Synthesis and nonlinear optical properties of novel T-type polyurethanes with high thermal stability of second harmonic generation

Methyl 2,5-di-(2′-hydroxyethoxy)benzylidenecyanoacetate (3) was prepared by hydrolysis of methyl 2,5-di-(2′-vinyloxyethoxy)benzylidenecyanoacetate (2). Diol 3 was condensed with 2,4-toluenediisocyanate (TDI) and 3,3′-dimethoxy-4,4′-biphenylenediisocyanate (DMBPI) to yield novel T-type polyurethanes (4-5) with 2,5-dioxybenzylidenecyanoacetate group as a nonlinear optical (NLO) chromophore, which constitutes a part of the polymer backbone. The resulting polyurethanes 4-5 were soluble in common organic solvents such as acetone and dimethylsulfoxide (DMSO). Polymers 4-5 showed a thermal stability up to 250 °C in thermogravimetric analysis (TGA) thermograms with glass transition temperature (Tg) values from differential scanning calorimetry (DSC) thermograms near 145 °C. The second harmonic generation (SHG) coefficients (d33) of poled polymer films were around 2.36 × 10−9 esu. These poled polymers exhibited a thermal stability of dipole alignment up to Tg, and no SHG decay was observed below 150 °C due to the partial main chain character of the polymer structure, which is acceptable for NLO device applications.