3.1. Structure of cellulose films
Fig. 1 shows the photographs of RC35 and RC0. The films prepared by coagulating with heated water were transparent, suggesting that good regeneration of the cellulose in the solution through the hydrogen-bonding rearrangement of the cellulose macromolecules. Fig. 2 shows the SEM images of the surface of the two sides: one is contacted with a glass plate (side A, bottom), and the other is contacted with the coagulant (side B, up). The films exhibited homogeneous porous structure, indicating a complete regeneration of cellulose. This was a physical regeneration during the coagulation. Interestingly, the pore size of the cellulose films at wet state was much larger in side B than that in side A. For example, the 2re (the average diameter of the pores from SEM
image) in RC-35 was changed from 800 nm to 200 nm. This could be explained that the coagulant diffusing much faster in side B than that in side A. With an increase of the water temperature, the pore size of both surfaces of the cellulose films increased, as a result of the enhancement of the removal of the solvent from the cellulose
solution, leading to the phase separation. The result of the elemental measurement indicated that there was no N in the films and the films were washed until pH 7, suggesting that NaOH and urea have been washed out. Fig. 3 shows the SEM images of the cross section of RC-25, RC-35 and RC-45. The 2re value increased from 100 nm to 350 nm with an increase of the temperature (Table 1). The cellulose solution was coagulated more slowly at low temperature than at relatively high temperature. However, high temperature (>50 ◦C) led to observable phase separation, resulting in the imperfect
regeneration of cellulose.