Synthesis and Characterization of Polyethylenimine-Based Urea-Containing Polymers and Their Application as Thermal Latent Curing Agents for One-Component Epoxy Resins

Linear polyethylenimines (L-PEIs) of varying molar masses were synthesized and modified with monoisocyanates to form urea-containing polymers, which were evaluated as thermal latent curing agents (TLCs) for one-component epoxy resins (OCERs). The incorporation of urea bonds enabled ambient-temperature stability and temperature-triggered reactivity. All TLCs and OCERs were thermally stable up to 100 °C. Differential scanning calorimetry (DSC) revealed that the presence of epoxy resin lowered the urea bond deblocking temperature via chemical interactions. The deblocking temperature was tunable through the L-PEI molar mass and the substituent group adjacent to the urea bond; notably, the 3,4-dichlorophenyl group provided the lowest deblocking temperature due to its strong electron-withdrawing nature. Among the formulations, PDCPU2320 demonstrated superior performance, effectively curing epoxy at 140 °C within 2 h. Optical, rheological, and thermomechanical analyses showed that PDCPU2320 had improved miscibility and curing behavior with increasing temperature in DGEBA. The viscosity and modulus transitions around 130–150 °C demonstrated temperature-triggered activation and gradual network formation. DMA and DSC results revealed a Tg of about 60–65 °C. This study introduces a versatile approach to designing OCERs with tunable curing profiles and long shelf life, offering strong potential for use in advanced composites and adhesive technologies.