Polyamides alternately substituted with an aliphatic cyclic moiety and an NLO-phore for second-order nonlinear optics

Abstract

This article describes the synthesis and second-order nonlinear optical (NLO) properties of polyamides containing an alternating arrangement of cholesteryl or menthyl moieties and an NLO-phore, i.e., N,N-dialkyl-4-mtroaniline in the side chain. The polyamides were prepared by condensation polymerization of 5-[2-(N-methyl-4-nitroanilino)ethoxy]isophthaloyl chloride with cholesteryl (P1) or L-menthyl (P2) 3,5-diaminobenzoate. The resulting polyamides P1 and P2 exhibit high glass transition temperatures (T g ) around 200 C and good solubilities in common organic polar solvents. Optical-quality films of P1 and P2 were obtained by spin-coating. The second harmonic generation (SHG) measurements of the spin-coated films of P1 and P2 were carried out by the Maker fringe method using a Q-switched Nd:YAG laser (1064 nm) as exciting beam after corona-poling: they indicate that P1 and P2 exhibit a second-order NLO coefficient. d 33 , of 10 pm/V. The orientational behavior of the NLO-phores in P1 is specific for the poling process. A noncentrosymmetric alignment of the NLO-phores in P1 was induced by corona-poling at a temperature lower than T g , i.e. at 100 C. Additionally, it was found that the noncentrosymmetric alignment of NLO-phores in P1 is inhibited over 160 C due to the para-crystalline aggregation of cholesteryl moieties as deduced from the SHG measurements and X-ray diffraction analysis.