Abstract
Colloidal silica spheres (diameter 88 nm) with a thick stericstabilization layer of polystyrene (PS; Mw = 26 600 g mol-1)were prepared. In cyclohexane, a marginal solvent for PS, particleaggregation and gelation were observed on lowering the temperature.Near the gelation temperature and at particle concentrations of afew per cent by weight, the gels were sufficiently weak to slowlycompact under gravity. On quenching to slightly lower temperatures,the gels still settled, but the top of the sediment did not becomeflat, as is usually the case. This seems to be related to anunusual mechanism for gel compaction, which starts by forming a moredense structure at the top of the sample. It is proposed that thisis related to the entangled polymer chains on neighbouring particlesresisting substantial rearrangement of the local structure. Thetransient gelation phenomenon, observed previously for mixtures ofcolloid and non-adsorbing polymer, has so far not been observed forour system.
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