Cross-sectional AFM Image

Abstract

A large number of SEM pictures have been taken to show the cross-sectional structure of integrally skinned asymmetric membranes and thin film composite membranes. Figures 6.1 [1] and 6.2 [2] are typical examples of such pictures, showing the top skin layer supported by a porous sublayer comprised of either sponge-like pores (Fig. 6.1) or finger-like pores (Fig. 6.2). Although these pictures can clearly distinguish the top skin layer from the support layer, whereby the measurement of the thickness of the top skin layer is enabled, they do not reveal the structure inside the top skin layer. Since the performance of the membrane is primarily governed by the top skin layer, whose structure will obviously control membrane performance such as flux and selectivity, it would be beneficial to have a closer view of the top skin layer. The cross-sectional pictures taken by Panar et al. [3] (see Chap. 4) revealing the nodular structure near the top edge of the membranes are probably the first attempt to examine the detailed structure inside the top skin layer. Similar attempts have been recorded since then in the literature, but the number is limited. There are many methods of cutting, slicing, and fractioning by using a razor blade, microtome, or ultramicrotome. Cutting can also be done at normal temperature or in liquid nitrogen. Usually hollow fibers or membranes are fractured at the liquid nitrogen temperature to keep the originality of the required testing area in cross-sectional imaging by SEM or TEM. It is, however, difficult to obtain a smooth cut of the cross section of the hollow fiber or membrane. Cross-sectional pictures have not been taken by AFM for a long time due to difficulties involved in the preparation of a smooth cross-sectional area of a reasonable size when the membrane is sliced or fractured. An attempt to overcome these difficulties was made recently by Khulbe [4], the results of which will be briefly outlined.