Coalescence and film formation of a low molecular weight polyurethane dispersions designed for radiation cure

The goal of this research was to investigate coalescence and film formation in a polyurethane dispersion (PUD) designed for radiation cure. The recipe for PUD synthesis was taken from a recent patent [US 20220112371A1, April 14, 2022], leading to uniform nanoparticles dispersed in water with N-(2-aminoethyl)-β-alaninate as the stabilizer and with a mean hydrodynamic diameter of 80 nm. The high content of 2-hydroxyethyl acrylate (HEA, 20 wt%) led to a low M_n (∼ 2.5 kDa) and a broad molecular weight distribution. Replacing a small fraction of the HEA with donor or acceptor dyes for fluorescence resonance energy transfer (FRET) experiments showed that the HEA was concentrated in a low molecular weight fraction of the polymer. This is an unexpected result. Introducing the dyes by pre-reacting them with the hexamethylene diisocyanate (HDI)-trimer component of the formulation led to a more uniform distribution of the dyes in the sample. We prepared components labeled with phenanthrene (Phen) as the fluorescent donor dye or with 1-(4-nitrophenyl)pyrrolidine (NPP) as the non-fluorescent acceptor dye. FRET experiments conducted on samples in the dispersed state showed that a significant extent of nanoparticle-nanoparticle polymer transfer took place through the water phase. Films formed at room temperature showed a significant amount of energy transfer only 1 h after drying, Φ_ET = 69 % for HDI-trimer-labeled samples and Φ_ET = 75 % for samples in which the dyes replaced HEA. These experiments indicate the high mobility of the polyurethane chains and substantial polymer diffusion occurs very quickly in the film.