| Aiming at the understanding of the structure-property relationships and the development of second-order nonlinear optical (NLO) polymers for electro-optic (EO) device applications, this thesis research explores the potential of a new class of zwitterionic chromophores, Z-beta-[1-substituted-4-pyridinium]-alpha-cyano-4-styryldicyanomethanide (trivial name: Pyridinium quinonedimethane, PQDM), as an active component in NLO polymers. Design and modification of NLO polymers were focused on enabling a high loading of chromophores in polymers and high poling efficiency. For the first time, the NLO polymers containing zwitterionic chromophores were demonstrated to show large EO coefficients (15--45 pm/V) at the telecommunication wavelength of 1550 nm. These NLO polymers also have good film-forming ability, good temporal stability of EO activity at elevated temperatures (e.g., 85°C), and adequate electrical resistivity (e.g., 10-15 O·cm), making them attractive for EO applications. Some important findings from this thesis work are highlighted below. (1) A hydroxyl-containing PQDM was readily prepared in gram quantity. From the Hyper-Rayleigh scattering experiment, a large, mubeta value of 27,000 x 10-48 esu at zero frequency was determined for PQDM. The chromophores exhibit a large negative solvatochromism, high thermal stability in nitrogen (Td, onset = 270°C), and good solubility in DMF, NMP and other amide-type solvents. (2) A polyimide was chosen as a host, owing to its good compatibility with the highly polar PQDM chromophores. A series of polyimides containing the carboxylic acid groups were designed to be different in chain flexibility, chain length, and topologies (linear and branched). The host polyimides have the glass transition temperatures (Tg) ranging from 208°C to 348°C. (3) A series of NLO polyimides were readily prepared by grafting the acid-containing polyimides with PQDM in the presence of a coupling agent. A new crosslinker [5-aminobenzocyclobutenone (BCBO)] and a site-blocking agent 3,5-di-tert-butylphenol (DBP) or 2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluoro-1-octanol (PDFO) were introduced as well into NLO polyimides. The resulting NLO polyimides contained the chromophores in the range of 4--29 wt.% (or number density, 6--52 x 1019 cm-3). The NLO polyimides with Tg in the range of 165--220°C are deemed to be suitable for further poling and EO studies, as thermal crosslinking of the BCBO moiety takes place at around 215°C. (4) The poling conditions were optimized through investigations into various factors such as voltage, poling temperature and poling time. High EO coefficients up to 45 pm/V were achieved, which is larger than that (31 pm/V) of lithium niobate being used in commercial EO modulators. The NLO polyimides retained the poling-induced EO activity at 85°C over 1000 h in air. The correlation between the polymer structure and EO activity was studied. |
