Time-temperature-pressure-concentration superposition and viscoelastic behavior of PS-CO2 mixtures at supercritical conditions

Abstract

A study of polystyrene resins containing supercritical CO2 was conducted to understand the thermodynamics of mixing to predict relevant viscoelastic properties for foaming applications. Three polystyrene resins with different molecular weight distribution were tested at three temperatures (170, 185 and 200 °C) and two pressures (6.89 MPa and 13.78 MPa) using a parallel plate rheometer equipped with a pressure cell. Effects of temperature, pressure and CO2 concentration on viscoelastic properties were evaluated and isolated according to a time-temperature-pressure-concentration superposition (TTCS) to describe the thermodynamic path of mixing. Finally, elongational viscosities of the mixtures were estimated using the Phan-Thien-Tanner model to speculate about the polymer's foaming behavior at different operating conditions. The analysis proved that while the viscosities of the resins are very similar at atmospheric pressure, the differences between resins' viscosities were stronger at high temperatures and pressures.