Ref Arxiv: 1212.3945
DOI: 10.1103/PhysRevE.87.032306
WoS: 000316393300007
Ref. & Cit.: NASA ADS
13 Citations
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)

Ref HAL: hal-00705673_v1
Ref Arxiv: 1205.4746
DOI: 10.1021/la301369z
WoS: 000304783300033
Ref. & Cit.: NASA ADS
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23 Citations
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.

Ref HAL: hal-00700912_v1
Ref Arxiv: 1203.6524
DOI: 10.1039/c2sm25488c
WoS: 000304309300004
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
28 Citations
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.

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.

Ref HAL: hal-00637578_v1
Exporter : BibTex | endNote
Résumé:

Composites materials comprising nanoparticles dispersed in a matrix are of great scientific and technological interest, since nanoparticles can enhance dramatically the matrix properties or even impart new functionalities, and because the matrix can act as a template that structures the particles at the nanoscopic level. However, controlling the three-dimensional spatial distribution of nanoparticles in a molecular or macromolecular matrix is a challenging task, as particle segregation usually depends crucially on the surface chemistry of the particles. Here, we present a model hybrid material, obtained by dispersing nanoparticles in a colloidal crystalline matrix, composed of thermoresponsive micelles. Using confocal microscopy, we show that the nanoparticles segregate in a network of thin sheets, in analogy to impurities confined in the grain boundaries of atomic polycrystals. We demonstrate that the size of the colloidal crystallites is tuned by varying independently the nanoparticle concentration (regardless of their composition and surface chemistry) and the crystallization rate, because they both determine the number of critical nuclei during the nucleation process and we quantify our findings using classical nucleation theory. Remarkably, we find that the efficiency of the segregation of the nanoparticles in the grain-boundaries is dictated solely by the typical size of the crystalline grains, due to the fact that the larger a grain can grow, the higher the concentration of the impurities progressively expelled from the crystallites during their growth and eventually trapped in the grain boundary, as we clearly show. Our method provides a general approach for confining nanoparticles in absence of any external field and in a controlled and tunable fashion in a three-dimensional soft colloidal matrix.

Résumé:

The structure and rheology of polycrystals is relevant in particular for metals, where post mortem studies are usually performed. In colloidal crystalline systems, the accessible size range makes in-situ investigations possible, under flow and at rest. In this project, termed ‘colloidal metallurgy’, a polycrystalline material is formed spontaneously at high concentrations (34%wt) of spherical Pluronics micelles (F108) in water. The crystalline grains have a local FCC structure (lattice constant 30 nm), and are separated from neighbouring grains of different orientation by grain boundaries (GB), cf. Fig 1, where fluorescent nanoparticles of size comparable to the lattice parameter have been added to visualize the GBs. We have determined the size distribution of grains for different crystallization speeds and nanoparticle contents (typically below 1%). In particular, a universal relationship between grains size and thermal history, and between grain size and amount of nanoparticles has been found, in agreement with theoretical considerations based on classical nucleation theory and constitutional under-cooling. The microscopic structure of silica-containing polycrystals has been studied by contrast-variation small angle neutron scattering, highlighting either the particles or the polymer micelles. The local micellar structure is found to be unaffected by the nanoparticles, which are shown to segregate predominantly in dense zones, presumably GBs. Very small-angle light scattering is currently used to characterize the mesostructure, and the influence of deformation and flow is investigated in a confocal rheo set-up. An alternative view of our findings is that we present here a general approach to confine nanoparticles in colloidal materials in a controlled fashion, independent of the details of the surface chemistry.