Synthesis and optical properties of polyurethanes containing a highly NLO active chromophore

Two types of new polyurethanes with a highly NLO active chromophore, 2-[4-[4-[bis(2-hydroxyethyl)amino]phenylazo]benzylidene]malonitrile (HAPM), in the polymer side chain were synthesized (PU1-DCN and PU2-DCN) and characterized. The diisocyanate components of the polymers are 2,4-tolylene diisocyanate and 3,3′-dimethoxybiphenyl-4,4′-ylene diisocyanate. The molecular weights of PU1-DCN and PU2-DCN were determined to be M̄_w = 23000 (M̄_w/M̄_n = 2.19) and M̄_w = 22000 (M̄_w/M̄_n = 2.34), respectively. The final products are readily soluble in polar aprotic organic solvents, like tetrahydrofuran, N,N-dimethyl-formamide, cyclohexanone, etc. Good optical quality films were obtained by spin coating. The glass transition temperatures of both polymers were found to be 158-159°C. They showed no melting points in the DSC curves, suggesting an amorphous phase. It was found that the electro-optic coefficient of PU1-DCN was r₃₃ = 62.7 pm · V^-1 at 633 nm wavelength. The macroscopic second-order hyperpolarizability at 1064 nm wavelength was determined to be χ⁽²⁾ = 136 pm · V^-1 for PU1-DCN and χ⁽²⁾ = 120 pm · V^-1 for PU2-DCN.