Non-solvent Induced Phase Inversion Research Articles

Kinetics of thermally induced phase separation in ternary polymer solutions. II. Comparison of theory and experiment

Light-scattering measurements and spinodal decomposition modeling have been used to quantify the kinetics of pore growth in thermally quenched polymer-solvent–nonsolvent [poly(methyl methacrylate) (PMMA)/1-methyl-2-pyrrolidinone (NMP)/glycerin] solutions. Solutions of fixed composition were quenched to a series of temperatures and light-scattering measurements and model calculations were performed to determine the temperature dependence of the pore growth rate. Both the experimental results and the model calculations show that the growth rate exhibits a maximum at an intermediate quench temperature that is related to an interplay between the thermodynamic and transport effects that govern pore growth. A similar growth-rate maximum is also observed when a series of solutions of varying nonsolvent composition are all quenched to the same temperature. The relevance of these experiments to the dynamics of pore growth and the eventual locking-in of the two-phase structure that forms during nonsolvent-induced phase inversion is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1461–1467, 1999

Kinetics of thermally induced phase separation in ternary polymer solutions. II. Comparison of theory and experiment

Light-scattering measurements and spinodal decomposition modeling have been used to quantify the kinetics of pore growth in thermally quenched polymer-solvent–nonsolvent [poly(methyl methacrylate) (PMMA)/1-methyl-2-pyrrolidinone (NMP)/glycerin] solutions. Solutions of fixed composition were quenched to a series of temperatures and light-scattering measurements and model calculations were performed to determine the temperature dependence of the pore growth rate. Both the experimental results and the model calculations show that the growth rate exhibits a maximum at an intermediate quench temperature that is related to an interplay between the thermodynamic and transport effects that govern pore growth. A similar growth-rate maximum is also observed when a series of solutions of varying nonsolvent composition are all quenched to the same temperature. The relevance of these experiments to the dynamics of pore growth and the eventual locking-in of the two-phase structure that forms during nonsolvent-induced phase inversion is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1461–1467, 1999

Kinetics of thermally induced phase separation in ternary polymer solutions. I. Modeling of phase separation dynamics

Simulations based on Cahn–Hilliard spinodal decomposition theory for phase separation in thermally quenched polymer/solvent/nonsolvent systems are presented. Two common membrane-forming systems are studied, cellulose acetate [CA]/acetone/water, and poly(ethersulfone) [PES]/dimethylsulfoxide [DMSO]/water. The effects of initial polymer and nonsolvent composition on the structure-formation dynamics are elucidated, and growth rates at specific points within the ternary phase diagram are quantified. Predicted pore growth rate curves exhibit a relative maximum with nonsolvent composition. For shallow quenches (lower nonsolvent content) near a phase boundary, the pore growth rate increases with increasing quench depth, whereas for deep quenches, where the composition of the polymer-rich phase approaches that of a glass, the pore growth rate decreases with increasing quench depth. With increasing initial polymer concentration, the overall rate of structure growth is lowered and the growth rate maximum shifts to higher nonsolvent compositions. This behavior appears to be a universal phenomenon in quenched polymer solutions which can undergo a glass transition, and is a result of an interplay between thermodynamic and kinetic driving forces. These results suggest a mechanism for the locking-in of the two-phase structure that occurs during nonsolvent-induced phase inversion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1449–1460, 1999

Kinetics of thermally induced phase separation in ternary polymer solutions. I. Modeling of phase separation dynamics

Simulations based on Cahn-Hilliard spinodal decomposition theory for phase separation in thermally quenched polymer/solvent/nonsolvent systems are pre- sented. Two common membrane-forming systems are studied, cellulose acetate (CA)/ acetone/water, and poly(ethersulfone) (PES)/dimethylsulfoxide (DMSO)/water. The ef- fects of initial polymer and nonsolvent composition on the structure-formation dynam- ics are elucidated, and growth rates at specific points within the ternary phase diagram are quantified. Predicted pore growth rate curves exhibit a relative maximum with nonsolvent composition. For shallow quenches (lower nonsolvent content) near a phase boundary, the pore growth rate increases with increasing quench depth, whereas for deep quenches, where the composition of the polymer-rich phase approaches that of a glass, the pore growth rate decreases with increasing quench depth. With increasing initial polymer concentration, the overall rate of structure growth is lowered and the growth rate maximum shifts to higher nonsolvent compositions. This behavior appears to be a universal phenomenon in quenched polymer solutions which can undergo a glass transition, and is a result of an interplay between thermodynamic and kinetic driving forces. These results suggest a mechanism for the locking-in of the two-phase structure that occurs during nonsolvent-induced phase inversion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1449 -1460, 1999

Observations on the dynamics of nonsolvent-induced phase inversion

The diffusion and gelation dynamics of nonsolvent-induced phase inversion in several polyethersulfone (PES)/solvent/nonsolvent systems are observed using a dark-ground optical technique. The observed dynamics are correlated with the resultant morphologies of the solidified gels obtained via scanning electron microscopy. In situ dynamic measurements show that rapid precipitations result in finger formation and delayed precipitations result in sponge formation. Rapid precipitations for some systems also exhibit an initial region of high, anomalous diffusion front motion which correlates well with the appearance of finger-like macrovoids in the film sublayer. Micrographs of both thin (200–300 μm) and thick (3 mm) films formed by liquid-liquid demixing clearly show that the overall morphologies scale with initial film thickness. However, as observed for the cellulose acetate (CA)/dimethylsulfoxide (DMSO)/H2O system, the possibility of crystallization can complicate the scaling analysis. A ternary diffusion model is also employed to describe the isothermal diffusion encountered during the formation of PES membranes. Binary thermodynamic and kinetic parameters needed for computations are determined from experimental data. Model results agree well with experimental observations. The model accurately predicts the transition from finger-to-sponge formation, as well as other observed trends in dynamics and morphology. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 569–585, 1997

The dynamics of diffusion and gel growth during nonsolvent-induced phase inversion of polyethersulfone

The in-situ, macroscopic dynamics of diffusion and gel growth during phase inversion of polyethersulfone (PES) solutions have been investigated using optical monitoring of the dark-ground and reflected light images which form on solution-nonsolvent contact. Data are reported for various ternary PES/solvent/nonsolvent systems. Analysis of two molecular weight systems shows that, under rapid quench conditions, a well-defined liquid demixing zone forms ahead of the gelation zone. Related to this, the occurrence of an initial region of anomalously high front motion followed by a more mild t 1 2 behavior suggests a dependence of the system diffusive transport properties on the relative time scales for gel formation and nonsolvent diffusion. The dependencies of the gel and diffusion front motions on solution concentration and nonsolvent solubility parameter are quantified in terms of the slope of the linear t 1 2 portion of the associated time-dependent curve. Rationales based on mass transfer models for both delayed and rapid precipitation systems support the concept of quantifying front motions in terms of an effective diffusion coefficient, D c, for either the homogeneous diffusion or gelation dynamics.