Photo-Orientation Phenomena in Photochromic Liquid Crystalline Azobenzene-Containing Polymethacrylates with Different Spacer Length

Synthesis and investigation of the phase behavior and photo-optical properties of two chiral liquid-crystalline polymers with azobenzene-containing side groups having different spacer lengths, 6 and 10 methylene groups (PMAzo-6 and PMAzo-10, respectively) are described. Formation of different smectic phases and high-temperature cholesteric phase is studied by polarizing optical microscope, differential scanning calorimetry, and X-ray investigations. It is shown that UV-irradiation induces effective reversible E-Z photoisomerization in polymer solutions and films. Atomic force microscopy (AFM) study reveals substantial changes in the surface topography of the polymer PMAzo-6 film after UV-irradiation, whereas PMAzo-10 surface remains the same. Irradiation by polarized light (457 nm) results in photo-orientation process in polymer films consequential in significant alignment of the chromophores in direction perpendicular to the polarization plane of the incident light. A significant difference is found in thermal stability of the photoinduced alignment; an annealing of PMAzo-10 irradiated samples results in a slight decrease of dichroism values (down to 0.57); whereas the dichroism increases up to very high values (0.91) for PMAzo-6.