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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MIMIC (Modeling microgels: from microscopic design to macroscopic description)

Teaser

Summary

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:
â€¢ To synthesize microgels in silico and to calculate swelling properties and single-particle elasticity;
â€¢ To calculate the effective interactions between microgel particles;
â€¢ To predict the phase behavior, glassy states and rheology of microgel suspensions;
â€¢ To compare with experimental measurements carried out by our collaborators both at the host institution and elsewhere.

Work performed

During the first 18 months, from the numerical point of view we have mainly focused on WP1 (In silico synthe-sis of microgels) and WP2 (Calculation of effective interactions). A few aspects of WP3 (Bulk behavior of microgel suspensions) have also been investigated, mainly from the experimental point of view, to provide data for the phase behavior, arrested states and rheology of the system in order to compare with simulations in the near future. Regular group meetings between all team members, favoring the experiment/simulation collaboration, take place every month.

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. The first results of this activity have been published in Macromolecules and selected, among all papers published in ACS journals, as ACS Editorâ€™s Choice for Oct 18, 2017. 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. In particular, we have put forward the role of the confining volume in which assembly takes place: this allows to tune the swelling curve of the microgels for a favorable comparison with experimental behavior. We have also calculated the form factors and successfully compared them with the widely-used fuzzy sphere model. 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 are currently calibrating the assembly in order to reproduce experimental form factors as well as completing the activities on individual microgels by setting up the calculation of elastic properties as a function of crosslinker concentration and of temperature. In addition, we are calculating the swelling and form factors (i) in the presence of an explicit solvent and (ii) for partially charged microgels in the presence of salt. The latter step is a necessary pre-requisite to tackle the case of interpenetrated polymer networks (IPN) microgels, experimentally investigated at the host institution.

WP2: We are currently calculating effective interactions between two of our realistic microgels to quantify deviations from the Hertzian behavior. In parallel, from the comparison with experiments, we are evaluating (in a phenomenological way) the effective interactions between microgels as a function of temperature in the swollen regime. This is done also in the presence of depletion interactions, in collaboration with Lund University.
We have 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 recent inelastic neutron scattering measurements performed by the PI and collaborators at ILL, Grenoble (experiment data doi: 10.5291/ILL-DATA.9-11-1736 and 10.5291/ILL-DATA.9-11-1770).

WP3: Using the effective interactions of WP2, we will assess bulk properties of the microgels in different conditions, to provide a thorough investigation of the phase diagram. Meanwhile, experimental investigations, mainly of IPN microgels, are being carried out by means of DLS, XPCS (performed at ESRF, Grenoble), calorimetry, rheology and AFM to provide the necessary ingredients for the numerical modeling.

General: The numerical team currently includes the PI, two researchers, one postdoc

Final results

The development of the microgel assembly protocol is already beyond the state-of-the-art and was the pillar of the proposal. With this achievement, we are now able to tackle the ambitious objectives of the project. However, apart from what has been published so far (see list of publications), at this stage it is too early to disseminate our ongoing activities. This will be done in the next periods.