2.3. Characterizations
The N contents in the cellulose films were measured with an elemental analyzer (Heraeus Co., Germany). Scanning electron micrographs (SEM) were taken on a Hitachi S-570 scanning electron microscope with 20 kV accelerating voltage and at magnification of
3000. The wet RC membranes were frozen in liquid nitrogen, immediately snapped and then vacuum-dried. The free surface (side in direct contact with the coagulant), the back surface (bottom side contact with the glass) and the fracture surface (cross-section)
of the films were sputtered with gold, then were observed and photographed. Solid-state 13C NMR spectra of the cellulose were recorded on a BRUKER spectrometer operated at a 13C frequency of 100 MHz using the combined technique of proton dipolar decoupling,
magic angle spinning (MAS) and cross-polarization (CP). The spinning speed was set at 12 MHz for all samples. The contact time was 2 ms, the acquisition time 30 ms and the recycle delay 4 s. A typical number of 7500 scans were acquired for each spectrum.
X-ray diffraction (XRD) measurement was carried out on an XRD diffract meter (D8-Advance, Bruker). The patterns with CuK radiation( = 0.15406 nm) at 40 kV and 30 mA were recorded in the region of 2 from 5 to 45◦. Samples were ground into powders and dried in a vacuum oven at 60 ◦C for 48 h. The crystallinity c (%) of the cellulose fibers and composite fibers were estimated by Rabek’s method, using the following relationship