Membrane morphological parameter

Before presenting the main microscopy techniques employed to determine membrane morphology, it is worth listing and briefly describing the membrane morphological parameters normally measured. 

Several approaches have been done to quantify membrane morphology parameters from SEM images. The first attempts were made in the early 1990s. They permitted more quantitative information to be obtained from membrane SEM micrographs and they showed the potential power of computer-aided SEM analysis [30, 31]. More recentiy, other approaches based on different model equations have been reported [32, 33]. The IFME software [32], was developed by our research group within the past years and permits numerical results of the main morphological parameters to be obtained from SEM micrographs. 

In some specific cases other parameters can be considered as important in the characterisation of membrane morphology like the surface roughness, pore anisotropy and porous network connectivity [16,17]. Concepts of percolation and fractal geometry are also of interest to better describe the statistical and random structures of many porous solids [14,18,19]. 

L. Zeman and L. Denault, Characterization of microfiltration membranes by image analysis of electron micrographs Part 1. Method develoment. /. Membr. Sci., 71 (1992) 221. L. Zeman, Characterization of microfiltration membranes by image analysis of electron micrographs Part II. Functional and morphological parameters. /. Membr. Sci., 71 (1992) 233. 

Various treatments of Eqs. (12.5) to (12.8) are proposed in the literature for those cases where charge effects cue negligible (microfiltration, ultrafiltration or nanofiltration of neutral solutes), and low concentrated solutions are considered, i.e. with negligible non-idealities and activities assimilated to concentrations. Most of the time, membrane morphology is just considered through simple parameters such as effective pore size, tortuosity accounting for the effect of fouling on measurable transport properties. A well-known model thus obtained is due to Kedem and Katchalsky [5] ... 

Many characterization techniques developed for the characterization of meso-porous and microporous materials have been adapted to membrane characterization (e.g., mercury porosimetry, adsorption and desorption isotherms, and thermoporometry). These techniques are related to morphological parameters... 

The question arises as to what parameters are important for membrane morphology and how can the latter be controlled In section in - 7 the influence of the most important membrane formation parameters will be described in relation to the membrane strucmre obtained. Other topics which have to be described is the determination of the various interaction parameters. The interaction parameter berween solvent and nonsolvent (x 12 O " S12 (concentration dependent) will be described in section III.7. Here, the determination of the other two parameters, polymer-nonsolvent (xi 3) and polymer-solvent (X23) will be described briefly. 

In the previous section the thermodynamic and kinetic relationships have been given to describe membrane formation by phase inversion processes. These relationships contain various parameters which have a large impact on the diffusion and demixing processes and hence on the ultimate membrane morphology. It has been shown that two different types of membranes may be obtained, the porous membrane (microfiltration and ultrafiltration) and the nonporous membrane (pervaporation and gas separation), depending on the type of formation mechanism, i.e. instantaneous demixing or delayed onset of demixing, involved. 

In this respect the choice of the polymer is not so important, although it directly influences the range solvents and nonsolvents that can be used. In this section the effect of various parameters on membrane morphology will be described. Two widely used polymers, polysulfone (PSf) and cellulose acetate (CA) will be taken as examples. The following factors will be described ... 

Characterisation data for porous membranes often give rise to misunderstandings and misinterpretations. It should be realised that even when the pore sizes and pore size distributions have been determined properly the morphological parameters have been determined. Howeven in actual separation processes the membrane performance is mainly controlled by other factors, e.g. concentration polarisation and fouling. 

It is often very difficult to relate the structure-related parameters directly to the permeation-related parameters because the pore size and shape is not very well defined. The configuration of the pores (cylindrical, packed-spheres) used in simple model descriptions deviate sometimes dramatically from the actual morphology, as depicted schematically in figure IV - 3. Nevenheless, a combination of well defined characterisation techniques can give information about membrane morphology which can be used as a first estimate in determining possible fields of application. In addition, it can serve as a feed-back for membrane preparation. 

This amazing versatility and usefulness of the controlled application of forces to vesicles motivated us to build a micropipette station as a standard tool to measure membrane material parameters (Figure 12.1). The technique has been improved by implementing real-time analysis of the morphological vesicle response to applied forces within an arrangement of computer-controlled components. 

The most important is the last one. To obtain it, the sample needs to be previously treated. The membrane has to be broken, but not cut in order to preserve the porous structure. To do this, the most used technique is to dip the sample in an ethanol bath (to ensure that all the pores of the membranes are filled with the alcohol) and afterwards to immerse it in a liquid nitrogen bath to freeze the ethanol. Then, the membrane can be broken. Once the micrograph is obtained, it has to be interpreted. It is desirable to obtain numerical results of the main morphological parameters, such as the pore size distribution, porosity, symmetry, regularity and tortuosity. To obtain these parameters in a systematic and fast way the IFME software can be used [32]. 

I 3 Microscopy Techniques for the Characterization of Membrane Morphology Table 3.1 Morphology parameters obtained from IFME. 

Most of the studies reviewed in this section apply CSLM to morphological membrane characterization and relate the results with the membrane formation parameters and transport properties. Considering that the type, nature, preparation method and final use of the membrane can differ significantly, the CSLM analysis protocol has to be particularized for each application. 

The following parameters were determined by analysing microscopic views of wear particles on membrane filters, where shadow images were used for morphological parameters while reflection images used for optical parameters [4-6]. Figure 4 illustrates the basic information for calculating the parameters. 

Solubility parameters are indexes usually employed to evaluate the interactions between the polymer, solvent, and nonsolvent, such as the solvent s ability to dissolve a given polymer, the miscibility between solvent and nonsolvent, and the coagulation power of a nonsolvent toward a polymer of interest. These interactions will strongly affect the path followed by the P/S/NS system during PI, and hence the final membrane morphology. 

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