Characterization of Membrane Morphology

Reproduced with permission from Chemical Pharmaceutical Bulletin, Vol. 34, No. 3. Copyright [1986] Pharmaceutical Society of Japan. 

Several methods can be used in the characterization of the morphology of porous membranes, like the bubble point method, mereury intrusion porometry, gas adsorption-desorption. 

In a scanning electron microscope (SEM, Table 32.3) a fine beam of electrons scans the membrane surface. This causes several kinds of interactions generating different signals, of which secondary electrons (SE) and backscattered electrons (BSE) are used in the image forming (Goldstein et al., 2003). 

Environmental scanning electron microscopy (ESEM) has been developed for studies of wet or nonconducting materials, and it enables the characterization of membrane samples also without coating (Yu et al., 2005). However, the resolution of ESEM is limited compared to conventional SEM and FESEM (Chan and Chen, 2004 Schossig-Tiedemann and Paul, 2001). In addition to morphology characterization, ESEM can also be used in the characterization of the hydrophUicity/hydrophobicity of a surface and in the determination of the swelling of a membrane (Yu et al., 2005, 2006). 

In atomic force microscopy (AFM, Table 32.3), also known as scanning force microscopy (SEM), a small, sharp tip scans the surface of a sample. The tip is located at the free end of a cantilever. Interaction forces between the tip and the sample surface cause the cantilever to bend or deflect. A detector measures the cantilever deflections, and a three-dimensional map of the surface topography of the sample is generated based on the measured deflections (Bowen et al., 1999 Reich et al., 2001 Yalamanchili et al., 1998). 

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I 3 Microscopy Techniques for the Characterization of Membrane Morphology Polysulfone... 

Ramaswamy, S. (2002). Development of an ultrasonic technique for the non-invasive characterization of membrane morphology. Ph.D. Dissertation, University of Colorado, Boulder, CO. 

Another possible approach to indirectly characterize the membrane morphology is based on the investigation of the free volume within the matrix. Density measurements [119,120] and positron annihilation lifetime spectroscopy evaluation [47] are common methods. Typically, the comparison between the theoretical density or free volume (calculated by simple additivity rules) and the experimental one can reveal the presence of a good interfacial morphology or the presence of interface voids or clustering formation. Fig. 7.13 shows the influence of filler content on the morphology of poly(trimethylsilyl propyne) (PTMSP)/Ti02 NCMs in terms of the volumetric fraction of interface voids as calculated from a comparison of the expected and measured membrane density [119],... 

The structure and morphology of the membrane also influence its permeability. XRD has been widely applied for the characterization of crystalline structure, especially for confirmation of alloy formation in multicomponent materials and identification of bulk corrosion products. SEM and atomic force microscopy are routinely applied to the characterization of membrane topography and are helpful for identification of failure mechanisms. More advanced methods, including... 

This chapter is divided in two sections. The first presents a review of the different techniques used for the morphological characterization of membranes. The second explains some cases studies developed by authors to carry out the mentioned characterization, adding emphasis on software developed by the authors to achieve these purposes, providing quantitative, fast and systematic analysis. 

Membrane characterization by CSLM has been rather limited when compared with other microscopic techniques such as SEM and atomic force microscopy (AFM). The earliest work found in the literature [13] records how van den Berg et al. used a combination of AFM and CSLM to study qualitative differences in the pore geometry of different brands of polypropylene membranes. The first reported applications that used only CSLM for membrane characterization [14,15] were by Charcosset et al. who used CSLM to characterize microporous membrane morphologies and to obtain values of surface porosity and pore size. The conclusions of those studies were that CSLM gave some characteristics on membrane morphology that SEM, which views only surfaces, cannot provide. However, as also mentioned previously in this chapter, they pointed out low resolution for membrane characterization as the main drawback of CSLM. This restricts the use of CSLM to the characterization of microfiltration membranes if measurements on pore size and surface porosity have to be performed. 

Ramaswamy, S., Greenberg, A. R., and Peterson, M. L. (2004). Non-invasive measurement of membrane morphology via UFDR Pore-size characterization. J. Membr. Sci. 239, 143. 

El Jastimi R, Edwards K, Lafleur M. Characterization of permeability and morphological perturbations induced by nisin on phosphatidylcholine membranes. Biophys J 1999(Aug) 77(2) 842-852. 

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