Surface-modified nanoparticles for ultrathin coatings

Nanoparticle modification and their utilization in the modification of planar substrates were examined. Emphasis was placed on two topics: the control of layer structure during formation and the alteration of the wetting characteristics of modified surfaces. Layer formation was investigated by adsorbing nanoparticles with a distinct shape and charge onto a nanofibrillated cellulose (NFC) substrate. In addition, nanosized silica particles and NFC were adsorbed sequentially with an oppositely charged polyelectrolyte onto an NFC substrate in order to explore the structures achievable using layer-by-layer assembly. Evidently, the utilization of nanoparticles in layer formation demands the control of the nanoparticle dispersion stability and particle affinity to the substrate. When combining nanoparticles with other substances, the properties of the particles define the layer structure; large fibrils were able to form a stratified layer, while silica nanoparticles were able to penetrate the preceding layer and transform the structure into a uniform network of polyelectrolyte and nanoparticles. The effect of nanoparticle surface modification on dispersion properties and on the structure and properties of the layers formed were also of interest. Modification of nanosized silica and precipitated calcium carbonate particles was conducted by treatment with oppositely charged substances. This treatment resulted in stable nanoparticle dispersions able to be further modified with hydrophobic sizing agents. In addition to enhanced stability and functionality, the polyelectrolyte treatment could be used to affect the interaction of the nanoparticles with the other dispersion constituents. Wetting of a smooth and dense substrate was not affected by the nanoscale roughness caused by the nanoparticle coating on the substrate. In order to affect substrate hydrophobicity, chemical hydrophobicity was deemed necessary. The combination of modified nanoparticles and a hydrophobic emulsion resulted in a nanostructure able to change the wetting characteristics of a planar substrate. Treatment of a smooth substrate with a hydrophobic dispersion resulted in slightly enhanced surface hydrophobicity. On paper, the combination of micron and nanoscale roughness with chemical hydrophobicity resulted in a significant increase in hydrophobicity. The coatings consisted of a thin nanoparticle structure with evenly distributed particles. In addition to use as a paper surface treatment, a layer, consisting of inexpensive particles allowing simple surface modification, could be used to functionalize planar substrates and enable the use of paper as a sustainable substrate, even in applications beyond its traditional use.