Original Investigation of the Thermodynamics and Kinetics of Phase Separation in Polymer Solution Systems Relevant to Industrial Membrane Fabrication in Nigeria

Phase separation by non-solvent induced phase separation is the dominant industrial process for fabricating asymmetric polymer membranes for water treatment and gas separation, yet the thermodynamics and kinetics of phase separation in commercially relevant polymer-solvent-non-solvent ternary systems have not been thoroughly characterised for industrial membrane production conditions in Nigeria. This research conducts an original investigation of phase separation thermodynamics and kinetics in polysulfone, polyethersulfone, and polyacrylonitrile membrane-forming systems, developing predictive frameworks applicable to membrane manufacturing process design. Ternary phase diagrams were determined experimentally by cloud point titration across a temperature range of 20 to 60 degrees Celsius, and Flory-Huggins interaction parameters were determined to enable thermodynamic modelling of phase boundary positions. Kinetics of phase separation were investigated by time-resolved light scattering and optical microscopy to characterise spinodal versus nucleation and growth phase separation mechanisms as a function of solution composition. Membrane morphology was characterised by scanning electron microscopy and mercury intrusion porosimetry. A process model integrating thermodynamics, mass transfer, and hydrodynamics of the coagulation bath was developed and applied to predict membrane morphology as a function of casting conditions. Experimental validation used a membrane casting facility established at a water treatment equipment manufacturer in Lagos State, South West Nigeria. Results demonstrate that accurate Flory-Huggins parameterisation with composition-dependent interaction parameters was essential for thermodynamic model predictive accuracy, reducing morphology prediction error from 34 to 11 percent compared to constant parameter models. The process model identified critical solution viscosity and non-solvent diffusivity as the primary kinetic parameters controlling membrane morphology. The research provides a theoretically rigorous and experimentally validated framework for optimising membrane fabrication processes in the Nigerian water treatment equipment sector. Keywords: membrane fabrication, phase separation thermodynamics, Flory-Huggins, polysulfone, non-solvent induced phase separation