Liquid-crystalline and photo-induced properties of P4VP quaternized with bromo derivative of dialkyloxy-phenyleneazobenzene groups

Poly(4-vinylpyridine) was partially (20% and 50%) and fully (100%) quaternized with a bromo-terminated 4,4-dialkyloxy-substituted cyanophenyleneazobenzene group (Azo-Br). The resulting liquid crystal azopolymers were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) to determine their thermotropic behavior. They showed only a smectic phase that extends over a broad temperature range. The smectic period was found to correspond to a double layer stacking where the pendant groups (mesogens) are partially interpenetrated. On the other hand, the azopolymers were irradiated (UV-vis wavelengths) to induce trans-to-cis conversion (photoisomerization), molecular alignment (birefringence), and mass migration (surface relief gratings). Results showed the lowest trans-to-cis photo conversion for the polymer with the highest content (100%) of chromophore units, and it was simply attributed to constrains arising from their close packing. On the contrary, this polymer showed the highest birefringence value suggesting a coordinated motion of the tightly held chromophore units at the smectic domains, as it was indeed reported for other photo-active liquid crystal azopolymers. Finally, surface relief gratings, having a sinusoidal profile with regular periodicity, were registered. (c) 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44819.