Abstract
A coarse-grained model for solutions of polymers in supercritical fluids is introduced andapplied to the system of hexadecane and carbon dioxide as a representative example.Fitting parameters of the model to the gas–liquid critical point properties of the puresystems, and allowing for a suitably chosen parameter that describes the deviation from theLorentz–Berthelot mixing rule, we model the liquid–gas and fluid–fluid unmixingtransitions of this system over a wide range of temperatures and pressures inreasonable agreement with experiment. Interfaces between the polymer-rich phaseand the gas can be studied at temperatures both above and below the end pointof the triple line where liquid and vapour carbon dioxide and a polymer-richphase coexist. In the first case interfacial adsorption of fluid carbon dioxide canbe demonstrated. Our model can also be used to simulate quenches from theone-phase to the two-phase region. A short animation and a series of snapshots helpto visualize the early stages of bubble nucleation and spinodal decomposition.Furthermore we discuss deviations from classical nucleation theory for small nuclei.
Highlights
The phase behavior of polymer-solvent systems has important application in the industry for the production and processing of many kinds of plastic materials [1, 2, 3, 4, 5, 6]
Interfaces between the polymer-rich phase and the gas can be studied both at temperatures above and below the end point of the triple line where liquid and vapor carbon dioxide and the polymer rich phase coexist
While for an incompressible polymer solution the early stages of phase separation can only be studied if one realizes a rapid temperature quench from a state in the one phase region into the miscibility gap
Summary
The phase behavior of polymer-solvent systems has important application in the industry for the production and processing of many kinds of plastic materials [1, 2, 3, 4, 5, 6]. An example is the formation of solid polystyrene foams [3, 4, 5, 6] Both the understanding of the equilibrium phase diagram of these systems and the kinetic mechanism of phase separation are challenging problems of statistical mechanics. The situation is more complicated, since one must treat both the densities of CO2 and hexadecane as two coupled order parameters, which both play a crucial role in the possible phase transitions (remember that for the pure solvent at its critical point gas-liquid phase separation starts). It can happen that both gas-liquid and liquid-liquid phase separation are present in the phase diagram, leading to the occurrence of a triple line (Fig. 2) [11].
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