High pressure miscibility studies are conducted in variable-volume view-cells equipped with sapphire windows that incorporate a movable piston. While fully miscible, homogeneous polymer solutions are transparent, they become cloudy when demixing occurs. The miscibility and phase separation may thus be assessed by simple visual observations. More detailed assessments are made by measuring the transmitted light intensity. Phase separation may be carried out by lowering the pressure at a given temperature, or by increasing or decreasing the temperature (depending upon the system showing LCST, or UCST behavior) at a given pressure. By recording the change in transmitted light intensity along the P or T change, the progress of phase separation may be fully documented.
In our laboratory, we conduct such measurements using a motorized pressure generator to increase and decrease the pressure in a controlled manner while recording the change in transmitted light intensity in going from homogeneous to phase separated condition. During these measurements, we also continually record the position of the movable piston with the aid of a linear variable differential transformer (LVDT) and thus assess the change in the density as the solution goes from homogeneous (one-phase) to phase separated (two-phase) conditions.
A close examination of the change in density of the solution along the phase separation path shows that density decreases as the system pressure is decreased (which would be as expected). However, the rate of decrease of density shows a change after entering the two-phase domain – it becomes higher. The pressure at which this shift in density reduction with pressure takes place correlates well with the pressure where the transmitted light intensity also indicates phase separation.
The possibility of assessing the phase separation from continuous density measurements provides an alternative methodology that can be used to monitor phase separation in systems that may not be amenable for assessments by visual inspection or transmitted light measurements, which may present challenges in systems that may be colored, or light transmittance may be obstructed due to the presence of fine particles.
This new methodology will be demonstrated in selected polymer-solvent systems.