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.
Highlights
In a different approach spherical particles of cross-linked polymer chains are used
This range of particle sizes allows us to check for a systematic variation of the swelling ratio QHS and other system characteristics with particle size – an issue raised in previous work.[54]
The thermodynamic properties and inner structure of PS microgels of different size and cross-link density were studied. Their suitability as hard sphere model systems was investigated by probing phase diagrams, rheological properties, structure factors and form factors
Summary
In a different approach spherical particles of cross-linked polymer chains are used. These so called microgels[19] swell in a good solvent, which leads to an easier matching of both refractive index and density. In the case of PS microgels homogeneously cross-linked microgels of low polydispersity can be prepared if 1,3-diisopropenylbenzene (DIPB) is used to form the polymer network,[35] leading to less soft particles than e.g. PNiPAMmicrogels For this reason PS microgels have been widely used as effective hard spheres to study e.g. glass transition,[24,25,26,27,36] crystallisation kinetics[7,28,37,38,39] or collective[33] and single[40] particle diffusion. We studied systematically the effect of varying the cross-link density on the static properties of PS microgels and compared their behaviour with the known one of hard sphere systems For this purpose we prepared a series of PS microgels with different amounts of cross-linking agents. We prepared particles via batch emulsion polymerisation and semibatch emulsion polymerisation using different feeding rates to study the influence of the polymerisation technique on the incorporation of DIPB, i.e. the fuzziness of the particles
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