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Domaines de Recherche: - Physique/Matière Condensée/Matière Molle
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Dernieres productions scientifiques :
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Nucleation and growth of micellar polycrystals under time-dependent volume fraction conditions 
Auteur(s): Louhichi A., TAMBORINI E., Ghofraniha N., Caton F., Roux D., OBERDISSE J., CIPELLETTI L., RAMOS L.
(Article) Publié:
Physical Review E: Statistical, Nonlinear, and Soft Matter Physics, vol. 87 p.032306 (2013)
Ref Arxiv: 1212.3945
DOI: 10.1103/PhysRevE.87.032306
Résumé: We study the freezing kinetics of colloidal polycrystals made of micelles of Pluronic F108, a thermosensitive copolymer, to which a small amount of silica nanoparticles of size comparable to that of the micelles are added. We use rheology and calorimetry to measure Tc, the crystallization temperature, and find that Tc increases with the heating rate \dot{T} used to crystallize the sample. To rationalize our results, we first use viscosity measurements to establish a linear mapping between temperature T and the effective volume fraction, {\phi}, of the micelles, treated as hard spheres. Next, we reproduce the experimental \dot{T} dependence of the crystallization temperature with numerical calculations based on standard models for the nucleation and growth of hard spheres crystals, classical nucleation theory and the Johnson-Mehl-Avrami-Kolmogorov theory. The models have been adapted to account for the peculiarities of our experiments: the presence of nanoparticles that are expelled in the grain boundaries, and the steady increase of T and hence {\phi} during the experiment. We moreover show that the polycrystal grain size obtained from the calculations is in good agreement with light microscopy data. Finally, we find that the {\phi} dependence of the nucleation rate for the micellar polycrystal is in remarkable quantitative agreement with that found in previous experiments on colloidal hard spheres. These results suggests that deep analogies exist between hard-sphere colloidal crystals and Pluronics micellar crystals, in spite of the difference in particle softness. More generally, our results demonstrate that crystallization processes can be quantitatively probed using standard rheometry.
Commentaires: Journal: Phys. Rev. E 87, 032306 (2013)
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Structure of nanoparticles embedded in micellar polycrystals
Auteur(s): TAMBORINI E., Ghofraniha N., OBERDISSE J., CIPELLETTI L., RAMOS L.
(Article) Publié:
Langmuir, vol. 28 p.8562−8570 (2012)
Ref HAL: hal-00705673_v1
Ref Arxiv: 1205.4746
DOI: 10.1021/la301369z
Résumé: We investigate by scattering techniques the structure of water-based soft composite materials comprising a crystal made of Pluronic block-copolymer micelles arranged in a face-centered cubic lattice and a small amount (at most 2% by volume) of silica nanoparticles, of size comparable to that of the micelles. The copolymer is thermosensitive: it is hydrophilic and fully dissolved in water at low temperature (T ~ 0{\deg}C), and self-assembles into micelles at room temperature, where the block-copolymer is amphiphilic. We use contrast matching small-angle neuron scattering experiments to probe independently the structure of the nanoparticles and that of the polymer. We find that the nanoparticles do not perturb the crystalline order. In addition, a structure peak is measured for the silica nanoparticles dispersed in the polycrystalline samples. This implies that the samples are spatially heterogeneous and comprise, without macroscopic phase separation, silica-poor and silica-rich regions. We show that the nanoparticle concentration in the silica-rich regions is about tenfold the average concentration. These regions are grain boundaries between crystallites, where nanoparticles concentrate, as shown by static light scattering and by light microscopy imaging of the samples. We show that the temperature rate at which the sample is prepared strongly influence the segregation of the nanoparticles in the grain-boundaries.
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Grain refinement and partitioning of impurities in the grain boundaries of a colloidal polycrystal
Auteur(s): GHOFRANIHA N., TAMBORINI E., OBERDISSE J., CIPELLETTI L., RAMOS L.
(Article) Publié:
Soft Matter, vol. 8 p.6214-6219 (2012)
Ref HAL: hal-00700912_v1
Ref Arxiv: 1203.6524
DOI: 10.1039/c2sm25488c
Résumé: We study the crystallization of a colloidal model system in presence of secondary nanoparticles acting as impurities. Using confocal microscopy, we show that the nanoparticles segregate in the grain boundaries of the colloidal polycrystal. We demonstrate that the texture of the polycrystal can be tuned by varying independently the nanoparticle volume fraction and the crystallization rate, and quantify our findings using standard models for the nucleation and growth of crystalline materials. Remarkably, we find that the efficiency of the segregation of the nanoparticles in the grain-boundaries is determined solely by the typical size of the crystalline grains.
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Grain refinement and partitioning of impurities in the grain boundary network of a crystal made of amphiphilic copolymers in water
Auteur(s): RAMOS L., Ghofraniha N., TAMBORINI E., OBERDISSE J., CIPELLETTI L.
Conference: CopAmphi 2012 (Toulouse, FR, 2012-06-05)
Résumé: Composite materials, comprising nanoparticles (NPs) dispersed in a matrix, are of great interest, since the NPs can enhance the matrix properties or impart new functionalities and because the matrix can act as a template that structures the particles at the nanometer scale. However, controlling the spatial distribution of NPs remains a challenging task, as it usually depends crucially on the particles surface chemistry. We present here a general approach to confine NPs in colloidal materials in a controlled fashion.
We design nanocomposite material obtained by dispersing small quantities of NPs (at most 2%) in a colloidal crystalline matrix composed of thermosensitive copolymer micelles. The volume fraction of the micelles increases with temperature T, until crystallization occurs due to entropic reasons. Hence our system allows crystallization to be induced at the desired rate simply by varying T. By analogy with atomic crystals, we expect the NPs, which act as impurities, to be partially expulsed from the growing lattice and to segregate in the grain boundaries.
We use scattering techniques and confocal microscopy to probe the microscopic and mesoscopic structures of the composite materials. We show that the NPs do not perturb the crystalline structure of the micelles but that different crystallization rates lead to different NPs rearrangements. We show that the NPs segregate in the grain-boundaries of the copolymer polycrystal. We demonstrate that the texture of the polycrystal can be tuned by varying independently the nanoparticle volume fraction and the crystallization rate, and quantify our findings using classical nucleation theory. Remarkably, we find that the efficiency of the segregation of the NPs in the grain-boundaries is determined solely by the typical size of the crystalline grains.
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