A new class of copolymer colloids with tunable, low refractive index for investigations of structure and dynamics in concentrated suspensions

Highly charged polymer colloids may serve as model systems for the investigation of condensed matter, if they self-organize to liquid-like, glassy or crystalline phases. Multiple scattering due to refractive index differences of colloidal particles and suspending medium is a serious problem when utilizing light scattering experiments for these investigations. In this work, a new class of monodisperse colloidal dispersions is prepared by means of emulsion copolymerization of the monomers n-butyl acrylate and 2,2,2-trifluoroethyl acrylate. By systematic variation of the molar ratio of fluorinated and non-fluorinated monomers, the refractive index n p of the colloidal copolymer particles is tuned in the range 1.38 < n p < 1.45. Thus, particles with any composition of both monomers can be index-matched by protic water/glycerol mixtures as demonstrated by minima of the relative transmission of suspensions in dependence on the refractive index of the suspending medium. Static and dynamic light scattering experiments as well as analysis of the static structure factors S(Q) by means of integral equations are employed to investigate the self-organization of the resulting colloidal copolymer particles. Hereby, the potential application of these new model systems to investigate dynamics in concentrated binary mixtures is demonstrated

Coexistence of hcp and bct Phases during In Situ Superlattice Assembly from Faceted Colloidal Nanocrystals

We study the in situ self-assembly of faceted PbS nanocrystals from colloidal suspensions upon controlled solvent evaporation using time-resolved small-angle X-ray scattering and X-ray cross-correlation analysis. In our bulk-sensitive experiment in transmission geometry, the superlattice crystallization is observed in real time, revealing a hexagonal closed-packed (hcp) structure followed by formation of a body-centered cubic (bcc) superlattice. The bcc superlattice undergoes continuous tetragonal distortion in the solvated state shortly after its formation, resulting in the body-centered tetragonal (bct) structure. Upon solvent evaporation, the bct superstructure becomes more pronounced with the still coexisting hcp phase. These findings corroborate the existing simulations of assembling cuboctahedral-shaped particles and illustrate that we observed the predicted equilibrium states. This work is essential for a deeper understanding of the fundamental forces that direct nanocrystal assembly including nanocrystal shape and ligand coverage

Monitoring Nanocrystal Self-Assembly in Real Time Using In Situ Small-Angle X-Ray Scattering

Self‐assembled nanocrystal superlattices have attracted large scientific attention due to their potential technological applications. However, the nucleation and growth mechanisms of superlattice assemblies remain largely unresolved due to experimental difficulties to monitor intermediate states. Here, the self‐assembly of colloidal PbS nanocrystals is studied in real time by a combination of controlled solvent evaporation from the bulk solution and in situ small‐angle X‐ray scattering (SAXS) in transmission geometry. For the first time for the investigated system a hexagonal closed‐packed (hcp) superlattice formed in a solvent vapor saturated atmosphere is observed during slow solvent evaporation from a colloidal suspension. The highly ordered hcp superlattice is followed by a transition into the final body‐centered cubic superlattice upon complete drying. Additionally, X‐ray cross‐correlation analysis of Bragg reflections is applied to access information on precursor structures in the assembly process, which is not evident from conventional SAXS analysis. The detailed evolution of the crystal structure with time provides key results for understanding the assembly mechanism and the role of ligand–solvent interactions, which is important both for fundamental research and for fabrication of superlattices with desired properties

In situ small-angle X-ray scattering environment for studying nanocrystal self-assembly upon controlled solvent evaporation

We present a sample environment for the investigation of nanoparticle self-assembly from a colloidal solution via controlled solvent evaporation using in situ small-angle X-ray scattering. Nanoparticles form ordered superlattices in the evaporative assembly along the X-ray transparent windows of a three-dimensional sample cell. The special design of the sample cell allows for monitoring the superlattice formation and transformation at different stages of the assembly process during the movement of the evaporation front in real time. The presented sample environment can be used to study the self-organization of a wide range of colloidal particles and other soft materials

The Phase Diagram of Colloidal Silica-PNIPAm core-shell Nanogels

We study the structure and dynamics of aqueous dispersions of densely packed core–shell nanoparticles composed of a silica core and a poly(N-isoproylacrylamide) (PNIPAm) shell as a function of temperature and concentration. With small angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS) we shed light on the structural and dynamical changes of the thermo-responsive colloidal nanogel during its volume phase transition at a lower critical solution temperature (LCST) of 32 °C. A transition of the dynamics and its distinct dependency on the particle number concentration could be determined by analysing the intensity autocorrelation function while the structural transition remains concentration independent. We found the dynamics of a jammed system beyond a critical concentration. In addition, by variation of the PNIPAm shell size we tuned both the effective volume fraction and the underlying nature of the dynamics in the system. With our results we can present a full phase diagram of a PNIPAm core–shell system that spans from diluted suspensions, where the system behaves like a liquid, to an effective volume fraction of more than ninety percent where after exceeding a critical concentration the system undergoes a temperature-induced transition from a liquid towards a colloidal gel

Influence of TMAO as co-solvent on the gelation of silica-PNIPAm core-shell nanogels at intermediate volume fractions

We study the structure and dynamics of poly(N‐isopropylacrylamide) (PNIPAm) core‐shell nanogels dispersed in aqueous trimethylamine N‐oxide (TMAO) solutions by means of small‐angle X‐ray scattering and X‐ray photon correlation spectroscopy (XPCS). Upon increasing the temperature above the lower critical solution temperature of PNIPAm at 33 °C, a colloidal gel is formed as identified by an increase of I (q ) at small q as well as a slowing down of sample dynamics by various orders of magnitude. With increasing TMAO concentration the gelation transition shifts linearly to lower temperatures. Above a TMAO concentration of approximately 0.40 mol/L corresponding to a 1 : 1 ratio of TMAO and NIPAm groups, collapsed PNIPAm states are found for all temperatures without any gelation transition. This suggests that reduction of PNIPAm‐water hydrogen bonds due to the presence of TMAO results in a stabilisation of the collapsed PNIPAm state and suppresses gelation of the nanogel

A new class of copolymer colloids with tunable, low refractive index for investigations of structure and dynamics in concentrated suspensions

Highly charged polymer colloids may serve as model systems for the investigation of condensed matter, if they self-organize to liquid-like, glassy or crystalline phases. Multiple scattering due to refractive index differences of colloidal particles and suspending medium is a serious problem when utilizing light scattering experiments for these investigations. In this work, a new class of monodisperse colloidal dispersions is prepared by means of emulsion copolymerization of the monomers n-butyl acrylate and 2,2,2-trifluoroethyl acrylate. By systematic variation of the molar ratio of fluorinated and non-fluorinated monomers, the refractive index n p of the colloidal copolymer particles is tuned in the range 1.38 < n p < 1.45. Thus, particles with any composition of both monomers can be index-matched by protic water/glycerol mixtures as demonstrated by minima of the relative transmission of suspensions in dependence on the refractive index of the suspending medium. Static and dynamic light scattering experiments as well as analysis of the static structure factors S(Q) by means of integral equations are employed to investigate the self-organization of the resulting colloidal copolymer particles. Hereby, the potential application of these new model systems to investigate dynamics in concentrated binary mixtures is demonstrated