Understanding the thermally induced phase separation process via a Maxwell–Stefan model

• A Maxwell–Stefan model was established to analyze TIPS process for the first time.
• The concentration and temperature profiles in TIPS process were calculated.
• The transport phenomena and phase separation were analyzed by the profiles.
• A better understanding on microporous materials formation via TIPS was obtained.

A Maxwell–Stefan model was firstly established to describe the mass and heat transfer in the air gap and coagulation bath stages via thermally induced phase separation (TIPS) process. A cylindrical configuration was considered with the polyvinylidene fluoride-diphenyl carbonate system. The model results showed that firstly, in the air gap, a polymer concentration gradient was induced in the polymer solution due to the diluent evaporation, and with an increase in the evaporation time, the gradient increased gradually. When the initial polymer concentration decreased, the polymer concentration gradient increased while the concentration gradient depth decreased due to a reduction in the diffusion coefficient. Secondly, the temperature of the polymer solution declined rapidly when it was immersed in the coagulation bath, which resulted in a temperature gradient in the solution. The temperature gradient diminished as the quenching time increased because of the heat transfer at the polymer solution–coagulation bath interface. In addition, with the initial polymer concentration decreasing, the solution heat transfer coefficient raised a little, which caused a slight rise in the temperature gradient. Finally, the model results were validated by the experiments to obtain a beneficial reference for the formation of polymeric microporous materials via TIPS.

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