Dynamic responses in nanocomposite hydrogels
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The shear response of a series of polymer-clay gels has been investigated by means of rheology and small-angle neutron scattering (SANS). The gels have the same composition by mass but different polymer molecular weights (M w). While long polymer chains can interconnect several platelets, which act as multifunctional cross-links, very short polymer chains should not be able to do so, allowing us to explore the effects of bridging on structure and dynamical responses. Increasing the polymer M w in the gels leads to increasingly strong anisotropy in the SANS data, indicating a larger and larger degree of shear orientation. This relative increase in orientation, however, is accompanied by a relatively lower amount of shear thinning. Simple solutions of anisotropic particles usually shear thin by alignment of the particles with the flow, allowing them for example to slide past each other more easily. In a connected gel, breaking of the connectivity should also lead to shear thinning. Thus, the inverse relationship between shear orientation and shear thinning, combined with the fact that the lowest M w, which exhibits the highest shear thinning, does not orient at all, supports our earlier hypothesis that the alignment mechanism in these systems stems from the coupling between the clay and the polymer mediated by the shear flow and nicely demonstrates the effect of bridging on the strength and dynamics of these gels. © 2006 American Chemical Society.