Free volume dependence of polymer viscosity

The Simha\u2013Somcynsky (S\u2013S) equation of state (eos) was used to compute the free volume parameter, h, from the pressure\u2013volume\u2013temperature (PVT) dependencies of eight molten polymers. The predicted by eos variation of h with T and P was confirmed by the positron annihilation lifetime spectroscopy; good agreement was found for h(P= constant)=h(T) as well as for h(T=constant)=h(P). Capillary shear viscosity (\u3b7) data of the same polymers (measured at three temperatures and six pressures up to 700 bars), were plotted as log\u3b7 vs 1/h, the latter computed for T and P at which \u3b7 was measured. In previous works, such a plot for solvents and silicone oils resulted in a \u201cmaster curve\u201d for the liquid, in a wide range of T and P. However, for molten polymers, no superposition of data onto a \u201cmaster curve\u201d could be found. The superposition could be obtained allowing the characteristic pressure reducing parameter, P*, to vary. The necessity for using a \u201crheological\u201d characteristic pressure reducing parameter, P*R=\u3baP*, with \u3ba=1 to 2.1 indicates that the free volume parameter extracted from the thermodynamic equilibrium data may not fully describe the dynamic behavior. After eliminating possibility of other sources for the deviation, the most likely culprit seems to be the presence of structures in polymer melts at temperatures above the glass transition, Tg. For example, it was observed that for amorphous polymers at T\u22451.52Tg the factor \u3ba=1, and the deviation vanish.Peer reviewed: YesNRC publication: Ye