Polymerization in oriented monolayers and vesicles

Interest in this area appears to have developed at round the same time as for lyotropic side chain polymers. Certainly the types of molecules incorporated into the various polymeric structures bear a close functional resemblance to one another. The driving force behind studies in this field, however, has been the biological interest in producing systems which are able to mimic the properties of cell membranes. For a more in-depth introduction to the subject the reader is directed to other reviews, such as those by Ringsdorf et and by Fendler.  

Biological membranes typically contain about 50% each of lipids and proteins. Perhaps the most familiar model of the membrane make-up has been that of Singer and Nicolson (shown in Fig. 5.19). In it the lipids form a bilayer similar to those found in lamellar liquid crystals. The bilayer structure is spanned by large globular protein complexes which serve as selective barriers to the transportation of metabolites in and out of the cell. In addition to this they also confer a degree of stability on the structure, preventing it from breaking apart. The exact nature of this stabilization is obviously rather complex and extremely difficult to mimic in the synthesis of artificial membranes. Therefore, polymerization of the 

The polymerizable groups and methods of polymerization are exactly the same as described in Section 5.3. Thus much use has been made of amphiphiles containing, for example, acryloyl, methacryloyl, vinyl and diacetyl groups. More recently, surfactants containing styrene moieties have also been polymerized. In theory, concentrated systems of the polymers produced should result in the formation of liquid crystalline structures. Such materials, however, have not been studied for LC behaviour as their prime interest lies within the field of polymer membranes. 

A number of methods have recently been applied to determining the properties of polymer membranes. These include the spreading of mono-layers at an air-water interface and also the production of vesicles or liposomes. The former method allows for the compression of the two-dimensional film between movable barriers. At the same time a film balance registers the change in surface pressure as a function of molecular area, thereby producing a pressure-area isotherm. Various states of molecular orientation can be determined from the shapes of the plot, typified by the diagram in Fig. 5.20. Such states are analogous to those in three-dimensional systems, i.e. solid, liquid, gas. 

The polymers are commonly produced by first spreading the film and then inducing the reaction process, rather than by spreading a layer of previously polymerized material. Many studies have been carried out on the variation of reaction kinetics with monolayer state. For example, certain diacetylenic lipids have been shown to undergo polymerization 

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