Small-angle neutron scattering study of temperature-induced emulsion gelation: The role of sticky microgel particles

In this work, small-angle neutron scattering (SANS) is used to probe the structural transformations that accompany temperature-induced gelation of emulsions stabilized by a temperature-responsive polymer. The latter is poly(NIPAM-co-PEGMa) (N-isopropylacrylamide and poly(ethyleneglycol) methacrylate) and contains 86 mol% NIPAM. Turbidity measurements revealed that poly(NIPAM-co-PEGMa) has a lower critical solution temperature (T LCST) of 36.5°C in D2O. Aqueous polymer solutions were used to prepare perfluorodecalin-in-water emulsions (average droplet size of 6.9 μm). These emulsions formed gels at 50°C. SANS measurements were performed on the poly(NIPAM-co-PEGMa) solutions and emulsions as a function of temperature. The emulsion was also prepared using a D2O/H 2O mixture containing 72 vol% D2O in order to make scattering from the droplets negligible (on-contrast). The SANS data were analyzed using a combination of Porod and Ornstein-Zernike form factors. The results showed that the correlation length (ξ) of the polymer scaled as ξ ∼ φp-0.68 at 32°C, where φp is the polymer volume fraction. The ξ value increased for all systems as the temperature increased, which was attributed to a spinodal transition. At temperatures greater than TLCST, the polymer solution changed to a polymer dispersion of poly(NIPAM-co-PEGMa) aggregates. The aggregates have features that are similar to microgel particles. The average size of these particles was estimated as 160-170 nm. The particles are "sticky" and are gel-forming. The on-contrast experiments performed using the emulsion indicated that the interfacial polymer chains condensed to give a relatively thick polymer layer at the perfluorodecalin-water interface at 50°C. The gelled emulsions appear to consist of perfluorodecalin droplets with an encapsulating layer of collapsed polymer to which sticky microgel particles are adsorbed. The latter act as a "glue" between coated droplets in the emulsion gel. © 2005 American Chemical Society.