Thermal and osmotic deswelling of poly(NIPAM) microgel particles

The effects of temperature and added free polymer on the swelling of poly(NIPAM) (NIPAM = N-isopropylacrylamide) microgel particles cross-linked with N,N'-methylene bisacrylamide have been investigated. The microgel particles were characterized by photocorrelation spectroscopy (PCS), TEM and SEM measurements. Particle sizes determined by TEM were invariably larger than those measured at 50 degrees C by PCS. SEM data suggest that dried particles have an oblate spheroid shape. The rate at which poly(NIPAM) particles collapse and swell in response to cycling of the temperature between 28 and 60 degrees C was examined and some hysteresis was observed. Excluded free polymer [PEG, where PEG = poly(ethyleneglycol)] also caused deswelling of the poly(NIPAM) particles at 25 degrees C due to osmotic pressure effects. Osmotic deswelling occurred in the presence of monodisperse PEG when the molecular weight of the latter was greater than or equal to 1470 g mol(-1). Higher molecular weight PEG polymers caused a general decrease in the extent of deswelling (at a fixed concentration of PEG).