THE GLASS-TRANSITION DYNAMICS OF POLYMER MICRONETWORK COLLOIDS - A MODE-COUPLING ANALYSIS

We studied the glass transition dynamics of polystyrene micronetwork c olloids 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 extra cting correct intermediate scattering functions in a system that might be termed as partially nonergodic. By using a charge-coupled device c amera as a detector and averaging-the intensity autocorrelation functi ons of 50 simultaneously monitored speckles the duration of the experi ment could be significantly reduced as compared to the conventional '' brute force'' ensemble averaging. Despite some striking similarities t o the behavior of hard sphere colloids the glass transition scenario i n our system differs in several respects when analyzing the dynamics i n the glass transition regime within the framework of mode coupling th eory. Besides the existence of structural relaxation processes above p hi(c) we find indications that additional dynamic processes modify the beta relaxation in the glassy phase. Our findings cannot be explained by the occurrence of hopping processes, but are rationalized via an i ncrease of the particle compressibility and the surface friction on de creasing the cross-link density from its hard sphere limit. (C) 1997 A merican Institute of Physics.