Mechanism of Shear Thickening in Reversibly Cross-linked Supramolecular Polymer Networks.
We report here the nonlinear rheological properties of metallo-supramolecular networks formed by the reversible cross-linking of semi-dilute unentangled solutions of poly(4-vinylpyridine) (PVP) in dimethyl sulfoxide (DMSO). The reversible cross-linkers are bis-Pd(II) or bis-Pt(II) complexes that coordinate to the pyridine functional groups on the PVP. Under steady shear, shear thickening is observed above a critical shear rate, and that critical shear rate is experimentally correlated with the lifetime of the metal-ligand bond. The onset and magnitude of the shear thickening depend on the amount of cross-linkers added. In contrast to the behavior observed in most transient networks, the time scale of network relaxation is found to increase during shear thickening. The primary mechanism of shear thickening is ascribed to the shear-induced transformation of intrachain cross-linking to interchain cross-linking, rather than nonlinear high tension along polymer chains that are stretched beyond the Gaussian range.
Xu
Donghua
D
Department of Chemistry and Center for Biologically Inspired Materials and Material Systems, Duke University, Durham, NC, 27708-0346, USA.
Hawk
Jennifer L
JL
Loveless
David M
DM
Jeon
Sung Lan
SL
Craig
Stephen L
SL