Dynamics and viscoelastic behavior of waterborne acrylic polymer/silica nanocomposite coatings

The dynamics and linear viscoelastic response of waterborne acrylic/silica nanocomposite coatings have been studied. In-situ nanostructured latex were prepared using emulsion polymerization in semi-batch mode and contained up to 3 wt% nanosilica of 7 nm diameter [Romo-Uribe et al., Europ. Polym. J. 2016, 76, 170-186]. High-resolution transmission electron microscopy (HRTEM) of the coatings demonstrated well-dispersed nanoparticles at low silica content. Increasing silica content up to 3 wt% reduced the interparticle distance and evidenced macromolecular confinement. The glass transition temperature T-g increased with silica content, suggesting modification to cooperative dynamics. The macromolecular dynamics was studied by constructing master curves at T-ref = T-g + 50 K using time-temperature superposition (TTS). The nanostructured melts were entangled and exhibited only rubber-like and transition regimes. Strikingly, the viscoelastic response exhibited entanglement dilution behavior, i.e. the rubber-like modulus G(e) decreased twofold, relative to the neat polymer. Then G(e) gradually increased with nanosilica content as the degree of confinement increased. Despite entanglement dilution, the rubber-like regime of the nanocomposites was more elastic than for the neat polymer. The decrease and then increase of entanglement density are nanoconfinement effects as the molecular weight of the polymer matrix and nanocomposites are of the same order of magnitude. The dilated entangled network produced smaller elastic tensile and shear modulus at room temperature.