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

Abstract

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).

Self-Lubricating and Shape-Stable Phase-Change Materials Based on Epoxy Resin and Vegetable Oils

“…The miscibility of epoxy resin with vegetable oils was evaluated by laser interferometry according to the method described in detail earlier [58,59]. The diffusion cell with samples was subjected to stepwise heating from 20 • C to 110 • C followed by slow cooling to 30 • C. Higher temperatures were not used since the experiment was not workable because of the strong interface curvature caused by the stirring of the resin and the oil due to their low viscosities.…”

Asphaltenes from Heavy Crude Oil as Ultraviolet Stabilizers against Polypropylene Aging

“…Meanwhile, films containing asphaltenes do not become brittle after UV exposure, which may indicate the anti-aging influence of asphaltenes and less damage of macromolecular chains in their presence. The rupture of macromolecules with the formation of reactive free radicals should substantially affect their rheological properties, which are extremely sensitive to the molecular weight, three-dimensional crosslinks, or branching of macromolecules [92][93][94]. Let us consider the rheology of the polypropylene melt containing asphaltenes before and after UV aging.…”