Synthesis and properties of thermostable polymers with high optical nonlinearity

Abstract

Two types of new polyurethanes with a highly NLO active chromophore, 2-[4-[4-[bis(3-hydroxyethyl)amino]phenylazo]benzylidene]malonitrile (HAPM) in the polymer side chain were synthesized (PU1-DCN and PU2-DCN) and characterized. With the same chromophore HAPM attached, PU1-DCN and PU2-DCN were of 2,4-toluene diisocyanate and 4,4````-diisocyanato-3,3````-dimethoxydiphenyl polymer backbone units, respectively. The molecular weights of PU1-DCN and PU2-DCN were determined to be $M_w$ = 23,000 ($M_w$/$M_n$ = 2.19) and $M_w$ = 22,000 ($M_w$/$M_n$ = 2.34), respectively. The final products are readily soluble in polar aprotic organic solvents, like tetrahydrofuran, N,N-dimethylformamide, cyclohexanone, etc. Good optical quality films were obtained by spin coating. The glass transition temperatures of both polymers were found to be 158-159℃. They showed no melting points in the DSC thermogram, suggesting an amorphous phase. It was found that the electro-optic coefficient of PU1-DCN was $r_{33}$=62.7 pm/V at a 633 nm wavelength. The macroscopic second-order nonlinear susceptibility was determined to be $X^{(2)}$=136 pm/V for PU1-DCN and $X^{(2)}$=120 pm/V for PU2-DCN at a 1064 nm wavelength. High thermal stability was observed up to ca. 140℃ in both polymers. Three different crosslinked polyurethanes were prepared by the reaction of poly[(phenylisocyanate)-co-formaldehyde], which has one crosslinking site at every phenylene moiety, and three different types of hydroxy-functionalized NLO chromophores. High nonlinear optical activity of the polymers was achieved by the introduction of a hemicyanine-type chromophore with high molecular hyperpolarizability into the polyurethane structure. For the poled and cured polymers the $X^{(2)}$ values were between 16.8 and 37.4 pm/V measured at a wavelength of 1064 nm. The thermal stability studies performed on PU-VP via second harmonic generation activity indicated a minimum decay of X(2) value due to the lattice hardening of the pol...