Skip to main content

Modeling microgels: from microscopic design to macroscopic description

Periodic Reporting for period 3 - MIMIC (Modeling microgels: from microscopic design to macroscopic description)

Reporting period: 2019-06-01 to 2020-11-30

This project aims at modeling in a realistic way a widely employed soft material, i.e. microgel particles. 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. This is the aim of the MIMIC project which will fill 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.

At the end of the project, a fine control on the behavior of the particles will be acquired, strongly enhancing their potentialities. This will not only improve the existing use of microgels but also open up new possibilities of exploitation in yet unexplored contexts.

The overall objectives of MIMIC are:
• To synthesize microgels in silico and to calculate swelling properties and single-particle elasticity;
• To calculate the effective interactions between microgel particles;
• To predict the phase behavior, glassy states and rheology of microgel suspensions;
• To compare with experimental measurements carried out by our collaborators both at the host institution and elsewhere.
During the first half of the project, several activities have already progressed. In more detail, the 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 are currently including the presence of charges in the microgels, also considering explicit counterions. The latter step is a necessary pre-requisite to tackle the case of interpenetrated polymer networks (IPN) microgels, experimentally investigated at the host institution, which will be addressed in the future.

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. We also set up a program of atomistic simulations, in collaboration with Ester Chiessi (Univ. Tor Vergata), where we investigate the dynamics and the interactions of PNIPAM chains and/or network portions with water as a function of temperature. We are comparing these results with the recent neutron scattering measurements we performed at ILL, both across the volume phase transition and at very low temperatures.

WP3: Using the effective interactions of WP2, in particular the Multi-Hertzian model, we are currently studying bulk properties of the microgels in different conditions, to provide a thorough investigation of the phase diagram and dynamics. However, numerical simulations of realistic microgels are unfeasible in bulk conditions. We recently proposed a simple model of elastic polymer rings, mimicking some features of microgels in 2D, in particular particle deformation and deswelling and we are studying their bulk behavior. These activities will increase in the second part of the project and later consider also the presence of charges for ionic or interpenetrated microgels. Eventually these will be compared with the extensive experimental investigations, mainly of IPN microgels, that are being carried at the host institution by means of a variety of techniques including DLS, XPCS (performed at ESRF, Grenoble), calorimetry, rheology and AFM.

General: The numerical team currently includes the PI, two researchers (one who recently moved to another position), two postdocs, three PhD students and 1 Master student. The experimental team includes four researchers, one of them devoted to the synthesis of the microgels. A technician is also employed to take care of the dedicated computational resources. The team has participated so far to more than 25 conferences/workshops around the world, delivering lectures, presenting posters and establishing new collaborations. The PI recently won the 2017 FARE call announced by the Italian Ministry of Education, University and Research dedicated to ERC grantees based in Italian host institutions with a project directly linked to the MIMIC project but extending its purpose with the specific aim to strengthen the activities taking place at the host institution. Our project entitled “Enhancing microgel potentialities: ultrasoftness and cultural heritage applications” has been selected for funding and, in large parts, will be dedicated to boost the experimental activities on microgels at the host institution and to improve the simulations/experiments synergy. The PI was also among the recipients of the 2017 LazioInnova grant for Research Projects with the project GELARTE, dedicated to the application of microgels to ancient paper preservation.

Meetings between team members happen on a regular basis, almost daily. The group is located in nearby offices. Regular group meetings between all team members, favoring the experiment/simulation collaboration, take place at least every month.
The development of the microgel assembly protocol is already 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 now have ample time to dedicate to other ambitious objectives of the project.

In particular, the recent introduction of the elastic polymer ring model provides us with a conveniente model to investigate bulk behavior of realistic-but-not-too-simple soft colloids and address important open questions regarding dynamics and jamming of the systems.
Project graphical abstract