Cluster-size distribution of cross-linked polymer chains across the gelation threshold

The evolution of cross-linked polymer chain clusters in a solvent from a monomer solution has been investigated by dynamic Light scattering. The characteristic decay time distribution, P(Gamma(-1)), of the gelling system has been examined as a function of the initial monomer concentration, C, where Gamma(-1) and Gamma are the characteristic decay time and the decay rate, respectively. When C is lower than the chain overlap concentration C*, P(Gamma(-1)) is dominated by a single peak related to clusters having a hydrodynamic radius of R-H. However, for C approximate to C*, P(Gamma(-1)) becomes broader, having a tail toward a larger value of Gamma(-1). This indicates a formation of an assemble of clusters with a broad size distribution. On the other hand, for C > C*, P(Gamma(-1)) becomes narrow and unimodal, which results from scattering by an infinite network and corresponds to the so-called gel mode scattering. This behavior is very different from that for the corresponding polymer solutions with the same concentrations. On the basis of these results, the time evolution of the dynamics of polymer gels will be discussed.