Synthesis and Characterization of Novel Multifunctional High-T_g Photorefractive Materials Obtained via Reactive Precursor Polymers

Abstract

We describe the synthesis of a new class of high-Tg multifunctional photorefractive polymers. They were obtained by radical copolymerization of methyl vinyl isocyanate and various N-substituted maleimides. Hole transporting carbazole moieties were attached through a variety of alkyl and phenyl spacers to the imide positions of the maleimides. In a polymer analogous reaction the reactive isocyanate groups of the precursor polymers were reacted with different hydroxyalkyl-terminated nonlinear optical chromophores. Two azo chromophores including dispersed red-1 and one pyrazolone dye were used. The resulting materials have been characterized by means of GPC, DSC, and UV/vis spectroscopy. All materials are amorphous and possess excellent solubility in common solvents such as chloroform and THF. Molecular weights range between 20 and 272 kg/mol. The glass-transition temperatures vary between 60 and 194 °C, depending on the length and the nature of the spacer groups between the maleimide rings and the carbazole moieties. Chromophore contents up to 44 mol % have been realized. Holographic experiments were performed on a device made from a multifunctionalized PR polymer plasticized such that the glass-transition temperature was Tg = 80 °C. The material was prepoled to break inversion symmetry and to induce macroscopic electrooptical properties. The steady-state gain coefficient (≈7 cm-1) and diffraction efficiency of this device (≈1%) were reduced compared with that of low-Tg materials. This is because orientational enhancement effects observed in low-Tg materials are excluded in the high-Tg regime. For the grating formation a dominant response time τ1 = 100 ms was found.