Effect of solvent choice on cellulose acetate membrane fabrication by phase inversion and deacetylation by alkaline hydrolysis

2022-8
Tekin, Fatma Seden
In this study, the effect of solvent choice on cellulose acetate (CA) membrane morphology and performance was investigated to relate this to the thermodynamics and kinetics of phase inversion. Three different solvent systems were used, which are dimethyl sulfoxide (DMSO), the mixture of DMSO: acetone (DA) and DMSO: acetic acid (DHAc) in the ratio of 1:1. Water was used as non-solvent. Acetone and acetic acid were chosen due to their similar solvent quality for cellulose acetate based on Hansen solubility parameters but their different viscosities. All solvents are less harmful to the environment and human health compared to the conventional solvents used in CA membrane fabrication. The thermodynamics of the systems were investigated by the binary and ternary interactions of the components. Phase inversion kinetics was investigated by phase inversion rate observation by optical microscope, light transmission measurement, and rheological analysis of solvents and polymer solutions. The performances of the membranes were characterized by pure water permeance (PWP) and molecular weight cut-off (MWCO), and the membrane morphology was observed by scanning electron microscopy (SEM). The phase inversion kinetics was observed to be the main parameter that controlled the morphology and performance of the membranes, even though the thermodynamic interactions between the components were also different. Phase inversion kinetics is mainly affected by solvent viscosity. The lower viscosity of DA led to a faster phase inversion, and asymmetric membrane structure, whereas adding acetic acid to the solvent system resulted in higher viscosity of solvent system and slower phase inversion, which made the membrane structure loose, porous, and symmetrical. The changes in the membrane properties were also investigated by applying the evaporation step to the cast polymer solution containing acetone as the volatile co-solvent for different durations. In addition, the presence of humidity in the evaporation bath was studied. As the evaporation time increased, the porosity and pore size of membranes decreased, leading to lower pure water permeance and MWCO. In addition, conducting evaporation in a humid medium resulted in a looser membrane structure. The effect of deacetylation via alkaline hydrolysis on the membranes with different porosity was investigated. There was no significant difference observed in the morphology after alkaline hydrolysis. However, the performance of membranes changed after alkaline hydrolysis, probably due to the partial degradation of cellulose chains in the aqueous alkaline solution affecting the pore size and membrane matrix. The narrowing of pores may be considered the dominant effect on the performance of membranes conducting separation based on the pore flow mechanism. On the other hand, the effect of a loosened membrane matrix also becomes important in the performance of membranes where separation occurs through the solution-diffusion mechanism. Consequently, when both pores and membrane matrix contribute to transport in comparable amounts, the water permeance increased while MWCO decreased due to water permeating through both pores and membrane matrix, while solutes essentially permeated through the pores only. When solution diffusion mechanism became dominant form both solvent and solutes, both permeance and MWCO increased. Hollow fiber membranes were fabricated by dry-wet spinning with the same polymer solutions to investigate the effect of membrane configuration on membrane performance. Compared with flat sheet membranes, denser selective layer, possibly resulting from molecular orientation due to shear rate in the spinning system, was obtained in the CA18-DHAc hollow fiber membrane. CA18-DA hollow fiber membrane showed similar morphology and MWCO, but higher pure water permeance. The difference in flat sheet and hollow fiber membranes with different solvent systems is attributed to the simultaneous impact of many spinning conditions in the fabrication of these membranes.

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Citation Formats
F. S. Tekin, “Effect of solvent choice on cellulose acetate membrane fabrication by phase inversion and deacetylation by alkaline hydrolysis,” M.S. - Master of Science, Middle East Technical University, 2022.