Nonlinear optical (NLO) polymers. 3. NLO polyimide with dipole moments aligned transverse to the imide linkage

This paper presents a new class of nonlinear optical (NLO) polyimide for second harmonic generation (SHG). This new class of NLO polyimide consists of 2,4-diamino-4'-nitroazobenzene (2R-DIAMINE) with 3,3'4,4'-benzophenonetetracarboxylic dianhydride, in which the NLO dipole moment is aligned transverse to the main chain. BR-DIAMINE was prepared from m-phenylenediamine with p-nitroaniline. The time dependence of the decay was found to be well-fitted by a Kohlrausch-Williams-Watts stretched exponential function. The difference of the orientational relaxation among the samples with different glass transition temperatures can be compared using the scaling relation of tau = A' exp[(T-0 - T)/T] in which tau is the relaxation time and T-0 is the effective transition temperature for SHG activity. T-0 is in good agreement with the corresponding glass transition temperature. Enhanced orientational stability of the aligned NLO chromophore is ascribed to the rigid structure of the polyimide backbone. For the orientational relaxation at 100 degrees C, a relaxation time of over 50 years could be estimated, and a relaxation time of 3.8 years could be obtained at a higher temperature of 150 degrees C.