Modification of homogeneous dense membranes

Chemical or physical modification of homogeneous membranes can drastically change their intrinsic properties, especially when ionic groups are introduced. Such charged membranes can be applied in eiectrodialysis, where ionic groups are necessary. Ionic membranes also show remarkable results in other processes. 

We shall describe two examples of modification of homogeneous dense films here, one by chemical and the other by physical means. The first example concerns polyethylene, which, although a very important bulk plastic, is only in limited use as 

Another method of modifying dense membranes is by means of grafting (e.g. radiation-induced grafting, see chapter II). This method allows a number of different kinds of groups to be introduced into the polymer, resulting in membranes with completely different properties [14]. A representation of this technique is given in figure III -16. 

A polymer film (III - I6a) is irradiated with electrons (= 200 keV) which leads to the generation of radicals(lll - l6b .The film is now immersed in amonomerbath where the monomers diffuse into the film (III- 16c). Polymerisation is initiated at the radicaliites in. the polymeric substrate and a graft polymer is covalently bound to the basic polymer. 

Not all kinds of low molecular weight monomers can be used for these polymerisations, since for example an unsaturated group RHC = CH. must be present. However, this technique will allow ionic groups (both acidic and basic) and neutral groups to be introduced. Table III.2 gives some examples of monomers which can be used. Very specific membranes can be developed by this technique because of the large number of possible variations. 

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