Shear thickening behavior of dilute poly(diallyl dimethyl ammonium chloride) aqueous solutions

Using dynamic light scattering (DLS) and capillary dynamic viscoelasticity (DVE) analyzer, we investigated dilute (0.5 mg/ml) poly(diallyl dimethyl ammonium chloride) (PDADMAC) aqueous solution properties for three different molecular weights of PDADMACs mixed with various concentrations of NaCl. The dependence of PDADMAC molecular chain conformations in aqueous solutions on polymer molecular weight and NaCl concentration were studied. By analyzing dynamic shear viscosity eta'(omega), viscoelastic relaxation times t(r) and shear rate at tube wall r'(a)(omega) of PDADMAC aqueous solutions in oscillatory flows, we proposed that polymer chain conformations varied with increasing shear frequency omega via the following steps: intra-polymer associations, dissociation of intra-polymer associations, stretching of polymer chains, inter-polymer aggregations, and dissociations of inter-polymer aggregations. The intra-polymer associations lowered the n' exponent of storage modulus G'(omega) (G'(omega) similar to omega(n')) with n' < 2, and the polymer chain stretching and inter-polymer aggregations caused shear thickening (i.e. upturn of eta'(omega)) of PDADMAC aqueous solutions. The behaviors of' the lowering of n' exponent with n' < 2 and the shear thickening were favored by increasing ionic strength of solutions. By comparing eta'(omega) data with DLS hydrodynamic radii (R-h) data, we also confirmed the possibility of inter-polymer aggregations in dilute solutions when polymer chains were stretched in oscillatory flows. (c) 2007 Elsevier Ltd. All rights reserved.