Rheology and Miscibility of Linear/Hyperbranched Polydimethylsiloxane Blends and an Abnormal Decrease in Their Viscosity

Linear polymers with the same monomer units and different molecular weights form miscible blends considered to be single polymers, but the situation changes if one is hyperbranched. Hyperbranched macromolecules can significantly change the rheology of a linear polymer due to a different flow mechanism and can even lead to the formation of immiscible or partially miscible blends with unusual mechanical properties due to the emulsion state and low droplet–matrix interfacial tension. Blends of hyperbranched polydimethylsiloxane (silicone MT resin, H-PDMS) with similar linear polymers (L-PDMS) of different molecular weights in a concentration range from 20 to 80% have been studied in this work. Laser interferometry of the diffusion zone between H-PDMS and L-PDMS showed their limited miscibility that is improving with decreasing molecular weight of the linear polymer, leading to a decrease in the upper critical solution temperature. The solubility of H-PDMS in L-PDMS is much higher than that of L-PDMS in H-PDMS (20–50% versus 1–2% at 25 °C, depending on the molecular weight of L-PDMS), and H-PDMS emulsifies in L-PDMS but not vice versa. As a result, the partially miscible blends are emulsions of almost pure H-PDMS in a saturated H-PDMS/L-PDMS solution, even at an H-PDMS concentration as high as 80%. The thermorheological behavior of blends was described using an averaged combination of the Arrhenius and Williams–Landel–Ferry equations, allowing for overcoming the large difference in the glass transition temperatures of hyperbranched and linear macromolecules. The viscosity of blends with a higher-molecular-weight L-PDMS has a positive deviation from the log-additivity rule due to macromolecular entanglements, while a decrease in the molecular weight of L-PDMS causes an abnormal drop in viscosities below those of pure polymers. The anomalous decrease in viscosity appears only in a specific temperature range and is explained by mixing a polymer with a high glass transition temperature and high flow activation energy (H-PDMS) with one having lower values of these characteristics (L-PDMS).