Polymerization-induced epitaxy of polylactones on graphite as probed by scanning tunnelling microscopy

Epitaxial films of poly(ε-caprolactone) and poly(δ-valerolactone) are formed on a basal plane of pyrolytic graphite by performing solution polymerization of cyclic lactones in the vicinity of the graphite surface. Scanning tunnelling microscopy (STM) of the film-covered surfaces shows a highly ordered, rectilinear array of strongly tunnelling regions with a detailed feature depending on the polymer. The period and symmetry of the observed patterns are compared with those of model images of proposed molecular structures by making a few simplifying assumptions on the STM contrast. For both polymers, the most probable epitaxial structures explain one of the two principal periods and the symmetry, but fail to reproduce the other periods. Because this paradox is common to both polylactones and has not been observed in similar STM analyses of polyethers and polyamide, the problem is strongly linked with the ester group. We propose an interference effect of tunnelling electrons through the spatially non-symmetric ester group to be responsible for this mismatch. Both polylactones are modelled by planar zigzag chains in an all-trans conformation with the adjacent chain orienting so as to disperse each ester group as far apart as possible, however, their orientations differ by the direction of the fibre axes relative to the graphite lattice.