Surface-Fluorinated Polyvinyl Alcohol Functional Membranes: Preparation, Morphology And Surface Property

Polyvinyl alcohol (PVA) is a widely used polymer with an ever-lasting interest and possesses various advantages including good membrane forming ability, high durability, non-toxicity to viable cells as well as a good mechanical strength. Thus PVA seems to be attractive in various fields, especially in preparing functional membrane materials, such as fuel cell, membrane separation, biomedical applications, etc. However, the water solubility sets a limit for PVA in practical using. A lot has been reported on the water resistance modification of PVA to get some specific properties, such as cross-linking, plasma, sputtering, laser evaporation, chemical vapor deposition, heat-treatment, incorporating with other hydrophobic moieties, etc. Surface fluorination was preferred as a particularly effective way in lots of previous work aiming to achieve enhanced performance without hardly any changes to the bulk properties of base materials.Considering all those above, a novel water resistant PVA membrane and an asymmetric PVA switchable membrane were elaborately designed and successfully prepared through surface fluorination with 2,3,3,3-tetrafluoro-2-[1,1,2,3,3,3-hexafluoro-2-(heptafluoropr-opoxy)propoxy] propionyl fluoride (n-C3F7OCF(CF3)CF2OCF(CF3The morphology of the surface-fluorinated PVA membranes was observed by SEM and AFM. The surface roughness was greatly increased by an island structure on the fluorinated layer. And the hydrophobicity of the fluorinated PVA membrane was evaluated by the water contact angle measurement. The results indicated that the water contact angle increased )CFO). The chemical composition of the fluorinated PVA membrane was characterized by ATR-FTIR and XPS. The results indicated that the pristine PVA membrane surface was successfully grafted with a perfluoroalkyl ether layer. And a high fluorine content of 49.08% was obtained on the fluorinated surface. Additionally, the reduction of the fluorine content with increase of X-ray take-off angle indicateed fluorine groups tended to enrich toward the outmost surface layer which could make great contributions to the excellent hydrophobicity of the membrane. with the fluorination time and finally reached a constant value of 135o after 2 hours.A novel asymmetric PVA membrane was designed and successfully prepared based on another type of PVA with a lower alcoholysis degree of 88%. The asymmetric membrane was proved to combine the water solubility of the PVA substrate with the excellent hydrophobicity and oil resistance of the grafted surface layer, and thus exhibited a special responsiveness to the direction of water flow. Meanwhile, the unique chemical structure and composition impart a gating property to the obtained membrane, for which the asymmetric PVA membrane could be used as a unidirectional switch in kinds of precise instruments, especially some micro-reactors.