Structural and photoresponsive behavior of a series of (phenylene)azobenzene-containing liquid-crystalline methacrylic copolymers

Abstract

A series of side-chain liquid-crystalline methacrylic copolymers, having various amounts of 4′-(4-decyloxyphenyl)-4-(hexyloxy)-azobenzene as the mesogenic group, were synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction to determine their mesomorphic properties, and by UV−visible spectrometry to elucidate their optical absorption characteristics. Also, birefringence and surface relief gratings were photoinduced in the films using a laser beam of 413 nm. Copolymers with high mesogen contents display a polymorphic behavior of the smectic type. The smectic order tends to disappear as the content of mesogens in the copolymer decreases, going from extended lamellar domains of chromophores packed in a parallel position, typical for smectic liquid crystals, to small aggregates of chromophores dispersed in an amorphous matrix. Important hypsochromic shifts (Δλmax ≈ 20 nm), associated with π interactions between neighboring aromatic groups, confirm the aggregation of chromophores in lamellar structures. The light-induced birefringence (0.005 − 0.06) is not as high as expected but it corroborates that there exists a tight interplay between photoinduced orientation and intrinsic molecular order. Surface relief gratings were recorded in copolymers with low mesogen content (15 − 30 mol%), showing by atomic force microscopy sinusoidal profiles with an average depth of around 150 nm. © 2013 Society of Chemical Industry