Development of polyurethanes with azo‐type chromophores for second‐order nonlinear optical applications

Abstract

AbstractActive nonlinear optical nitro‐substituted thiazole, benzothiazole, and thiadiazole chromophores were prepared and condensed with tolylene‐2,4‐diisocyanate (TDI) and 4,4′‐methylenedi(phenyl isocyanate) (MDI) to yield a series of polyurethanes. The resulting polyurethanes were characterized with Fourier transform infrared, proton nuclear magnetic resonance, and ultraviolet–visible spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and gel permeation chromatography. The weight‐average molecular weights of the polyurethanes ranged between 19,500 and 28,000 (weight‐average molecular weight/number‐average molecular weight = 1.71–2.15). All the polyurethanes exhibited excellent solubility in most common organic solvents, and this indicated that these polyurethanes offered good processability. The glass‐transition temperatures (Tg's) of the polyurethanes were in the range of 166–204°C. Among the polyurethanes, chromophores containing the nitrothiazole moiety exhibited lower Tg values in comparison with those of chromophores containing nitrobenzothiazole and nitrothiadiazole moieties. This was attributed to the small size of the nitrothiazole moiety in the polyurethane matrix. The polyurethanes containing a TDI backbone demonstrated relatively high Tg values in comparison with those of the polyurethanes containing an MDI backbone. This was a result of an enhancement of the rigidity caused by the incorporation of a toluene ring into the polyurethane backbone. The second harmonic generation (SHG) coefficients of the poled polyurethane films ranged from 67.29 to 105.45 pm/V at 1064 nm. High thermal endurance of the poled dipoles was observed for all the polyurethanes. This was attributed to the formation of extensive hydrogen bonds between urethane linkages. Furthermore, none of the developed polyurethanes showed SHG decay below 150°C, and this signified their acceptability for nonlinear optical devices. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009