NMR Investigations of Self-Diffusion and Shear Thinning for Semidilute Polymer Solutions near the Demixing Transition

Pulsed gradient spin echo and rheo-NMR methods are used to investigate semidilute solution properties of high molecular mass polystyrene in cyclohexane, in the vicinity of a demixing transition. The dependence of polymer self-diffusion on temperature and molar mass is measured using PGSE NMR. The molar mass variation is consistent with M -2 scaling while the temperature variation in approach to demixing can be modeled as a glass transition process using the WLF equation. The measured values of the demixing transition temperature T p are consistent with known thermodynamic properties. By using NMR velocimetry, we are able to measure the velocity profiles for the polystyrene solutions when sheared in a cylindrical Couette cell. These profiles, which are consistent with strong shear thinning, are fitted using a power law fluid model, and entanglement renewal times obtained from the power law exponents are consistent with and complementary to tube disengagement times obtained from self-diffusion measurements, this family exhibiting M 3 scaling.