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A series of NLO polyamides based on the new monomer 2',5'-diamino-4-(dimethylamino)-4'-nitrostilbene (DDANS) has been synthesized. The polymers are amorphous and show glass transition temperatures up to 206 "C. They can easily be processed by spin-coating into thin films of optical quality. These films have been oriented by a corona discharge poling process. Second-order nonlinear optical coefficients (du) of up to approximately40 pm/Vat a fundamental wavelength of 1542 nm have been measured. Preliminary results indicate that the orientation of the dipoles shows no significant relaxation at ambient conditions in 120 days after poling. These polymers represent a new approach to the design of polymers with large and stable second-order nonlinear optical properties, where the nonlinear optical units are fixed in the polymer backbone with their dipole moments oriented transverse to the main chain. I. IntroductionThere has been considerable interest in organic nonlinear optical (NLO) materials, because of their potential application in integrated electrooptical devices.' A promising approach to the development of new second-order NLO materials is that of poled polymers. The advantages of poled polymers are large susceptibilities, fast response times, easy processability, and high physical and mechanical stability, but their NLO properties are usually not stable, due to the thermal relaxation of the NLO phore Orientation.' Different design strategies have been worked out to synthesize polymers with desirable NLO properties. Typically the NLO phores have been incorporated by d~p i n g~-~ (guest-host systems) or attaching them covalentlyP8 (side-chain systems) into amorphous or liquid-crystalline polymers. To enhance the orientational stability of the NLO phores, cross-linked polymers7?* or polymers which incorporate the NLO phores with their dipole momenta head-to-tail in the main chaingJO have been synthesized. However, problems such as relaxation of the NLO phores' orientation, optical losses, or intractability still exist.' In this paper, we present a new approach to the design of amorphous polymers with large and stable second-order nonlinear optical properties.To reach a maximum of NLO phore concentration, the bulky nonlinear optically active units should be linked with small spacer units to or in the polymer main chain. In our approach, the NLO phores are placed into the main chain, and their dipole moments are fixed transverse to it, based on the assumption that in this arrangement the NLO phores are easier to orient by an external field than in structures where their dipole moments are pointing along the polymer main chain. We further surmise that the local chain mobility is predominantly governed and limited by the bulkiness of the NLO phores and that mobility can be influenced by the chain length of flexible spacer units, similar to the behavior observed in liquidcrystal polymers.ll Hence, we hope to obtain systems with high NLO phore concentration, good orientability, and adjustable stability of properties.

A comparison is made between the second-order polarizabilities of nitropyridine derivatives determined with two different methods: electric-field-induced second-harmonic generation (EFISH) and a solvatochromic method. Both techniques yield the same values within the experimental errors, showing that the main contribution of the microscopic nonlinearity arises from a single intramolecular charge-transfer transition. Based on the quantum-mechanical two-level model, the solvatochromic method also yields important molecular parameters, such as transition and excited-state dipole moments. The second-order polarizabilities determined with the EFISH method are shown to follow the theoretical two-level dispersion.

Relaxation processes in nonlinear optical modified polyimide polymers with side-chain azo chromophores having glass transition temperatures in the range of 140 < T g < 170 °C have been studied. The relaxational mechanisms of the side-chain chromophores in these polymers have been investigated above and below the glass transition by second-harmonic decay, dielectric relaxation, and differential scanning calorimetry measurements. The nonexponential relaxation in both the time and frequency domains was modeled by the Kohlrausch−Williams−Watts (KWW) function. The nonlinear relaxational behavior of these polymers can be modeled in terms of the Tool−Narayanaswamy description of glassy state behavior. It allows for the nonlinear extension of the liquid equilibrium state behavior into and below the glass transition region with an accurate prediction of the relaxation times over more than 15 orders of magnitude in time. Time−temperature scaling of the relaxation times with (T g − T)/T as the relevant scaling parameter is observed below the glass transition.

New modified polyimide polymers with pendent side group nonlinear optical (NLO) azo chromophores and moderate to high glass transition temperatures (140 °C < Te < 190 °C) have been prepared. Corona poled films of these polymers possess large nonlinear optical susceptibilities of d3J = 23 pm/V and electrooptic (EO) coefficients of ri3 -6.5 pm/V at a wavelength of X = 1.3 gm. The structural properties (glass transition, molecular weight, chromophore density) and optical properties (refractive index, optical nonlinearity) of these polyimides can easily be varied to fulfill the requirements of potential electrooptic devices. Due to the relatively high glass transition temperatures of these polymers, long-term stability of the optical nonlinearity of typically one to hundreds of years at operating temperatures of 80-100 °C is predicted from accelerated time-temperature measurements. Using a development of a phenomenological theory of the glass transition, a normalized relaxation law is proposed with (Tg -T)/T as the relevant scaling parameter.