Diblock copolymers composed of a liquid crystalline azo block and a poly(dimethylsiloxane) block: synthesis, morphology and photoresponsive properties

Abstract

A series of diblock copolymers, composed of a soft poly(dimethylsiloxane) (PDMS) block with a defined length and a liquid crystalline poly(6-(4-(4′-cyanophenylazo)phenoxy)hexyl acrylate) (POCN) block with different lengths, was synthesized by the atom transfer radical polymerization (ATRP). A homopolymer POCN was also prepared by the same method for comparison. The polymer structures, liquid crystalline phase, microphase-separated morphology, and photoisomerization were then investigated by ¹H NMR, FT-IR, GPC, UV-Vis, POM, DSC, XRD and TEM. The results showed that the well-defined diblock copolymers (PDMS_n-b-POCN_m) possess four different soft/rigid ratios (n = 58, m = 8, 15, 24, 36) and relatively narrow molecular distributions (PDI ≤ 1.30). POCN blocks of the copolymers form a smectic-A sub-phase, which is identical to the mesomorphic behaviour of POCN. After being annealed at 140 °C in a vacuum for 24 h, the copolymers form a lamellar morphology when W_PDMS is 42.3–57.8% and a morphology of PDMS spheres embedded in an azo matrix when W_PDMS is 23.3–31.4%. POCN and PDMS_n-b-POCN_m in solution show typical photoisomerization behaviour of the azobenzene moieties. Although H-aggregation is observed to a certain extent, photochemical processes of the as-cast films of the polymers are similar to those of the solutions. On the other hand, the photoisomerization behaviour of annealed PDMS_n-b-POCN_m films is significantly different from that of the annealed POCN film, which is closely correlated with the phase-separated morphologies. The observations can provide deep understanding of the phase-separated structures and be used to develop new materials.