Miscibility, density and viscosity of polystyrene in n-hexane at high pressures

Abstract

The demixing pressures, densities and viscosities of solutions of nearly monodisperse polystyrene samples ( M w/ M n < 1.1) in n-hexane have been determined. Measurements were conducted with samples of different molecular weights ( M w = 4000–50 000) at concentrations up to 8 mass% over a pressure range from 5 to 70 MPa and a temperature range from 323 to 423 K. The demixing pressures were observed to increase with molecular weight (from about 10 MPa for M w = 4000 to 50 MPa for M w = 50 000) and show high sensitivity to temperature (Δ P/Δ T being about −1 MPa°C −1) in the range investigated. Density and viscosity data were collected in the one-phase homogeneous regions, and viscosity was correlated with an equation of the form η = Aexp{ B/(1 − V 0 ρ ), which is based on the free-volume considerations. For a given system, the viscosities increase with solution density (or pressure). At a fixed solution density, viscosity increases with polymer concentration or molecular weight. Depending upon the system, viscosities have ranged from 0.15 to 0.45 mPa s. Flow activation volumes and flow activation energies were about 40 cm 3 mol −1 and 6 kJ mol −1, respectively. The demixing and viscosity data have been also compared with the results obtained in n-butane and n-pentane for one sample ( M w = 9000). In going from n-butane to n-hexane, the demixing pressures decrease while the viscosities become higher.