Polarised light-induced orientation and reorientation processes and unexpected 'memory effect' in side-chain azobenzene-containing LC polymers

Photoorientation phenomena in thin films of side-chain azobenzene-containing polymethacrylates having different length of spacer (4, 6 and 10 methylene units) were studied in detail. Irradiation of these films with polarised blue light (473 nm) leads to photoorientation of photochromic side groups in a direction perpendicular to the polarisation plane of the incident light. It was shown that values of photoinduced dichroism increase with the lengthening of the methylene spacer. Annealing of the polymers films at a temperature higher than glass transition results in an additional growth of dichroism. However, the degree of uniaxial order of the annealed films was almost the same. The reorientation phenomena in irradiated and annealed films were studied by the irradiation of these films with the light of the same wavelength (473 nm) but polarisation rotated by 90° with respect to the first irradiation cycle. A study of the kinetics of the reorientation showed that the polymers with a spacer having four and six methylenic groups undergo the fast complete reorientation, whereas the polymer with 10 methylenic units is characterised by a much slower reorientation process. Moreover, an unusual ‘memory effect’ was found for the polymer films deposited on quartz substrates; annealing of the films after reorientation restores an initial orientation state achieved by the first irradiation cycle, whereas the films spin-coated on mica surface do not demonstrate this phenomenon. The ‘memory effect’ is probably associated with the roughness of the quartz substrates.