Phase behavior and rheology of miscible and immiscible blends of linear and hyperbranched siloxane macromolecules

It is obvious that polymers of the similar composition but with different molecular weights are miscible, but what will happen if one of them is linear and another one has branched architecture? In this paper, mixtures of linear polydimethylsiloxane (PDMS) with methyl-terminated silicone MT resins are considered. Since these resins consist of hyperbranched macromolecules with nearly spherical shape and nanoscale diameters, they are both macromolecules and colloid particles at the same time. The phase states of mixtures were studied by laser interferometry method accompanied with rheological tests. The effect of molecular weight of resins and their terminal groups on miscibility was demonstrated. The methyl-terminated resins with low molecular weight are well miscible with linear PDMS macromolecules but immiscible blends are formed in the case of resin's high molecular weight. The rheology of the blends is determined by their phase state, which depends on the ratio of components and temperature. For homogeneous resin/PDMS mixtures, a slight positive deviation of viscosity from the log-additivity rule is inherent, whereas heterogeneous blends are similar in rheological behavior to a gum. Various empirical equations are considered for describing the viscosity-composition dependence; as a result, their unification to characterize blends with different phase states was proposed.