Synthesis of azobenzene-containing side chain liquid crystalline diblock copolymers using RAFT polymerization and photo-responsive behavior

Well-defined azobenzene-containing side chain liquid crystalline diblock copolymers composed of poly[6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate] (PAzoMA) and poly(glycidyl methacrylate) (PGMA) were synthesized by a two-step reversible addition-fragmentation chain transfer polymerization (RAFT). The thermal liquid-crystalline phase behavior of the PGMA-b-PAzoMA diblock copolymers in bulk were measured by differential scanning calorimetry (DSC) and polarized light microscopy (POM). The synthesized diblock copolymers exhibited a smectic and nematic liquid crystalline phase over a relatively wide temperature range. With increasing the weight fraction of the PAzoMA block, the phase transition temperatures, and corresponding enthalpy changes increased. Atomic force microscope (AFM) measurements confirmed the formation of the microphase separation in PGMA-b-PAzoMA diblock copolymer thin films and the microphase separation became more obvious after cross-linking the PGMA block. The photochemical transition behavior of the PGMA-b-PAzoMA diblock copolymers in solution and in thin films were investigated by UV-vis spectrometry. It was found that the trans-cis isomerization of diblock copolymers was slower than that of the corresponding PAzoMA homopolymer and the photoisomerization rates decreased with increasing either the length of PAzoMA block or PGMA block. The photo-induced isomerization in solid films was quite different with that in CHCl₃ solution due to the aggregation of the azobenzene chromophore. The cross-linking structures severely suppressed the photoisomerization of azobenzene chromophore. These results may provide guidelines for the design of effective photo-responsive anisotropic materials.