The Nanostructures of Native Celluloses, Their Transformations upon Isolation, and Their Implications for Production of Nanocelluloses

Native celluloses in plant cell walls occur in a variety of highly periodic fibrillar forms that have curvature and varying degrees of twist about their longitudinal axes. Though X-ray measurements reveal diffraction patterns, the celluloses are not crystalline in the traditional sense. The diffraction patterns rather are a consequence of the high degree of spatial periodicity inherent in the fibrils and in their hierarchic organization at different levels in the cell walls. Upon dehydration and exposure to elevated temperatures, the fibrils tend to collapse into linearly parallel structures that are not unlike traditional crystals. However, in alleviating the inherent twist of the fibrils during formation of linear domains, less tightly ordered terminal points are formed at the connection between adjacent linear domains. These domains are less tightly aggregated than the linear domains between which they occur, and are therefore more readily accessible to hydrolytic agents than the tightly aggregated linear domains between which they occur. Hydrolysis at the less tightly aggregated points transforms the tightly aggregated linear domains between them into nanocellulose particles. In consequence nanocelluloses isolated from different native forms, are likely to have structures that are as much a function of the isolation history as they are of the native source. We present data reflecting the effects of different thermal histories on the states of aggregation of cellulose. We also present data that confirm a previously proposed model, which we believe to be representative of nanoscale transformations of higher plant celluloses that occur upon dehydration and exposure to elevated temperatures. We anticipate that they provide a basis for understanding observations made upon preparation of nanocelluloses from higher plant tissues.