Photoresponsive side-chain liquid crystalline polymers with amide group-substituted azobenzene mesogens: effects of hydrogen bonding, flexible spacers, and terminal tails

Abstract

The synthesis of a series of new photoresponsive side-chain liquid crystalline polymethacrylates with amide group-substituted azobenzene (azo) mesogens and different length of flexible spacers and terminal tails via conventional free radical polymerization is described. The resulting azo polymers proved to have high thermal stability and good solubility in common organic solvents (e.g., tetrahydrofuran and chloroform). Differential scanning calorimetry, polarizing optical microscopy, and small angle X-ray scattering studies confirmed the presence of obvious enantiotropic smectic C liquid crystalline phases (with a bilayer lamellar structure) for all these polymers. The introduction of an amide group into the azo mesogen led to the formation of strong hydrogen bonding among the side chains of the polymers (as revealed by variable temperature FT-IR), which played a decisive role in forming and stabilizing the liquid crystalline mesophases of the polymers. In addition, the length of the flexible spacers and terminal tails also significantly influenced their phase transition behaviors. Furthermore, the photoresponsivity of the polymer solutions was verified and the effects of the molecular structures of the polymers on their photoresponsive properties were also studied.