Physical aging and phase behavior of multiresponsive microgel colloidal dispersions

Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions; this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles (i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage size) decreases with time, indicating that the physical aging process does not end following assembly, but instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the aging process that we associate with an evolution of attractive interparticle interactions during aging. These attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion, the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function of pH and volume fraction following approximately 1 month of aging. This diagram reveals highly complex dispersion characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that, at pH < pK(a), the final dispersions behave in a manner that can be associated with attractive interparticle interactions, whereas at pH > pK(a), repulsive interactions appear to be dominant. These results are discussed in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.