Structure of the Casting Solution

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. 

Figure 7. Photo-micrographs of the casting solution/precipitant interphase at (a) the beginning of the precipitation and after (b) 12 sec., (c) 2 f sec. and (d) 5 min. Series I giving a "finger"-type structure and series II giving a "sponge"-type structure.

Rationalization of the Verious Membrane Preparation Procedures. To discuss the formation of membrane structures it is useful to describe the concentration profiles of the casting solution components in the precipitating casting solution. The schematic diagram of Figure 1.21 shows the concentration profiles of polymer, solvent and precipitant at some intermediate time during the precipitation of a polymer film cast on a glass plate. 

Usually, a flat sheet membrane is prepared by spreading a casting solution on a flat surface and evaporating the solvent. A thin polymeric layer that is formed between air and the bulk of the casting solution is called the active or the top layer of the membrane. The performance of the membrane depends largely on the physical or molecular structure of the active layer. 

Other preparation parameters are polymer concentration, evaporation time, humidity, temperature, and the composition of the casting solution (e.g. additives). These parameters are mainly determining the ultimate membrane perfonnance (flux and selectivity) and hence for its application. The relation between these parameters and membrane structure will be described in greater detail in section IH - 6. The membranes obtained after precipitation can be used directly or a post treatment (e.g. heat treatment) can be applied. 

It was shown by an electron micrograph that a membrane consists of spherical micelles packed densely at the membrane surface and loosely in the interior of the membrane. It was also shown that the structure of the polymer in a film casting solution is essentially the same as that of the membrane. In other words, the microscopic structure of a membrane is intrinsically governed by that of the casting solution structure. It is therefore interesting to know about the structure of the polymer in a film casting solution, and particularly about the size of individual polymeric molecules and their packing mode. 

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