Structure-property relationships in biopolyurethane/acrylate hybrid films: Role of chain extender and acrylic content

Abstract

Waterborne polyurethane/acrylic (PU/AC) hybrid dispersions are widely used in coatings applications due to their low environmental impact and tailorable thermomechanical performance, and are typically composed of particles with a core-shell morphology. Controlling phase morphology and interdomain interactions through formulation parameters is still a key challenge. In order to examine the combined effects of acrylic content and chain extender chemistry on dispersion stability, morphology, and film properties, PU/AC hybrids were synthesized with varying acrylic contents (30–70 wt%) and three different chain extenders: ethylenediamine (EDA), allylamine, and 2-hydroxyethyl methacrylate (HEMA). Stable dispersions were only obtained for acrylic contents of 40 and 50 wt%, with particle sizes ranging from 84 to 126 nm, while acrylic monomer conversions higher than 95% were achieved in all stable formulations. Increasing the acrylic content resulted in particles with thinner polyurethane shells, and an increased tendency for particle coalescence, as observed by TEM and AFM, indicating reduced ability of the polyurethane phase to stabilize the acrylic-rich domains. The nature of the chain extender strongly influenced hard segment organisation and phase cohesion: EDA promoted the formation of urea-rich, highly cohesive domains, leading to films with higher storage modulus values and three distinct relaxation processes, with Tanδ maxima up to 63–65 °C, whereas allylamine and HEMA promoted more heterogeneous, partially mixed morphologies with two main relaxation processes, the first one in the range of −40 to −33 °C, and the second one at around 0 to 5 °C. These results demonstrate that the morphology and thermomechanical response of PU/AC hybrid films can be effectively controlled by changing both acrylic content and chain extender chemistry, providing valuable insights for the development of advanced waterborne coating systems.