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

Abstract

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.

Wide-angle static and dynamic light scattering under shear

“…Thus the motions of the particles at any height, follow a shear deformation, but that a higher shear rate forms close to the lower surface . This suggests that shear banding, generally observed for suspensions with attractive interactions 51 , occurs in our system. These measurements show the ability of our setup to measurement local shear rates, with spatial resolution of ≈ 100 µm.…”

“…Here, we consider polystyrene microgel particles suspended in a good solvent, 2-ethyl-naphtalene (2EN), of refractive index n = 1.599. The structure and dynamics of these suspensions at volume fractions corresponding to a glass phase have been studied in detail in the absence of shear 50,51 . Using measurements of the elastic modulus 51,52 , it has been shown that their interaction potential is well described by u(r) ∼ r r 0 n with n as large as 99 51 .…”

“…The structure and dynamics of these suspensions at volume fractions corresponding to a glass phase have been studied in detail in the absence of shear 50,51 . Using measurements of the elastic modulus 51,52 , it has been shown that their interaction potential is well described by u(r) ∼ r r 0 n with n as large as 99 51 . These suspensions may thus be considered as good models of hard sphere suspensions 53 .…”

Pathways and challenges towards a complete characterization of microgels

“…Colloidal hydrogels are a common subtype where the continuous phase is water. Often, but not always, these networks exhibit conformational changes in response to changes in their environment, such as the solvent composition or temperature 1 , 4 , 5 . The sensitivity to external stimuli, such as temperature, pH, or solvent conditions, is an essential feature of many microgels, in particular for the most widely studied type of microgels, based on pNIPAM (poly-N-isopropylacrylamide) and its derivatives 1 , 6 (Fig.…”

Correction: Polymer-enforced crystallization of a eutectic binary hard sphere mixture

“…In a recent paper, 3 we have systematically synthesized a large variety of polystyrene micro-gels of different sizes and degrees of cross-linking. For these, we studied the relation of dry particle sizes, swelling ratios in the good solvent 2-ethylnaphthalene, phase boundaries and light scattering properties.…”