Nucleation and growth of micellar polycrystals under time-dependent volume fraction conditions

Abstract

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 T c , the crystallization temperature, and find that T c increases with the heating rateṪ 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, ϕ, of the micelles, treated as hard spheres. Next, we reproduce the experimentalṪ 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 ϕ 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 ϕ 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.

Crystallization kinetics of colloidal model suspensions: recent achievements and new perspectives

“…Remaining seeds then are either distributed throughout the sample or populate the grain boundaries of the emerging crystals. Their accumulation there is assisted by annealing processes [81,365] and a foam-like microstructure appears similar to that observed for long lasting annealing of AS crystals [298,299]. Cleaning of contaminated crystals can be achieved by processes analogous to zone melting [366].…”

Viscoelasticity of colloidal polycrystals doped with impurities

“…The colloidal crystals have been described elsewhere [29][30][31]. They comprise a commercial blockcopolymer, water and nanoparticles.…”

“…3B), a trend opposite to that measured without NPs. Microscopy and scattering experiments [29,30] indicate that when Ṫ increases the average size of the crystallite decreases. Hence, the total surface of grain boundaries, where we expect dissipation to occur predominantly, increases.…”

Mechanical strength enhancement by grain size reduction in a soft colloidal polycrystal

“…Their extensive use is also motivated by the facile means that leads to their synthesis and manipulation, coupled to their direct characterization by several non-destructive techniques [25]. Among the recent studies in this field, a new strategy emerged and consisted of adding small amounts of nanoparticles (NPs) to a system of different nature [26][27][28][29][30][31][32][33][34]. It was successfully shown that when a phase transition is induced in such a system, microscopic segregation occurs and results in polycrystalline grains separated by nanoparticle (NP) rich interstices.…”

“…Moreover, as the phase transition temperature is reached, the NPs are being segregated outside the micrograins whose equilibrium size is, in turn, controlled by the NPs. This effect was explained by studying the evolution of grain size as a function of the two tuning parameters: NP volume fraction and crystallization rate in a nearly similar colloidal system [28,29]. Accordingly, it was shown that the presence of NPs decreases the energy barrier to nucleation and favors the formation of smaller crystalline nuclei through reduction of the surface term in Gibbs energy.…”