Adsorption studies on cellulose surfaces by combinations of interfacial techniques

Abstract

In this work, the adsorption of various polymers on cellulose surfaces was studied in detail at molecular level. Special attention was paid on the interactions between renewable polysaccharides and different nanofibrillated cellulose (NFC) grades. Polymer or nanoparticle adsorption in aqueous medium was explored as a strategy to functionalize NFC. The role of pulp raw material and chemical pre-treatment on the NFC properties was clarified via indirect adsorption studies with ultrathin NFC films. Atomic force microscopy (AFM) -in different imaging and force detection modes-, quartz crystal microbalance with dissipation (QCM-D), Raman spectroscopy and surface plasmon resonance (SPR) were combined to carry out this research. The similar backbone having polysaccharides had natural affinity on NFC substrates. Comparison between NFC from different origin (hardwood vs. softwood), although of similar morphology, revealed differences in the conformation of adsorbed polysaccharide layer. The polysaccharide structure rather than NFC origin had more notable effect on adsorbed polysaccharide amount and layer properties. The attachment of the very thin (only few nm thick) polysaccharide layer was uniform without aggregates. They nevertheless were able to change the surface properties of cellulosic materials. One example was the lowered friction co-efficient with one polysaccharide (chitosan) determined for regenerated cellulose spheres in low pH aqueous solution. In addition, NFCs prepared after chemical pre-treatments were compared to unmodified NFC. Increasing the anionicity prevented the interfibril association by electrostatic repulsion. As a consequence the fibrillation efficiency was enhanced and very thin nanofibrils were achieved. The surface interactions were systematically probed and compared with different cationic counterparts using layer-by-layer (LbL)-technique. The high charged, chemically modified NFC behaved differently compared to low charged, unmodified NFC: they bound more water and the layer formation and stabilization was faster; and the adsorbed amount increased as the function of layer number. Nevertheless, multilayers could also be formed with the lower charged NFCs to some extent. Non-electrostatic interactions were significant between oppositely charged all-cellulosic materials. A considerable increase in adhesive forces during multilayer build-up due to high compressibility of the high charged NFC was also detected. The information obtained in this study for the interactions of emerging, renewable, bio-based materials can be used to create more sustainable material applications in the future.