Light scattering, electrokinetic measurements and flow rheology have been used to characterise the dispersion behaviour a responsive core-shell latex particle system. The system of choice is polystyrene latex which has a stimulus responsive steric polymeric layer at its surface: the responsive polymer used here is a block copolymer of poly(N,N-dimethylaminoethyl methacrylate)-b-poly(methylmethacrylate) [pDMAEMA-b-pMMA], where the pMMA block acts as an anchor to the latex particle and the pDMAEMA block projects into solution. pDMAEMA is a weakly basic polymer and its solubility is therefore dependent upon the pH, it also has a lower critical solution temperature (LCST) of around 40-50 °C in water. Zeta potential measurements showed an iso-electric point (iep) at between pH 7.5 and 8.2, dependent on the background salt concentration, with the core-shell particles having a positive charge at low pH as a result of the strong protonation of the weakly basic pDMAEMA shell. The dimensions of the pDMAEMA shell as a function of pH were measured using dynamic light scattering. At low pH, the steric pDMAEMA shell was seen to extend strongly from the surface of the core PS latex as a result of charge repulsions within this layer. Near to the iep, the steric polymer shell was seen to shrink as a result of the discharging of the weakly basic amine functionalities, finally collapsing back to the latex particle surface close to the iep. Flow rheology data indicated a significant shear-thinning behaviour at all measured particle volume fractions and at all pH. High shear rate data of the viscosity as a function of pH correlated directly with changes in the hydrodynamic volume of the particles; the measured viscosity decreasing as the apparent size of the core-shell particles decreased due to shell collapse. Evidence for the aggregation of the dispersion at high pH near to and above the iep were also seen.
|Number of pages||6|
|Journal||Colloids and Surfaces A: Physicochemical and Engineering Aspects|
|Publication status||Published - 15 Jan 2010|