Hydrogen-Bond Interactions between Ester and Urethane Linkages in Small Model Compounds and Polyurethanes

Four model compounds (Us, Uu, UEex, and UEin) and two polyurethanes (PU1 and PU2) were prepared to monitor the hydrogen-bond (H-bond) interactions of urethane−urethane and urethane−ester (including external and internal esters). The results from differential scanning calorimetry (DSC) and X-ray diffraction suggested that the infrared absorption patterns in the −NH and −CO regions are closely related to the thermal history of the test specimen. Except for the incapability of the internal ester −CO to form an H-bond with urethane −NH, five −CO absorption bands can be assigned and attributed to free internal ester, free urethane, free external ester, bonded external ester, and disordered and ordered bonded urethane absorptions. The frequencies of the respective −CO band maxima obtained from the model compounds were given. In cases of polyurethanes, the −CO absorption of the amorphous PU1 was assigned on the basis of the deconvoluted result of a heat-treated UEex, while the liquid crystalline PU2 has its −CO absorptions assigned on the basis of the physical states of the test specimen. The as-reacted PU2 has its −CO absorptions closely related to the heat-treated UEin; however, as PU2 was heated into the mesophase, the orientation order caused the corresponding −CO absorptions similar to those for the as-reacted UEin.