Preparation of Novel Y-Type Nonlinear Optical Polyimides with High Thermal Stability of Second Harmonic Generation

Abstract

2,4-Bis-(3,4-dicarboxyphenylcarboxyethoxy)-1-(2,2-dicyanovinyl)benzene dianhydride (4) was prepared and reacted with 1,4-phenylenediamine and 4,4′-oxydianiline to yield novel Y-type polyimides 5-6 containing 2,4-dioxybenzylidenemalononitrile groups as NLO-chromophores, which constitute parts of the polymer backbone. The resulting polyimides 5-6 are soluble in polar solvents such as dimethylsulfoxide and N,N-dimethylformamide. Polymers 5-6 showed a thermal stability up to 330°C in thermogravimetric analysis thermograms with T _g values obtained from differential scanning calorimetry thermograms in the range 179–188°C. The second harmonic generation (SHG) coefficients (d ₃₃) of poled polymer films at the 1064 nm fundamental wavelength were around 6.32 × 10⁻⁹ esu. The dipole alignment exhibited exceptionally high thermal stability even at 20°C higher than T _g , there was no SHG decay below 200°C because of the partial main chain character of polymer structure, which is acceptable for nonlinear optical device applications.

Keywords:

• differential scanning calorimetry (DSC)
• NLO
• polyimides
• relaxation of dipole alignment
• SHG coefficient
• thermogravimetric analysis (TGA)

Acknowledgments

This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund, KRF-2007-313-C00498).

Notes

^a Determined from DSC curves measured on a TA 2920 differential scanning calorimeter with a heating rate of 10°C/min under nitrogen atmosphere.

^b Determined from TGA curves measured on a DuPont 951 thermogravimetric analyzer with a heating rate of 10°C/min under nitrogen atmosphere.

^a Polymer film after corona poling.

^b SHG coefficients (d ₃₃) were derived from the analysis of measured Maker-fringes [16].

^c Order parameter Φ = 1 − A₁/A₀, where A₀ and A₁ are the absorbances of the polymer film before and after corona poling, respectively.

^d Film thickness was determined by the optical transmission technique [15].