Crosslinked Epoxy Polymers With Large And Stable Nonlinear Optical Susceptibilities

Abstract

A novel type of second-order nonlinear optical (NLO) polymer has been prepared by the reaction between tetrafunctional nitro-substituted aromatic diamines and difunctional epoxy monomers. In these materials the NLO-active moieties are incorporated covalently into the network of a crosslinked polymer. For comparison the corresponding linear (not crosslinked) polymers have also been prepared. The noncentrosymmetry was induced by poling under strong electric fields either with electrodes in liquid crystal display type cells or by corona discharge poling of spin-coated films. The resulting nonlinear optical susceptibilities were evaluated from second harmonic generation experiments. The linear polymer prepared from 4-nitroaniline and bisphenol-A diglycidylether initially exhibits a nonlinear coefficient d₃₃ of 31 pm/V measured at 532 nm. However, 28 days after the poling, d₃₃ decayed to 22 pm/V. This relaxation is comparable to that of a NLO polymer containing a similar chromophore as a side group that we investigated earlier. A significant improvment of the stability of the nonlinear optical coefficients was achieved with the crosslinking epoxy NLO polymers. These materials can be easily melt- or solution-processed in the form of soluble, low molecular weight prepolymcrs. Subsequently, the prepolyiners are simultaneously poled and cured to give crosslinked polymers with high glass transition temperatures. For a crosslinked polymer synthe-sized from 4-nitro 1,2-phenylenediamine and bisphenol-A diglycidylether, a d₃₃ value of 14 pm/V was found. This high nonlinear optical coefficient remained unchanged under ambient conditions for at least 500h. Even at 85°C no signs of relaxation were detected.