Tiina Nypelö

Microbeads and Hollow Microcapsules Obtained by Self-Assembly of Pickering Magneto-Responsive Cellulose Nanocrystals

Cellulose microbeads can be used as immobilization supports. We report on the design and preparation of magneto-responsive cellulose microbeads and microcapsules by self-assembled shells of cellulose nanocrystals (CNC) carrying magnetic CoFe2O4 nanoparticles, that is, a mixture of isotropic and anisotropic nanomaterials. The magnetic CNCs formed a structured layer, a mesh, consisting of CNCs and magnetic particles bound together on the surface of distinct droplets of hexadecane and styrene dispersed in water. Because of the presence of CNCs the highly crystalline mesh was targeted to provide an improved barrier property of the microbead shell compared to neat polymer shells, while the magnetic particles provided the magnetic response. In situ polymerization of the styrene phase led to the formation of solid microbeads (∼8 μm diameter) consisting of polystyrene (PS) cores encapsulated in the magnetic CNC shells (shell-to-core mass ratio of 4:96). The obtained solid microbeads were ferromagnetic (saturation magnetization of ∼60 emu per gram of the magnetic phase). The magnetic functionality enables easy separation of substances immobilized on the beads. Such a functionality was tested in removal of a dye from water. The microbeads were further utilized to synthesize hollow microcapsules by solubilization of the PS core. The CNC-based, magneto-responsive solid microbeads and hollow microcapsules were characterized by electron microscopy (morphology), X-ray diffraction (phase composition), and magnetometry (magnetic properties). Such hybrid systems can be used in the design of materials and devices for application in colloidal stabilization, concentration, separation, and delivery, among others.

Interactions between inorganic nanoparticles and cellulose nanofibrils

Nanofibrillated cellulose (NFC) is increasingly utilized in materials and biomedical applications consequently increasing interest in the modification of its surface properties. Besides modification using polyelectrolytes and polysaccharides, NFC can be combined with solid particles enabling formation of fibril network loaded with particles. Use of particles enabling easy functionalization could be beneficial for the development of hybrid structures, and lead to preparation of nanocomposites and functional materials. In order to explore interactions related to preparation of such structures, the interactions between nanosized precipitated calcium carbonate (nanoPCC) and nanoclay particles and NFC were examined by observing adsorption of the particles on NFC substrate using a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) imaging. By a treatment with carboxymethylated cellulose (CMC), the anionicity of the NFC substrate could be increased, providing an additional tool to affect the interplay between NFC and the inorganic particles. For slightly cationic nanoPCC particles an increase in the anionicity of the NFC by the CMC treatment increased the affinity, while the opposite was true for anionic nanoclay. Additionally, for interactions between nanoclay and NFC, dispersion stability was an important factor. QCM-D was successfully used to examine the adsorption characteristics of nanoparticles although the technique is commonly used to study the adsorption of thin polymer layers. Distinct adsorption characteristics were observed depending on the nanoparticle used; nanoclay particles deposited as a thin layer, whereas nanoPCC particles formed clusters.

Tailoring surface properties of paper using nanosized precipitated calcium carbonate particles

Pigment particles used in paper coatings are typically of micrometer size and consequently the thickness of the coatings is, even at its lowest, in micrometer scale. Progress in nanotechnology has given way to the development of nanosized materials to be used in coatings, yet their exploitation has not been studied to a great extent. This study examines utilization of nanosized precipitated calcium carbonate (nanoPCC) particles in nanoscale thin coating layers. In contrast to commonly used coatings, a thin nanoparticle-based coating was targeted to change the substrate surface characteristics via controlled surface structure rather than via high coat weight. A novel approach for stabilizing and modifying the nanoPCC particles with pectin and alkenyl succinic anhydride (ASA) was utilized and a nanoparticle coating with uniform particle distribution was created. The coating applied on paper substrate was hydrophobic, having a water contact angle of 125°. Particle surface modification provided dispersion stability, enabling control of the coating layer structure. The introduced concept provides a new approach to paper coatings utilizing controlled deposition of nanoparticles with extremely low coat weight, yet having high impact on substrate surface properties. Additionally, as paper is an environmentally sound product, the approach to form a controllable nanostructure on a green substrate has potential in applications outside the traditional paper products.

Modifying the Wettability of Surfaces by Nanoparticles: Experiments and Modeling Using the Wenzel Law

The control of the wettability by nanoparticles (NPs) was studied by treating lignin model surfaces with silica NPs at various NP concentrations. The experimental contact angles (CAs) were compared to a modified Wenzel equation, which takes into account the changes in the effective surface area and the surface roughness. The conclusion was drawn that the change of effective roughness is not visible, which implies that the NP-induced geometric heterogeneity is irrelevant for the CA change. This is attributed to dynamics of contact line in the weak pinning regime.