Extending the novel method of Eich et al. [J. Appl. Phys. 66, 3241 (1989)] to prepare highly stable second-order nonlinear optical (NLO) polymers via chemical cross-linking under electric field, we have obtained a new polymer exhibiting large and stable second-order optical nonlinearities after relaxation at 80 °C. This was achieved by forming a network polymer from two NLO-active monomers, bifunctional N,N-(diglycidyl)-4-nitroaniline and trifunctional N-(2-aminophenyl)-4-nitroaniline. Here, every NLO moiety is connected to the network by a single covalent bond. After full cure under corona poling at 120 °C, the sample exhibited at ambient conditions d33≂50 pm/V and d31≂16 pm/V at 1064 nm fundamental wavelength, as estimated from the Maker fringe data. Upon heating to 80 °C, the nonlinearities decreased somewhat initially, but leveled off and remained stable at 80 °C. This stable sample gave d33≂42 pm/V and d31≂14 pm/V at ambient conditions. Furthermore, the linear electro-optical coefficient of this sample, measured with a Mach–Zehnder interferometer, was r13≂6.5 pm/V at 530.9 nm. From this r13 coefficient one estimates d31≂11 pm/V at 1064 nm, in good agreement with the Maker fringe value.

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We have studied second-order nonlinear optical properties and dielectric relaxation characteristics for two amorphous epoxy polymers based on bisphenol-A with nitroaniline-type nonlinear optical moieties covalently attached as part of the main chain. The nonlinearities, rather large immediately after corona poling, undergo a slow decay at ambient temperature (e.g., ∼29% reduction in the nonlinear coefficient d33 in 28 days), even though the nominal glass transition temperature Tg is ∼80 °C. In dielectric relaxation measurements, both the unpoled and poled samples exhibit two relaxation modes: an α relaxation attributed to glass transition and a β relaxation due to local segmental motions of the main chain. Surprisingly, the critical temperature T∞ associated with the glass transition, obtained by a WLF [M. L. Williams, R. F. Landel, and J. P. Ferry, J. Am. Chem. Soc. 77, 3701 (1955)] fit of the α relaxation characteristics, shows a significant decrease (as much as 40 °C) with increasing poling field. Moreover, the relaxation times of temporal decay of field-induced alignment at ambient temperature, as determined from the stretched exponential function fit of birefringence changes, are found to be consistent with extrapolations from the the dielectric α relaxation characteristics of poled samples.

A new linear epoxy polymer containing 4-amino-4′-nitrotolane chromophores attached to the chain backbone was synthesized in an attempt to enhance both the poling-induced nonlinear optical susceptibility and its long-term stability. The polymer films poled by corona discharge showed very large nonlinear optical doubling coefficients of d33 ≂ 89 pm/V and d31≂25 pm/V for incident light of 1.06 μm wavelength, and a linear electro-optic coefficient r13≂8 pm/V at 633 nm wavelength. Poling-induced alignment of the nonlinear optical moieties showed no detectable decay in two weeks at ambient temperature, as studied by the birefringence. Even at 100 °C the relaxation time estimated from the birefringence decay was ca. 450 h, consistent with roughly 16% decrease in dij coefficients in 20 h at this temperature and also the extrapolation from the dielectric α relaxation characteristics. The dielectric α relaxation temperatures of the poled polymer samples exhibited higher values than that of the unpoled counterpart. This unusual behavior may be due to a significant contribution of Maier–Saupe thermotropic interactions among the tolane moieties that favor parallel alignment.

Linear and crosslinked epoxy polymers containing high concentrations (ca. 86 wt.-Yo) of pendant 4-(4-nitrophenylethynyl)aniline [4-amino-4'-nitrotolane] chromophores were synthesized. They exhibit a smectic phase in the bulk once heated above the glass transition temperature (ca. 70°C). The appearance of the highly scattering smectic phase could be avoided by starting the crosslinking process of thin polymer films in the presence of solvent. Application of corona poling during crosslinking resulted in a highly polar polymer, exhibiting a very large birefringence of ca. 0,17 at 633 nm. Moreover, the birefringence remained stable during annealing at 80-90 "C for ca. 30 days. The electro-optic coefficient, determined from the Pockels-effect, was roughly 8 pm/V for r,13 at 633 nm. a) IMB-visiting scientist, present address: