Nodule aggregates

Important membrane surface properties include the size of nodules and nodule aggregates, the shape of pores, the pore size and pore size distribution, and the surface roughness. In this chapter, the focus will be on nodules and nodular aggregates since AFM seems most suitable for those. Moreover, there is evidence that nodular structure has some relationship to membrane performance. 

Khayet et al. [31] studied the surfaces of asymmetric poly(etherimide) (PEI) ultrafiltration membranes by AFM. Membranes were prepared by casting mixtures of PEI, hydroxybutyric acid y-lactone (GBL, y-butyrolactone) as nonsolvent and N,N-dimethylacetamide (DMAc) at 50 °C. The average size of the nodule aggregates versus PEI concentration is given in Fig. 4.16. From Fig. 4.16, it is clear that the nodule aggregate diameter increases as the PEI concentration increases. 

Samples Nodules (nm) Nodule aggregates (nm) Supernodular aggregates (nm)... 

The diameter of the nodules appears to be 20 and 16 nm for S and M membranes, respectively. For the H membrane, supernodular aggregates of 620 nm contained 6-8 nodule aggregates of 140 nm (Fig. 4.27). In addition to their small diameter (12 nm), the Si02 nodules are more discernable than those of M and S membranes. 

Figure 4.36a-d is for samples taken from inner surface of a hollow fiber spun at a 10-cm air gap. From the surface images a and b, the diameters of the nodule aggregates were obtained and the results shown in Table 4.9. Again, no significant difference in diameter was observed between samples a and b. However, Fig. 4.36a appears very similar to Fig. 4.36b. 

Figures 4.38 and 4.39 show, respectively, the AFM images of the outer surfaces of the hollow fibers spun at the air gaps of 1 cm and 10 cm. Again, a and b are the surface images of two samples taken from two different sites along a hoUow fiber, while c and d are the three-dimensional images of another two sites. The sizes (diameters) of the nodule aggregates summarized in Table 4.9 show that ...

Table 5.18. Roughness parameters and curvature radii of nodule aggregates (parameters were calculated on a 2 X 2 pm area scan) on plasma-treated cellulose ester membranes...

Figure 6.10 shows the AFM image (scan range 2 im) of the midsection of the wall of a hollow fiber that was fabricated at a 50-cm air gap. From Fig. 6.10 it seems there are spheres in the cross section, which are designated as nodules due to their dimensions, as suggested by Resting [15]. Some nodules are fused with each other to form nodule aggregates. In Fig. 6.10 there are dark areas, which maybe macrovoids, or paths of pores that are commonly observed by SEM. 

The cross section near to the inner layer contains well-defined nodules with a smaller number of nodules and nodule aggregates. 

In flat sheet membranes, the nodules/nodule aggregates observed in the cross section are aligned parallel to the surface-forming layers. In hollow fibers, the aUgnment of the nodules was observed in the angular direction. In hollow fiber formation, the rate of the bore fluid influences the alignment of the nodules. 

A dense homogeneous cellulose acetate membrane was studied by AFM in its dry and wet state [50], Figures 8.14, 8.15, and 8.16 show the three-dimensional AFM images of the CA membrane in a dry state, after contacting the surface with a few drops of water, and after drying the water-treated membrane at room temperature for 4 days, respectively. Table 8.10 summarizes the mean nodule aggregate sizes and the roughness parameters of the surfaces. 

CA membrane Roughness. Ra (nm) Nodule aggregate size (nm) Mean Max. Min. ... 

The atomic force microscope work has shown the size of the aggregates of nodule aggregates (ANA s) vary with the spinning conditions and also... 

It was also revealed that the sizes of nodule aggregates on the top layer were smaller than the nodule aggregates in the inner layer. Figure 37 shows the AFM image of the PPO-TCE membrane after the membrane has been etched for 3,200 second. 

Figure 38. Arrangement and size of supemodular aggregates and nodule aggregates in dense PPO-CS2 membrane.

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