On the use of pressure‐volume‐temperature data of polyethylene liquids for the determination of their solubility and interaction parameters

Abstract

AbstractSpecific volumes of high‐density and low‐density polyethylene liquids at several elevated temperatures and pressures were measured. The measured specific volumes were then used to estimate the thermal expansion coefficients \documentclass{article}\pagestyle{empty}\begin{document}$\left( {{\rm \alpha = }\frac{{\rm 1}}{v}\left( {\frac{{\partial v}}{{\partial T}}} \right)_P } \right)$\end{document} and isothermal compressibility \documentclass{article}\pagestyle{empty}\begin{document}$\left( {{\rm \beta = } - \frac{{\rm 1}}{v}\left( {\frac{{\partial v}}{{\partial P}}} \right)_T } \right)$\end{document} of the polymers. Two different approaches were used in which one was simply to fit the raw data by second order polynomials to obtain (∂v/∂T)P and (∂v/∂P)T, while the other by the Sanchez‐Lacombe (S‐L) equation of state. It was found that the resultant α and β obtained from the above methods differ significantly, indicating that the S‐L equation of state may not be suitable for determining α and β at elevated temperatures. When these two sets of α and β were used to calculate the corresponding solubility parameters and then the Flory‐Huggins interaction parameters (χ) of the polymers, the results also differ considerably. Nonetheless, χ obtained from the first method agrees well with the results obtained from small angle neutron scattering measurements while the S‐L equation of state method does not. The current results suggest that solubility and interaction parameters obtained from pressure‐volume‐temperature experiments depend critically on the manner by which the data analysis is performed. Polym. Eng. Sci. 44:853–860, 2004. © 2004 Society of Plastics Engineers.