Acrylate hybrid nanocomposite coatings based on SiO2 nanoparticles. In-situ semi-batch emulsion polymerization

• Semi-batch synthesis at high solids of acrylate/untreated SiO2 nanoparticles.
• Optically transparent film coatings with well-dispersed SiO2 nanoclusters.
• Enhanced thermal stability, glass transition temperature and water contact angle.
• Increased decomposition temperature by 20 °C.
• Increased Young’s modulus by 100% at only 3% concentration of SiO2.

Acrylate–SiO2 nanocomposites were synthesized by in situ emulsion semi-batch polymerization in aqueous dispersion utilizing untreated nanoparticles. Butyl acrylate (BA), methyl methacrylate (MMA) and acrylic acid (AA) with composition 56:42:2 were copolymerized at 50% solids in the presence of SiO2 nanoparticles of 7 nm size. SiO2 nanoparticles were added up to 3 wt% and the emulsions thus obtained were stable for at least 6 months in storage at room temperature. The semi-batch polymerization was carried out for 2.5 h. and yielded conversions as high as 92% and not lower than 84%, superior to batch polymerization of the same system. The synthesized emulsion hybrids were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, scanning and transmission electron microscopy and atomic force microscopy. As cast films about 1 mm thick were optically transparent suggesting good nanoparticle dispersion. However, dispersed SiO2 nano aggregates were detected by SEM, TEM and AFM, and the identity of the nanoclusters was confirmed by EDS and EFTEM. Thus, the morphology of cast films consisted of SiO2 nanoclusters dispersed throughout the acrylate matrix. Strikingly, the untreated SiO2 nanoparticles induced significantly higher thermal decomposition temperatures, Tdec increased by as much as 20 °C relative to the neat acrylate. There was also an increase of glass transition temperature Tg and tensile modulus increased nearly 100% relative to the neat acrylate suggesting modification of macromolecular dynamics by the presence of the SiO2 nanoparticles, and this was confirmed by shear rheometry of the molten coatings where a rubber-like plateau was clearly defined. The results are contrasted for the same system but using batch polymerization.

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