A Novel Polymer With Cyclic-Azobenzene Pendants:Fine Synthesis And Investigation On Its Properties

In this paper, a novel cyclic-azobenzene and a methacrylate (MMA) monomer containing the cyclic-azobenzene (cyclic-AzoMMA) were synthesized successfully. And the monomer cyclic-AzoMMA was used to generate a well-defined polymer (Pcyclic-AzoMMA) with cyclic-azobenzene pendants via reversible addition-fragmentation chain transfer (RAFT) polymerization. The thermal properties, photoisomerization, fluorescence emission and surface-relief-grating (SRG) of cyclic-azobenzene and Pcyclic-AzoMMA were investigated respectively. The detailed researches were summarized as the following:(1) The cyclic-azobenzene was synthesized via the Cu-catalyzed azide/alkyne cycloaddition (CuAAC)"click" cyclization from α-alkyne-ω-azide bifunctional linear-azobenzene. Then, a methacrylate (MMA) monomer containing the cyclic-azobenzene (cyclic-AzoMMA) was synthesized and the corresponding polymer Pcyclic-AzoMMA with cyclic-azobenzene pendants was prepared by RAFT polymerization in THF solution using2-cyanoprop-2-yl1-dithionaphthalate (CPDN) as the RAFT agent and2,2’-azobis(isobutyronitrile)(AIBN) as an initiator. The molecular weights and molecular weight distributions were measured by gel permeation chromatograpgy (GPC) and the results indicated that the polymerization showed living and controlled features.(2) Properties of linear-azobenzene and cyclic-azobenzene as well as Pcyclic-AzoMMA were investigated, such as thermal properties, photoisomerization, fluorescence and surface-relief-grating (SRG) properties. Detailed results are summarized as follows:(A) The melting points (Tms) of the cyc/zc-azobenzene and linear-azobenzene were determined by modulated differential scanning calorimetry (MDSC) measurement, and the results indicated that the cyclic-azobenzene had higher Tm than its linear precursor linear-azobenzene. Besides, the glass transition temperature (Tg) of Pcyclic-AzoMMA was measured by differential scanning calorimetry (DSC). Compared with linear azobenzene side chains Plinear-AzoMMA possessed similar structure, Pcyclic-AzoMMA showed higher Tg than Plinear-AzoMMA.(B) The UV-vis absorption changes of photo-induced trans-to-cis isomerization of the linear-azobenzene, cyclic-azobenzene and Pcyclic-AzoMMA in THF were investigated by irradiation with365nm UV light, and their thermally-driven cis-to-trans recovery were carried out through heating irradiated samples in the water-bath at40°C. Cyclic-azobenzene had higher trans-to-cis rates and slower cis-to-trans rates than its precursor. Besides, unlike the cyclic-azobenzene, the Pcyclic-AzoMMA shows much smaller cis-to-trans rates.(C) The fluorescent emission of linear-azobenzene and cyclic-azobenzene in THF solutions were investigated before and after irradiation with365nm UV light. The cyclizc-azobenzene has stronger fluorescence emission than its linear precursor at room temperature. After the UV light irradiation, the fluorescence intensities of the cyclic-and linear-azobenzene solutions increased with the irradiation time. However, Pcyclic-AzoMMA in THF solution showed no fluorescence emission before or after irradiation with365nm UV light.(D) The SRG formation and optical switching effects of the film of Pcyclic-AzoMMA were carried out in comparison with Plinear-AzoMMA under the same conditions. The average modulation depth (M.D.) of the Pcyclic-AzoMMA was thirty times larger than that of the Plinear-AzoMMA. The SRG and optical switching of the Pcyclic-AzoMMA and cyclic-AzoMMA were determined under identical conditions. The M.D. of the Pcyclic-AzoMMA was larger than that of cyclic-AzoMMA.