Origin of the low-frequency plateau and the light-scattering slow mode in semidilute poly(ethylene glycol) solutions

A low-frequency plateau is often found in the rheological spectra of various kinds of semidilute solutions of polymers and other colloids; also, many such solutions have been reported to show slow-modes in their dynamic light scattering autocorrelation functions. Both these observations may lead to the hypothesis of weak associative network structures built by the dissolved polymer chains or colloidal building blocks. To challenge this hypothesis, we conduct a series of comparative studies on semidilute solutions of poly(ethylene glycol) by using classical rheology as well as passive microrheology based on dynamic light scattering, along with structural studies using static light scattering. Although we indeed find a low-frequency plateau using classical shear rheology, even at elevated temperatures where potential polymer aggregates should be broken, no such plateau is observed in any of our microrheology experiments. Also, dynamic and static light scattering studies on the polymer solutions do not confirm the presence of larger structural entities: no slow mode can be detected in the autocorrelation function of the scattering intensity signal, and this signal is angle independent if the samples are purified by a thorough procedure of filtration. Based on these findings, we conclude that the low-frequency plateau in classical rheology results is an instrument effect caused by erroneous recording of the phase angle, although the magnitude of the torque lies well within the resolution of the rheometer. We also conclude that slow modes in dynamic light scattering on solutions of poly(ethylene glycol) are impurity-based artifacts rather than due to actual associated structures.