Novel Gelling Behavior of Poly(N-isopropylacrylamide-co-vinyl laurate) Microgel Dispersions

Aqueous colloidal microgel particles prepared from N -isopropylacrylamide (NIPAM) co-polymerized with different ratios of the comonomer vinyl laurate have been investigated with respect to their physicochemical properties and colloid stability. The hydrodynamic diameters of microgel particles synthesized using 10% and 50% w/v vinyl laurate have been examined. The poly(NIPAM)- co -vinyl laurate microgel particles, suspended in water, show similar conformational behavior to poly(NIPAM) microgels in that they reversibly shrink and swell (undergo a reversible volume phase transition) in response to heating and cooling. Turbidity measurements have been used to study the colloid stability of poly(NIPAM) and poly(NIPAM)- co -vinyl laurate microgel particles as a function of electrolyte concentration. When heated to 40 °C in a NaCl electrolyte solution, the poly(NIPAM)- co -vinyl laurate microgels form irreversible flocs and macroscopic gels above a critical electrolyte concentration. This novel flocculation behavior is in contrast to poly(NIPAM) microgels that reversibly flocculate when heated/cooled in an electrolyte solution (above a critical electrolyte concentration). Photon correlation spectroscopy (PCS) and turbidity measurements have been used to study the swelling (volume phase transition) behavior of the poly(NIPAM)- co -vinyl laurate microgels and good agreement was observed between the data obtained from both these techniques.