Honeycomb-like microstructured materials for mechanically strong polymer films. Author: Ms. Nerea Jiménez
TEM micrograph of a cross-section of an acrylic polymer film for coatings applications. In the image, the darkest component forms a honeycomb microstructure around the lightest phase. The dark phase was composed of tannic acid, a naturally occurring polyphenol, and the light phase was consisting of poly(N-vinyl pyrrolidone-co-butyl acrylate-co-methyl methacrylate) (P(NVP/BA/MMA) polymer particles produced by seeded semibatch emulsion polymerization. For the preparation of the sample, 5% based on polymer of tannic acid was added as an aqueous solution to the P(NVP/BA/MMA) polymer dispersion. The blend was stirred for 30 minutes, cast into a silicone mould and dried for 7 days at 55 % relative humidity and 25 ºC. The resulting polymer film was cut employing a microtome and stained with ruthenium tetraoxide (RuO4) in order to increase the contrast of the aromatic rings of tannic acid. The staining process allowed the observation of the honeycomb microstructure formed by tannic acid around the polymer particles. The honeycomb microstructure was formed as a result of the strong hydrogen bonds between the aromatic hydroxyl groups of tannic acid and the amide groups of the pyrrolidones present in the polymer, which promoted the homogeneous distribution of tannic acid around the polymer particles. This conclusion was made by comparing the microstructure of the materials prepared without the pyrrolidone: The tannic acid formed aggregates instead of the continuous honeycomb microstructure. The P(NVP/BA/MMA)/tannic acid hydrogen bonded materials presented greater Young’s modulus and yield stress as a result of the hard honeycomb structure. The high stiffness presented by these materials make them excellent candidates for applications that require hard polymer films and opens the way for the complete replacement of solventborne coatings.