Divergent Shear Thinning and Shear Thickening Behavior of Supramolecular Polymer Networks in Semidilute Entangled Polymer Solutions.

The steady shear behavior of metallo-supramolecular polymer networks formed by bis-Pd(II) cross-linkers and semidilute entangled solutions of poly(4-vinylpyridine) (PVP) in dimethyl sulfoxide (DMSO) or N,N-dimethyl formamide (DMF) is reported. The steady shear behavior of the networks depends on the dissociation rate and association rate of the cross-linkers, the concentration of cross-linkers, and the concentration of the polymer solution. The divergent steady shear behavior-shear thinning versus shear thickening-of samples with identical structure but different cross-linker dynamics (J. Phys. Chem. Lett. 2010, 1, 1683-1686) is further explored in this paper. The divergent steady shear behavior for networks with different cross-linkers is connected to a competition between different time scales: the average time that a cross-linker remains open (τ(1)) and the local relaxation time of a segment of polymer chain (τ(segment)). When τ(1) is larger than τ(segment), shear thickening is observed. When τ(1) is smaller than τ(segment), only shear thinning is observed.