Towards tailored hierarchical structures in cellulose nanocomposite biofoams prepared by freezing/freeze-drying

Cellulose nanofiber (MFC) reinforced starch-based foams, prepared by the freezing/freeze-drying route, are very interesting porous materials due to the strong MFC reinforcement of the cell wall itself. However, in order to fully realize the potential of these nanocomposite biofoams, both cell wall composition and cell structure must be controlled. The effect of starch-MFC-water suspension composition, together with preparation temperature (-27, -78, and -196 degrees C) on the foam cell structure is investigated. NMR-analysis of bound water content, DSC and freezing experiments in combination with freeze-drying experiments and FE-SEM microscopy are used to determine a suitable freeze-drying temperature. The freeze-drying temperature is critical in order to avoid cell structure collapse, as found from FE-SEM studies. By varying the cell-wall composition and preparation temperature, the foam morphology can be manipulated. The connection between cell size and starch content is considered to depend on the inherent properties of starch and a mechanism for ice crystal formation is suggested. Based on improved preparation conditions, foams with mixed open and closed cell structures and as much as 70 wt% MFC in the cell wall are created successfully.