Azobenzene-containing liquid crystal triblock copolymers: Synthesis, characterization, and self-assembly behavior

A series of azobenzene-containing isotropic/nematic/isotropic liquid crystal (LC) triblock copolymers (PBMA-b-PMAazo444-b-PBMA) with different block ratios was synthesized by atom transfer radical polymerization (ATRP). The central block PMAazo444 is an azobenzene-containing side-on nematic liquid crystalline polymer and PBMA a coil polymer. These azo-triblock copolymers were prepared and studied with the aim of producing, in the future, photoresponsive elastomers with lamellar structure. Kinetic Studies on the polymerization of the azobenzene-containing LC monomer demonstrate that its polymerization is a controlled process. A LC homopolymer with narrow molecular weight distribution (M-w/M-n = 1. 13) was used as a difunctional macroinitiator to prepare the triblock copolymers by ATRP. The triblock copolymers were characterized by NMR, SEC, DSC, and POM. A triblock copolymer with 47 wt % of LC part self-assembles into a lamellar phase as evidenced both by SAXS and by TEM. The surface alignment of this lamellar phase on a silicon substrate was studied by AFM and compared with its nonphotosensitive triblock homologue (PBA-b-PA444-b-PBA). For this nonphotosensitive triblock copolymer the lamellas aligned parallel to the substrate, while for the azobenzene-containing triblock copolymer they organized perpendicular to the substrate.