Waterborne layered silicate/acrylate nanocomposites by in-situ emulsion polymerization: Thermal and mechanical reinforcement

• Synthesis of waterborne acrylate-bentonite nanocomposites.
• Enhanced thermal stability and increased glass transition temperature.
• Optically transparent coatings, intercalated nanostructure.
• Dynamic modulus E′ increased over two orders of magnitude at only 3% bentonite content.

Layered silicate acrylate/bentonite nanocomposites were synthesized by seeded batch emulsion polymerization. The acrylates were based on butyl acrylate (BA), methyl methacrylate (MMA), and acrylic acid (AA) with composition 56:42:2. Surface untreated nanoclay, bentonite, was incorporated in situ up to 3 wt%. Coating films obtained from the aqueous dispersions were optically transparent. Transmission and scanning electron microscopy (TEM, SEM) showed that bentonite was aggregated into clusters dispersed throughout the polymer matrix, and TEM and wide-angle X-ray scattering (WAXS) confirmed that the silicate layers were intercalated by the macromolecules. The nanocomposites exhibited higher onset for thermal degradation temperature, Tonset, and higher glass transition temperature, Tg, increasing up to 20 °C and 6 °C, respectively, relative to the neat acrylate. Strikingly, the dynamic elastic modulus E’ increased over two orders of magnitude at only 3 wt% concentration of bentonite. Furthermore, the α relaxation, denoted by maximum tan δ (=E″/E′) was shifted to higher temperatures and its strength significantly attenuated thus reflecting a decrease in toughening, and evidencing the slowdown of long range molecular motions associated to the glass transition. The mechanical reinforcement was confirmed by uniaxial tensile tests, the Younǵs modulus E exhibited a 5-fold increase at only 1 wt% bentonite content. The modification of long range molecular motions evidenced by dynamic mechanical analysis would be associated to the restricted motions of the macromolecules confined within the clay nanoplatelets.

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