Customizing the mechanical performance of water stable TEMPO oxidized cellulose nanofibril films

Hygroscopic nature of cellulose is one of the most difficult limiting factors to overcome in utilization of nanocellulose. When unmodified nanocellulose structure is subjected to humidity or immersed in water it loses its structural integrity very quickly. Nanoscaled cellulose fibrils form very tightly packed films, which can be used, for example, as excellent barriers or membranes. Water stability of nanocellulose films has been improved by preparation of multilayered structures, reduction of free volume, surface modification and additives. Polyvinyl alcohol (PVA) can be used to significantly enhance water stability of films prepared from TEMPO oxidized cellulose nanofibrils (TOCNs). In water swollen TOCN films, PVA has two functions; it can either improve the wet strength by forming interfibrillar bridges or act as a plasticizer. The interfibrillar bridging is result of two mechanisms (Fig. 1): 1) hemiacetal bonding between PVA hydroxyls and aldehydes in TOCNs and 2) acid catalysed formation of ester bonds between PVA hydroxyls and carboxylic groups in TOCNs. Sufficient bonding between TOCNs to achieve water stability could not be observed. Mechanical properties of the films can be tuned without losing the water stability merely by changing the amount of PVA. Even after being immersed in water for several months, TOCN/PVA films maintain their structural integrity. The water stability increases to such extent that TOCN/PVA films can be used as water filtration membranes