Design and characterization of cellulose nanofibril-based freestanding films prepared by layer-by-layer deposition technique

Freestanding layer-by-layer (LbL) films of anionic carboxymethylated cellulose nanofibrils (NFC) and a cationic branched polyelectrolyte, polyethyleneimine (PEI) have been prepared and characterized in terms of their structural and mechanical properties. The consecutive build-up of PEI and NFC on a hydroxylated and trifunctional organosilane-coated silicon substrate was monitored with X-ray photoelectron spectroscopy (XPS), quartz crystal microbalance with dissipation (QCM-D) and dual polarization interferometry (DPI) techniques. QCM-D and DPI measurements showed that the formation of each layer was fairly rapid and that the thickness of the NFC layers was larger than that of the PEI layers. The results also showed a linear to exponential growth with increasing layer number. The functionalization of the surface with trichlorosilanes did not significantly change the build-up of the LbL structures but it made it possible to easily peel off the formed films from the substrate. The stratified cross-sectional image of the (PEI/NFC)(150) freestanding film was imaged with field-emission scanning electron microscopy (FE-SEM) and the thickness of (PEI/NFC)(150) was measured to be about 5 mu m. Surface morphologies of the LbL films showed a randomly oriented nanofibrillar structure with an average surface roughness of ca. 9 nm. Uniaxial tensile tests on the freestanding LbL films showed that the introduction of polyelectrolytes into the nanofibrillar network increased the modulus, strain-at-break and stress-at-break, probably as a consequence of the softening of the cellulose film under absolutely dry conditions allowing for some movement of the fibrils before breakage of the film.