Leaching of the extractant from polymer inclusion membranes (PIMs) into the feed and receiving aqueous solutions shortens their life. Therefore, when a particular PIM extractant has been selected, it is important to choose a base polymer that will minimize to the greatest extent extractant leaching compared to other base polymers, thus providing the best stability of the PIM. However, comparisons of the stability of PIMs composed of the same extractant and different base polymers is usually conducted by multiple cycles of extraction and back-extraction steps, which are time-consuming and labor-intensive. An alternative approach based on thermal analysis (thermogravimetric analysis (TGA) and differential thermal analysis (DTA)) was developed and applied to PIMs containing 40 wt.% Aliquat 336, one of the most frequently used PIM extractants, and the three most frequently used PIM base polymers, i.e., poly(vinyl chloride) (PVC), cellulose triacetate (CTA), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The temperatures and enthalpies associated with Aliquat 336 release were compared, with PVDF-HFP exhibiting the highest values, indicating the strongest interaction between the extractant and the polymer matrix and, thus, the highest stability. The PVC-based PIM was predicted to be the most prone to extractant leaching among the PIMs studied. This stability ranking was confirmed theoretically by quantum chemistry (DFT) calculations, which provided molecular-level insights into the likely interaction sites between Aliquat 336 and the polymer chains. An experimental validation of the above leaching order was also provided by PIM leaching experiments in aqueous 0.1 M and 0.05 M NaCl solutions, where membrane mass losses over a 24 h period were determined. The results of the current study demonstrated thermal analysis to be a fast and viable approach in comparing the stability of PIMs with the same extractant but different base polymers.

Polymer inclusion membranes (PIMs) based on PVDF-HFP as the base polymer and Aliquat 336 as the carrier in a mass ratio of 6:4 with concentrations of embedded glass fibers up to 5 wt% were successfully fabricated. Their microstructure, as well as their mechanical and thermal properties, were characterized using scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), differential thermal analysis/thermogravimetric analysis (DTA/TGA), and tensile testing. Membrane performance and long-term stability in transporting thiocyanate ions were evaluated in a two-compartment transport cell. The results showed that the membranes retained their amorphous structure even with glass-fiber loadings of up to 5 wt%. The addition of glass fibers was found to primarily enhance the elastic modulus and tensile strength, while causing a moderate reduction in plasticity without negatively affecting membrane transport properties and long-term stability. Therefore, it was concluded that the incorporation of glass fibers could improve the suitability of PIMs for industrial applications.