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SUMMARY:Some properties of microgel particles - Dr Alex Routh\, Department
  of Chemical Engineering and BP Institute\, University of Cambridge
DTSTART:20061027T131500Z
DTEND:20061027T141500Z
UID:TALK5204@talks.cam.ac.uk
CONTACT:Dr Kalin Dragnevski
DESCRIPTION:Microgel particles are cross-linked polymer colloids which dis
 play a swelling under certain conditions. Particles based on poly(2-vinylp
 yridine) swell in acidic conditions and those based on poly(N-isopropylacr
 ylamide) swell at temperatures below 32oC.  A number of properties of thes
 e particles will be discussed:\n\nThe flow of solvent around and through t
 he porous particles is solved in the limit of low Reynolds number and the 
 diffusion coefficient of particles is derived in terms of the particle per
 meability. 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.\n\nRheology of dilute disper
 sions of temperature sensitive particles is sensitive to the particle swel
 ling. Heating a dispersion of PNIPAM particles under controlled stress con
 ditions leads to a decrease in the dispersion viscosity as the particles c
 ollapse and the volume fraction decreases. Under certain stresses the poly
 mer chains entangle and a sharp increase in the dispersion viscosity is se
 en. This structuring is not observed when the dispersions are subsequently
  cooled. Hence we observe a temperature induced hystersis.\n\nThe dynamics
  of swelling of poly(2-vinylpyridine) particles has been followed using st
 opped flow. The results show that higher external salt concentrations lead
  to a faster swelling\, which is in contradiction to the accepted theory o
 f gel swelling\, by Tanaka. An alternative view\, where the flow of solven
 t into the microgel is the time limiting step will be proposed and the res
 ulting scaling predictions shown to conform with the experimental observat
 ions.\n
LOCATION:IRC in Superconductivity Seminar Room\, Cavendish Laboratory
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