De la teoría estructuralista a la sociología del individuo y la experiencia

In this section, structural characteristics of FO membranes are discussed, fol- lowed by a brief overview of membrane synthesis methods. FO membranes are asymmetric membranes: a thin active layer provides the separating capability of the membrane, which is supported by a much thicker support layer provid- ing mechanical strength. Membrane characteristics can be divided by active layer and support layer characteristics, while membranes are synthesized us- ing predominantly phase inversion (PI) or interfacial polymerization, leading to thin film composite (TFC) membranes. Polymers predominantly used for FO membrane synthesis are polyamide (PA) and polyethersulfone (PES) for the active layer and support layer respectively of TFC membranes, and cellulose triacetate (CTA) for PI membranes.

1.4.1

FO membrane synthesis

Phase inversion is a precipitation process in which a polymer is rapidly pre- cipitated, causing the formation of a dense film. The process starts by cast- ing a polymer solution onto a plate and spreading out the solution to attain certain thickness. Subsequently, the solvent in which the polymer is soluble is replaced by another solvent in which the polymer is insoluble, called the non-solvent, which is typically done by immersing the plate in a non-solvent bath. The solvent and non-solvent are mutually soluble or miscible. Due to the introduction of the non-solvent and removal of the solvent, the polymer will start to precipitate, forming a dense film at the polymer - non-solvent interface which becomes the active layer of the membrane. As the process continues, the non-solvent diffuses into the polymer film: due to kinetic effects, precipitation deeper into the polymer film will create a porous structure comprising of zones of dense polymer alternating with pores. This porous zone is the support layer of the membrane.

TFC membranes are formed using interfacial polymerization which takes place on the active layer of a preformed membrane, often a UF membrane, becom- ing the support layer of the TFC membrane. TFC membranes hold some ad- vantages over PI membranes: the separate production of the support layer and active layer allows the tailoring of both layers separately. For TFC membranes having a PA active layer, which is most common, the PA layer is synthesized

FO Membranes

in situ at the active layer of what will become the support layer. This is done by contacting two solutions containing different monomers, causing the for- mation of a PA film. For PA TFC membranes, the monomers are trimesoyl chloride (TMC), a reactive and aromatic tricarboxylic acid derivative, and a diamine, such as phenylene diamine or piperazine. TMC and the diamine are dissolved in an apolar solvent and water respectively, with the solvents being non-miscible, in order to maintain an interface at the site of polymerization. Films are self-closing during synthesis: monomers diffuse into the solution of the other monomer type, condensing at the interface of both solutions, thereby closing pores through which the monomers were diffusing. Once a closed film is formed, monomer diffusion is strongly hindered and the reaction is termi- nated. The use of the aromatic phenylene diamine yields fully aromatic PA films in which the polymer strands can be stacked more efficiently compared to when the non-aromatic piperazine is used; the former resulting in films with reduced permeability compared to the latter [35]. Consequently, fully aromatic films find use in RO membranes, while semi-aromatic polyamide films find use in NF membranes. Many variations are possible, such as blending different monomers or varying the reaction time, again showing the versatility of this process. For FO membranes, the most widely used membrane was a PI CTA membrane produced by HTI (Albany, OR, USA). In recent years, TFC FO mem- branes have become commercially available from companies such as Porifera, Toray or Aquaporin.

1.4.2

Active layer

The active layer characteristics which determine the permeation rate of water and solutes are the amount and size of free volume within the active layer poly- mer and the thickness of the active layer. The active layer of FO membranes is similar to those of RO and NF membranes, which is logical considering that FO, NF and RO are related processes. Consequently, some of the research cited in this section pertains to other dense membranes. Dense membranes, such as FO, RO, NF, and gas separation membranes, are considered to be non-porous mem- branes: the active layer does not contain discrete, permanent pores. Rather, their active layer contains voids in between polymer chains, called free vol- ume, which constantly fluctuate in size due to random movement of polymer moieties. The diameter of the free volume voids is in the order of 0.1 to 0.5

nm for PA [36, 37], a slightly larger value of 0.65 nm has been reported for CTA [38]. For NF membranes, the free volume voids are larger and are in the transition zone towards permanent pores. The free volume fraction within a polymer is reported to be in the order of 7 - 9% for PA as measured by PALS (Positron Annihilation Lifetime Spectroscopy) [36]. These results agree well with those obtained by Freger [39] who studied swelling of isolated PA active layers in water, finding swelling ratios of 5 - 12 % for RO membranes. Free volume and the degree of swelling are strongly correlated [40] but are how- ever not completely interchangeable as swelling causes the polymer chains to extend thereby increasing the free volume [38]. For CTA and other cellulose esters, the free volume fraction as measured by PALS is somewhat lower: 2% has been reported for CTA [38] and 4 - 5% for a number of other cellulose esters [41, 42].

