Atomic force microscopy study of surface topography of films of cholesteric oligomer- and polymer-based mixtures with photovariable helix pitch

The surface topography of glass-forming polymers and oligomer cholesteric systems with a phototunable helix pitch was studied. For this purpose several mixtures based on nematic polyacrylate and cholesteric cyclosiloxanes doped with chiral-photochromic dopant were prepared and investigated. The molecules of chiral-photochromic dopant consist of isosorbide chiral moiety and cinnamic C=C double bonds capable of E-Z photoisomerizing. UV irradiation of the planarly oriented films of mixtures leads to dopant photoisomerization and changes of its helical twisting power. During this process irreversible changes of helix pitch values take place, which allows one to obtain the same cholesteric systems with different helix pitch values. The films of the annealed mixtures were studied by atomic force microscopy and transmission electron microscopy. The correlations between the features of surface topography and helix pitch of cholesteric supramolecular structure were found and discussed.