"Wet-to-Dry" Conformational Transition of Polymer Layers Grafted to Nanoparticles in Nanocomposite

The present communication reports the first direct measurement of the conformation of a polymer corona grafted around silica nano-particles dispersed inside a nanocomposite, a matrix of the same polymer. This measurement constitutes an experimental breakthrough based on a refined combination of chemical synthesis, which permits to match the contribution of the neutron silica signal inside the composite, and the use of complementary scattering methods SANS and SAXS to extract the grafted polymer layer form factor from the inter-particles silica structure factor. The modelization of the signal of the grafted polymer on nanoparticles inside the matrix and the direct comparison with the form factor of the same particles in solution show a clear-cut change of the polymer conformation from bulk to the nanocomposite: a transition from a stretched and swollen form in solution to a Gaussian conformation in the matrix followed with a compression of a factor two of the grafted corona. In the probed range, increasing the interactions between the grafted particles (by increasing the particle volume fraction) or between the grafted and the free matrix chains (decreasing the grafted-free chain length ratio) does not influence the amplitude of the grafted brush compression. This is the first direct observation of the wet-to-dry conformational transition theoretically expected to minimize the free energy of swelling of grafted chains in interaction with free matrix chains, illustrating the competition between the mixing entropy of grafted and free chains, and the elastic deformation of the grafted chains. In addition to the experimental validation of the theoretical prediction, this result constitutes a new insight for the nderstanding of the general problem of dispersion of nanoparticles inside a polymer matrix for the design of new nanocomposites materials

Self-assembly of Microcapsules via Colloidal Bond Hybridization and Anisotropy

Particles with directional interactions are promising building blocks for new functional materials and may serve as models for biological structures. Mutually attractive nanoparticles that are deformable due to flexible surface groups, for example, may spontaneously order themselves into strings, sheets and large vesicles. Furthermore, anisotropic colloids with attractive patches can self-assemble into open lattices and colloidal equivalents of molecules and micelles. However, model systems that combine mutual attraction, anisotropy, and deformability have---to the best of our knowledge---not been realized. Here, we synthesize colloidal particles that combine these three characteristics and obtain self-assembled microcapsules. We propose that mutual attraction and deformability induce directional interactions via colloidal bond hybridization. Our particles contain both mutually attractive and repulsive surface groups that are flexible. Analogous to the simplest chemical bond, where two isotropic orbitals hybridize into the molecular orbital of H2, these flexible groups redistribute upon binding. Via colloidal bond hybridization, isotropic spheres self-assemble into planar monolayers, while anisotropic snowman-like particles self-assemble into hollow monolayer microcapsules. A modest change of the building blocks thus results in a significant leap in the complexity of the self-assembled structures. In other words, these relatively simple building blocks self-assemble into dramatically more complex structures than similar particles that are isotropic or non-deformable

Solvation in ionic liquids with polymer-grafted nanoparticles

Polymer-grafted nanoparticles stabilized in ionic liquid (IL)-solvent mixtures are investigated using transmission electron microscopy, dynamic light scattering and electrochemical impedance spectroscopy. The ionic conductivity of IL/solvent mixtures with the polymer-grafted nanoparticles is found to be higher than nanoparticles in the IL. These particles offer additional interactions between polymer and IL, which can mitigate solvation in ILs with solvents. Motivated by this, we present the conductivity data of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM-TFSI) with PMMA-grafted particles in good and bad solvents and further discuss how graft density influences the swelling of PMMA and solvation characteristics of HMIM-TFSI. We found that HMIM-TFSI/acetonitrile containing high grafting density particles has higher conductivity than HMIM-TFSI/methanol mixture with grafted particles. Thus, solubility of PMMA in acetonitrile and preferential interactions between PMMA/HMIM-TFSI are shown to govern the swelling, solvation and conductive properties of IL with the polymer-grafted nanoparticles

Dynamics of ionic liquids in the presence of polymer-grafted nanoparticles

We incorporated polymer-grafted nanoparticles into ionic and zwitterionic liquids to explore the solvation and confinement effects on their heterogeneous dynamics using quasi-elastic neutron scattering (QENS). 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM-TFSI) mixed with deuterated poly(methyl methacrylate) (d-PMMA)-grafted nanoparticles is studied to unravel how dynamic coupling between PMMA and HMIM-TFSI influence the fast and slow diffusion characteristics of the HMIM+ cations. The zwitterionic liquid, 1-butyl-3-methyl imidazole-2-ylidene borane (BMIM-BH3) is critically selected and mixed with PMMA-grafted nanoparticles for comparison in this work as its ions do not selfdissociate and it does not couple with PMMA through ion-dipole interactions as HMIM-TFSI does. We find that long-range unrestricted diffusion of HMIM+ cations is higher in well-dispersed particles than in aggregated particle systems, whereas the localized diffusion of HMIM+ is measured to be higher in closepacked particles. Translational diffusion dynamics of BMIM-BH3 is not influenced by any particle structures suggesting that zwitterions do not interact with PMMA. This difference between two ionic liquid types enables us to decouple polymer effects from the diffusion of ionic liquids, which is integral to understand the ionic transport mechanism in ionic liquids confined in polymer-grafted nanoparticle electrolytes