5 - Structure of multilayer films

Summary

Organization in long-chain materials

Long-chain aliphatic materials pack together with their hydrocarbon chains parallel. The simplest scheme is an hexagonal array, with the molecules freely rotating as rigid rods about their long axes. The diameter of the cylinder into which one molecule fits is about 0.48 nm (Kitaigorodskii, 1961). Such a plastic crystalline state (section 1.2), originally known as a rotator phase (Ungar, 1983), may be exhibited by straight chain alkanes and some monolayers just below their melting point. There may even be some similarity with the LS phase in floating monolayer films (section 2.4.3). However, there is still considerable debate about this (Ulman, 1991).

For infinite aliphatic molecules, the hydrocarbon chain takes the form of a zig-zag, repeating at 0.254 nm intervals along the chain axis. In the most stable state, all the CH₂ group carbon atoms lie in a plane to give a flat zigzag (appendix A). Close-packed structures result from the hydrogen atoms in a CH₂ group on one molecule fitting into depressions between hydrogen atoms on adjacent molecules. Different packing arrangements of the C₂H₄ repeat units define the crystallographic nature of the subcell or sublattice. There are three possible close-packed structures with similar packing densities: orthorhombic (R), monoclinic (M) and triclinic (T) (appendix C) (Kitaigorodskii, 1961).