Delineating crosslink density gradients via in-situ solvation of immiscibly blended polyurethane thermosets

The cure of a blended resin thermoset coating comprised of two immiscible acrylic polyols, and an isophorone diisocyanate (IPDI) compatibilizer was analyzed via vibrational spectroscopy and TEM. The polyol reactants differed in their relative quantities of OH groups (referred to as high OH and low OH), which rendered them incompatible. The reaction yielded a coating with a surface roughened by droplet domains. Previous analysis revealed that domains consisted of the high OH resin. Carbonyl peaks originating from IPDI exhibited intensities commensurate with IPDI concentration for single resin coatings. However, commensuration was absent from the blend phases due to a diffusion gradient. The gradient is evidenced by the domains’ resistance toward in-situ caprolactam and introduced styrene monomer. These conclusions support the theory that caprolactam acts an internal solvent to reduce the coating viscosity for even application on a substrate. Domains are less solvated due to their higher crosslink density.