New modified polyimide polymers with pendent side group nonlinear optical (NLO) azo chromophores and moderate to high glass transition temperatures (140-degrees-C < T(g) < 190-degrees-C) have been prepared. Corona poled films of these polymers possess large nonlinear optical susceptibilities of d31 = 23 pm/V and electrooptic (EO) coefficients of r13 = 6.5 pm/V at a wavelength of lambda = 1.3 mum. 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-degrees-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 (T(g) - T)/T as the relevant scaling parameter.