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Polymeric microspheres have a shell whose density is dependent on the temperature
Nanoparticles whose outer shells and inner cores are made of different materials are useful for many industrial and biomedical applications. In order for nanoparticles to be used as sensors or for the controlled release of substances held within their shell, for example, an important requirement must be met: the shell must be more dense than the core to form a barrier for the external medium.
W. Richtering and I. Berndt in Aachen, in collaboration with J.S. Pedersen in Århus, Denmark, have now found an elegant solution to this problem. In a two-step process, they synthesized polymeric microspheres with a core made of poly-N-isopropylacrylamide and a shell of poly-N-isopropylmethacrylamide. Both polymers are known for a particular characteristic: they swell in water, forming microgels. Because of the different polymer building blocks used in the shell and core, these differ in the absorption of water.
At 70 °C, the temperature at which the microspheres are synthesized, both polymers are densely packed. They cannot take up much water and thus no substances dissolved in the water either. When they are cooled to 25 °C, the core and shell have the highest water content and the lowest density. Dissolved molecules can pass through the shell into the core, where they disperse. Things get especially interesting when the temperature is raised to 39 °C: at this temperature, only the swelling properties of the shell are changed. It expels water, shrinks together, and becomes denser than the core. Substances dissolved in the core can no longer pass through the shell and are now locked inside. Aside from their sensitivity to temperature, polymeric microgels with core–shell architectures have another advantage too. Selection of the basic components and the reaction conditions also allow other properties of the polymers to be controlled. In addition, the synthesis of particles with multiple shells is also a possibility; these could separate different reaction chambers within the particle. The possible applications are many and varied.
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