Morphology asymmetric membranes

MV growth is an example of several questions that can be addressed by membrane processing in low gravity. It is quite possible that entirely new membrane structures may be created in low-g. Indeed, such low-g studies may lead to modified manufacturing practices that can be used to control asymmetric membrane morphologies. 

It has been shown (, , 2.) that a membrane casting dope is a strongly structurlzed polymer solution, and that the morphology of the membrane surface layer can be correlated to the structure of the casting solution. The latter parameter affects the nature and details of the phase inversion process occuring in the upper part of the cast solution, in an incipient skin. Thus the solution structure is one of the factors responsible for the skin properties, and consequently for the performance of the ultimately formed asymmetric membrane. 

Membrane morphology is studied with scanning electron microscopy (SEM) thereby providing an Insight into the relationship between asymmetric membrane preparation, structure, and performance (29,3A). The extent of ion exchange of the salt form of the SPSF membranes is studied with atomic absorption spectroscopy (AAS), neutron activation analysis (NAA), and ESCA. AAS is used for solution analysis, NAA for the bulk membrane analysis, and ESCA for the surface analysis. 

An important point to consider about hollow fiber membranes is their morphology. Hollow fiber membranes can be either symmetric or asymmetric.16 Symmetric membranes have continuous pore structure throughout. Asymmetric membranes have a dense upper layer or skin layer that is then supported with a sublayer that is significantly more porous. Figure 6.2 shows SEM images of... 

Asymmetric membrane Membrane constituted of two or more structural planes of non-identical morphologies. 

In case of complex membrane morphology such as asymmetric or composite membranes, or when Fickian diffusion is not valid, evaluating will be more complex. Individual mass transfer coefficients in Equation 2.2 depend on multiple factors such as temperarnre, pressure, flow rates, and diffusion coefficients and could often be estimated from empirical correlations available in literature [1,2,6]. 

Membrane morphology (or structure) is generally classified into two groups porous and dense. Dense fibers utilize the chemical and the physical characteristics of their structure to provide separation depending on the diffusivity or solubility of the solute species. In the case of porous membranes (Fig. 1), internal structure could be symmetric, asymmetric, or composite. Asymmetric membranes feature a very thin active layer, responsible for the separation process, supported... 

The flux of 0.03 gfd for the homogeneous polyamide membrane was more than two orders of magnitude too low for commercial desalination. The flux was increased 175 fold with no decrease in salt rejection by casting the membrane with asynmetric morphology. Even higher fluxes, up to 3.5 times that observed for the asymmetric MPD-l/T (100-70/30) polyamide membrane, were obtained with asymmetric membranes cast from polyhydrazides and polyamide-hydrazides. Permeation properties for the three types of aromatic polyamides are shown in Table IX. The RO properties of this group of membranes illustrate the combined effects of Structure Levels I, II and III on membrane performance. 

One can actually consider the trapped solution morphology as a functional definition of the asymmetric membranes. It should be emphasized that this viewpoint clearly differentiates asymmetric membranes that have shown the highest reverse osmosis fluxes from membranes with a thin dense layer of normal solid morphology. 

Further approaches to meet the requirement of high selectivity may Include the blending of glassy and rubbery polymers, the chemical alteration of the dense skin-layer of Integral-asymmetric membranes and morphological variations of dense polymer films by proper post-treatment—as exemplified In this paper for CA blend membranes. 

The permeability and udectivjty of an asymmetric membrane are determined by its complex morphology... 

Technology development on the fabrication of asymmetric membranes with an ultrathin dense layer has received much attention due to the fact that the thinner the dense layer is, the higher is the productivity. The fabrication of a hollow fiber with a desirable pore-size distribution and performance is not a trivial process as many factors influence fiber morphology during the phase inversion. 

Kawakami et al. prepared dense and asymmetric membranes from 2,2 -bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and bis[4-(4-aminophen-oxy)phenyl]sulfone (APPS) by solvent evaporation (dense) and by the dry-wet phase inversion technique [47]. The surface morphology was studied by AFM. They reported that the solvent evaporation method adopted for the preparation of the dense membrane influenced the formation of nodules, while the dry-wet process in which solvent/nonsolvent exchange was involved determined the roughness of the skin layer. 

The studies of membrane morphology by SEM have produced a large number of cross-sectional pictures for polymeric membranes since the onset of asymmetric cellulose acetate membranes by Loeb and Sourirajan. The contribution of those pictures to the design of novel membranes with improved performance was truly phenomenal. SEM requires cumbersome sample preparation, which may hinder true images. AFM does not need such sample preparation, and the pictures taken by AFM are considered to reflect the true nature of membrane morphology. 

Park HC, Moon YS, Rhee HW, Won J> Kang YS> Kim UY (1999) Effect of solvent exchange on the morphology of asymmetric membranes. In Pinnanau I> Freeman BD (eds) Membrane formation and modification. ACS Symposium Series 744. American Chemical Society, Washington, DC, p 121... 

Although models and techniques such as those described in preceding sections permit characterization of dense-film transport properties, a need exists for improved characterization of the transport behavior of asymmetric membrane properties. Currently, performance tests such as gas permeability and selectivity using standardized feed streams provide useful tools for quality control but are ambiguous for fiindamental characterizations of the type possible for dense membranes. The following discussion presents information pertinent to the characterization of porosity and morphology of asymmetric membranes and to analysis of their flux decline as a function of time in service. These are two of the most important additional characteristics that asymmetric structures introduce into the description of transport behavior of the separator module. 

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