A dialysis cell for microscopy was used to study the phase behavior of colloidal suspensions of poly-N-isopropylacrylamide-co-acrylic acid hydrogel particles (pNiPAm-co-AAc; diameter ca. 400nm). In addition to the well-known temperature sensitivity of pNiPAm particles, pNiPAm-co-AAc particles also exhibit pH responsiveness due to presence of acrylic acid monomers. At room temperature, when pNiPAm particles are fully swollen, the particle size of pNiPAm-co-AAc particles can be manipulated by varying the solution pH. As a result, the effective particle volume fraction of a hydrogel suspension can be controlled without adding or removing solvent.

The dialysis cell enables direct visualization of the colloidal hydrogel particles via differential interference contrast microscopy during these changes in solvent composition. Standard image analysis and particle tracking algorithms were applied to monitor the particle dynamics (mean squared displacement) and structural evolution (pair distribution function, bond orientation order parameter) in both dilute and concentrated suspensions. In dilute suspensions, the Stokes-Einstein relation can be used to relate diffusion coefficient and particle size, and the (de)swelling of pNiPAm-co-AAc hydrogels in response to pH changes can be measured directly. In concentrated suspensions, sudden swelling of hydrogel particles causes jamming of the suspension because of excluded volume effects. After this sudden decrease in particle mobility, the particles slowly reorganize in glassy and crystalline structures, depending on the effective particle volume fraction. We will present data about this structural evolution as a function of hydrogel concentration for both jamming (swelling) and melting (deswelling) experiments.