AFM study of two-dimensional epitaxial arrays of poly(γ-L-glutamates) with long n-alkyl side chains on graphite

Two-dimensional (2D) arrays of poly(gamma-n-alkyl L-glutamates) (PGn) on HOPG are found to be formed by casting under conditions producing submonolayer coverage. Direct observation of the arrays in a series of polymers using atomic force microscopy allowed clear visualization of the island structures, which consisted of parallel running rods. The helical polymer chains were epitaxially aligned to form rods with a "head to head" or a "head to tail" association. In the array composed of rods, the extended alkyl side chains with all-trans conformation are divided to both sides of a helical main chain and align perpendicularly to the main chain. The interval distances between parallel rods in the island increased linearly by about 2.2 Angstrom/CH2 unit with increasing alkyl side chain length. Epitaxial adsorption of alkyl chain moieties onto graphite is the primary driving force in the formation of the 2D array structure of these helical polymers.