Synthesis of Novel Y-type Nonlinear Optical Polyesters with Enhanced Thermal Stability of Dipole Alignment

Abstract

2,4-Di-(2'-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and polymerized with terephthaloyl chloride and adipoyl chloride to yield novel Y-type polyesters 4 and 5 containing dioxybenzylidenemalononitrile groups as NLO-chromophores, which constituted parts of the polymer backbone. The resulting polymers 4 and 5 are soluble in common organic solvents such as acetone and N,N-dimethylformamide. Polymers 4 and 5 showed thermal stability up to 300 <TEX>${^{\circ}C}$</TEX> in thermogravimetric analysis with glass-transition temperatures obtained from differential scanning calorimetry in the range 83-94 <TEX>${^{\circ}C}$</TEX>. The second harmonic generation (SHG) coefficients (<TEX>$d_{33}$</TEX>) of poled polymer films at the 1064 nm fundamental wavelength were around <TEX>$6.48\;{\times}\;10^{-9}$</TEX> esu. The dipole alignment exhibited high thermal stability even at 10 <TEX>${^{\circ}C}$</TEX> higher than <TEX>$T_g$</TEX> and no significant SHG decay was observed below 105 <TEX>${^{\circ}C}$</TEX> partially due to the main-chain character of polymer structure, which is acceptable for NLO device applications.