AbstractThe viscosities obtained for the copolymer blends of the previous paper were correlated with several relations derived to describe more fundamental behavior of polymer–diluent mixtures at both infinite dilution and finite concentrations. Only the most efficient blends showed any appreciable expansion of hydrodynamic volume as temperature increased from 25° to 98.9°C. However, in spite of restricted coil expansion, all of the copolymers were effective viscosity index improvers. The mechanism of viscosity index improvement in multigrade oils was shown to be largely regulated by the translational friction generated by the polymer coils. This greatly increased the apparent negative entropy change of the blends; the enthalpy change characteristic of the base oil was retained. Efficiency resulted from coil contraction at low temperatures, but enthalpy decrease below that of the base oil was small. In contrast, viscosity index improvement using higher molecular weight solvents was accompanied by large enthalpy increases. Thus, undesirably high viscosities resulted at low temperatures. The structure of these blends was uncomplicated by polymer chain entanglements; unit values of the Fox‐Flory exponent were obtained for the relation between viscosity and weight‐average carbon backbone length. The lack of evidence for coil compression in the thermodynamically miscible blends above a critical reduced concentration was anomalous. Intermingling of side chains and their interaction may have overcome normal excluded volume
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