Molecularly smooth cellulose surfaces for adhesion studies

Cellulose is deposited on silicon wafer surfaces via spin coating from a solution of cellulose in dimethylacetamide (+7%+7% lithium chloride). The resulting cellulose layers were analyzed by ellipsometry, AFM, FTIR, ICP-MS, X-ray reflectivity, and contact angle measurements. For cellulose concentrations below 0.07 wt% the wafer surfaces are covered with a network of cellulose fibrils. For concentrations between 0.07 and 0.5 wt%, closed films with thicknesses between 1.5 and ≈10 nm are obtained. These films are molecularly smooth (rms roughness < 2 nm). Higher concentrations result in thicker films with significantly rougher surfaces (rms roughness > 2 nm). The cellulose layers were used to investigate cellulose/cellulose adhesion and their modification by polyelectrolytes. To this end the sticking behavior of cellulose beads was analyzed. It is demonstrated that the sticking of the beads depends on the type, amount, and adsorption symmetry of adsorbed polyelectrolyte. Low, incomplete polyelectrolyte coverage always enhances sticking, whereas for high coverage the symmetry of the polyelectrolyte coating is very important. In this case, sticking (adhesion) is enhanced if only one surface is covered with polyelectrolyte prior to contact. If both surfaces were fully covered with polyelectrolytes before contact, sticking (adhesion) is decreased.

Graphical abstract3D-view and thickness profile of a region with and without cellulose film (scratched off with the tips of tweezers). The thickness profile is taken from an averaged box of 2×3 μm2×3 μm along the edge.Figure optionsDownload full-size imageDownload as PowerPoint slide