Composite membranes fabrication techniques

The technology to fabricate ultrathin high-performance membranes into high-surface-area membrane modules has steadily improved during the modem membrane era. As a result the inflation-adjusted cost of membrane separation processes has decreased dramatically over the years. The first anisotropic membranes made by Loeb-Sourirajan processes had an effective thickness of 0.2-0.4 xm. Currently, various techniques are used to produce commercial membranes with a thickness of 0.1 i m or less. The permeability and selectivity of membrane materials have also increased two to three fold during the same period. As a result, today s membranes have 5 to 10 times the flux and better selectivity than membranes available 30 years ago. These trends are continuing. Membranes with an effective thickness of less than 0.05 xm have been made in the laboratory using advanced composite membrane preparation techniques or surface treatment methods. 

These developments result from the introduction of composite membranes, originally developed in the 1970s primarily, for desalination by reverse osmosis. Application of the same membrane fabrication techniques to pervaporation membranes radically improved their performance and spurred commercial utilization. Today, pervaporation and vapor permeation plants are widely used to dehydrate volatile organics and separate other mixtures, primarily in the pharmaceutical and fine chemical industries. 

In a similar work, the synthesis of a composite with iron oxide particles and sul-fonated cross-linked polystyrene (SXLPS) for application in the PEMs for fuel cells was described (Brijmohan and Shaw 2007). The technique used for the polymerization was similar to the miniemulsion polymerization (Ramirez and Landfester 2003). However, some modification to the procedure was required to make the cross-linked and functional polymer-iron oxide composites. Also reported was the membrane fabrication process, which inclndes the alignment of synthesized particles in a high-performance snlfonated poly(etherketoneketone) (SPEKK) matrix (Gasa et al. 2006), and the properties of such PEMs for fuel cell applications. The final properties of the membrane depend on various factors, such as the lEC, the matrix, and the size of the particles. However, the main emphasis of the research was to demonstrate a useful membrane-fabrication technique that can be utilized to enhance the conductivity of the PEMs. 

The majority of the commereial gas separation membranes are made by wet phase inversion method whieh results in an integrally skinned asymmetrie membrane. This method was first used by Loeb and Sourirajan to produee cellulose acetate membranes for desalination of sea water. An alternative method for making gas separation membranes uses an ultra-porous skinned asymmetric membrane over which a thin polymer film is deposited by either coating or by interfacial polymerization. This method was developed by Cadotte for the creation of in situ dense skin thin film composite membranes for water desalination. These membrane fabrication techniques were made commercially successful for gas separation membranes by a brilliant empirical discovery for in situ sealing of the tiny pinhole defects on the skin of the membrane. 

Limited testing on chlorine sensitivity of poly(ether/amidel and poly(ether/urea) thin film composite membranes have been reported by Fluid Systems Division of UOP [4]. Poly(ether/amide] membrane (PA-300] exposed to 1 ppm chlorine in feedwater for 24 hours showed a significant decline in salt rejection. Additional experiments at Fluid Systems were directed toward improvement of membrane resistance to chlorine. Different amide polymers and fabrication techniques were attempted but these variations had little effect on chlorine resistance [5]. Chlorine sensitivity of polyamide membranes was also demonstrated by Spatz and Fried-lander [3]. It is generally concluded that polyamide type membranes deteriorate rapidly when exposed to low chlorine concentrations in water solution. 

The initial studies by Cadotte on interfacially formed composite polyamide membranes indicated that monomeric amines behaved poorly in this membrane fabrication approach. This is illustrated in the data listed in Table 5.2, taken from the first public report on the NS-100 membrane.22 Only the polymeric amine polyethylenimine showed development of high rejection membranes at that time. For several years, it was thought that polymeric amine was required to achieve formation of a film that would span the pores in the surface of the microporous polysulfone sheet and resist blowout under pressure However, in 1976, Cadotte and coworkers reported that a monomeric amiri piperazine, could be interfacially reacted with isophthaloyl chloride to give a polyamide barrier layer with salt rejections of 90 to 98% in simulated seawater tests at 1,500 psi.4s This improved membrane formation was achieved through optimization of the interfacial reaction conditions (reactant concentrations, acid acceptors, surfactants). Improved technique after several years of experience in interfacial membrane formation was probably also a factor. 

