Polymer Network Evolution Induced by Dual-Crosslinking Architectures in UV-Curable Waterborne Polyurethanes

Dual-crosslinked waterborne polyurethane (WPU) coatings offer high performance, yet how sequential crosslinking governs the dynamic evolution of internal hydrogen bonding remains poorly understood. Herein, a dual-crosslinking strategy was developed by coupling castor oil (CO) thermal pre-branching with photo-curing monomers of contrasting functionalities: hydroxyl-bearing monofunctional acrylate (2-hydroxyethyl methacrylate, HEMA) and trifunctional acrylate (pentaerythritol triacrylate, PETA). The interplay between covalent crosslinking and physical hydrogen-bonding reorganization was systematically investigated using FTIR carbonyl deconvolution, gel fraction, crosslinking density, and dynamic mechanical analysis. Spectral deconvolution demonstrated that dual-crosslinking reorganizes hard-segment microenvironments rather than merely increasing hydrogen-bond density. Specifically, CO pre-branching facilitated supramolecular association, where CO1 selectively promoted localized, highly ordered microdomains (33.8% strongly bonded carbonyls in WPU-CO1-TA) and CO2 expanded the overall hydrogen-bonded population. DMA further revealed that these topological variations directly dictate segmental relaxation and network integrity across the glass transition. Overall, this preliminary study establishes that tailoring photo-monomer functionality alongside bio-based branching regulates physical hydrogen-bonding redistribution, providing a rational design strategy for advanced dual-crosslinked WPU films.