Microgel Dispersions: Colloidal Forces and Phase Behavior

In many aspects, colloidal particles suspended in a liquid behave like large idealized atoms that exhibit liquid, glass, and crystal phases similar to those observed in atomic systems. Phase transitions in colloidal systems have been intensively studied over the last decade not only because of the theoretical interest for addressing fundamental questions about the nature of liquids, crystals, and glasses, but also for many practical applications of colloids. In recent years, colloidal dispersions have been used extensively for the fabrication of nanostructured materials such as photonic crystals, catalysts, membranes, and ceramics, and for device applications. Current uses of colloidal particles as the building blocks of materials rely mostly on empirical approaches. Meanwhile, present knowledge on the structural and thermodynamic properties of colloidal dispersions is primarily based on an effective one-component model (OCM) where colloidal particles are represented by hard spheres and all remaining components in the dispersion, including solvent molecules, small ions, and polymers, are represented as a continuous medium. Although the OCM approach is attractive because of its simplicity, application to practical systems is often limited by incomplete understanding of colloidal forces. Amid numerous conventional colloids, aqueous dispersions of poly-N-isopropylacrylamide (PNIPAM) microgel particles, first synthesized by Pelton and Chibante in 1986, are of special interest for studying phase transitions and for the fabrication of colloid-based advanced materials. Nearly monodispersed PNIPAM particles can now be routinely prepared in a wide range of colloidal sizes (50 nm up to 1 mm) and with a variety of physiochemical characterizations. Because the size of PNIPAM particles is temperature-sensitive, crystallization at different colloidal volume fractions can be conveniently measured by varying temperatures. By tuning the preparation conditions and the composition of the aqueous solution, the interaction potential between microgel particles can vary from star-polymer-like to hard-sphere-like potential for short-range repulsion, from electrostatically neutral to highly ionizable for long-range electrostatic interactions, and from essentially no attraction to strong attraction for van der Waals forces. Furthermore, steric repulsion can be introduced by grafting polymers on the surface of PNIPAM particles. The versatility in interaction potential makes PNIPAM microgel particles attractive for studying a broad variety of interesting phenomena in colloidal systems. Although the practical values of PNIPAM particles have been long recognized, most previous studies on the physiochemical properties of PNIPAM dispersions have focused on particle preparations, swelling, rheology, and light (neutron) scattering measurements. Little work has been reported on the relationship between the temperature-dependent interparticle potential and the phase behavior of PNIPAM dispersions. Unlike that in a conventional colloidal system, the interparticle potential in aqueous dispersions of PNIPAM microgel particles is sensitive to temperature changes. Consequently, the phase diagram of PNIPAM dispersions may be noticeably different from those for ordinary colloids where, in most cases, the interparticle potential is essentially invariant with temperature. In this chapter, we report our recent investigations on the colloidal forces and phase behavior of neutral PNIPAM particles dispersed in pure water. We will first discuss thermodynamic methods for the characterization of colloidal forces based on dynamic and static light scattering measurements. The analytical expression of colloidal forces allows us to construct a theoretical phase diagram that can be compared with that obtained from spectroscopic measurements. In particular, we will illustrate how the volume transition of PNIPAM particles affects the interaction potential and determines a novel phase diagram that has not been observed in conventional colloids. Finally, we discuss briefly the kinetics of crystallization in an aqueous dispersion of PNIPAM particles.