EQUILIBRIUM AND SHEAR-INDUCED NONEQUILIBRIUM PHASE-BEHAVIOR OF PMMA MICROGEL SPHERES

Polymethylmethacrylate (PMMA) spheres suspended in benzyl alcohol are found to swell to more than two times that of their dry radius and hav e been observed to undergo an equilibrium phase transition from liquid to crystalline structure with increasing concentration. The width of the coexistence region is found to be narrower by nearly half compared to simulation results for hard sphere systems. Comparison of the meas ured freezing point and fractional density change upon melting with th ose calculated from soft sphere simulations are consistent with a pure ly repulsive interparticle potential on the order of 1/r(20). Analysis of powder pattern scattering profiles from samples in the crystallize d region of the equilibrium phase diagram indicates crystallites made up of a registered random stacking of hexagonal close packed planes, s imilar to that found in monodisperse suspensions of hard spheres. With the application of oscillatory shear, nonequilibrium microstructures similar to those found in model hard sphere systems have been observed in these suspensions. (C) 1996 Academic Press, Inc.