(278a) Miscibility and Dynamics of Pressure-Induced Phase Separation in Polymers Solutions: Poly(epsilon-caprolactone) + Acetone + CO2

We have recently developed a new experimental system which permits determination of the volumetric properties, phase conditions, and also the dynamics of phase separation in terms of phase settlement times and mechanism being Nucleation and Growth of Spinodal Decomposition [1]. The system is a unique high pressure view-cell (Figure 1) equipped with a dual set of pistons and dual set of sapphire windows. One set of the windows separated by 25.4 mm allows the assessment of the phase state and is used to monitor the transmitted light intensities. The second set of windows separated by 50 µm is used to monitor the scattered light intensities over a wide range of scattering vector q (from 0- 4 µm^-1) which allows the assessment of the mechanism of phase separation. The dual set of pistons that are employed which are synchronized and actuated by motorized pressure generators creates a churn-like action in the cell insuring effective mixing, even at high polymer concentrations by translating the cell content across a magnetically-coupled rotating mixer impeller. The piston actions assure also that the solution is effectively introduced into the narrow gap between the scattering windows, and refreshed. Piston positions are determined with real-time positions sensors that help determine the internal volume at any given temperature and pressure, hence permit assessment of density and the volumetric  (P, ρ, T) properties of the solutions.

Using this new system, we have investigated the miscibility and the kinetics of pressure-induced phase separation in solutions of poly(e-caprolactone) (PCL) in acetone + CO₂ binary fluid mixtures has been studied Investigations have been carried out for a wide range of polymer concentrations, from 2.0 to 35 wt %, while holding the acetone-to-CO₂ (wt:wt) ratio in each solution at a constant value of 2:1. The results show that these solutions all display LCST type behavior. In the concentration range from 9-15 wt %, phase separation after crossing the phase boundary, proceeds by spinodal decomposition which is characterized by the formation and evolution of the spinodal ring patterns corresponding to a maximum in the angular variation of the scattered light intensities. At concentrations below 9 % and above 15 %, the solutions were observed to undergo phase separation via nucleation and growth mechanism which is characterized by circular symmetric patterns in their light scattering patterns.  In the nucleation and growth regime, Debye-Bueche type scattering function was used to analyze the domain size of the new phase that forms and develops after a pressure quench.  The early stage of the spinodal decomposition was captured with the wide angular range that the present system permits, and its time evolutions was found to be describable by the linearized Cahn theory. The variation of the scattered light intensity maximum I_m and its location in scattering vectors q_m with time in the later stage of the spinodal decomposition was found to obey power-law scaling according to I_m ~ t^b and q_m ~ t^-a. The results for the 9.0 and 12.0 wt % solutions show that b/a changes its value from b/a > 3 to b/a≈ 3 with time, indicating the progression of the spinodal decomposition from intermediate to late stage.

[1] E. Kiran, J. C. Hassler and R. Srivastava, Miscibility, phase separation and phase settlement dynamics in solutions of EPDM in propane + n-octane binary fluid mixtures at high pressures, Industrial & Engineering Chemistry-Research, 2013, 52, 1806-1818.