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 degreesC (Theta) similar to 50 degreesC; concentration = 59 kg m(-3); molecular weight range 20020 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 Theta 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 Theta 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 Theta 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 Theta solutions may retard the entanglement dynamics.