More than ever polymer science focuses on complex molecular structures and supramolecular assemblies. Microgels are responsive polymer materials and structures, which can be manipulated in e.g. charge or size by external parameters like pH or temperature variation. The investigated microgels are soft particulate polymer networks that can be dispersed in an aqueos medium. They reveal unique features providing new opportunities to develop smart bio-inspired materials. In contrast to rigid colloidal particles, which lack the possibility to adapt their size and shape to enviromental requirements, microgels have switchable properties of form and function that make them very useful in a wide range of e.g. biological sciences and medical applications. They combine properties of dissolved macromolecules with those of colloidal particles.
The direct visualization of the internal structure of materials is very important to analyse the spatial distribution of different compartments and, thus, to design novel materials with tailored properties. Microgels can be prepared with various morphologies and functions in different compartments. Careful analysis of the correlation between architecture and function requires powerful methods to visualize inner structure and compartmentalization in the nanometer range.
Here, the direct visualization of different compartments within microgels using a combination of in situ and cryo transmission electron microscopy methods is shown. In particular, the challenge of determing the radial distribution of appropriately labeled compartments within single microgels and particles from 2D projections is adressed.
Microgels with core-shell architecture were obtained by precipitation polymerization. First a particle was synthesized and purified before a shell was synthesized on top by the seed and feed method. Core and shell have oppositely charged copolymers to create a two compartment amphoteric microgel system, that is alternately stained with gold and magnetite nanoparticles. , 
For in situ liquid cell experiments, a thin layer of liquid was embedded between two hermetically sealed, electron transparent Si3N4-windows. The used holder is an in situ-liquid cell holder manufactured by Hummingbird Company and the microscope is a Zeiss Libra 200FE with an acceleration voltage of 200 kV. The resolution is mainly limited by the thickness of the liquid.
Figure 1 shows a comparison between cryo TEM and in situ STEM. Due to the liquid layer thickness the resolution is limited in (b). Also the Brown emotion leads to a defocused and smudged image.
Figure 2 shows the comparison of the radial distribution of nanoparticles according to the two images above calculated by a MATLAB routine. For the cryo TEM image in (b) the relative particle density as a function of the relative particle radius is plotted. (c) shows a 3D reconstruction of the model.
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The authors kindly acknowledge the financial support by the DFG through the SFB 985 “Functional Microgels and Microgel Systems”.
To cite this abstract:Tobias Caumanns, Arjan Gelissen, Alexander Oppermann, Pascal Hebbeker, Rahul Tiwari, Sarah Turnhoff, Dominik Wöll, Andreas Walther, Joachim Mayer, Walter Richtering; In situ and cryo (S)TEM imaging of internal microgel architectures. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/in-situ-and-cryo-stem-imaging-of-internal-microgel-architectures/. Accessed: September 25, 2023
EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-and-cryo-stem-imaging-of-internal-microgel-architectures/