Atoms and molecules are the building blocks for all matter in the universe and understanding how they behave are crucial to understanding our world. The study of atoms and molecules, which are very small and move very fast, however requires special, highly sophisticated equipment as well as very low temperatures. To circumvent these limitations, one can develop model systems that exhibit the same behavior as atoms and molecules, but that are easier to study. Often, spherical so-called colloids have been used as model systems. These are particles that are many magnitudes larger than atoms and molecules (in some dimensions colloids have a size between one nanometer and one micrometer), which allows them to be studied with more readily available techniques such as microscopy and light scattering.
The use of colloids as model systems is based on the fact that they obey the same physical laws as atoms and molecules. This has proven to be valid for, for example, phase transitions, which are completely analogous on colloidal and atomic levels. However, (the vast majority of) atoms and molecules are not spherical, and spherical colloids are therefore not entirely ideal as model systems. In pursuit of more realistic models, researchers have therefore focused on colloids with a non-spherical shape, such as platelets, ellipsoids, cubes, peanut- and bowl-shaped particles. Over the past decade, a new type of non-spherical colloids, so-called colloidal molecules, has received much attention. Colloidal molecules consist of spherical colloids assembled into structures with a shape that precisely mimics that of the small molecules to be studied.
In my research project I work with synthesis of colloidal building blocks and the development of methods for their assembly into colloidal moelcules. Unique to this project in comparison with previous reports on the same topic is the choice of building blocks. Here we have chosen to work with temperature-responsive so-called microgel colloids based on the polymers PNIPAM and PNIPMAM. Polymers are very long chain molecules, and in the microgel, which are overall spherical, they are held together by crosslinks. The microgels are suspended in water that penetrates and swells the microgels.
PNIPAM and PNIPMAM microgels are temperature-responsive with respect to several properties. For example, they shrink in size as the temperature exceeds a certain threshold temperature (32°C for PNIPAM and 45°C for PNIPMAM). This transition from swollen to shrunk is due to the polymer going from a "water loving" to a "water avoiding" state where polymer-polymer interactions are favoured over polymer-water ones. This behaviour is furthermore linked of the interactions between the microgels, which behave repulsively below the threshold temperature but aggregate above as a result of a change in behaviour to attractive. This means that the microgel interactions can be manipulated from repulsive to attractive by increasing the temperature.
The temperature dependent behaviour of the individual microgels means that each individual microgel building block in the colloidal molecule can act as an interaction site, and can interact either repulsively or attractive with sites on surrounding colloidal molecules depending on the temperature. This makes our colloidal molecules very interesting as model systems, where it is often desirable to manipulate the interactions. Furthermore, our temperature responsive colloidal molecules can be used as building blocks to create three-dimensional structures and materials where one can easily change a variety of properties with temperature. This makes our colloidal molecules very attractive objects for the materials industry.
Månsson, L. K., Immink, J. N., Mihut, A. M., Schurtenberger, P., & Crassous, J. J. (2015). A new route towards colloidal molecules with externally tunable interaction sites. Faraday discussions, 181, 49-69.
Crassous, J. J., Mihut, A. M., Månsson, L. K., & Schurtenberger, P. (2015). Anisotropic responsive microgels with tuneable shape and interactions. Nanoscale, 7(38), 15971-15982.
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