Mechanics of cellulose nanopapers

Cellulose nanopaper is a fibrous network composed of cellulose nanofibres connected by hydrogen bonds, which shows pronounced mechanical and physical properties. This thesis investigates the mechanics of cellulose nanopaper from various aspects. First, the fracture properties of cellulose nanopaper were investigated using experimental and modelling approaches. It was found that the fracture strength of notched nanopaper is insensitive to notch length. Cohesive zone models were used to describe the fracture behaviour of notched cellulose nanopaper. Fracture energy was extracted from the cohesive zone models and divided into an energy component consumed by damage in materials and a component related to pull-out and bridging of nanofibres between cracked surfaces which is not facilitated by short nanofibres in nanopaper. Strain mapping revealed a small region of highly localized strain ahead of the notch tip with multiple stress concentration sites which are indicative of a stress delocalization mechanism. Secondly the inelastic deformation mechanisms of cellulose nanopaper were investigated. Results indicate that the inelastic deformation of cellulose nanopaper does not originate from fibre slippage and shearing as often suggested in literature but originates from inelastic deformation in amorphous regions in the cellulose nanofibres itself. It is proposed that this mechanism is associated with segmental motion of cellulose molecules facilitated by the breakage of hydrogen bonds within these amorphous regions. Thirdly, the effect of preparation methods on the mechanical properties of cellulose nanopaper was investigated. The influence of processing parameters such as compaction pressure and temperature was investigated and the mechanical properties of these nanopapers were compared with nanopaper prepared by a suspension casting method. Finally, a micromechanical fibrous network model was used to investigate the parameters that determine the elastic modulus of cellulose nanopaper. The effect of fibre size, waviness and modulus, inter-fibre bond density as well as network density on elastic modulus was investigated.