DYNAMIC LIGHT-SCATTERING STUDY OF CONCENTRATED MICROGEL SOLUTIONS AS MESOSCOPIC MODEL OF THE GLASS-TRANSITION IN QUASI-ATOMIC FLUIDS

This paper presents a light scattering study of the dynamics of concentrated solutions of nearly monodisperse (sigma almost-equal-to 0.16) spherical micronetwork particles consisting of highly cross-linked linked polystyrene dissolved in carbon disulfide, i.e., a "good" solvent. Above volume fractions of phi = 0.50 the intermediate scattering function, measured over a time window of 10(-7) to 10(3) s using the ALV5000 correlator, decays in two steps and shows indications of nonergodic behavior for phi greater-than-or-equal-to 0.64. Such behavior is typical for glass forming systems and has recently been found close to the glass transition of a hard sphere colloidal system [W. van Megen and P. N. Pusey, Phys. Rev. A 43, 5429 (1991)]. Thus the introduced system can be used for modeling the glass transition of atoms on a mesoscopic scale. The traditional analysis of structural relaxation in terms of a Kohlrausch-Williams-Watts distribution yields a mean relaxation time which follows the empirical Mooney equation as a function of concentration and thus corresponds to Vogel-Fulcher-Tammann behavior. However, the necessity to add an unspecified "intermediate" process between the short and long time KWW decays demonstrates the limitations of this "pragmatic" approach. The mode coupling theory of the glass transition interprets the intermediate scattering function consistently over nearly seven decades in time, the intermediate region corresponding to the crossover from beta to alpha relaxation (von Schweidler law). The critical volume fraction of 0.636 derived by this analysis corresponds to a value of 0.59 for an ideal monodisperse system which is well in accord with other experimental and computer simulation studies of the glass transition of atomic systems.