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

Linear polymers with the same monomer units and differentmolecularweights form miscible blends considered to be single polymers, butthe situation changes if one is hyperbranched. Hyperbranched macromoleculescan significantly change the rheology of a linear polymer due to adifferent flow mechanism and can even lead to the formation of immiscibleor partially miscible blends with unusual mechanical properties dueto 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 weightsin a concentration range from 20 to 80% have been studied in thiswork. Laser interferometry of the diffusion zone between H-PDMS andL-PDMS showed their limited miscibility that is improving with decreasingmolecular weight of the linear polymer, leading to a decrease in theupper critical solution temperature. The solubility of H-PDMS in L-PDMSis much higher than that of L-PDMS in H-PDMS (20-50% versus1-2% at 25 & DEG;C, depending on the molecular weight of L-PDMS),and H-PDMS emulsifies in L-PDMS but not vice versa. As a result, thepartially miscible blends are emulsions of almost pure H-PDMS in asaturated H-PDMS/L-PDMS solution, even at an H-PDMS concentrationas high as 80%. The thermorheological behavior of blends was describedusing an averaged combination of the Arrhenius and Williams-Landel-Ferryequations, allowing for overcoming the large difference in the glasstransition temperatures of hyperbranched and linear macromolecules.The viscosity of blends with a higher-molecular-weight L-PDMS hasa positive deviation from the log-additivity rule due to macromolecularentanglements, while a decrease in the molecular weight of L-PDMScauses an abnormal drop in viscosities below those of pure polymers.The anomalous decrease in viscosity appears only in a specific temperaturerange and is explained by mixing a polymer with a high glass transitiontemperature and high flow activation energy (H-PDMS) with one havinglower values of these characteristics (L-PDMS).