Water–methanol separation characteristics of annealed SA/PVA complex membranes

Polymeric membranes based on sodium alginate (SA)/polyvinyl alcohol (PVA) were prepared with the SA/PVA blending ratio of 80/20, 90/10 and 95/05 wt% and complexed with divalent calcium ions. The prepared membranes were modulated by post-heat treatment to achieve good vapor permeation (VP) separation characteristics of water–methanol mixture. Membranes performances for water–methanol separation were investigated in terms of their flux and permselectivity. The effect of various parameters like operating temperature, annealing time and PVA contents on membranes performances was also investigated. To study the effect of operating temperature on flux and permselectivity, vapor permeation experiments were performed in the range of 40–70 °C. As a whole, the maximum selectivity was observed at minimum flux conditions and vice versa. The only water component was penetrated through SA/PVA (90/10) annealed complex membrane for the operating temperature of 40–60 °C. A comparable performance was shown by the SA/PVA (80/20) annealed complex membrane at 40 °C. The structural characteristics, morphology and stability of complex membranes were investigated using FTIR, XRD, SEM, TGA and DTG analysis. The results confirmed that annealing process affects the structure morphology and leads to free volume shrinkage between polymer chains. So, it is concluded that the Ca-complex SA/PVA annealed membranes exhibited the molecular sieving mechanism for separation of water–methanol mixtures because of decrease in free volume and strong intermolecular interactions between the membranes components.