Origin of Fluoropolymer Affinity toward Water and Its Impact on Membrane Performance

Fluoropolymers exhibit very high hydrophobicity because of the presence of fluorine groups. However, they sometimes exhibit peculiar polarity—referred to as “polar hydrophobicity”—depending on their crystal polymorphs. This intriguing phenomenon is not yet clearly understood, and it remains a challenge to tailor this rare property for desired applications. Particularly, water desalination via membrane distillation (MD) is an emerging process, which requires hydrophobic membranes with extremely low affinity toward water molecules. In this study, we investigated the water affinity of fluoropolymers, namely, vinylidene fluoride (VDF) and hexafluoropropylene (HFP) groups, and their correlation with membrane performance. The molecular origin of fluoropolymer polarity was examined via density functional theory analysis, which unveiled that β-phase VDF crystals have twice as many water-favorable sites as the α-phase, whereas HFP groups exhibit lowest interaction energy with water. To maximize membrane hydrophobicity, we synthesized P(VDF-co-HFP) polymers with varying VDF-to-HFP ratios and fabricated porous membranes with similar pore sizes using the electrospinning technique. Upon thorough characterization, a clear inverse correlation was observed between the β-phase VDF crystals and the observed hydrophobicity and performance, confirming the source of fluoropolymer polarity. The polymorph-controlled membranes exhibited a unique optimum at 8% HFP content with 35-fold enhancement in MD long-term stability compared to that of pristine polyvinylidene fluoride membranes. It is highlighted in this work that the polymer mechanical properties cannot be compromised over the desired hydrophobic properties in membrane contactor processes.