Polymer shell growth around anisotropic Layered Double Hydroxides (LDH) particles by RAFT-mediated emulsion polymerization. – PolHydRaft

To achieve the goal of our project, the first part was devoted to the preparation of stable colloidal suspensions of macroRAFT-LDH hybrid particles. The synthesis of the two precursors: 1- LDH platelets with a large range of aspect ratios and charge densities prepared by hydrothermal process, and 2- RAFT and macroRAFT agents carrying suitable carboxylic or phosphonic acid groups to permit their incorporation within the inorganic layers by electrostatic interactions, has been carried out. Then we performed to the assembly of the above-mentioned partners to form the expected macroRAFT-LDH hybrid particles. Three different processes were tested to prepare the hybrid materials: i) the macroRAFT agents intercalation by direct anion exchange on LDH platelets, ii) the direct coprecipitation of LDH in presence of macroRAFT agents and iii) the direct coprecipitation of LDH in presence of RAFT agent followed by in situ polymerization. According to the LDH charge density and the process used, various amounts of macroRAFT agents were immobilized, influencing likewise the colloidal stability of the resulting macroRAFT-LDH hybrid particle suspensions.

Synthetic conditions for LDH nanoparticles preparation were optimized to directly access to colloïdal suspensions of nanoparticles in the range of 60 nm to 200nm, displaying various chemical compositions.In parallel, RAFT and statistic macro-RAFT agents based on acrylic acid (AA) and butyl acrylate (ABu), typically P(AA50%-co-ABu50%)-RAFT, bearing anionic charge were prepared. AA units are in favor of adsoprtion phenomenon on LDH surface while ABu units enhance the hydrophobic feature of modified LDH. Such hydrophobicity is of great interest to improve the affinity with the monomer during polymerization process. Then, LDH-RAFT and LDH-macroRAft phases were elaborated in using different approches, respectively direct coprecipitation and anionic exchange. Systematically, a special attention was paid to hte maintain of the colloidal stability of the suspensions. In the case of macro-RAFT agent, adsoprtion isotherms allowed to define the macro-RAFT optimal concentration. Note the macro-Raft agent should be present in exces to induce a charge inversion phenomenon and avoid an agglomeration of the particles. The preparation of LDH based nanocomposite colloids in aqueous phase was able to be developed by combining a controlled radical polymerization process and an emulsion polymerization. The RAFT copolymer adsorption on the surface of inorganic platelet allows to initiate the polymerization from the surface of the particle inducing its encapsulation. Maintaining the colloidal character is paramount throughout the process while parameters such as the nature of the copolymers, the type of initiator , monomer composition modulate obtained morphologies. An alternative strategy by colloidal particles heterocagulation has also been implemented and has permitted to access to films having mechanical reinforcement properties.

PolHydRaft project has lead to two major advances , the first is the formation of HDL -based nancomposites particles by combining the controlled radical polymerization and emulsion polymerization. The second has been to update a process heterocoagulation to access the nanocomposite films nanostructured with highly improved mechanical properties.
It is now necessary to develop films based on the latex -based LDH particles encapsulated, to study their thermomechanical properties and permeation to connect to different microstructures.

In terms of scientific production, for now two articles appeared one in Langmuir (2015) and the other in Applied Clay Science ( 2016) respectively on the colloidal stability of LDH in the presence of copolymers and the development of nanocomposite film properties obtained by heterocoagulation . Two publications are being submitted on results concerning intercalation of macroRAFT agents and encapsulation and two others are in preparation on the influence of encapsulation parameters and in situ growth of macroRAFT copolymers into the LDH. Finally a seventh publication should appear soon on the influence of particle size on the mechanical properties of films obtained by heterocoagulation. Meanwhile, these results have given rise to nine oral presentations at international conferences and presentation in a national conference , as well as through four posters.

The main aim of our project is to develop an innovative RAFT-mediated emulsion polymerization process enabling to grow a thin polymer shell adopting the contours of Layered Double Hydroxides (LDH) nanoplatelets. The unique morphology of the resulting plate-like core-shell polymer-LDH-particles should result upon water evaporation into nanostructured composite films with LDH platelets oriented parallel to the plane of film formation. Such a microstructure should not only provide the film with enhanced mechanical properties, but should also strongly improve the barrier properties to water or gases by creating a tortuosity of the diffusion path way.

To achieve the goal of this ambitious project, this project will be divided in two main parts:

- The first part of our project will be devoted to the preparation of stable colloidal suspensions of macroRAFT-LDH hybrid particles. The synthesis of the two precursors: 1- LDH platelets with a large range of aspect ratios and charge densities prepared by soft chemistry, and 2- RAFT and macroRAFT agents carrying suitable carboxylic or phosphonic acid groups to permit their incorporation within the inorganic layers by electrostatic interactions, will be carried out in a first task. The second task will consist in assembling the above-mentioned partners to form the expected macroRAFT-LDH hybrid particles. Three different processes will be tested to prepare the hybrid materials: i) the macroRAFT agents intercalation by direct anion exchange on LDH platelets, ii) the direct coprecipitation of LDH in presence of macroRAFT agents and iii) the direct coprecipitation of LDH in presence of RAFT agent followed by in situ polymerization. According to the LDH charge density and the process used, various amounts of macroRAFT agents will be immobilized, influencing likewise the colloidal stability of the resulting macroRAFT-LDH hybrid particle suspensions.
- The second part of the project will focus on the elaboration of LDH-based nanocomposite latex particles and the corresponding nanostructured films. The encapsulation of the LDH platelets will be carried out by RAFT-mediated emulsion polymerization. Careful selection of the experimental conditions and notably the nature and concentration of initiator, monomers and stabilizer will be realized to form a thin and uniform film-forming polymer shell of controlled thickness and composition following the contours of the particle shape. Particular care will be taken in the choice of compositions and concentrations of the different macroRAFT agents to avoid the formation of pure polymer particles by secondary nucleation and ensure a successful encapsulation. Finally, the microstructure and properties of the nanostructured films issued from the nanocomposite latexes will be characterized. In particular, the influence of the filler aspect ratio will be studied as well as that of the filler orientation. Taking into account such structural parameters, the mechanical behavior will be compared to classical modeling approaches in order to clearly establish the microstructure-mechanical properties relationships. This will allow optimizing the synthesis of the LDH-polymer core-shell particles owing to a quick feedback on the properties of the resulting films.

The multidisciplinary nature of the project requires a combination of specific know-how. Accordingly, a team of young researchers from C2P2 and MATEIS (M. Lansalot and L. Chazeau, respectively) will be formed around Vanessa Prévot (LMI-Aubière), coordinator of this project to combine the necessary competences in the LDH chemistry and reactivity, in the RAFT and emulsion polymerization process and in characterization of nanocomposite properties.