Progress in the development of a new class of multi-functional polyimides for use in electro-optical devices is reported.
These polyimides contain hydroxymethyl-functional side-groups attached to the polymer backbone, allowing for the
attachment of a wide variety of molecular species. It is shown that multiple types of organic molecules may be attached
to the polymer simultaneously, with a quantitatively controllable distribution, to tailor the physical properties of the
material. Methods for cross-linking the polyimides are presented, based on both modification to the backbone and the
addition of difunctional additives (such as isocyanates) to solutions of the polymer during spin casting. Processing
studies using spectroscopy to track the cross-linking reaction and its effects on organic nonlinear optical materials
indicate that the latter method is compatible with poling processes for polymer guest/host systems with high nonlinear
optical activities. Further studies using a novel thermomechanical analysis method demonstrate that the cross-linking
reactions increase the glass transition temperature and inhibit physical relaxation processes in the cross-linked
Mach-Zehnder optical modulators were fabricated using the CLD and FTC chromophores in polymer-on-silicon optical
waveguides. Up to 17 months of oven-ageing stability are reported for the poled polymer films. Modulators containing
an FTC-polyimide had the best over all aging performance. To model and extrapolate the ageing data, a relaxation
correlation function attributed to A. K. Jonscher was compared to the well-established stretched exponential correlation
function. Both models gave a good fit to the data. The Jonscher model predicted a slower relaxation rate in the out
years. Analysis showed that collecting data for a longer period relative to the relaxation time was more important for
generating useful predictions than the precision with which individual model parameters could be estimated. Thus from
a practical standpoint, time-temperature superposition must be assumed in order to generate meaningful predictions.
For this purpose, Arrhenius-type expressions were found to relate the model time constants to the ageing temperatures.
A method of chemical synthesis that allows for the facile attachment of a wide variety of chemical compounds, including highly active nonlinear optical chromophores, to polyimides has been developed recently at the Naval Air Warfare Center, Weapons Division. The synthesis of these compounds is presented, along with a discussion of their relevant physical and chemical properties, alone and in comparison to equivalent guest/host materials. Examples of attached chromophores include the well-known Disperse Red 1, along with high-activity chromophores of more recent interest such as FTC and CLD. The synthesis of structures that contain both attached chromophores and chemical functionalities that enable thermal cross-linking of the polyimides is also discussed.
Methods that successfully predict the refractive index at near-infrared wavelengths of negatively birefringent polymer films for optical waveguide applications are presented. The starting point for these methods is a correlation based on connectivity indexes originally developed by Bicerano for the refractive index of isotropic polymers at visible wavelengths. This correlation is applied to a set of polyimides at near infrared wavelengths with modifications in order to improve its predictive power. The polyimides were synthesized by condensation of monomers to form the precursor poly(amic acid)s followed by imidization in solution. Solutions of the polyimides were then spin coated onto glass substrates and baked to produce films of 2-3 microns in thickness with a variable negative birefringence. The refractive index profiles of these films near 1320 nm were then measured in both the TE- and TM- modes using a prism-coupling technique. The average refractive index of these films was then compared to the prediction generated by the model. The agreement between the predicted and observed values has been sufficient to enable the rapid development of materials for optical waveguides without the need for many rounds of trial-and-error investigation. These techniques facilitate the development of specialized polymers for optical waveguide applications.