Toughening mechanisms in cellulose nanopaper: the contribution of amorphous regions

Cellulose nanopaper is a strong and tough fibrous network composed of hydrogen bonded cellulose nanofibres. Upon loading, cellulose nanopaper exhibits a long inelastic portion of the stress–strain curve which imparts high toughness into the material. Toughening mechanisms in cellulose nanopaper have been studied in the past but mechanisms proposed were often rather speculative. In this paper, we aim to study potential toughening mechanisms in a systematic manner at multiple hierarchical levels in cellulose nanopaper. It was proposed that the toughness of cellulose nanopaper is not, as is often assumed, entirely caused by large scale inter-fibre slippage and reorientation of cellulose nanofibres. Here it is suggested that dominant toughening mechanism in cellulose nanopaper is associated with segmental motion of molecules facilitated by the breakage of hydrogen bonds within amorphous regions .