Reexamination of the slow mode in semidilute polymer solutions: The effect of solvent quality

Dust-free semidilute and concentrated polystyrene (PS) solutions in two solvents were prepared by conventional slow evaporation and in situ living anionic polymerization that removes previously troublesome artifacts, such as dust contamination and a concentration gradient. Their dynamics was reexamined by a combination of static and dynamic laser light scattering. As expected, only one fast diffusive mode was observed in benzene (an athermal solvent for PS) up to 20% concentration (C), attributed to thermally agitated fluctuation of chain segments (blobs) confined in a noninteracting tube made of its surrounding chains. In cyclohexane, whose quality as a solvent for PS decreases with the temperature in the range 32-50 degrees C, we observed an additional slow mode. The slow mode is enhanced as the solvent quality decreases. The scaling exponent (as) between its related line-width (F,10,) and the scattering vector (q), i.e.,Gamma(slow) similar to q(alpha s), decreases from 3 to 0 as C increases, which suggests that the slow mode has different natures, depending on C. When C is only a few times larger than the overlap concentration (C*) but lower than the entanglement concentration (C-e i.e., not all the chains are entangled together, the slow mode is related to transient interchain segment-segment interaction-induced clusters. When CIC, > I it is attributed to the confinement of each chain inside an inhomogeneous tube with a handlike structure due to relatively stronger segment-segment interaction near the entanglement points.