Polymer Films from Cellulose Nanofibrils—Effects from Interfibrillar Interphase on Mechanical Behavior

Dense polymeric films based on network-forming cellulose nanofibrils (CNFs) have excellent mechanical properties but are limited by moisture sensitivity. Here, interfibrillar effects from CNF surface properties are investigated. TEMPO-oxidized CNFs and two native CNFs are prepared with a similar length and width, to exclude geometrical effects. The CNFs have different surface properties in terms of sorbed hemicellulose content, hemicellulose molar mass, and surface charge. Moisture sorption, structural changes, and mechanical properties at different relative humidities are characterized. The presence of sorbed hemicelluloses in the interfibrillar interphase has favorable effects on the mechanical tensile properties. Surface-charged carboxyls increased moisture sorption and film thickness swelling and reduced the mechanical properties. A comparison with biaxially oriented polyethylene terephthalate films provides a perspective into the structure and properties of CNF films. The present study shows the importance of the interfibrillar interface and interphase region for mechanical film properties, including moisture effects.