ABSTRACT
Polyamides based on 2’,5′-diamino-4-(dimethylamino)-4′-nitrostilbene (DDANS) represent a new approach to the design of NLO polymers in which the nonlinear optical units are fixed in the polymer backbone with their dipole moments oriented transversely to the main chain. Two different series of polymers have been synthesized and investigated. Semiflexible polyamides based on DDANS and linear aliphatic diacid chlorides 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. Second-order nonlinear optical coefficients (d 33) of up to 40 pm/V at λ=1.54µm have been measured on films which have been oriented by a corona discharge poling process. Orientational relaxation experiments reveal a remarkable orientational stability for these polymers. At ambient conditions, no significant relaxation of the NLO coefficients can be observed within 240 days after poling. Rigid-rod polyamides (aramids) based on DDANS and aromatic diacid chlorides are a class of novel liquid-crystalline polymers, where different mechanisms of orientation can be combined. In the nematic phase, the rigid-rod molecules form highly aligned domains that can be oriented using mechanical processes such as shearing. In addition, the stilbene units, which are fixed in the polymer backbone with their dipole moments oriented transversely to the main chain, can be oriented in an electric field. The combination of shearing and corona-discharge poling was found to yield highly oriented films. The two orientation mechanisms seem to cause a synergistic effect, probably since each affects different levels of the polymer microstructure in the solid.
