Structure–property relationships in polyurethane-urea coatings: Role of aromatic and aliphatic amine-based chain extenders

The chemical structure of the chain extenders used in the synthesis of poly(urethane-urea) coatings directly influences the physical and mechanical properties of the coating. In this study, poly(urethane-urea) coatings were synthesized using polyethylene glycol 600 (PEG600) and isophorone diisocyanate (IPDI) precursors, along with two different amine-based chain extenders: aromatic para-phenylenediamine (PU1) and cyclic aliphatic piperazine (PU2), respectively. The effect of the structural differences between the chain extenders on the properties of the coatings was investigated. Based on the Fourier transform infrared spectroscopy analysis, the structural differences between the two coatings were primarily interpreted through changes in the carbonyl peak profile. Thermal behaviour of the coatings was investigated by TGA (Thermal Gravimetric Analysis), revealing that the presence of aromatic benzene ring in PU1 chain supplied the conjugate stability which increases bond strength and promotes the thermal stability. PU1 exhibited superior adhesion performance, attributed to better bonding with the stainless steel surface through hydrogen bonding interactions. Mechanical testing revealed a significant difference in the flexibility of the coatings under applied stress. The use of a cyclic aliphatic amine as the chain extender enhanced flexibility compared to the aromatic diamine, due to its lower rigidity and greater conformational freedom. Conversely, the higher stiffness of the aromatic chain extender restricted polymer chain mobility, resulting in reduced elongation capacity of the coating under mechanical stress. Overall, the results highlight how the chemical nature of chain extenders governs the performance of poly(urethane-urea) coatings.