Thermally stable NLO poly(amide-imide)s via sequential self-repetitive reaction

A series of thermally stable side-chain second-order nonlinear optical (NLO) poly(amide-imide)s via sequential self-repetitive reaction (SSRR) have been developed. This SSRR is based on carbodiimide (CDI) chemistry. Three difunctional azo chromophores (DR19, NDPD and DNDA) were, respectively, reacted with excessive amount of 4,4′-methylene-diphenylisocyanate (MDI) to form poly-CDI, and subsequently trimellitic anhydride (TMA) was added to obtain an intermediate, poly(N-acylurea). Poly(N-acylurea) exhibits excellent organosolubility, which enables the fabrication of high quality optical thin films. Moreover, its moderate glass transition temperature (T_g) characteristic allows the NLO-active polymer to exhibit high poling efficiency. After in situ poling and curing process, N-acylurea moieties were converted to amide-imide structures via SSRR, and the T_gs of the polymers were elevated significantly up to 70 °C higher than that of the poly-CDI sample. Electro-optical coefficients, r₃₃ of about 5.2-25.2 pm/V at 830 nm were obtained. Good temporal stability (80 °C) and waveguide optical losses (3.8-6.6 dB/cm at 830 nm) were also obtained for these polymers.