Lay summary

Microgels are polymer particles with sizes in the range from 100 up to 1000nm that are immersed in a aqueous solvent. They respond reversibly to changes in their environment by changing their size. E.g. a temperature change form 20 to 40 degrees Celsius brings them from a swollen to a de-swollen state. Due to their responsiveness, microgels have great potential for applications, including drug delivery. In the swollen state the particles are soft, as they can be easily deformed. By contrast, in the de-swollen state, the particles behave very much like hard spheres. As a result, microgels are ideal systems to address the transition from hard to soft sphere behavior in one and the same system, and to study the behavior of soft particles, which has many unexplored and unknown aspects mainly due to the combination of colloid- and polymer-like behaviors that arise in these systems.

We explore the particle properties and their collective behavior when they are highly concentrated in water. Microgels offer the interesting possibility to reach very high particle concentrations that are much higher than those that can be reached with hard spheres, as microgels deform easily, can interpenetrate and shrink, and therefore can fill space much more efficiently. So far, fundamental properties like their phase behavior are not well known in this range of concentrations. The transition from the liquid to the crystal and the occurrence of an arrested state, a glass, are not well understood. Theoretical work suggests that crystal structures different from those of hard spheres should be expected for particles that are not too soft, while very soft particles are predicted not to crystallize and to stay in the liquid state. Furthermore, it has been shown that a large microgel particle in a crystal of small ones can shrink spontaneously to fit into the crystal to prevent the creation of a defect -- a behavior that cannot occur in hard-sphere systems.

The behavior of microgel particles in a glassy state at high concentrations is also far from understood, despite it has been studied to some extent with certain types of particles. With molecular substances that form glasses a wide range of behaviors has been observed as the glassy state is approached. Interestingly, recent experimental results have shown that such a wide range of behaviors is also found in microgel suspensions approaching the glassy state. However, the reasons for why this is the case are still unclear and largely unexplored.

This research project is carried out in close collaboration of the Laboratory for Neutron Scattering at Paul Scherrer Institut, Switzerland, and the Soft Condensed Matter Laboratory at the Georgia Institute of Technology, Atlanta USA. The experience and know-how of the two laboratories with complemental facilities are combined to strengthen and expand the ongoing collaboration on the properties of microgels at high concentrations. The many unexplored aspects mentioned above will be tackled using a wide range of techniques such as direct measurements of osmotic pressure, various scattering techniques, and real space imaging, which are all available. Furthermore, the PhD student working on the project profits from the international collaboration and the access to various experimental techniques.