The thermally stable 2nd order NLO polymeric materials were successfully prepared by sol-gel process using functionalized stilbene derivative 2nd-order nonlinear optic (NLO) chromophore attached to silicon atoms by covalent bonding. The dye-containing alkoxysilane monomers were synthesized by coupling of alcoholic chromophores and 3-isopropyl triethoxysilane in dimethylacetamide (DMAc) solvent. The monomers (dye-containing alkoxysilane) were homopolymerized and copolymerized with teraethyoxysilane (TEOS) by acid catalyst. The various precursor polymers have good solubility in polar solvent such as THF, dioxane, DMF, and DMAc. The precursor polymers (sol) were spin-coated on the various substrates such as ITO glass and KBr disc. Films (gel) were further polycondensed and poled by corona discharge and electrode contact method at 170℃ for 3 hours. When the cured samples were soaked in DMAc, which is a good solvent for monomer, the solvent did not extract any measurable concentration of dye from the films, suggesting that the NLO dyes are firmly incorporated in the silicon-oxygen network. The chemical structure of the resulting polymeric material was determined by FT-IR, UV-visible, and solid state $^29Si-NMR$ spectroscopy. The FT-IR results showed that the polymerizations of monomers and TEOS were successfully achieved, and the cross-linked silicon-oxygen network was well developed. The silica existed mono-hydroxy $(T^2, Q^3)$ and non-hydroxy$(T^3, Q^4)$ states after curing at 170℃, which suggest formation of developed silica-oxygen network. The UV-visible spectra showed that the NLO dyes were aligned after poling process, and the dyes were not degraded or sublimed during poling/curing process. All of the cured films are transparent and any phase separation was not detected by SEM. The AFM photograph showed the films have good surface flatness. X-ray diffraction showed that NLO polymeric film has an amorphous structure. The polymers were quite stable at elevated temperatures (>250℃).
The results of the polymer P-2 showed that the values of electro-optic coefficients, $τ_{33}$, for poled samples have values of 3.7, 12.4, 7.8, and 9.1 pm/V for 100, 75, 50, and 25 weight percent, respectively. All of the $τ_{33}$ values unchanged at room temperature after 20 days. However, the $τ_{33}$ signal of homopolymer P-2 vanished after annealing at 100℃ for 2 hours because the effective cross-linked network was not developed. The temporal stability of the EO coefficient for the film of 50wt% copolymer shows excellent thermal stability. The $τ_{33}$ signal remained 80% of initial value after heating even at 150℃ for 3hours. This means that the cross-linked silicon-oxygen network was developed and the Tg of film was higher than 150℃. The polymers P-3 have the values of electrooptic coefficients, $τ_{33}$, for poled samples of 4.7 and 7.6 pm/V for 100 and 50 weight percent of monomer in copolymer, respectively. The electrooptic coefficients, $τ_{33}$, for poled P-5 have values of 2.7 and 4.3 pm/V for 100 and 50 weight percent of monomer in polymer, respectively. The both end anchored P-5 was conjectured to have excellent thermal stability of EO coefficients.
With baking the polymer films (P-2) at 180℃ for 20 minutes, we have succeeded in the electric contact poling for the sample. The EO coefficient of 7.36 pm/V was obtained by this method. The refractive index was decreased after photobleaching experiment. The photobleaching technique makes the channel patterning possible.
