| Waterborne polyurethanes (WB PUR's) are the object of ongoing research efforts, as they offer high-performance coatings with significantly reduced volatile organic content (VOC). Due to the inherent sensitivity of WB PUR's to a number of synthetic, process, and environmental variables, the understanding of their film-formation mechanisms as well as surface/interfacial processes during crosslinking are of great interest. Since waterborne polyurethanes often exhibit heterogeneous distribution of components, internal reflection infrared imaging (IRIRI) was developed to allow surface-selective, microscopic chemical imaging with unprecedented 1 mum/pixel spatial resolution. Using IRIRI and a variety of other powerful spectroscopic and microscopic approaches, these studies investigated phase separation, stratification, and crosslinking reactions in waterborne two-component polyurethanes (WB 2K-PUR's) and methyl methacrylate/n-butyl acrylate/methacrylic acid hybrid polyurethane dispersions (MMA/nBA/MAA-PUD's) as well as UV crosslinkable thiol-ene polyurethane dispersions (TE-PUD's). WB 2K-PUR's exhibited heterogeneous crosslinking which was manifested by partial phase separation and formation of rough surface topographies when crosslinked at elevated relative humidity (RH), which was related to migration of PEG-modified polyisocyanate crosslinkers to the F-A interface. Surface morphological studies indicated that surface roughness at elevated RH was strongly affected by solution dynamics in which polyisocyanate hydration and hydrolysis reactions triggered aggregation of reactant droplets, which resulted in an increase of the particle size with time, but ultimately limited phase separation during film formation by locking in a nano-structured colloidal morphology. Crosslinking reactions conducted at low RH resulted in smooth, transparent films with stratified crosslink density with higher Tg values at the film-air (F-A) interface than at the film-substrate (F-S) interface. This behavior was attributed to the evolution of H2 O concentration gradients during film formation. Novel MMA/nBA/MAA-PUD's were synthesized and displayed heterogeneous film formation when polymerization conditions did not facilitate uniform monomer transport, which was evidenced by phase separated domains approx. 3--20 mum in films examined using IRIRI. These experimental observations, in conjunction with Monte Carlo simulations of heterogeneous film formation prompted the synthesis of novel one-component TE-PUD's which undergo crosslinking upon 254 nm UV irradiation to produce uniform, crosslinked films. Such materials offer a number advantages including low VOC content, fast drying and crosslinking times as well as low sensitivity to O2 inhibition. |
