Re-examination of the slow mode in semidilute solutions

Dynamics of semidilute solutions has been well described by two different motions, respectively, related to the ``blobs'' and the reptation of an entire chain. In the past, dynamic LLS results revealed that besides the fast relaxation related to the ``blobs,'' there existed an additional slow relaxation. In the earlier time, such a slow mode was wrongly identified as the reptation. Later, this slow mode was attributed to possible problems in the sample preparation, such as dust particles or a concentration gradient. Whether this slow mode is real has remained a challenging problem since 80's. Recently, we found that it appears only when the solvent quality is less good. To avoid problems in the sample preparation, we, respectively, used the coil-to-globule transition of long polystyrene chains, the high-vacuum anionic polymerization of styrene in cyclohexane, and the living bulk polymerization of MMA to alternate the size of polymer chains (i.e., the overlap concentration) to induce an \textit{in-situ} dilute-semidilute transition. Our results confirm that this slow mode is real with no ambiguity. In theory, we can demonstrate that this slow mode appears whenever segments (monomers) in different ``blobs'' start to interact with each other and its characteristic relaxation time is related to the correlation length of these interacting ``blobs.''