A spectroscopic analysis of the phase evolution in polyurethane foams

Infrared spectroscopy has been used to study the evolution of polyurethane foam structure, providing measures of relative reaction kinetics, hard segment growth, the onset of phase separation, the formation of order, and the development of final morphology. Changes in free, monodentate, and bidentate hydrogen-bonded urea groups dominate the organization of the entire ensemble. Hard segments formed by reaction of 2,6-toluene diisocyanate (2,6-TDI) and by a mixture of 80% 2,4-TDI and 20% 2,6-TDI displayed very different local segmental alignment, a factor crucial in the development of morphology. Phase separation occurred faster, with fewer and shorter hard segments, in the systems with well-ordered straight chains. The formation and time evolution of monodentate ureas suggest that phase development may be incomplete, or trapped, in systems with ill-defined urea structures. A low degree of spatial order exists in the systems containing these structures.