Surface/interfacial changes during polyurethane crosslinking: A spectroscopic study. V

Crosslinking reactions and stratification processes in polyurethane films were investigated using attenuated total reflectance (ATR) FTIR spectroscopy. The HDI trimer levels near the film–substrate (F–S) interface appear to increase at the early stages of reactions, and after reaching a maximum, decrease. This is attributed to solvent depletion and subsequent film shrinkage, thus causing local NCO concentration changes. At the later stages of crosslinking reactions, stratification of the 1,6‐hexamethylene diisocyanate (HDI) trimer occurs, with higher concentration levels at shallower penetration depths from the F–S interface. At the same time, NCO‐containing groups assume preferentially parallel orientation to the F–S interface. On the other hand, H‐bonded carbonyl groups tend to orient themselves in a perpendicular direction. Quantitative analysis indicates that at extended reaction times the amount of H‐bonded CO groups at shallower penetration depths increases and their orientation tends to be more random, which is correlated with migration toward the F–S interface. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2045–2054, 2001