8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.168Wed 711:10-14:00The phase behaviour of pNIPAM microgel and colloidmixturesJeroen van Duijneveldt, 1 Katie Bayliss, 1 Malcolm Faers, 2 and Ronald Vermeer 21 University of Bristol, School of Chemistry, Cantock’s Close BS8 1TS, Bristol,United Kingdom2 Bayer CropScience, Monheim am Rhein, GermanyDepletion interactions give rise to phase separation in colloid-polymer mixtures at sufficientlyhigh concentrations of both species. Similarly a fluid-solid phase separation is expected for binaryhard sphere (BHS) mixtures, however in practice it appears such mixtures are very prone to formmetastable (jammed) states. Recently it was shown that the addition of microgel particles caninduce depletion attractions between colloidal particles [1]. These colloid-microgel mixtures areof particular interest, as the stimuli responsive nature of the microgels enables us to reversiblytune particle interactions by changing external parameters. Here, temperature responsive pNIPAMmicrogel particles are used to induce depletion attractions between polystyrene latex spheres. Suchmicrogels can be modelled as hard spheres with a polymer brush-like outer layer [2]. Mixtureswith a microgel / polystyrene size ratio of 0.11 showed a fluid-solid phase separation, in goodagreement with predictions for BHS. Gel states were obtained on further increase of concentration,around the position of the predicted metastable BHS fluid-fluid binodal, echoing previous claimsof the significance of a hidden fluid-fluid phase boundary as the precursor to gelation. It thusappears that the slight deformability of the microgel particles reduces the tendency for mixturesto remain in jammed states. Nevertheless the mixtures did tend to form gels more easily thanmixtures of the same polystyrene spheres with linear polymer coils at a similar polymer / colloidsize ratio.[1] For example: Fernandes, G. et al. Langmuir (2008). 24, 10776[2] Scheffold, F. et al. , Phys. Rev. Lett. (2010) 104, 128304.168