Reexamination of slow dynamics in semidilute solutions: From correlated concentration fluctuation to collective diffusion

Two different polymer systems, poly( N-isopropylacrylamide) (PNIPAM)/H2O and poly( ethylene oxide)-poly(propylene oxide)-poly( ethylene oxide) (PEO-PPO-PEO)/H2O, were examined by laser light scattering( LLS). In both cases, a single relaxation mode was observed in dilute solution which is related to the mutual diffusion of separated polymer chains. As the polymer concentration increases and enters the semidilute regime, one fast and one slow relaxation modes were observed. The fast mode corresponds to cooperative diffusion of chain segments inside each blob. Utilizing the thermal-sensitive properties of these two systems, we followed the disentanglement of transient network in the semidilute solutions through temperature-induced chain shrinkage, but without changing the overall concentrations. Meanwhile, we can follow the slow mode of the semidilute solution changes from long-range correlated concentration fluctuation of transient network to collective diffusion of aggregates or micelles. The present results clearly reveal that the slow modes in these two different systems have the same nature.