The glass transition dynamics of polymer micronetwork colloids. A mode coupling analysis

Abstract

We studied the glass transition dynamics of polystyrene micronetwork colloids with an average cross-link density of 1:50 (inverse number of monomer units between cross-links) and a hydrodynamic radius of about 100 nm by dynamic light scattering. Special emphasis was put on extracting correct intermediate scattering functions in a system that might be termed as partially nonergodic. By using a charge-coupled device camera as a detector and averaging the intensity autocorrelation functions of 50 simultaneously monitored speckles the duration of the experiment could be significantly reduced as compared to the conventional “brute force’’ ensemble averaging. Despite some striking similarities to the behavior of hard sphere colloids the glass transition scenario in our system differs in several respects when analyzing the dynamics in the glass transition regime within the framework of mode coupling theory. Besides the existence of structural relaxation processes above φc we find indications that additional dynamic processes modify the β relaxation in the glassy phase. Our findings cannot be explained by the occurrence of hopping processes, but are rationalized via an increase of the particle compressibility and the surface friction on decreasing the cross-link density from its hard sphere limit.