Structural Ordering and Phase Behavior of Charged Microgels

Recent theoretical phase diagrams for loosely cross-linked ionic microgels with a low monomer volume fraction (Gottwald; et al. Phys. Rev. Lett. 2004, 92, 068301) have predicted a re-entrant order-disorder transition (i.e., fluid-FCC-BCC-fluid) as a function of concentration and so far there has been no experimental verifications of these theoretical predictions. Here, we present experimental results on phase behavior of loosely cross-linked charged poly(N-isopropylacrylamide co acrylic acid) (PNIPAm-co-AAc) microgesls with a low monomer volume fraction (similar to 0.003) for a wide range of concentrations (0.02-0.6 wt%) using static and dynamic light scattering methods. These microgel dispersions exhibit a short-range liquid order at low concentration (<0.03 wt %), a FCC crystalline order at intermediate concentrations (0.03- 0.3 wt %). In addition, we suggested a possible coexistence of BCC and FCC phases at higher concentration crystalline suspension (similar to 0.34 wt %). These results clearly demonstrate the experimental verification of above theoretical prediction below the overlap concentration and also reveal that the interaction potential between the microgel particles is of screened Coulomb repulsive type within these concentration ranges. At further higher concentration (similar to 0.57 wt %), we once again observed a disordered state and this disordered state from dynamic light scattering was confirmed to be a glass. These initial results are discussed in the light of previously reported results on the phase behavior of ionic microgel colloidal dispersions.