Effects of non-adsorbing polymer molecular weight on the rheology and microstructure of dense suspensions

Abstract

Dense suspensions with high solid volume fractions (\(\phi \ge 0.5)\) are prevalent in nature and throughout industry. These highly loaded suspensions can exhibit complex rheological behaviors, including both shear thinning and subsequent thickening with increasing shear rate. Understanding the mechanisms behind these rheological behaviors can improve the design of materials systems to behave as predicted and desired under process flows. Here, we present a study on an industrially relevant, dense (\(\phi =0.55)\), colloidal alumina suspension and show how the addition and loading of a non-adsorbing polyvinylpyrrolidone (PVP) at different molecular weights can be used to tune and control its rheological properties. PVP was added at varying concentrations spanning the dilute and semi-dilute non-entangled regimes for each molecular weight. The addition of PVP at concentrations in the dilute regime was shown to increase the viscosity of the suspension and induce discontinuous shear thickening (DST). However, further increases in PVP loading particularly within the semi-dilute non-entangled regime and at higher PVP molecular weights also increased the dynamic yield stress of the material, made the suspension more shear thinning, and delayed the onset of DST. Rheological measurements were coupled with insights on the relevant particle and polymer length scales from small angle neutron scattering (SANS), including Rheo-SANS measurements, to inform on the mechanisms by which non-adsorbing polymers influence suspension rheology across multiple flow regimes.

Graphical abstract