H. Kawamura, R. Yoshiie, H. Moritomi, M. Nishimura, Fabrication of a palladium composite membrane using supercritical wet plating technique in Proceedings of the Sth International Conference on Inorganic Membranes, Jul. 18-22, 2004, Cincinnati, OH, Adams Press, Chicago. IL. 

Mohammad et al. [29] fabricated NF composite membranes by the interfacial polymerization technique and studied the membrane s surface by AFM. The membrane support was prepared from a dope containing polysulfone (PSf) (P1835-BP Amoco) and poly(vinylpyrrolidone) (PVP) (Fluka) with JV-methyl-2-pyrrolidone (NMP) as the solvent. The top active layer was obtained through interfacial polymerization between trimesoyl chloride (TMC) in hexane and the aqueous phase containing bisphenol A (BPA). Table 5.9 shows the summary of the membrane preparation conditions. The first three membranes identified as PT-30, PT-45, and PT-60 differ in the period of interfacial reaction. The other three membranes identified as PC-05, PC-1, and PC-2 differ in terms of the concentration of BPA in the aqueous phase. The pore sizes determined by AFM and also calculated using the Donnan-steric-... 

The first generation of the NF membranes can be traced back to the early 1970s when most of the membranes were made of cellulose acetate (CA) and other cellulose esters. These cellulose-based membranes, however, severely limited the range of industrial applications due to their poor chemical and biological resistances coupled with insufficient water permeation. This consequently resulted in the development of a second generation of noncellulosic NF-composite membranes made of polyamide (PA) and polyurea (PU) with the aim to improve water permeability and selectivity, together with better pH and solvent stability (Schafer et al. 2003). This section does not intend to provide an exhaustive review of all the NF membranes developed to date. It simply aims to give the latest development of NF membranes in the past decade. Attention is paid to the research and development of NF prepared from two different fabrication techniques. 

Excimer lasers have also been used to manufacture novel composite membranes to be used as an effective transducer for the selective transfer and sensing of molecular ions [39]. Matson et al. [40] also employed excimer laser direct patterning at 248 nm to produce membranes for solvent separators by a step and drill method but they also developed a mask patterning process to create multiple pores of small size. McNeely et al. [41] developed a rapid prototyping technique to fabricate passive hydrophobic microfluidic systems integrated with macroscopic external devices aimed at highly parallel sample analysis. Sabbert et al. [42] machined cydoolefin copolymer (COC) with no redeposition effects, smooth surface and ablation rates smaller than for PMMA using an ArF excimer laser (193 nm). 

Watanabe et al. [26] and Antonucci et al. [27] investigated the reduction of water above 100°C to maintain proton conductivity by incorporating hydrophilic micronsized metal oxide particles such as SiO and TiO into Nafion with limited success. Peak power densities of 250 and 150 mW-cm" in oxygen and air, respectively, were reported. Mauritz et al. used an in situ sol-gel technique to intfoduce a polymeric form of SiO into Nafion to form composite membranes [28]. Recently, Adjemian et al. utilized Mauritz synthetic procedure to fabricate Nafion/SiOj membranes and demonstrated that water management within Nafion improved at elevated temperatures in a 3-atm pressurized H /O PEMFC [29]. Various PFSAs, including Nafion and Aciplex, were studied as pure and in the SiO composite membranes for operation in H /O PEMFCs from 80 to 140°C. These cells demonstrated acceptable current densities, for instance, Nafion-IH/SiO and Nafion-112/Si02 achieved 850 and 1280 mA cm", respectively, at 0.4 V up to at least 130°C, 3 atm pressure, and 100% relative humidity. 

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