Supported membranes structure transport properties

We report here on the structure and gas transport properties of asymmetric membranes created by the Langmuir-Blodgett deposition of ultra-thin polymeric lipid films on porous supports. Transmission and grazing angle FTIR spectroscopy provide a measure of the level of molecular order in the n-alkyl side-chains of the polymeric lipid. The level of orientational order was monitored as a function of the temperature. Gas permeation studies as a function of membrane temperature are correlated to the FTIR results. 

We report here on the structure and gas transport properties of asymmetric membranes produced by the LB deposition of a polymeric lipid on porous supports. The effects of temperature on the structure and gas transport is described. The selectivity of CO2 over N2 permeation through the LB polymer films is determined. The polymerized lipid used in this study contains tertiary amines which may influence the CO2 selectivity over N2. The long term objective of our work is to understand how structure and chemistry of ultrathin films influence the gas permeation. 

The efforts and advances during the last 15 years in zeolite membrane and coating research have made it possible to synthesize many zeolitic and related-type materials on a wide variety of supports of different composition, geometry, and structure and also to predict their transport properties. Additionally, the widely exploited adsorption and catalytic properties of zeolites have undoubtedly opened up their scope of application beyond traditional separation and pervaporation processes. As a matter-of-fact, zeolite membranes have already been used in the field of membrane reactors (chemical specialties and commodities) and microchemical systems (microreactors, microseparators, and microsensors). 

In this chapter, pressure driven processes involving porous inorganic or hybrid membranes are particularly examined. As recently shown by Bhave [1], differences with traditional organic elements mainly result from the structure and intrinsic properties of materials, either regarding flow (with ceramic membranes, the transport occiu-s through the intergranular spaces within the top layer, porous sublayers and support, while across polymeric barriers it develops... 

Although no commercial examples exist currently in the gas separation field, thin film composite membranes such as those pioneered by Cadotte and co-workers (10) may ultimately permit the use of novel materials with unique transport properties supported on standard porous membranes. Therefore, the focus in this paper will be on suggesting a basis for understanding differences in the permeability and selectivity properties of glassy polymers. Presumably, if such materials prove to be difficult to fabricate into conventional monolithic asymmetric structures, they could be produced in a composite form. Even if thin film composite structures are used, however, the chemical resistance of the material remains an important consideration. For this reason, a brief discussion of this topic will be offered. 

Potential direction in the SLM gas separation and biochemical conver-sion/separation techniques is the immobilization by selective facilitating agents at the stage of membrane supports fabrication. There is a need for both fundamental and apphed research to develop new membrane supports, incorporating improved transport, stabihty and resistivity properties with specific affinity to separating solutes. The main idea for the membrane production researchers and developers is to produce membrane supports not only with improved transport and stabUity characteristics but also permselective to different gas and hquid solutes. The lamination methods permit to obtain ultrathin films of suitably permselective material to be deposited on prefabricated porous support structures of different composition. 

Most of the measurements to be performed in a membrane-testing program involve asymmetric membranes after the first phase of woric to screen candidate polymers. Nevertheless, it is wise to peribim a benchmark study using dense films of the polymer of interest. This approach permits an unambiguous determination of the sorption and transport properties of the inherent polymer, unobscured by minuscule, but possibly significant, porous defects in the asymmetric structure. Moreover, if such data are available, drifts in tte permeability and selectivity of asymmetric membranes composed of the polymer can be understood better. By making comparisons between the asymmetric merttbrane results and thoM of the dense film, which are free of time-dependent support effects, drifts due solely to support aging can be cpiantified. 

Abstract One of the most critical fuel cell components is the catalyst layer, where electrochemical reduction and oxidation of the reactants and fuels take place kinetics and transport properties influence cell jjerformance. Fundamentals of fuel cell catalysis are explain, concurrent reaction pathways of the methanol oxidation reaction are discussed and a variety of catalysts for applications in low temperature fuel cells is described. The chapter highlights the most common polymer electrolyte membrane fuel cell (PEMFC) anode and cathode catalysts, core shell particles, de-alloyed structures and platinum-free materials, reducing platinum content while ensuring electrochemical activity, concluding with a description of different catalyst supports. The role of direct methanol fuel cell (DMFC) bi-fimctional catalysts is explained and optimization strategies towards a reduction of the overall platinum content are presented. 

Further development of supported membrane struemres will benefit from the availability of suitable characterization tools. The acquisition of accurate fundamental transport parameters, reproducibility, and lifetime behavior requires that several membrane parameters are obtained, preferably in a nondestmetive way. Development of membrane properties is best followed for the same membrane. Membranes and intermediate layers can be very thin and often show considerable penetration in their supporting structure. This hinders unambiguous determination of their thickness and accurate determination of the parameters... 

In some other successful examples, zeolite nanoparticles have been incorporated into a polymer matrix to form a thin-film nanocomposite RO membrane and to create a preferential flow path for water molecules, leading to enhanced water transport through the membrane [64,65]. Use of zeolite in the development of TFN for RO was first reported by Hoek and co-workers [66]. Similarly, Jeong et al. [64] prepared a thin-film RO nanocomposite membrane by interfacial in situ polymerization on porous polysulfone support, in which NaA zeolite nanoparticles were incorporated into a thin PA film. Introduction of zeolite nanoparticles into a conventional PA RO thin film has enhanced flux to more than double of the conventional membrane with a salt rejection of 99.7%, which is attributed to the smoother and more hydrophilic negatively charged surface. Silica nanoparticles of various sizes have also been incorporated into a PA polymer matrix for RO desalination [67]. Presence of silica nanoparticles was found to remarkably modify the PA network structure, and subsequently the pore structure and transport properties with only 1-2 wt% of silica, a membrane was fabricated with significantly enhanced flux and salt rejection. 

Parameters, such as carrier and solvent properties, membrane support, temperature, etc., that influence transport kinetics are analyzed. Structural modifications and kinetic parameters of the carriers that improve the performance of LM are presented. Examples of carrier modifications are given. 

Dacron pressed paper was found to be highly adequate for permeate transport while possessing the wet strength plastic properties needed in winding operations. Dacron paper was also used in casting membranes directly on support structures. 

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