Shear-Induced Network Formation in Colloid/Polymer Mixtures: A Molecular Dynamics Study

It is well-known that rheological properties of latex dispersions are drastically changed by the addition of small amounts of polymers with associative interactions. This kind of technique has been widely used in application areas, especially in paints, coatings, and cosmetics. The associative polymers are also known as associative thickeners and rheology modifiers. In most cases, since large colloids with micrometer-size diameter strongly associate with polymers at quiescent states, the time-scale of the association and dissociation processes is quite long compared to the typical experimental time scale. Therefore, it is diffcult to observe the Newtonian regime in such systems. Recently, Otsubo and his collaborators studied rheological properties of mixtures of silica nanoparticles and poly(ethylene oxide) (PEO), and showed that the nonlinear steady shear viscosity exhibits Newtonian behavior in the small shear-rate region, and drastically increases with increasing shear rate, where the value of the viscosity is one order of magnitude larger than that in the Newtonian regime. This strong thickening is considered to be attributed to the shear-induced gelation. In the case of mixtures of telechelic associating polymers and colloids, one of the authors reported the study on rheological behavior by using a molecular dynamics (MD) simulation method. In the above-mentioned case, however, PEO chains have many hydrogen bonding sites along the chain, which associate with colloid particles. To understand the molecular mechanism of the rheological behavior of such nanocolloid/polymer mixtures, it is necessary to study the system of associating polymers with many associative groups along the chain. Therefore, in the present study, we perform MD simulations of mixtures of associating polymers with many associative groups and colloid particles.