Novel Y-type polyimide with highly enhanced thermal stability of second harmonic generation

Abstract

3,4-Bis-(3,4-dicarboxyphenylcarboxyethoxy)-4′-nitrostilbene dianhydride was prepared and reacted with 4,4′-(hexafluoroisopropylidene)dianiline to yield a novel Y-type polyimide containing the 3,4-dioxynitrostilbenyl group as an NLO-chromophore, which constituted part of the polymer backbone. The resulting polyimide was soluble in polar solvents such as acetone andN,N-dimethylformamide. The polymer exhibited good thermal stability up to 370 °C in the thermogravimetric analysis. The glass-transition temperature (T _g ) obtained from the differential scanning calorimetry thermogram was near to 153 °C. The second harmonic generation (SHG) coefficient (d ₃₃) of the poled polymer film at the fundamental wavelength of 1064 cm⁻¹ was around 2.15 × 10⁻⁸ esu (9.01 pm/V). The dipole alignment exhibited exceptionally high thermal stability even at a temperature 30 °C above theT _g , and there was no SHG decay below 180 °C because of the partial main chain character of the polymer structure.