Exploiting Fluoropolymers Immiscibility to Tune Surface Properties and Mass Transfer in Blend Membranes for Membrane Contactor Applications

Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and polyvinylidene fluoride (PVDF) blend membranes, with interconnected channels decorated with polymer crystallites, were produced by a two-step phase separation technique, using nontoxic solvents and without any chemical additive as pore forming. Results demonstrated that the mass-transport and the interface properties of the membranes can be tailored by a synergic combination of immiscible blend components and an adequate manufacturing procedure, that exploit the slow diffusion of the nonsolvent by vapor induced phase separation. In this way, multilevel hierarchical surfaces, with raspberry-and cauliflower-like substructures, were obtained. The optimal blending ratio between the two components was assessed by chemical physical and mass-transport property characterizations of the prepared membranes. The beneficial effect of blending was evidenced for all the properties investigated (crystallinity, surface roughness, wettability, surface charge, strength, toughness, and flux), indicating a good physical interaction between the two components, despite their thermodynamically stated immiscibility. This physical interaction was also proved by the presence of PVDF crystallites embedded in the PVDF-HFP network of blend membranes, which act as reinforcing agents rather than as independent fillers with respect to the pure copolymer, providing enhanced mechanical properties.