Cellulose thin films: degree of cellulose ordering and its influence on adhesion.

Adhesion measurements have been performed with thin cellulose films using continuum contact mechanics with application of the JKR theory. Three different cellulose surfaces were prepared, one crystalline and two surfaces with a lower degree of crystalline order. Adhesion between two cross-linked poly(dimethylsiloxane) (PDMS) caps, as well as the adhesion between PDMS and the various cellulose surfaces, was measured. The work of adhesion (from loading) was found to be similar for all three surfaces, and from contact angle measurement with methylene iodide it was concluded that dispersive interactions dominate. However, the adhesion hysteresis differed significantly, being larger for a less ordered cellulose surface and decreasing with increasing degree of crystalline order. This is suggested to be due to the surface groups' ability to orient themselves and participate in specific or nonspecific interactions, where a surface with a lower degree of crystalline order has a higher possibility for reorientation of the surface groups. The mobility of cellulose chains increases with water uptake, resulting in stronger adhesive joints. These films will hence allow for determination of the contributions of hydrogen bonding and inter-diffusion on the adhesion, determined from the unloading data, as the thermodynamic work of adhesion was found to be independent of the cellulose surface used.