Nucleation and crystal growth in a suspension of charged colloidal silica spheres with bi-modal size distribution studied by time-resolved ultra-small-angle X-ray scattering

Formation of a transient amorphous solid in low density aqueous charged sphere suspensions

Colloidal glasses formed from hard spheres, nearly hard spheres, ellipsoids and platelets or their attractive variants, have been studied in great detail. Complementing and constraining theoretical approaches and simulations, the many different types of model systems have significantly advanced our understanding of the glass transition in general. Despite their early prediction, however, no experimental charged sphere glasses have been found at low density, where the competing process of crystallization prevails. We here report the formation of a transient amorphous solid formed from charged polymer spheres suspended in thoroughly deionized water at volume fractions of 0.0002-0.01. From optical experiments, we observe the presence of short-range order and an enhanced shear rigidity as compared to the stable polycrystalline solid of body centred cubic structure. On a density dependent time scale of hours to days, the amorphous solid transforms into this stable structure. We further present preliminary dynamic light scattering data showing the evolution of a second slow relaxation process possibly pointing to a dynamic heterogeneity known from other colloidal glasses and gels. We compare our findings to the predicted phase behaviour of charged sphere suspensions and discuss possible mechanisms for the formation of this peculiar type of colloidal glass.

On tuning microgel character and softness of cross-linked polystyrene particles

Polystyrene (PS) microgel colloids have often been used successfully to model hard sphere behaviour even though the term "gel" invokes inevitably the notion of a more or less soft, deformable object. Here we systematically study the effect of reducing the cross-link density from 1 : 10 (1 cross-link per 10 monomers) to 1 : 100 on particle interactions and "softness". We report on the synthesis and purification of 1 : 10, 1 : 25, 1 : 50, 1 : 75 and 1 : 100 cross-linked PS particles and their characterization in terms of single particle properties, as well as the behaviour of concentrated dispersions. We are able to tune particle softness in the range between soft PNiPAM-microgels and hard PMMA particles while still allowing the mapping of the microgels onto hard sphere behavior with respect to phase diagram and static structure factors. This is mainly due to a rather homogeneous radial distribution of cross-links in contrast to PNiPAM microgels where the cross-link density decreases radially. We find that up to a cross-link density of 1 : 50 particle form factors are perfectly described by a homogeneous sphere model whereas 1 : 75 and 1 : 100 cross-linked spheres are slightly better described as fuzzy spheres. However the fuzziness is rather small compared to typical PNiPAM microgels so that a hard sphere mapping still holds even for these low cross-link densities. Finally, by varying the reaction conditions - changing from batch to semibatch emulsion polymerization and varying the feed rate or by adjusting the monomer to initiator ratio we can tune the fuzziness or significantly alter the inner structure to a more open, star-like architecture.

Probing the absolute scattering intensity by means of a laboratory-based small-angle X-ray scattering camera using an imaging plate detector

This paper deals with the application of an imaging plate (IP) detector for probing absolute intensities in small-angle X-ray scattering experiments. The IP detector is used with an in-house modified Kratky camera. It is shown that the normalization of scattering data to the transmitted primary beam intensity properly compensates for various unwanted features typically associated with the IP detector,e.g.the fading effect and poorly reproducible scanning times, promoting precise absolute intensity calibration. The absolute intensity calibration was performed by measuring a large number of different silica sols serving as secondary standards. The calibration factor, which converts the measured data into absolute units, was determined by the average over the data obtained for different silica sols, providing a precise calibration of the IP detector. In particular, it is shown that silica sols do not require application of synchrotron radiation or a long exposure time as typically needed if pure liquids or standard proteins are used as secondary standards. To check the applicability of the IP detector in a real synthesis, Stöber silica particles were synthesized under various reaction conditions and the scattering intensity was probed and converted into absolute units. The volume fractions and the number densities of silica particles provided by the absolute intensities showed a good agreement with theoretical values predicted for the different reaction conditions. Therefore, the IP detector can be used for accurate probing of absolute intensities.