition kinetics of smart N-n-propylacrylamide microgels studied by time-resolved pressure jump small angle neutron scattering
doi: 10.1038/s41598-018-31976-4

Formation and Growth of Mesoglobules in Aqueous Poly(N-isopropylacrylamide) Solutions Revealed with Kinetic Small-Angle Neutron Scattering and Fast Pressure Jumps. 
doi: 10.1021/acsmacrolett.8b00605