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Cellulose can be dissolved in precooled (-12 °C) 7 wt % NaOH-12 wt % urea aqueous solutionwith in 2 min. This interesting process, to our knowledge, represents the most rapid dissolution of native cellulose.The results from 13C NMR, 15N NMR, 1H NMR, FT-IR, small-angle neutron scattering (SANS), transmissionelectron microscopy (TEM), and wide-angle X-ray diffraction (WAXD) suggested that NaOH “hydrates” couldbe more easily attracted to cellulose chains through the formation of new hydrogen-bonded networks at lowtemperatures, while the urea hydrates could not be associated directly with cellulose. However, the urea hydratescould possibly be self-assembled at the surface of the NaOH hydrogen-bonded cellulose to form an inclusioncomplex (IC), leading to the dissolution of cellulose. Scattering experiments, including dynamic and static lightscattering, indicated that most cellulose molecules, with limited amounts of aggregation, could exist as extendedrigid chains in dilute solution. Further, the cellulose solution was relatively unstable and could be very sensitiveto temperature, polymer concentration, and storage time, leading to additional aggregations. TEM images andWAXD provided experimental evidence on the formation of a wormlike cellulose IC being surrounded withurea. Therefore, we propose that the cellulose dissolution at -12 °C could arise as a result of a fast dynamic
self-assembly process among solvent small molecules (NaOH, urea, and water) and the cellulose macromolecules. |
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