Spectroscopic Characterization of Model Urea, Urethane Compound, and Diamine Extender for Polyurethane−Urea

During the reaction injecting molding process of polyurethane−urea (PUU) based on 4,4‘-methylenebisphenyl diisocyanate (MDI), urethane as well as urea linkages are produced when a highly fluorescent aromatic diamine is used as a chain extender. To explore intrinsic fluorescence as a cure characterization technique, model urethane and urea compounds with similar chemical structures to MDI-based PUU were synthesized. Their fluorescence behavior were compared with that of diethyltoluenediamine (DETDA), which is often employed as a chain extender. The emission maxima for DETDA in polypropylene oxide (PPO) polyol occurred at 336 nm, while the urea (diphenylurea) and the urethane (ethyl N-phenylcarbamate) model compounds showed emission maxima at 301 and 303 nm, respectively. The relative quantum yield for DETDA was determined to be 57 times greater than the urea compound, while the urethane compound possessed a relative quantum yield 11 times greater than the urea compound. The viscosity effect increased DETDA fluorescence intensity greater than the observed fluorescence intensity decrease associated with a temperature increase of 55 °C. DETDA was found to be an effective collisional quencher for the urea and the urethane model compounds. The urea compound does not appear to quench the urethane compound's fluorescence significantly. Study of FT-IR spectra of model urea, urethane compounds, and DETDA provided characteristic peaks for each compound: 1727 cm-1 for urethane, 1647 cm-1 for urea, and 1623.7 cm-1 for DETDA, which were also proportional to their concentration in PPO. NIR spectra of model compounds and DETDA indicate characteristic peaks for the urea, the urethane compounds, and DETDA. However, the urea peak absorbance was difficult to use for quantitative analysis, while the urethane and DETDA peaks could be readily applied for such analysis.