Mechanism of structure formation and dissolution of regenerated cellulose from cellulose/aqueous sodium hydroxide solution and formation of molecular sheets deduced from the mechanism

The structure formation of regenerated cellulose from aqueous sodium hydroxide solution was investigated using synchrotron X-ray radiation. The diffraction shoulder, which was possibly assigned to the molecular sheet structure, was observed in the wide angle scattering region during the gelation process of the solution. Molecular sheets were progressively piled to more than three layers at the final stage of gelation, as deduced from small angle X-ray scattering. These results suggested that the molecular sheet was formed at the initial stage of structure formation. The model of the molecular sheet structure is reasonably stable in aqueous media because of its hydrophilic exterior and hydrophobic interior. The dissolution of cellulose in aqueous sodium hydroxide solution was also investigated and the molecular sheet was found to remain in the dissolving system until the last stage of dissolving. These results, namely, the molecular sheet firstly appeared from the cellulose solution in the coagulating process and finally disappeared in the dissolving process, indicates that structure formation and dissolution are “two sides of the same coin”. The X-ray diffraction peak, which could not be assigned to any crystalline diffraction, was observed for the regenerated cellulose films prepared from aqueous sodium hydroxide solution. The scattering vector q of the diffraction was 14 nm⁻¹, which was equivalent to the spacing between cellulose chains in the molecular sheet, 0.45 nm. The trace of the molecular sheet possibly exists in the regenerated cellulose.