High‐pressure viscosity and density of polyethylene solutions in n‐pentane

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Abstract
Viscosity and density of homogeneous one‐phase solutions (1 wt %) of polyethylenes with narrow molecular weight distributions (Mw = 2150, 15,520, 108,000, and 420,000) in n‐pentane were determined at 398, 413, and 428 K over a pressure range from 20 to 60 MPa. Measurements were done with a special falling cylinder type viscometer that permits simultaneous determination of viscosity, density, and phase state of the solutions. It is shown that the viscosities of these solutions can be correlated with density (p) using the exponential relationships μ − A exp {B/(1 − Voρ)} or μ = C1 exp (C2ρ), which are based on free‐volume considerations. Analysis of the temperature dependence of viscosity at fixed pressures, and its pressure dependence at fixed temperatures show that the flow‐activation energies for these solutions are in the range 8–12 kJ/mol, and the apparent activation volumes are in the range 30–45 cm3/mol. Evaluation of the specific viscosity and analysis of the molecular weight dependence of intrinsic viscosity in accordance with Mark–Houwink type relationship μ = KMa suggest a value of 0.5 for the exponent a, which is typical of poor or theta solvents. © 1995 John Wiley & Sons, Inc.