IR spectroscopy is one of the most widely and successfully applied analytical techniques to polyurethane chemistry and structure [49-52]. The chemistry of polyurethanes is the story of isocyanate and active hydrogen conversion to urethanes, ureas, and isocyanate-isocyanate reactions. During polymerization, strongly IR absorbing functional groups are consumed, and other strongly IR absorbing functional groups are formed. This makes IR spectroscopy particularly useful for following reaction kinetics and perturbations as a function of composition. Table 5.5 is a reference compendium of structures commonly encountered in polyurethane chemistry and associated resonance frequencies. At the same time, the final product structures can exist in various states of interchain interaction, which modify the IR resonances in highly predictable ways. Furthermore, since polyurethanes involve active hydrogen reactants and produce hydrogens in IR-active functional groups, experiments using deuterium isotope affects can be contemplated for information about reaction mechanisms and removing ambiguity in final structures . The systems are also amenable to study by model compounds that can be designed to isolate certain chemical and structural aspects of interest and mitigating confounding or complicating features [54-57]. Lastly, IR measurements are adaptable to other analytical techniques such as X-ray analyses allowing simultaneous measurements of structure formation providing understanding of the order of events, especially phase separation,
TABLE 5.5 Reference for resonance frequencies for functional groups commonly encountered in polyurethanes analysis
Figure 5.10 ATR-FTIR of isocyanurate foam. These foams are made with a significant excess in isocyanate to assure that a large amount of trimer is formed. Isocyanate functionality is clearly visible as an isolated peak at 2270 cm-1. The insert details the IR resonance associated with the isocyanurate ring at 1410 cm-1.
during reaction . Thus, by combining X-ray and IR analyses, polyurethane reaction dynamics can be combined with structural details of the final product.
The power of IR spectroscopy applied to polyurethane chemistry and structure does not imply that reliable data using this technique is easily obtained. Many aspects of polyurethane chemistry hinder obtaining quantitatively reliable data. In the case of polyurethane foaming and foams, the chemistry is very rapid, is exothermic, and involves very large changes in sample density. The product itself is opaque and so is best studied by ATR spectroscopy using Fourier transform instruments. Figures 5.10, 5.11, and 5.12 are examples of foam spectra illustrating interesting resonance features and a complication associated with operating in a transmission versus a surface reflectance mode.
Figure 5.11 ATR-FTIR of flexible foams showing the sensitivity of urea/urethane populations of foaming formulation details. The TDI peaks show nearly identical levels of TDI in all 4 foams.
Figure 5.12 Transmission FTIR of TDI-PU flexible foam. Notice the difference in absorbance values and baseline offset due to complications associated with working in transmission mode.