Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

Giovanni Del Monte (1, 2, 3) , Domenico Truzzolillo (4) , Fabrizio Camerin (1, 2) , Andrea Ninarello (1, 2) , Edouard Chauveau (1, 2) , Letizia Tavagnacco (1, 2) , Nicoletta Gnan (1, 2) , Lorenzo Rovigatti (1, 2) , Simona Sennato (1, 2) , Emanuela Zaccarelli (1, 2)

1 Dipartimento di Fisica [Roma La Sapienza]
2 National Research Council–Institute for Complex Systems (CNR-ISC), Sapienza University of Rome, 00185 Rome, Italy
3 Center for Life Nano- and Neuro-Science, Fondazione Istituto Italiano di Tecnologia, 00161 Rome, Italy
4 L2C - Laboratoire Charles Coulomb

Thermoresponsive microgels are one of the most investigated types of soft colloids, thanks to their ability to undergo a VolumePhase Transition (VPT) close to ambient temperature. However, this fundamental phenomenon still lacks a detailed microscopicunderstanding, particularly regarding the presence and the role of charges in the deswelling process. This is particularly important for the widely used poly(N-isopropylacrylamide)–based microgels, where the constituent monomers are neutral but charged groups arise due to the initiator molecules used in the synthesis. Here, we address this point combining experiments with state-of-the-art simulations to show that the microgel collapse does not happen in a homogeneous fashion, but through a two-step mechanism, entirely attributable to electrostatic effects.The signature of this phenomenon is the emergence of a minimum in the ratio between gyration and hydrodynamic radii at the VPT. Thanks to simulations of microgels with different cross-linker concentrations, charge contents, and charge distributions, we provide evidence that peripheral charges arising from the synthesis are responsible for this behavior and we further build a universal master curve able to predict the twostep deswelling. Our results have direct relevance on fundamental soft condensed matter science and on applications where microgels are involved, ranging from materials to biomedical technologies.

Domaines: Matière Molle [cond-mat.soft]

Giovanni Del Monte, Domenico Truzzolillo, Fabrizio Camerin, Andrea Ninarello, Edouard Chauveau, et al.. Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels. Proceedings of the National Academy of Sciences of the United States of America, 2021, 118 (37), pp.e2109560118. ⟨10.1073/pnas.2109560118⟩. ⟨hal-03341513⟩