Synthesis and nonlinear optical properties of branched copolymers with covalently attached azochromophores

Soluble branched chromophore-containing nonlinear optical (NLO) methacrylate copolymers were synthesized by two-stage procedure. At the first stage aniline-containing precursors were synthesized by radical copolymerization of ММА with N-methyl,N-phenylamino-2-hydroxypropyl methacrylate (АМА) in various ratios (90:10; 80:20; 70:30) with ethylene glycol dimethacrylate, EGDMA, being used as branching agent and 1-decanethiol, DT, as a chain-transfer agent. At the second stage these precursors were functionalized by 4-nitrobenzene-diazonium tetrafluoroborate giving the azofunctionalized methacrylate units, MAZ; and copolymers with NLO groups in the side chains were obtained. The synthesized copolymers were studied and some physical–chemical characteristics were determined. On the base of the synthesized copolymers thin films were spin-cast, their thickness being measured by AFM technique. The polymer films were corona-poled and the NLO characteristics were determined by second harmonic generation technique. The values of d33 are shown to depend noticeably on the chromophores concentration in the polymer; for various ratios of MMA:MAZ units (90:10; 80:20; 70:30) they are equal to 77 pm/V, 45 pm/V and 28 pm/V, respectively. The values of NLO coefficients of branched chromophore-containing polymers are essentially higher than those of linear analogues obtained earlier in spite of an essentially lower concentration of chromophore groups.