Characterization of a series of azobenzene-containing liquid crystalline methacrylate polymers and their thermochromic properties

A series of Poly-{lcub}4′-{lcub}(X-methacryloyloxyalkyl)methylamino{rcub}-4-nitroazobenzene{rcub} polymers were characterized for their liquid crystalline (LC) properties. The polymers are abbreviated pXMAN, where X is the number of methylene units in the spacer and varies from 2 to 12. All of the polymers exhibit a smectic A phase and only p2MAN also contains a nematic phase. After spin-casting the polymer films are red, heating to the Tg range causes the polymers to become orange and continued heating to the isotropization range causes the polymers to become red once again. Kinetics experiments showed that the processes driving the colour changes are polymer backbone relaxation and mesogen relaxation. Birefringence results showed how the polymers with shorter spacers typically exhibit higher levels of induced birefringence at high laser intensities. Birefringence experiments also showed that induced anisotropy can be enhanced by annealing optically aligned polymer films above Tg. In another set of experiments a small molecule (4′-{lcub}(4-isobutyrylolyoxybutyl)methylamino{rcub}-4-nitroazobenzene or 4MOD) with structure analogous to that of the pXMAN polymers was synthesized and characterized for LC properties. Although it does not possess a low ordered liquid crystalline phase it undergoes a crystal to crystal transition. X-ray also showed that the crystal structure of this material occurs with an anti-parallel alignment of azobenzene molecules.