Mercerization of primary wall cellulose and its implication for the conversion of cellulose I → cellulose II

The mercerization of homogenized primary wall cellulose extracted from sugar beet pulp was investigated by transmission electron microscopy (TEM), X-ray diffraction together with (13)C CP-MAS NMR, and FT-IR spectroscopy. For samples resulting from acid extraction, mercerization began at 9% NaOH, whereas for samples purified by alkaline treatment, the mercerization started at 10% NaOH. The change in morphology when going from cellulose I to cellulose II was spectacular, as all the microfibrillar cellulose morphology disappeared during the treatment. This change in morphology was very drastic as soon as the NaOH concentrations were increased beyond 8 and 9% for the acid and alkaline prepared samples, respectively. On the other hand, the conversion was found to be more progressive in terms of increasing NaOH concentration when the transformation was analyzed by X-ray diffraction or spectroscopy. Our observations of the mercerization of isolated cellulose microfibrils are consistent with the concept of cellulose microfibrils made of parallel chains in cellulose I and crystals of cellulose II consisting of antiparallel chains.