A slow relaxation mode of polymer chains in a semidilute solution

Dust- free semidilute and concentrated polystyrene (PS) solutions in different solvents were prepared by slow evaporation and in situ anionic polymerization, which removes the effects of troublesome artifacts such as dust contamination and concentration gradient. The dynamics was reexamined by a combination of static and dynamic laser light scattering. In benzene and toluene (good solvents for PS), only one fast diffusive mode of polymer chains can be observed when the concentration (c) is up to 20%, which is attributed to thermally agitated fluctuation of blobs or chain segments. Static and dynamic correlation lengths (xi(s) and xi(D)) are scaled with c as xi(S) (or xi(D)) similar to c(-0.72 +/- 0.02). In cyclohexane, whose quality decreases with temperature in the range 32-50 degrees C, an additional slow mode of polymer chains can be observed. Such a slow mode is viewed more obviously at a large scattering angle even in a concentrated solution with high chain entanglement. The present study indicates that the slow mode is due to the solvent quality.