The tricarboxylic monomer TMC used in PA films enables the formation of crosslinks, which creates a macro-molecular 3D-polymer network rather than individual polymer chains. Both simulation and membrane characterization results [43, 39] suggest that a 3D-polymer network is inherently more perme- able than an array of unlinked linear polymer chains, such as CTA: it is theo- rized that 3D-polymer networks contain a much larger permanent void fraction within the polymer, where diffusivity of solvent and solutes is relatively high, while the separation of solvent from solutes takes place in thin zones of high polymer density [39]. In arrays of unlinked polymer chains however, a much smaller permanent void fraction causes hindrance against diffusion for solutes and solvent over a longer distance. This can be seen in the free volume results presented above as well. Separation is then achieved by increased hindrance of solutes compared to the solvent, at a cost of decreased solvent permeabil- ity. Crosslinked polymer networks cannot be produced using phase inversion (disregarding post-processing): PI membranes are produced from polymer so- lutions, while crosslinked polymers are inherently insoluble. In a crosslinked, macro-molecular polymer network, a solvent cannot completely wet and en- velop polymer chains, which is needed for solubilization, because the polymer chains are covalently bound to each other. The above reasoning again shows why TFC membranes are superior to PI membranes; consequently, their mar- ket share dominates over PI membranes [44]. CTA is an uncharged polymer, however, the surface charge of CTA membranes has been shown to be slightly negative, which could be due to surface oxidation resulting in carboxylic acid

FO Membranes

groups or due to the adsorption of poorly hydrated anions [45]. TFC mem- branes on the other hand, contain both amine and carboxylic acid functional groups, and, due to the higher concentration of carboxylic acid groups in the PA polymer, TFC membranes have a net negative surface charge [46, 47]. PA TFC RO and NF membranes have an active layer thickness of around 200 and 20 nm respectively based on AFM measurements of active layers isolated from their support [39]. This isolation procedure is only possible for TFC mem- branes: PI membranes are composed of a single polymer with the active layer gradually transitioning into the support layer. The active layer thickness of PI membranes can however be determined by PALS: for cellulose acetate FO membranes prepared using different PI conditions, the resulting active layer was found to vary from 100 to 800 nm [41].

1.4.3

Support layer

The support layer has no direct influence over the separating properties of a membrane, as these are determined by the active layer, but it has a pro- found influence on mass transfer, especially for FO. Important characteristics are the support thickness, porosity, tortuosity and hydrophilicity. Compared to NF and RO membranes, the support layer in FO membranes is much thinner: the support does not need to be able to withstand high pressure because no hydrostatic pressure is applied; support thickness is generally between 50 and 100 µm [9, 48]. Porosity and tortuosity are somewhat related: theoretical and empirical study on porous media has shown that tortuosity is inversely related to porosity [49]. Tortuosity can furthermore be limited by producing support layers having finger-like macrovoids perpendicular to the active layer [50, 51], these macrovoids can be produced by tweaking the process parameters of PI. Tortuosity cannot be measured directly, but can be inferred from mass transfer modeling, which will be discussed in section 1.5. Huang and McCutcheon have shown that increasing the support pore size subsequently increases FO water flux as well, although an optimal pore size exists beyond which the active layer is no longer supported, causing the membrane to fail [52]. Potentially very porous and low tortuosity support layers can be produced using electrospin- ning. Using this technique, a non-woven fabric of fibers less than 1 µm can be produced. This contrasts with a more sponge-like structure of support layers produced by phase inversion. However, poor adhesion between active and sup-

port layers has been reported as well [53]. Finally, McCutcheon and Elimelech have shown that hydrophobicity of the support layer reduces water flux, likely due to incomplete hydration of the support layer [54]: small air bubbles would remain trapped in the support layer after hydration of the membrane, thereby blocking liquid mass transfer in the support layer.