Pore Structure Control of PVDF Membranes Using Dual Coagulation Bath Phase Inversion Process for Application in Membrane Distillation (MD)

Membrane fabrication by phase inversion is a sophisticated process that involves interplay of different processing parameters, including the choice of a coagulation bath; its composition, temperature and immersion time. The phase inversion path of a membrane forming system is dictated by the composition of the coagulation and casting solution [1], which influences the solvent exchange rate. Coagulation bath liquids are categorized into soft and harsh solvents, based on the extent of the de-mixing stage, crucial to pore formation in the phase inversion pathway. Systems with a rapid phase inversion rate tend to form macro-voids with finger-like structures, whereas systems with a slow phase inversion rate yield sponge-like structures [2]. The employment of non-aqueous solvent in the coagulation bath or the addition of such solvent into a water bath may induce delayed liquid–liquid de-mixing of the polymer and eventually result in the formation of membranes with sponge-like structure or even spherulite formation [3,4].

In this research, the use of a dual (2-stage) coagulation bath was investigated. The first bath contained water (harsh solvent), while the second contained ethanol (soft solvent). A variation of process combinations was evaluated, including bath temperature and immersion times, polymer solution concentration, and thickness of the casted PVDF membranes. While the majority of previous studies in this area have been done using pore forming agents to control the porosity of the PVDF membranes, this study has exclusively relied on coagulation baths parameters to achieve pore size control, in the absence of pore forming agents. This procedure was also found to control the shrinkage of the membranes, thereby keeping the surface porosity unaffected. The modified membranes have been characterized for their contact angle, liquid entry pressure (LEP), microstructure, maximum pore size and pore size distribution, mechanical strength, and porosity. They were finally tested for their performances in the MD process.

We found that, by increasing the polymer concentration, the morphology of the prepared membranes was improved due to the smaller pores and highly porous structure formed. The membranes formed were found to have a very uniform and open structure on the surface and an asymmetric interconnected pore structure all through the thickness of the membranes. Superior membrane properties, including a narrow pore size distribution (PSD) (0.14- 0.2 microns), high LEP (49- 83 psi), high porosity (57-79%), and a high contact angle (100-127˚) were achieved in the fabricated membranes. With the synergy of hydrophobicity, mechanical strength, high LEP and a very narrow PSD, the membranes were found very suitable for MD applications. Experimental studies were then carried out to determine the best operational MD configuration for maximum vapor flux, based on the obtained membrane properties.

References

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