Thesis
Fabrication of polyvinylidene fluoride membranes using gamma-valerolactone as an alternative green solvent
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2022
- Thesis identifier
- T16558
- Person Identifier (Local)
- 201887187
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Polyvinylidene fluoride (PVDF) membranes were synthesised using the nonsolvent phase inversion method (NIPS) to design tailored separation films suitable for different membrane processes. Based on the green chemistry principles “safer solvents and auxiliaries” and “use of renewable feedstocks”, this study proposes the possibility of using an alternative green solvent - gamma-valerolactone - as a solution for a more sustainable production of PVDF membranes. Gamma-valerolactone (GVL), a bio-derived green and non-toxic solvent with eco-friendlier properties, was used as an alternative solvent to replace the commonly used, toxic and problematic, petroleum-derived solvent N-methyl-2-pyrrolidone (NMP). Preliminary studies examined the thermodynamic/kinetic behaviour of the PVDF, gamma valerolactone and water ternary system using a developed “polymer dissolution in a vial” (PDV) method. Results obtained from cloud points analysis and literature studies were decisive in determining the location of the demixing boundary curve and suitable casting compositions. Furthermore, comparative analysis at different temperatures of the PVDF/GVL/water system reveals the location and transient nature of the demixing boundary/miscibility gap due to the effect of temperature and time. Finally, the proposed PDV method was validated with the known PVDF/DMSO/water ternary system, with experimental results demonstrating reasonable agreement with results published in the literature. PVDF membranes fabricated using PVDF/GVL binary solutions were compared to PVDF membranes fabricated using PVDF/NMP binary solutions (direct solvent swap), with the membranes prepared using GVL exhibiting significantly different membrane structures and permeation performance (low gas and no pure water permeation) when compared with the membranes prepared using the traditional NMP. The possibility of regulating the PVDF dope using GVL as a solvent to obtain improved membrane properties was further investigated. Different polymer dope/casting solutions based on different concentrations of PVDF, cosolvent (DMSO), polymer additive polyvinylpyrrolidone (PVP), as well as preparation/operating temperatures were prepared to investigate their effect on the morphology and properties of the fabricated PVDF membranes. The membranes fabricated with different PVDF concentrations, different solution preparation temperatures, and different casting temperatures all demonstrated a very thin film, with a unique membrane structure of large thick, dense top layer and globular/spongy substructure. The prepared membranes’ morphology demonstrated that the PVDF/GVL system exhibits a delayed demixing phenomenon, leading to the typically widely recognised observed dense/spongy membrane structures. Additionally, the examined ranges of solution preparation temperature (Tdissol) and casting temperature (Tcast), showed a correlation between the membrane morphology and the experimental ternary PVDF ystem phase diagram. The analysis of the results shows that the location of the demixing boundary differs significantly for the PVDF/GVL/water system for conditions at different solution preparation temperatures (Tdissol). In contrast, by varying the examined Tcast parameters, the location of the demixing boundary showed very little or no shift change. PVDF membranes prepared with lower PVDF concentrations or by employing lower temperatures (Tdissol and Tcast), showed high gas permeation fluxes and no pure water permeation, making them unsuitable for ultrafiltration membrane processes, contrary to the ones obtained using the traditional solvent NMP. PVDF membranes prepared by adding either a non-toxic cosolvent (DMSO) or a polymer additive (PVP) resulted in a modification of the morphology and improved PVDF membrane performance properties. The PVDF membranes produced as a result of combining GVL with different concentrations of a DMSO as cosolvent or by adding PVP (acting as a pore former) to the polymer dope, displayed promising results and is a good development in the fabrication of PVDF membrane using more sustainable materials. The formation of finger like, macrovoid and spongy membrane sub-structures resulted in improved membrane properties in terms of porosity, membrane thickness, wettability, gas and water performance. Furthermore, the assessment and analysis of the fabricated PVDF membranes indicate that the concentration of these additives (cosolvent or pore former) played a significant role in the membrane formation process. Finally, a holistic view of the performance of all fabricated PVDF membranes provides a suitability indicator to identify membranes that can be used in different membrane processes such as membrane contactors, membrane distillation and gas-liquid separation applications. Nevertheless, as suggested in the section “Recommendations for Future Work”, a better understanding of the thermodynamic and kinetic behaviour of the PVDF systems containing additives should be addressed to improve further. Overall, this study highlights that it is possible to fabricate PVDF membranes with specific characteristics using the bio derived and more environmentally friendly solvent gamma-valerolactone, and that these membranes can be tailored to perform similarly to membranes produced using traditional polar aprotic solvents.
- Advisor / supervisor
- Magueijo, Vitor
- Resource Type
- DOI
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