Abstract

Microgel particles are cross-linked polymer colloids which display a swelling under certain conditions. Particles based on poly(2-vinylpyridine) swell in acidic conditions and those based on poly(N-isopropylacrylamide) swell at temperatures below 32oC. A number of properties of these particles will be discussed:

The flow of solvent around and through the porous particles is solved in the limit of low Reynolds number and the diffusion coefficient of particles is derived in terms of the particle permeability. The result is that the Stokes-Einstein diffusion coefficient is a valid estimate for porous particles and hence dynamic light scattering is a valid method to size microgel particles.

Rheology of dilute dispersions of temperature sensitive particles is sensitive to the particle swelling. Heating a dispersion of PNIPAM particles under controlled stress conditions leads to a decrease in the dispersion viscosity as the particles collapse and the volume fraction decreases. Under certain stresses the polymer chains entangle and a sharp increase in the dispersion viscosity is seen. This structuring is not observed when the dispersions are subsequently cooled. Hence we observe a temperature induced hystersis.

The dynamics of swelling of poly(2-vinylpyridine) particles has been followed using stopped flow. The results show that higher external salt concentrations lead to a faster swelling, which is in contradiction to the accepted theory of gel swelling, by Tanaka. An alternative view, where the flow of solvent into the microgel is the time limiting step will be proposed and the resulting scaling predictions shown to conform with the experimental observations.