Glasslike Polymer Nanocomposite Coating via High Loading of Mixed-Size Nanoparticles

Achieving glasslike polymer nanocomposite (PNC) coatings via very high nanoparticle (NP) loading on transparent polymer substrates addresses unmet needs in industrial and consumer electronics applications. However, very high NP loadings while maintaining the optical transparency of PNC coatings are not yet demonstrated without complicated coating processes. Here, we demonstrate glasslike PNC coatings on transparent polymer films via mixed-size SiO2 NP loading above random close packing limit for monodisperse particles. We probed the effects of NP size and NP volume fraction, Vp, on the optical and mechanical properties of urethane acrylate-based PNC coatings. The optimum ratio of NP diameters to achieve high loading was calculated based on the amorphous packing structure of the binary sphere system. When a mixture of 50 and 10 nm diameter NPs (ratio of 11.6:1) was used, the PNC coating maintained optical transparency and exhibited the indentation modulus and hardness values of 19.6 and 1.52 GPa, respectively, at Vp = 65.4%. To our knowledge, these values are the highest among PNC coatings with SiO2 NPs. In contrast, PNC coatings with single-size NPs exhibited a drop in mechanical properties at the similar Vp due to NP agglomeration. Unlike most of the previous studies using complex and time-consuming processes, our PNC coatings were fabricated through simple mixing and web coating processes. We believe that this simplicity to achieve glasslike PNC coatings has broad implications for emerging industrial and consumer electronics applications.