3.3. Regeneration process of cellulose in water
From the above results, the regeneration of cellulose depended on the rate of coagulation, namely the rate of solvent diffusing out of, and the rate of non-solvent diffusing into the cellulose solution. As shown in SEM images of Figs. 2 and 3, relatively large pore size of the films displayed at higher temperature. Namely, the non-solvent (water) entering into cellulose solution and the solvent (NaOH and urea) diffusing out were faster at higher temperature than at lower temperature. Cellulose inclusion complex (IC) was destroyed by heating, resulting in the cellulose gelation through the formation of new hydrogen bonding networks (Cai & Zhang, 2006). Actually, by raising temperature, the inclusion complex(IC) associated with NaOH and urea hydrates bound on the cellulose chains were perturbed, and water was the non-solvent, so the junction between cellulose molecules occurred as a result of the strong self-association force of cellulose, which supported the viewpoint above. In view of these results, a model describing the regeneration process of the cellulose in NaOH/urea aqueous solution is proposed in Scheme 1. The cellulose inclusion complex hosted by urea existed in NaOH/urea aqueous solution to form homogeneous cellulose solution, as shown in Scheme 1a. By adding water (nonsolvent), the IC consisted of NaOH and urea, which was bound on
the cellulose molecules, were broken. Thus, the cellulose aggregates occurred, as a result of the self-association of cellulose chains. In this case, NaOH and urea molecules bonded on the –OH groups of cellulose were removed out in water, and the new –OH groups of cellulose were regenerated through the exchanging with water to form regenerated cellulose gel sheets, as shown in Scheme 1b. The rearrangement of the hydrogen bonds of cellulose occurred here, leading to the regeneration. There were many pores in the cellulose gel sheets, as a result of the phase separation between solvent and non-solvent.