THE PREPARATION AND PHYSICOCHEMICAL PROPERTIES OF POLY(N-ETHYLACRYLAMIDE) MICROGELS

The preparation and physico-chemical characterisation of a novel therm osensitive microgel dispersion of anionic poly(N-ethylacrylamide), pol y(NEAM), is reported. Dispersions were prepared by the single-step emu lsion polymerisation of N-ethylacrylamide in water, in the presence of N,N'-methylenebisacrylamide as a cross-linking agent and ammonium per sulfate as the initiator. Transmission electron micrographs of the mic rogel dispersions showed that the reaction time greatly influenced the physical characteristics of the resultant microgel particles, with mo nodisperse spheres, having a mean diameter of 414 +/- 21 nm, being obt ained following a 24-h preparation. Using high sensitivity differentia l scanning calorimetry (HSDSC) a kinetically limited phase transition was observed for a 2.6% (w/w) dispersion, having an excess specific he at capacity maximum at 78.2 degrees C (at a scan rate of 60 K h(-1)). The thermodynamic parameters and turbidimetric changes (as a function of temperature, pH and electrolyte concentration) associated with this transition are reported. In the presence of 1 mol dm(-3) sodium chlor ide, the temperature at which this conformational transition occurs de creases with a corresponding increase in the calorimetric enthalpy of the transition. A reversible aggregation of the microgel particles was also observed on heating a 0.7% (w/w) dispersion to 80 degrees C in t he presence of 3 mol dm(-3) sodium chloride. Experimental measurements of the reduced viscosity, eta(red), of a 3% (w/w) dispersion, as a fu nction of temperature and electrolyte concentration, provided further evidence of a temperature-induced conformational change with respect t o the poly(NEAM) chains, resulting in a more compact conformation. (C) 1997 Elsevier Science Ltd. All rights reserved.