Photo-orientation Processes in Liquid Crystalline Polymethacrylates with Side Azobenzene Groups Having Lateral Methyl Substituents

Design of new photoactive materials exhibiting nano-organization aiming for smart practical applications remains an actual and highlighted task. The synthesis and comparative study of chiral liquid crystalline polymethacrylates with azobenzene groups having lateral methyl substituents were carried out to contribute to a better understanding of their structure–property relationship. The synthesized polymers possess the same photochromic fragment and differ in the length of the spacer (m) and the terminal chiral group (n, 6 and 10 methylene units). Racemic analogues of the chiral polymethacrylates were also synthesized and studied. The effects of chirality and length of the spacer of the terminal alkyl groups on the mesomorphic behavior and photo-optical properties were established. Depending on the chemical structure of the variable fragments and chirality, the polymers exhibit the nematic, the cholesteric, the orthogonal smectic A, and other smectic phases. The presence of the lateral methyl substituent in the photochrome/mesogen fragment leads to a significant decrease in isotropization temperature compared with the same azobenzene-containing polymers without lateral substitution. The photo-orientation processes under illumination with polarized visible light (457 nm) are studied for the amorphousized films of the investigated liquid crystalline polymethacrylates. It was found that an increase in the spacer length leads to an increase in the value of photoinduced dichroism (ΔD = 0.28). The maximal photoinduced dichroism is independent of the length of the terminal chiral group. The maximal dichroism (D = 0.78) under polarized visible light irradiation is found for the polymers with m = 10 and n = 6. Upon annealing the irradiated polymer films at room temperature, a reversible change in the dichroism value was observed. Chirality plays a significant role in the relaxation behavior of the irradiated enantiopure and racemic films of polymers with a long spacer and alkyl tail (10 methylene units).