Electrolytes suppress segmental movement in microgels

Microgels of various structures can be designed to respond to changes in temperature, pressure, and acidity, making them promising candidates for drug delivery and nanoreactors. Better understanding of the interactions between the polymer chains of microgels and added guest molecules, such as electrolytes, could help improve microgel design for applications.

Pasini et al. studied the interaction of a cationic microgel, called PNIPAM-co-MAPTAC, dissolved in water with the anionic electrolyte polystyrene sulfonate (PSS). Unlike other techniques, a combination of small angle neutron scattering and neutron spin echo allowed the authors to observe the local density fluctuations of the microgel and the electrolyte on molecular length- and time-scales.

They found the electrolyte suppresses the segmental dynamics of the microgel’s polymer chains. In the presence of PSS, the microgel transforms from its original swollen state to a more compressed state, inhibiting polymer segment movements between the crosslinks.

Prior to the addition of PSS, the microgel’s polymer chains exhibit both density fluctuations and Zimm dynamics, which are segmental fluctuations in response to temperature. However, after the addition of PSS, the polymer chains show only density fluctuations and no Zimm dynamics.

“The experiments gave first insights with a model microgel and electrolyte,” said author Olaf Holderer. “An important point in this work is the interdisciplinary collaboration between chemistry, physical chemistry, and physics, and between synthesis laboratories and large scale facilities.”

The authors will explore the microgel-electrolyte interaction, when their charges have been altered, to further advance the design of microgels with advantageous properties, such as the controlled release of guest molecules.

Source: “Fluctuation suppression in microgels by polymer electrolytes,” by S. Pasini, S. Maccarrone, N. K. Székely, L. R. Stingaciu, A. P. H. Gelissen, W. Richtering, M. Monkenbusch, and O. Holderer, Structural Dynamics (2020). The article can be accessed at https://aca.scitation.org/doi/full/10.1063/4.0000014 .