Dynamics during a transient gelation process studied by XPCS

Photon correlation spectroscopy with partially coherent X-ray beams (XPCS) available at third generation synchrotron sources is an experimental technique that allows the direct measurement of the low frequency microstructural dynamics that are often present in a large class of soft-condensed matter systems. In many such systems and in particular in concentrated disordered systems, at least two distinct relaxation mechanisms can usually be found. The fast(er) ones correspond to the ``trapped'' motion of individual particles or aggregates in ``cages'' created by other particles/aggregates. The slow relaxation modes correspond to the structural re-arrangements of the ``cages''. In this work we report the XPCS study of the structural dynamics associated with the slow collapse of transient gels consisting of mixtures of sterically-stablised polymethylmethacrylate (PMMA) particles and random-coil polystyrene (PS) dispersed in cis-decalin. The intermediate scattering functions change during the process from stretched to compressed exponential decays indicating a jamming of the system in the full aging regime. A complex aging behavior towards the final collapse of the gel is observed and we propose that large scale network deformations trigger an un-jamming process leading to the collapse.