Photochromic liquid crystal polymers

Copolymers containing both spiropyran and mesogenic groups were synthesized either by free radical polymerization or by a polymer analogous reaction. The thermal back reaction leading to equilibrium between spiropyran and merocyanine forms of the photochrome proceeds in a mesophase with much lower rate than in an amorphous phase and is controlled by the viscosity of a polymer. An interesting feature of these systems is the physical crosslinking of the macramolecules due to dimerization of the merocyanine side groups. This results in formation of a network on heating or irradiation and leads to a new interesting rheo-optical effect above the clearing point. The copolymer films above the clearing point exhibit very strong dynamic birefringence on mechanical disturbance.