Characterization of polymeric membranes for non-aqueous separations

The industrial application of solvent resistant nanofiltration (SRNF) membranes for solvent purification has grown considerably in the last decade. The number of solvent/membrane systems is increasing and consequently, the need for characterization of chemical and physical phenomena, i.e. sorption, swelling, compaction and mobility. This thesis provides a tool box with characterization methods that are aimed to be independent of the solvent and membrane material and to provide results focused on the transport determining layer of the membrane. In Chapter 2 the determining transport mechanism of three similar solvents (methanol, ethanol and 1-butanol) in a silicon-type SRNF membrane has been studied. The results suggest that diffusion alone can not explain the transport of alcohols and an additional transport mechanism takes place. Assuming the additional mechanism to be viscous in nature allows a fit of a flux expression to the experimental data using one single fitting parameter. For 1-butanol, the contribution of the additional mechanism is only small. In contrast, for the smaller methanol and ethanol the contribution of viscous flow is very high. This is in contrast with observations in literature for similar systems, signifying a limited applicability of state-of-the-art models for predicting solvent transport through SRNF membranes. Chapter 3 presents a generic method that allows the determination of retention behavior in non-aqueous solvent filtration. The method has limited restrictions with respect to solute solubility and is independent of the solvent studied. Using a low concentration of polyethylene glycol 1000 with a broad molecular weight distribution, combined with a powerful analytical technique such as mass spectroscopy, a single retention experiment produces a full retention curve. In this way, the molecular weight cut-off (MWCO) of a membrane can be directly determined and the cost and time of the experimental procedure is reduced. Retention has been shown to be mostly dependent on the rate and mechanism of solvent transport. For transport that is solely or partially viscous, dragging effects cause the retention of small PEG oligomers to be low. For transport determined by diffusion, dragging effects are insignificant and retention is high (98%) irrespective of the size and shape of the solute molecule. Chapters 4 and 6 show the versatility of attenuated total reflectance spectroscopy infrared (ATR-IR) to determine in-situ sorption and compaction in the active layer of a cellulose acetate (CA) membrane and a silicon-type SRNF membrane, respectively. Preferential sorption of water has been measured in a CA membrane for water/methanol and water/ethanol mixtures. The results are in accordance with literature. The extent of compaction of the transport determining layer of the commercial SRNF membrane has been determined under different pressure conditions. Results show that reduction in thickness of the entire membrane can not be directly related to the reduction in density of the active layer. Comparison with permeation experiments under the same conditions show the importance of measuring both swelling and compaction simultaneously. Chapter 5 shows the development of two straightforward techniques to measure swelling: mechanically using a micrometer and optically using an interferometer. Both methods allow the study of the dynamic swelling behavior and yield similar results. Different polymer layers present in the membrane show different contributions to swelling. Comparison of the dynamics of swelling and sorption reveals that these two phenomena, although interdependent, do not start simultaneously and are not comparable in magnitude. For this reason, if the extent of sorption is interpreted as a direct measure for the extent of swelling, incorrect conclusions can be drawn. Chapter 7 concludes the thesis by taking a look at the impact of the work developed. Furthermore, the future possibilities for developments regarding the characterization, prediction and process monitoring are discussed.