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

Abstract

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(VDFco-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 polymorphcontrolled 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.

Fluorine-containing bio-inert polymers: Roles of intermediate water

“…This result shows that the PVDF-β phase has all-trans planar conformation with six sites that can interact with water, while in the PVDF-α phase, the fluorine atoms are alternately positioned on both sides of the chain axis with only three sites. Hence, the water molecules can adhere more strongly to the PVDF-β phase than to the PVDF-α phase [62].…”

Preparation and Characterization of a Janus Membrane with an “Integrated” Structure and Adjustable Hydrophilic Layer Thickness

“…Most existing research on Janus membranes involves the post-modification of porous matrix membranes to prepare hydrophilic-hydrophobic double-layer or multilayer composite membranes. Single-sided coating/spraying/deposition [24][25][26], surface grafting [27,28], hydrogel [29,30], and electrospinning [31][32][33] have been used to construct hydrophilic (hydrophobic) coatings on the surfaces of other hydrophobic (hydrophilic) substrate membranes. In a study by Yang et al [26], a polydopamine/polyethyleneimine hydrophilic solution was deposited in multiple cycles on the inner lumen side of a hollow fiber that was originally hydrophobic to create a serviceable hydrophilic coating.…”