Progress on Nonlinear Optical Chromophores and Polymers with Useful Nonlinearity and Thermal Stability
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ABSTRACT Organic nonlinear optical (NLO) polymers must possess large and stable bulk nonlinear properties if they are to be of practical use. Numerous classes of NLO chromophores and polymers have already been evaluated but the selection process, particularly as related to stability issues, has been very stochastic in nature. If suitable chromophores are to be successfully identified their thermochemical degradation mechanisms must be elucidated and structural modifications introduced in a rational fashion. We have devised a protocol for the evaluation of NLO chromophores which involves initial screening by thermal analysis (DSC and TGA) and selected molecular hyperpolarizability (EFISH) analysis. The most promising candidates are incorporated into thermoplastic polymer hosts where bulk nonlinear properties (dj and rj) and additional thermal stability properties are evaluated by spectroscopic means (UV-VIS). Cyclicvoltammetry (CV) has been identified as a useful tool for evaluation of these chromophores; in the case of azobenzenes with aliphatic amine donor and a nitro acceptor a correlation between the thermal stability and oxidation potential has been found. The most facile thermal degradation mechanism which occurs in these dyes has been identified and structural changes to the donor group have been introduced to avert the process.
INTRODUCTION A variety of outstanding materials and processing issues must be resolved prior to substantiation of the anticipated attributes of organic and polymeric nonlinear optical media.1, 2 Poled polymer waveguides have been intensively scrutinized as media for electro-optical devices and among the many materials properties which have been examined the magnitude of the bulk nonlinearity created by electric field poling has attracted the most attention. However, more recently there has been significant additional emphasis on enhancement of the temporal stability of this electro-optic coefficient. 3 Evidence has accumulated that the temporal stability of poled order in a glassy polymer waveguide is primarily linked to the difference between the glass transition Tg of the chromophore/polymer composition and the subsequent thermal history experienced by the device. The exact relationships between the thermal history profile and the kinetics of poled order decay are very complicated and depend on the exact composition of chromophore, polymer and their connectivity. 4 -6 It appears that a (Tg - T) > 200--,2500 C must be realized if the device is to possess an electrooptic coefficient which is sufficiently large and stable over a 421 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
period of years. As typical processing and manufacturing temperatures may exceed 250 0C and continuous use temperatures may exceed 80'C it appears that chemical stability and a Tg > 300'C is mandated (Figure 1).7
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Time (sec)
Figure 1 Projected long term stabilities of poled order (d/d0 ) as a function of the difference between the glass transition tem
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