The structure of celluloses

X-ray and neutron fiber diffraction has been used to study cellulose as it is converted from its naturally occurring crystal phase, cellulose I, to an activated crystal phase, cellulose III I , by ammonia treatment. The detailed crystal structures determined for cellulose I β , an intermediate ammonia-cellulose I complex, and cellulose III I , reveal a structural transition pathway: hydrogen bonded sheets of chains in cellulose I β slip with respect to each other to accommodate the penetrating ammonia guest molecules in the intermediate complex. On evaporation of ammonia from the intermediate complex, there is a relative small change in chain packing as an inter-sheet ammonia bridge is replaced by an inter-sheet hydrogen bond in cellulose III I . When cellulose III I is heated it converts back to cellulose I β . Both ammonia-cellulose I and cellulose III I have extended chains of cooperative hydrogen bonds in relatively open crystal structures that may add to their susceptibility to rapid change.