Viscoelasticity of Polymers in ϑ Solvents around the Semidilute Regime

The complex shear modulus, G *, was studied for polystyrene solutions in dioctyl phthalate (DOP); temperature = 22 °C (ϑ) ∼ 50 °C; concentration = 59 kg m -3; molecular weight range 200−20 000 kg mol -1 . The data were compared with those for PS in a good solvent, tricresyl phosphate (TCP). The solvent viscosity of the two neat solvents was found to be the same within experimental accuracy in the range of studied temperatures. For high- M samples, the rubbery plateau modulus of the two systems, in ϑ and good solvents, agrees with each other. This result is inconsistent with the prediction of the original scaling theory, which predicts a lower rubber plateau modulus in a ϑ solvent, but agrees well with the two-parameter scaling theory by Colby and Rubinstein [ Macromolecules 1990, 23, 2753]. The location of the terminal flow region was almost the same for low- M samples, M < M e . However, with increasing M, the terminal flow region in DOP was located at lower frequencies than in TCP. These results indicate that G * in ϑ and good solvents cannot be described with a universal function of N e = M / M e, as is usually assumed in scaling theories. We speculate that larger concentration fluctuation in ϑ solutions may retard the entanglement dynamics.