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Modeling microgels: from microscopic design to macroscopic description

Periodic Reporting for period 4 - MIMIC (Modeling microgels: from microscopic design to macroscopic description)

Reporting period: 2020-12-01 to 2022-01-31

This project aimed to model a widely employed soft matter system, i.e. microgel particles, in a realistic way. Currently, microgels are used in a great variety of applications, such as drug delivery, surface coating, cosmetics, sensoring, etc. However, fundamental knowledge on their behaviour has been limited so far by the complexity of the system. Indeed, microgels have a dual colloid/polymeric nature, combining together several different time and length scales. Thus, a multiscale, coarse-grained approach able to span several orders of magnitude of description --- from the atomic to the material scale --- is needed. The MIMIC project has now filled this gap by modeling, at first, microgels at the individual particle level, then microgel-microgel effective interactions, in order to finally gain a predictive description of the bulk materials that they form.

The overall objectives of MIMIC were:
1) To synthesize microgels in silico and to calculate swelling properties and single-particle elasticity;
2) To calculate the effective interactions between microgel particles;
3) To predict the phase behavior, glassy states and rheology of microgel suspensions;
4) To compare with experimental measurements carried out by our collaborators both at the host institution and elsewhere.

At the end of the project, we can undoubtedly claim that we have accomplished three of these objectives (1,2 and 4). Regarding objective 3, since the standard effective potentials like the Hertzian one were not found to be accurate, especially at high densities for bulk suspensions, we need to continue our work in the near future in order to be able to predict the microgel phase behavior, glassy states and rheology.
The main activities that have been carried out for each workpackage are listed below:

WP1: we have developed an efficient protocol to assemble realistic individual microgel particles. This work represents a concrete step forward with respect to any previous numerical study of microgels, because it provides fully-bonded, disordered networks, differently from other works which were all based on ordered (diamond-like) structures. Thanks to this achievement we can now study the dependence of the microgel properties and their swelling behavior on the crosslinker concentration and on the parameters that can be tuned in the assembly protocol. We can further tune the internal density profile of the microgels by driving the arrangement of the crosslinkers through an external force, being able to reproduce a wide variety of profiles, from homogeneous to heterogeneous (core-corona) structures. We have been able to calibrate the assembly in order to reproduce experimental form factors obtained by our collaborators in Lund University and at the host insitution. In addition, we have been able to calculate all the elastic moduli of individual microgels as a function of crosslinker concentration and of temperature. We found the presence of a minimum in the Poisson's ratio occurring at the Volume Phase Transition, in agreement with experiments. We have also included explicitly the solvent in the treatment and, as a new perspective deriving from this WP, we are now studying microgels at liquid-liquid interfaces.
We have also included the presence of charges in the microgels, considering explicit counterions. In this way, we were able to assess the role of peripheral charges in the volume phase transition, which leads to a two-step deswelling. Being able to correctly model also ionic contributions, we have now finally started to tackle interpenetrated polymer networks (IPN) microgels, that are extensively investigated experimentally at the host institution.

WP2: we have calculated effective interactions between two of our realistic microgels in the swollen regime and tested the Hertzian potential. We also proposed the Multi-Hertzian model which works well in the case of microgel mixtures, where depletion interactions are at work, in collaboration with Lund University. More recently, we also calculated effective interactions between two microgels at the interface and also for composite microgels made of PNIPAM and PEG. In addition, we also set up a program of atomistic simulations where we investigate the dynamics and the interactions of PNIPAM chains and/or network portions with water as a function of temperature. We have compared these results with recent neutron scattering measurements performed at ILL, both across the volume phase transition and at very low temperatures. Recently we also evaluated the effect of pressure and of co-solvents on the VPT, as well as the role played by polymer architecture. Finally, we are currently evaluating effective forces between atomistically-resolved microgel portions.

WP3: using the effective interactions of WP2, we are currently studying bulk properties of the microgels in different conditions, to provide a thorough investigation of the phase diagram and dynamics. In parallel, we also started numerical simulations of few realistic microgels. In addition, we recently proposed a simple model of elastic polymer rings, mimicking some features of microgels in 2D, in particular particle deformation and deswelling. Experimental investigations at the host institution have also continued and we are currently trying to compare results. Additional calorimetry, rheology and AFM studies have been performed.
The development of the microgel assembly protocol is beyond the state-of-the-art and was the pillar of the proposal. Having reached this achievement realatively early in the duration of the project, we then had ample time to dedicate to other ambitious objectives of the project. This allowed us to establish fruitful collaborations with European-leading experimental groups working in the field, including Lund University (Prof. Schurtenberger), Aachen University (Prof. Crassous, Prof. Richtering, Dr. Scotti), Montpellier University (Dr. Truzzolillo), Fribourg University (Prof. Scheffold), Florence University (Prof. Laurati) and ETH Zurich (Prof. Isa).


In addition, we opened new research lines, not initially planned in proposal: namely (i) microgels at liquid-liquid interfaces, (ii) the study of hydrogels, their phase behavior and elastic properties, (iii) cultural heritage applications for which we also recently recived a Proof of Concept grant from the ERC.
Project graphical abstract