Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels
Soft particles often combine colloidal and polymeric aspects, making them very valuable for both fundamental and applied science. Microgels, which are colloidal-scale polymer networks, are one of the most important examples in class of systems. Their properties can be tuned by varying the constituent polymers and, for example, their affinity to the solvent in which they are dispersed. This is the case of thermoresponsive microgels, whose remarkable property to reversibly change their size by tuning the temperature of the suspension stems from the occurrence of a Volume Phase Transition (VPT) at a characteristic temperature. Such a fundamental phenomenon is exploited for many applications, ranging from biomedicine to sensors. Despite the impressive research effort spent on this topic, the microscopic nature of the VPT is not yet understood, because the role of the charges borne by all microgels synthesized in polar solvents has been neglected so far.
Combining scattering experiments with state-of-the-art simulations of realistic microgels, a group of researchers of the Sapienza Physics Department, of the Institute for Complex Systems of CNR and of the Center for Life Nano- & Neuro-Science if IIT, in collaboration with CNRS-University of Montpellier, has recently shown that electrostatic effects are of paramount importance since they determine a two-step, rather than a homogeneous colloidal collapse of the particle at the VPT, indicated by the appearance of a minimum in the ratio between gyration and hydrodynamic radius, as reported in Figure. These results, just published in PNAS, thus highlight the role of charges within the network and demonstrate that there exists a general criterion dictating the onset of this two-step process, which is able to yield a universal master-curve for microgels of varying crosslinker concentration, charge content and charge distribution.
To conclude, the results presented in this work put forward the idea that charge effects cannot be overlooked in a proper description of microgel-microgel interactions, and therefore of the collective behavior of the suspensions, providing new avenues to further develop the many potential applications of microgels.
Giovanni Del Monte, Domenico Truzzolillo, Fabrizio Camerin, Andrea Ninarello, Edouard Chauveau, Letizia Tavagnacco, Nicoletta Gnan, Lorenzo Rovigatti, Simona Sennato, Emanuela Zaccarelli
PNAS 2021 Vol. 118 No. 37 e2109560118