Thermodynamic analysis of polymer solutions for the production of polymeric membranes

Modification of the Surface Morphology and Properties of Graphene Oxide and Multi-Walled Carbon Nanotube-Based Polyvinylidene Fluoride Membranes According to Changes in Non-Solvent Temperature

The effect of changes in non-solvent coagulation bath temperature on surface properties such as morphology and hydrophilicity were investigated in multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO)-based polyvinylidene fluoride (PVDF) membranes. The properties of pores (size, shape, and number) as well as membrane hydrophilicity were investigated using field emission scanning electron microscopy, Raman spectroscopy, optical microscopy, water contact angle, and water flux. Results showed that the pore size increased with an increase in coagulation temperature. The hydrophilic functional groups of the added carbon materials increased the solvent and non-solvent diffusion rate, which significantly increased the number of pores by 700% as compared to pure PVDF. Additionally, these functional groups changed the hydrophobic properties of pure PVDF into hydrophilic properties.

Thermodynamic Modeling and Validation of the Temperature Influence in Ternary Phase Polymer Systems

The effect of the temperature, as a process variable in the fabrication of polymeric membranes by the non-solvent induced phase separation (NIPS) technique, has been scarcely studied. In the present work, we studied the influence of temperature, working at 293, 313 and 333 K, on the experimental binodal curves of four ternary systems composed of PVDF and PES as the polymers, DMAc and NMP as the solvents and water as the non-solvent. The increase of the temperature caused an increase on the solubility gap of the ternary system, as expected. The shift of the binodal curve with the temperature was more evident in PVDF systems than in PES systems indicating the influence of the rubbery or glassy state of the polymer on the thermodynamics of phase separation. As a novelty, the present work has introduced the temperature influence on the Flory-Huggins model to fit the experimental cloud points. Binary interaction parameters were calculated as a function of the temperature: (i) non-solvent/solvent (g₁₂) expressions with UNIFAC-Dortmund methodology and (ii) non-solvent/polymer (χ₁₃) and solvent/polymer (χ₂₃) using Hansen solubility parameters. Additionally, the effect of the ternary interaction term was not negligible in the model. Estimated ternary interaction parameters (χ₁₂₃) presented a linear relation with temperature and negative values, indicating that the solubility of the polymers in mixtures of solvent/non-solvent was higher than expected for single binary interaction. Finally, PES ternary systems exhibited higher influence of the ternary interaction parameter than PVDF systems.