Interaction parameters and miscibility limits of poly(dimethylsiloxane) and dinonyl phthalate or squalane determined from studies of the absorption of hexane by the binary liquid mixtures

Abstract

Hexane has been used as a probe molecule to determine the interactions in liquid mixtures consisting of poly(dimethylsiloxane)(PDMS) and a large monomeric molecule, dinonyl phthalate (DNP) or squalane (SQ), by measuring the absorption of hexane at 303 K into mixtures of the involatile liquids on a vacuum microbalance. Five samples of PDMS of number-average molecule weights varying from 3350 to 89000 have been studied. The measurements for the ternary liquid mixtures formed have been analysed and shown to give consistent interaction parameters between the involatile components. These are used to predict miscibility limits to within on average 3% for the DNP + PDMS systems and 4% for the SQ + PDMS systems. In general, Flory–Huggins theory based on volume fractions gave a better fit to the experimental results and a better estimation of the cloud points of the partially miscible mixtures than that using segment fractions as in the later Flory theory.

In predicting the miscibility limits it was assumed that the polymer concentration in the polymer-dilute phase was zero, so that a tangent could be drawn to the free-energy curve for the system through the origin to find the composition of the polymer-rich phase. The approach is shown to be valid for the systems studied.