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 non-toxic 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 non-solvent by vapor induced phase separation. In this way, multi-level 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 properties 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 co-polymer, providing enhanced mechanical properties.