Aromatic versus Aliphatic: Hydrogen Bonding Pattern in Chain-Extended High-Performance Polyurea

Polyurea, a reaction product of functional isocyanates and amines, is finding increasing application in the field of protective coatings, especially for retrofitting purposes. The properties of polyurea can be tuned by the prudent choice of reactants, specifically chain extenders. In this work, an experimental study into the consequences of chain extension of a representative polyurea formulation with different type of chain extenders (aromatic and aliphatic) has been undertaken, with an aim to gain an insight into their role in improving properties. Rheological studies were performed to understand the effect of their inclusion on the processing conditions and quantification of the gelation time. Aromatic chain extenders are far more reactive than their aliphatic counterparts, which lead to significantly short 'gel-time' as quantified by rheometry. The degree of H-bonding was qualitatively established by the red-shift associated with N-H and >C=O bands in the FTIR spectra of the polymer. Introduction of either type of chain extender lead to remarked processability of polyurea formulations through spray coating technique. An optimal aromatic: aliphatic chain extender ratio was found to result in optimal H-bonding, which in turn reflected in terms of mechanical properties. Dynamic studies were performed and all formulations were found to exhibit sub-ambient T-g which is appreciably affected by the type of chain extender used. Higher aromatic: aliphatic chain extender ratio was found to result in higher storage modulus and lower dissipation potential.