Effect of filler incorporation route on the properties of polysulfone–silver nanocomposite membranes of different porosities
Introduction
Porous polymer nanocomposite membranes can be prepared using a number of nanoparticle incorporation strategies. These strategies can be grouped into four categories based on whether particles and membranes are pre-formed or synthesized in situ during the formation of the nanocomposite. Methods that involve attachment or in situ synthesis of nanoparticles on the pore surface of existing membranes [1], [2], [3], [4], [5], [6], [7] have the advantages of improved reproducibility of nanocomposite preparation, the possibility to reuse the membrane matrix or to regenerate nanoparticles, and better accessibility of immobilized nanoparticles to reactants in the permeate flow. In contrast, incorporation of nanoparticles during membrane formation [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] is appealing because it should allow the design of nanocomposite structures with improved mechanical and separation properties along with embedded nanoparticle-based functionalities. The structure of the resulting membranes is generally a function of the physical and chemical properties of the polymer matrix and nanoparticles as well as the method of nanoparticle incorporation.
A number of published studies describe the formation of organic–inorganic porous (10–200 kDa) composite membranes by adding particles (Al2O3 [18], [19], ZrO2 [11], [14], TiO2 [15], [20], SiO2 [8], [9], [12], [22], Ag0 [13], [21], [23]) to the solution of the membrane-forming polymer (cellulose acetate (CA) [13], [18], [23], polysulfone (PSf) [8], [9], [14], [17], [20], polyvinylidene fluoride (PVDF) [11], [12], [19], polyacrylonitrile (PAN) [21], polyamide-imide (PAI) [24], poly(vinylidenefluoride-co-hexafluoropropylene) [22], or poly(phthalazinone ether sulfone ketone) (PPESK) [15]). These studies indicate that skin layer properties (porosity, surface porosity, thickness) as well as the macrovoid morphology of the support layer can be affected by nanoparticle incorporation. Both of these membrane regions are important; the skin layer determines the permeability, rejection, and selectivity of membranes, while the structure of the support layer affects its compaction behavior [25], [26]. Although nanoparticle-induced changes in membrane structure are specific to the particular filler/matrix combination, certain common trends can be identified. Increasing nanoparticle loading led to: (1) increased skin layer thicknesses (SiO2/PSf [8], [9], TiO2/PSf [20], TiO2/PPESK [15]), (2) higher surface porosity of the skin (ZrO2/PSf [14], TiO2/PSf [20], TiO2/PPESK [15], SiO2/PSf [9]), (3) suppressed macrovoid formation (SiO2/PSf [8], [9], TiO2/PSf [20], Al2O3/CA [18], TiO2/PPESK [15]), and (4) higher permeability of the membrane (ZrO2/PSf [14], SiO2/PVDF [12], ZrO2/PVDF [11], Al2O3/PVDF [19], Ag/CA [13], SiO2/PSf [9], TiO2/PSf [20]. In a few cases, decreased permeability (Ag/PAN [21]) or a maximum permeability at (TiO2/PPESK [15]) intermediate nanoparticle loadings was also reported. The rejection either peaked at intermediate loadings (TiO2/PSf [20]), or decreased after a threshold in filler loading was exceeded (SiO2/PSf [9], SiO2/PVDF [12], ZrO2/PVDF [11], TiO2/PPESK [15]), or remained unchanged (ZrO2/PSf [14], Al2O3/PVDF [19]). In a recent study, nanotube fillers were used to prepare a nanocomposite membrane when polysulfone was embedded with (acid-treated) multiwall carbon nanotubes [27]. The average porosity of the composite membranes peaked at a nanotube loading of 1.5%, which also corresponded to the highest permeate flux. Interestingly, the only pronounced change in membrane macroscopic morphology was an increase in surface roughness.
Examples of simultaneous formation of the polymer matrix and the nanoparticles include chemical reduction of metal ions by a component of the membrane casting solution [13], [21], [28]. Meyer and Bhattacharyya [28] reported on the formation of Fe/Ni nanoparticles in porous cellulose acetate membranes by in situ reduction wherein the reducing agent (NaBH4) was a component of the non-solvent bath. Yu et al. [13], [21] prepared silver-loaded cellulose acetate and polyacrylonitrile hollow fibers by the reduction of Ag+ by N,N-dimethylformamide, a polymer solvent, with the objective of producing biofouling-resistant ultrafiltration membranes.
The results of the studies published to-date strongly imply that the microstructural changes induced by nanoparticle incorporation can potentially be controlled to prepare membranes with desirable properties. To enable the rational design of porous nanocomposite membranes, the role of essential process variables such as nanofiller size (relative to the characteristic size of matrix pores), the affinity of the filler material for the components of the casting mixture, and the effect of the matrix pores functioning as reactors for forming filler nanoparticles need to be understood.
The overall goal of this study was to understand how the preparation conditions (nanoparticle incorporation route, porosity of the host matrix, etc.) influence the structure of the prepared nanocomposite membranes. To this end, we attempted to map nanocomposite membrane properties to preparation conditions for a series of membrane morphologies and two nanoparticle incorporation routes. Three types of membranes differing in skin porosity and macrovoid structure were prepared, and silver nanoparticles were either synthesized ex situ and then added to the casting solution as an organosol or produced in the casting solution via in situ reduction of ionic silver during the phase inversion process. Polysulfone was chosen as the matrix material due to its practical importance and wide use in the preparation of ultrafiltration membranes, while nanoscale silver was chosen for two reasons. First, there are established protocols for the preparation and characterization of silver sols in various dispersion media, including the organic solvents used in membrane preparation. Second, the presence and availability of silver can be detected by the extent of its antibacterial effect; this provides a convenient framework for the evaluation of the accessibility of silver in polymer matrices and serves as additional motivation in view of the potential application of silver-filled nanocomposites as biofouling-resistant materials.
Section snippets
Materials
Silver nitrate (GR ACS crystals), N,N-dimethylformamide (99.8% anhydrous), N,N-dimethylacetamide (99.8% anhydrous), polyethylene glycol (MW avg. 400), 2-propanol (99.5%), and polysulfone (Udel P-3500 LCD pellets, MB8, 79 kDa) were all used as received. The ultrapure water used in all experiments was supplied by a commercial ultrapure water system equipped with a terminal 0.2 μm capsule microfilter; the resistivity of water was greater than 16 MΩ cm. The standard constituents of the Luria-Bertani
Silver-free polysulfone membranes
Three strategies were used to prepare more porous PSf membranes of Types II and III: (a) decreasing the polymer content of the casting mixture, (b) increasing the temperature of the non-solvent bath, and (c) altering the composition of the non-solvent bath (Table 1). Relative to the Type I system, the polymer content of the casting solutions was decreased by a factor of 2 for both Type II and Type III membranes. The viscosity of the casting mixture has a direct influence on the gelation rate of
Conclusions
The effects of casting mixture composition and nanoparticle incorporation route on the morphological and separation properties of prepared porous nanocomposite membranes were studied using silver as the model filler material. It was demonstrated that:
- (1)
The reduction of ionic silver by polymer solvents prior to or during the phase inversion process can be employed to synthesize silver–polymer nanocomposites of a range of porosities and nanoscale silver distributions.
- (2)
Ex situ and in situ routes of
Acknowledgements
This material is based upon work supported by the National Science Foundation under Grant No. OISE-0530174. Robin Diets is acknowledged for his assistance with membrane permeability and rejection measurements. We also thank Solvay Polymers for providing the polysulfone for membrane synthesis. JST acknowledges support by Nordberg and McCowan fellowships.
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