Linear and nonlinear optical properties of Langmuir-Blodgett multilayers from chromophore-containing maleic acid anhydride polymers

Organized polymer films prepared from amphiphilic side-chain polymers were investigated with respect to their application in passive or active photoni c devices, The polymers were synthesized by means of polymer analogous reac tion of azo chromophores with a maleic acid anhydride polymer which bears s aturated hydrophobic side chains. The resulting polymer contains, besides h ydrophilic groups and hydrophobic side chains, azo chromophores with high s econd-order hyperpolarizability which are attached to the main chain via a spacer. in order to increase the hydrophobic character of the chromophore, a fluorinated moiety was chosen as acceptor of the chromophore, Langmuir-Bl odgett (LB) technique was applied for preparation of multilayer structures with distinct supramolecular architecture from these polymers. They an char acterized by high anisotropy and a special combination of linear and nonlin ear optical (NLO) properties. The polymers were deposited both in Y-type an d Z-type LB layers. The Y-type layers of the azopolymer show high birefring ence and dichroism but small nonlinear response which originates from the f irst deposited layer only. Multilayers of the azopolymer deposited in Y-typ e configuration no characterized by high internal order which can be conclu ded from a pronounced Bragg-peak and Kiessig-fringe patterns as obtained fr om gracing incidence X-ray scattering. Experiments to deposit the active po lymer directly in Z-type configuration by passing a blank water compartment before layer deposition in a double compartment LB-trough led to the resul t that well-ordered, anisotropic films are obtained only if the number of d eposited monolayers is limited to a value of about 30. The layers deposited in Z-type manner show low order. This can be concluded from the fact that only a weak Bragg-peak is observed in the X-ray scattering curves, and Kies sig-fringes are very weak or missing. The same conclusion can be drawn from ellipsometric and second-harmonic generation (SHG) experiments. A second k ind of Z-type multilayers was prepared by alternate deposition of the activ e azopolymer and of another maleic acid anhydride polymer without chromopho res. They show similar linear optical properties like Y-type layers but due to the noncentrosymmetric internal structure, a NLO second-order response grows proportional to the square of the sample thickness. Again, thick laye rs of the azopolymer prepared by alternate deposition LB-technique were pre pared, which can be used as active waveguide for second-order NLO applicati on. (C) 1999 Elsevier Science S.A. All rights reserved.