The effect of silica nanoparticles on transient microemulsion networks made of microemulsion droplets and telechelic copolymer molecules in water is studied, as a function of droplet size and concentration, amount of copolymer, and nanoparticle volume fraction.The phase diagram is found to be affected, and in particular the percolation threshold characterized by rheology is shifted upon addition of nanoparticles, suggesting participation of the particles in the network. This leads to a peculiar reinforcement behaviour of such microemulsion nanocomposites, the silica influencing both the modulus and the relaxation time. The reinforcement is modelled based on nanoparticles connected to the network via droplet adsorption. Contrast-variation Small Angle Neutron Scattering coupled to a reverse Monte Carlo approach is used to analyse the microstructure. The rather surprising intensity curves are shown to be in good agreement with the adsorption of droplets on the nanoparticle surface.
Figures: 11Tables: 1 * author for correspondence: oberdisse@lcvn.univ-montp2.fr the influence of the nanoparticle-to-droplet size ratio. The observed shifts in the phase diagrams are related to those of the rheological properties, and in particular to the shift of the percolation line, presented in section 3.2. It will be shown that the silica nanoparticles induce an increase in the Maxwell modulus G, and a decrease of the relaxation time τ. The apparition of a second, longer relaxation time related to the silica particles is also reported in this section.The structure of the silica-loaded microemulsion gels has been studied by contrast-variation Small Angle Neutron Scattering (SANS), and the results are presented in section 3.3. In the discussion of the structure, section 4, the analysis of the rather complex intensity curves has been backed up by reverse Monte Carlo simulations. Details of the latter are outlined in the appendix. In section 5, finally, simple models for the reinforcement on these microemulsion nanocomposites are developed, before concluding in the last section.
Experimental section
Preparation of the microemulsion gels with silica nanoparticlesSynthesis of the triblock copolymer CH 3 -(CH 2 ) 17 -NH-CO-O-(CH 2 -CH 2 -O) n -CO-NH-(CH 2 ) 17 -CH 3 has been described elsewhere [19,28]. Here, we use a 10k-poly(ethylene oxide) chain (220 EO units) which has a hydrophobic C 18 -sticker at each extremity, connected through a urethane group. The radius of gyration of the PEO-block is about 35 Å. Bindzil silica (B30/220, 30% wt, radius 100 Å, log-normal polydispersity ≈ 20%) suspended in water was a gift from Akzo Nobel. The pH of the delivered stock solutions was set between 9 and 10 in order to ensure maximal colloidal stability to the silica by electrostatic repulsion. All silicacontaining samples have thus been prepared at pH 10, all others at pH 7. The order of mixing of the components is of importance due to the high viscosity of the microemulsion networks.First, microemulsions were formed with a surfactant (Triton X-100),...