Thermodynamic analysis of polymeric membrane formation by non-solvent induced phase separation in the presence of different nanoparticles

Abstract

Thermodynamic phase diagrams are powerful tools for predicting the thermodynamic behavior and finally, the morphology of phase inversion membranes. Addition of nanoparticles into a polymeric membrane forming system significantly affects the system phase diagram. Using a lattice-based thermodynamic model, we study the phase diagram of polymeric membrane formation by non-solvent induced phase separation (NIPS) in the presence of three types of nanoparticles (spherical, rod-like or sheet-like). The concept of “nanoparticle-out effect” is introduced to justify the influence of nanoparticles on the binodal curves of the systems. Due to this effect, nanoparticles attract the solvent molecules and reduce the polymer solubility, thereby increasing the thermodynamic instability of the system and moving the binodal curves toward the polymer–solvent axis. This effect is found to be more pronounced for nanorods and nanosheets than for nanospheres. The effect is intensified with increasing the nanoparticles size faster for anisotropic nanosheet and nanorod than for isotropic nanospheres, which results from their different surface fraction values. The system thermodynamic instability gradually rises with decreasing the binary interaction parameters between nanorods or nanosheets and other components while it is nearly unchanged by varying the interaction parameters for nanospheres, which is consistent with their smaller surface fraction value. The accuracy of the phase diagram predictions is demonstrated by comparison with published experimental data on related systems. This thermodynamic description along with the governing kinetic features of membrane forming systems containing nanoparticles could be applied for fabricating polymeric membranes with the desired morphology and function.