Behavior and stability of PNIPAM hydrogel particles in liquid phase under electron irradiation

Abstract number
European Microscopy Congress 2020
Corresponding Email
[email protected]
PST.6 - In-situ and in-operando microscopy
Albert Grau-Caronell (1), Sina Sadighikia (1), Tom A. J. Welling (1), Prof. Dr. Alfons van Blaaderen (1), Dr. Marijn A. van Huis (1)
1. Utrecht University

In-situ,  Liquid Phase, Polymers

Abstract text

The direct observation of pure, unlabeled microgels relies on imaging techniques that operate either in dry or in cryogenic conditions. Liquid Cell (Scanning) Transmission Electron Microscopy (LC-(S)TEM) imaging allows the investigation of the behavior of materials with nanometric resolution in liquid environments and offers then an ideal platform to image microgels. However, the LC-(S)TEM technique comes with its own challenges, as the interaction between the electron beam and the liquid results in the generation of many radiolitic species that can interact with the microgels. We studied Poly(N-isopropylacrylamide) (PNIPAM) colloids in aqueous environments via LC-(S)TEM. PNIPAM particles were confined in a minuscule volume of water by using a Poseidon dedicated Liquid Cell holder. PNIPAM microgels were successfully characterized in their most swollen state. We report that the electron irradiation compromised the stability of the particles, and we propose a mechanism for this beam induced damage. This mechanism relates the reducing radicals resulting of the interaction of the electron beam with the water volume and the chemistry that drives structural changes in polymers. We explored whether the addition of different radical scavengers reduced the impact of our imaging method on the integrity of the microgels. The non-homogeneous core-shell internal structure of our PNIPAM microgels also allowed us to explore the limits of LC-(S)TEM to obtain enough contrast to image polymeric particles in water. The ability of LC-(S)TEM to characterize a fully hydrated microgel crucially depended on the volume fraction of the swollen polymer. In conclusion, we show that LC-(S)TEM can image fully hydrated unlabeled microgels in their native liquid phase. Our results also characterize the detrimental effects of the electron beam irradiation on microgel particles and we discuss the damage mechanism behind our observations. Finally, we show that the ability to directly visualize microgels critically depends on the polymer volume fraction.