Polyurethane surface and interfacial spectroscopic studies

Studies were undertaken to analyze and understand surfaces and interfaces of polyurethanes during the film formation process. A particular emphasis was given to attenuated total reflectance (ATR) and step-scan photoacoustic (SS-PAS) Fourier transform infrared (FT-IR) spectroscopies to qualitatively and quantitatively examine the reactions, migration, stratification, orientation, catalysis, additives, and environmental influence near the film-substrate (F-S) and film-air (F-A) interfaces.;These studies showed that there is an NCO concentration maximum during the crosslinking process due to the solvent evaporation. At later stages of reactions, stratification of NCO functionality occurs. Although C=O groups tend to align in the parallel direction, H-bonding prefers the perpendicular direction near the F-S interface. Quantitative analysis indicated that the amount of H-bonding increases with time. There is a kinetic difference between noncatalyzed and organotin-catalyzed isocyanate-alcohol reactions, suggesting that organotin-alcohol association is involved in the reactions in the latter system.;In addition to developing a spectroscopic method to study diffusion in non-reacting films, a spectroscopic approach has been established for and applied in the separation of reactions and migration in reactive polyurethane systems. The diffusion type of small molecules changes during film formation, indicating that the relaxation of polyurethane macromolecules becomes slower at the later stages of film formation and that the film changes from a viscous state to an elastic/glassy state. Reaction kinetics showed a reaction rate constant change corresponding to this transition.;Investigation of the interactions between related reagents indicated that there is complexation between polyol and organotin catalysts in tin-catalyzed solvent-borne and water-borne polyurethane systems. In addition, there are structural changes and stratification of the catalysts during the polyurethane formation process.;Polyether modified polysiloxanes were studied as important surface-modifying agents. Both long-chain polydimethylsiloxane (used in solvent-borne systems) and short-chain silicone surfactant (for water-borne systems) migrate to the F-A and F-S interfaces. To satisfy the lowest-energy requirement, the Si-O-Si segments were oriented in certain directions near the F-A and F-S interfaces, depending on systems. The structure-property relationship of acrylic polyurethane films was also investigated with a focus on the relative humidity (RH) influence. Studies indicate that more urea structure forms at a higher RH.