Absolute Viscosity and Density of Trisubstituted Phosphoric Esters

This paper presents measurements on the absolute viscosity (η) and density (ρ) of trisubstituted phosphoric esters which are useful in understanding their flow mechanism necessary for accessing their role as plasticizers. The effect of chain length and branching has been examined on the η and ρ trends. From η data, by using the Vogel−Tammann−Fulchur (VTF) equation, the VTF temperature ( T o ) has been obtained which also represents the ideal glass transition temperature. T o is related to the flexibility of the molecules. It is observed that T o initially decreases with molecular weight, reaches a minimum, and increases thereafter. The initial decrease in T o has been attributed to the enhanced flexibility of the phosphate esters. Reversal of flexibility with relative molar mass beyond 400 is due to the gentle collision of the arms of the trisubstituted phosphoric esters. This has been further corroborated from the molar mass exponent as exhibited in the η−molar mass plot. The isomeric effect on η has also been investigated in tricresyl phosphates, hitherto for the first time. The ortho isomer has highest η among the isomers. The para isomer was found to have lowest T o and hence highest flexibility compared to the ortho and meta isomers.