Temperature-dependent electrophoretic mobility and hydrodynamic radius measurements of poly(N-isopropylacrylamide) microgel particles:: structural insights

Electrophoretic mobility and hydrodynamic radius measurements of poly(N-isopropylacrylamide) [poly(NIPAM)] microgel particles dispersed in water were made as a function of temperature and ionic strength. The data reveal differences between the temperatures at which the particle volume and electrophoretic mobility undergo significant changes. The temperature at which the volume phase transition (VPTT) occurred was 5-6 degrees C lower than the temperature at which pronounced electrophoretic mobility changes occurred. The data appear to be consistent with three stages of collapse for the microgel particles over the temperature range 25-50 degrees C. Ohshima's equations for the electrophoretic mobility of particles covered by an ion-impenetrable surface charge layer were successfully used to fit the data. Electrokinetic equations for hard-sphere systems could not be used for our system. The successful application of the Ohshima theory is evidence that poly(NIPAM) microgel particles prepared by surfactant-free emulsion polymerisation consist of a core-shell structure. It is proposed that the core is relatively highly cross-linked and has a low concentration of ionic groups; whereas, the shell is lightly cross-linked and can be considered as a polyelectrolyte layer. The softness parameter obtained from fitting the Ohshima model to data obtained at 46 degrees C was much higher than expected for a rigid particle and suggests incomplete collapse of the shell at this temperature.