Critical dynamics of cross-linked polymer chains near the gelation threshold

The dynamics of cross-linked polymer chains in solution near the gelation t hreshold has been investigated by dynamic light scattering. The time-intens ity correlation function (ICF) was obtained as a function of initial monome r concentration, C-NIPA, and cross-linker concentration, C-BIS, where NIPA and BIS denote N-isopropylacrylamide and N,N'-methylenebis(acrylamide), res pectively. The ICF exhibited a distinct change at a unique set of (C-NIPA, C-BIS) for a given R-BIS (=2C(BIS)/(C-NIPA + 2C(BIS))), i.e., (1) from a st retched exponential to a clear power-law behavior and (2) depression of the initial amplitude of ICF. According to the previous work [Norisuye et al. Macromolecules 1998, 31, 5316], this point was determined to be the gelatio n threshold, (C-NIPA(th), C-BIS(th)). The locus of(C-NIPA(th), C-BIS(th)) w as a decreasing function of C-NIPA(th) With the lower cutoff at C-NIPA(th) approximate to C-NIPA* and was in good agreement with the prediction of the site-bond percolation theory, where C-NIPA* is the so-called chain overlap concentration. Several characteristic features at this connectivity transi tion point, such as anomaly in the scattered intensity and the power-law ex ponent, are discussed as critical dynamics.