Cross-Linkable Zwitterionic Polyimides with High Electrooptic Coefficients at Telecommunication Wavelengths

Abstract

A series of nonlinear optical (NLO) polyimides were prepared by grafting 4−16 wt % zwitterionic pyridinium tricyanoquinonedimethane chromophore with a large negative hyperpolarizability (β0 = −590 × 10-30 esu) onto the host polyimides through an ester bond formation. To enhance the temporal stability of the poled NLO polymers, a new thermally reactive group, i.e., 5-aminobenzocylcobutenone, was introduced into the NLO polymer systems to effect the cross-linking during the thermal poling process. The resulting NLO polyimides showed good solubility in polar aprotic solvents such as N,N-dimethylformamide and could form uniform films by casting or spin coating without any noticeable chromophore aggregation or phase separation as examined under an optical microscope. These NLO polyimides had glass transition temperatures in the range of 170−213 °C and were thermally stable up to 270 °C. The electrical resistivity of NLO polyimides was found to be on the order of 1015 Ω·cm at 80 °C and dependent on the applied voltages. As the temperature increased, the resistivity dropped monotonically to 2.4 × 1011 Ω·cm at 200 °C. With a 10 wt % chromophore loading (or a number density of 15 × 1019 cm-3) in the polymer, the electrooptic (EO) coefficient (r33 at 1550 nm) reached 45 pm/V. No substantial decrease in EO coefficients was observed after the test cell was kept in the dark at 85 °C under nitrogen after 1200 h nor when the test cell was irradiated with 1550 nm light (50 μW/∼4 mm2) for 10 h at ambient temperature in air.