Thermodynamic behavior of a ternary system with one low molecular weight component, formic acid (FA) as the solvent, water (non-solvent), and two high molecular weight polymers [polyamide-6 (PA-6) and cellulose acetate (CA)] was investigated using an extended modified Flory-Huggins model. Where, all chemicals were purchased from Leuna Werke AG (Germany). The model was solved by MATLAB SIMULINK software manufactured in the USA. The predicted results from the model explained that the miscibility of the two blend polymers, PA-6 and CA, completed over all compositions at room temperature, and the minimum point where the miscibility of the two polymers completed was in the composition of 0.2 volume fraction of PA-6 at Gibbs free energy change on mixing (Delta G(m)) of -1.74015 kJ/mole. The critical temperature (T-c) for superiority properties of the polymer blend solution are in the range between the upper critical saturation temperature (UCST) 323K and the lower critical saturation temperature (LCST) 338K. The diffusion model on the solution of the immersion precipitation process in the coagulation bath indicates that the solvent volume fractions increase with time, while the polymer solution volume fraction decreases, due to solvent removal from the polymer solution and membrane formation. According to the mathematical model, it was found that the annealing temperature can affect the densification of the membrane top layer. However, the heat treatment process leads to a decrease in thickness of the membrane bottom layer, as a result of reduced and distributed membrane pores.
