Influence of composition on the morphology of polyurethane/acrylic latex particles and adhesive films

Polyurethane (PU)/acrylic hybrid particles with different PU contents were synthesized by miniemulsion polymerization and subsequently dried to give solid adhesive films. The morphologies of the particles and the morphologies and mechanical properties of the resulting films were investigated by Transmission electron microscopy combined with selective staining of the PU and by uniaxial tension tests. Morphological investigations showed a clear change in the particle morphology as the PU weight fraction increased. While at 5 wt% and 25 wt% PU (with respect to total organic content) the particles were relatively homogeneous and mechanical properties of the films could be readily interpreted with molecular arguments, at 50 wt% PU a core–shell structure was observed. This heterogeneous structure of the 50 wt% PU particles persisted in the films, resulting in a percolating network of the harder PU phase. The low deformability and strain at failure of the 50% PU films suggest that, unlike the adhesives with lower PU content, the relatively weak interfaces between the original latex particles dominate the mechanical properties.