Qualifications for strength: Tensile properties of bacterial cellulose nanopapers

Nanosized cellulose has recently established itself as a promising material to be used as reinforcement in the production of high performance renewable composite materials. Bacterial cellulose (BC), i.e., nanofibres synthesized by bacteria, such from the Acetobacter species, consists of long threads of pure cellulose with a fibre diameter of 50 nm and several micrometres in length. When a continuous network is prepared from BC nanofibres or other nanocellulosic species by water removal, it is called nanopaper. Nanopapers are materials with exceptionally high mechanical properties in their own right and they can also be utilized as reinforcing phases in composite materials. The influence of the basic properties within the nanopaper network has received surprisingly little attention. In this work, the tensile properties of BC nanopaper were examined as a function of the amount of material in the sheet, that is, by varying the nanopaper grammage. We observe that there exists a minimum threshold grammage, whereby the nanopapers possess sufficient mechanical strength. This elucidates the transition from a loose nanofibrous network to nanopapers, i.e., the point where nanopaper starts to be regarded as paper and not just a random agglomerate of nanofibres. In addition to analysis of the tensile properties, mercury intrusion porosimetry and helium pycnometry were applied to attain data for quantitative assessment of the correlation between the film structure and mechanical integrity.