Synthesis and non-linear optical properties of four polyurethanes containing different chromophore groups

Four kinds of polyurethanes (PUs) containing azo chromophore groups with different molecular hyperpolarizability (β) as side chains were synthesized and their second-order non-linear optical (NLO) properties studied. PU-1, PU-3 and PU-4 were prepared from the reaction of toluene 2,4-diisocyanate with N,N-di(2-hydroxyethyl)aniline containing azobenzene (Disperse Red-19, DR-19), azo-benzothiazole and bisazobenzene, respectively, whereas PU-2 was obtained from the reaction of hexamethylene 1,6-diisocyanate with DR-19. Thin transparent, poled films of PU were prepared by spin coating, followed by thermal corona poling. Glass transition temperatures (Tg) of PU determined by differential scanning calorimetry decreases in the order: PU-2>PU-1=PU-4>PU-3, corresponding to 163°C, 155°C, 154°C and 125°C, respectively. The second-order NLO coefficient χ2 of all the poled PU films can reach 10−7 esu and decreases in the order: PU-4>PU-3>PU-2>PU-1, which depends mainly on the molecular hyperpolarizability of the chromophore groups. The higher the β value of chromophore group, the larger is the χ2 of the poled PU. The temporal stability of NLO properties for the four PUs increases with Tg of the PU. The poled PU-2 film shows the best temporal stability of NLO properties and maintains 98% of the order parameter at 120°C for more than 400 h.