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

Abstract

Thermoresponsive microgels are one of the most investigated types of soft colloids, thanks to their ability to undergo a Volume Phase Transition (VPT) close to ambient temperature. However, this fundamental phenomenon still lacks a detailed microscopic understanding, 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 two-step 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.

The role of charge content in the two-step deswelling of Poly(N-isopropylacrylamide)-based microgels

“…All samples have been purified via three consecutive centrifugation/supernatant removal cycles 28,36 and 19.5 mg (2 mM) of Sodium Azide have been added to prevent bacterial growth.…”

“…To match with the experimental systems, we study different charge fractions f and distributions. In particular, we consider that charged groups coming from the initiator are distributed within the corona of the microgels, 28,50 so that we assign a fraction of charged beads f s = 3.2%, representing initiator molecules, to be located at a distance from the microgel center of mass greater than its radius of gyration. On the other hand, in principle, we do not know where the AAc monomers are located.…”

“…The fingerprint of such two-step deswelling has been identified in the onset of a minimum of the ratio R g /R H between the gyration and the hydrodynamic radius of the microgels as a function of temperature. 28 Simulations have further shown that this feature does not take place in the absence of charges. 28 Previous works have also demonstrated that the properties of the VPT, in particular the critical temperature T c , can be altered by incorporating a charged co-monomer such as Acrylic acid (AAc) 29 into the PNIPAm network.…”

“…28 Simulations have further shown that this feature does not take place in the absence of charges. 28 Previous works have also demonstrated that the properties of the VPT, in particular the critical temperature T c , can be altered by incorporating a charged co-monomer such as Acrylic acid (AAc) 29 into the PNIPAm network. In the presence of acrylic acid and for pH>pKa (AAc) , the swelling/deswelling behaviour of microgels depends on the subtle balance between hydrophobic attraction and electrostatic repulsion due to the carboxylate groups borne by the charged co-monomers.…”

“…Very recently, some of us 28 have shown that crosslinked networks of PNIPAm homopolymers undergo a two-step volume phase transition, where the residual charges carried by the ionic initiators, mainly located in the outer corona, play a pivotal role. During an ascending temperature ramp, PNIPAm microgels deswell with their initiator-free core collapsing first, namely at temperatures lower than the initiator-rich corona.…”