Viscosity and viscoelasticity of liquid nanoparticles with polymeric matrix

Abstract

The subject of this study is polymethylsilsesquioxane nanoparticles of different sizes and molecular weights (MW). Unlike numerous solid nanoparticles, these objects form liquids. They could be considered as an intermediate structure between macromolecules and colloidal particles. Their structure can be described as a solid core surrounded by a soft cover. The flow happens by relaxation mechanism and the high viscosity is not due to entanglements (as it takes place in linear polymers) but due to interparticle friction. The study of these liquids revealed several rather special rheological phenomena. The characteristic size of these nanoparticles is proportional to their MW. It means that the looseness of these nano-objects increases along with their MW. The dependence of viscosity on MW is unusually strong in comparison to all known cases and can be described by a power law with an exponent of the order of 8.5. In opposite to the known Cox-Merz rule having the universal value for flexible polymers, there is no correlation between the dynamic viscosity in oscillations and the Newtonian viscosity in steady flows. These evidences point to deep differences in the mechanisms of flow and viscoelasticity between nanoliquids and polymer melts. Temperature dependence of viscosity is described by the equation typical for amorphous liquids and at some critical temperature, the transition to a glassy state (or gelation) of these nanoliquids takes place. The transition temperature depends on MW and the viscosity is determined by remoteness from the transition temperature. The flow of nanoliquids is Newtonian though they demonstrate viscoelastic behavior with a rather wide relaxation spectrum. In general, the rheology of the nanoparticles under study is close to the behavior of the so-called Boger liquids without any nonlinear phenomena. Interpretation of the rheological behavior of nanoparticles/polymer melt mixtures is based on the concept of dualism of nanoparticles which are simultaneously liquids and colloidal objects. Depending on the composition, mixtures can be homogeneous or two-phase systems while phase separation at a high shear rate can be a deformation-induced phenomenon.