Nanocellulose materials

This work was focused on four typical nanocellulose samples: CNC I, CNC II, CNF I, and CNF II. The preparation procedures for these four nanocellulose samples were clearly reported by Han et al.³⁴; therefore only a brief description of their preparation is provided here. For CNC I, dried bleached wood pulp (W-50 grade of KC Flock, Nippon Paper Chemicals Co., Tokyo, Japan) was raw material and it was hydrolyzed for 1 h with 64 wt % sulfuric acid and then was filtered under vacuum. The filtered material was mixed with distilled water thoroughly for 20 min and then centrifuged at 26 °C for three cycles. The suspension was obtained by centrifuging after each washing and dialyzed against distilled water in regenerated cellulose dialysis tubes with a molecular weight cutoff of 12000–14000 (Fisher Scientific, Pittsburgh, PA, U.S.A.). This process was repeated for several days until a pH neutral solution was obtained. This material was then put into a high-pressure homogenizer (Microfluidizer M-110P, Microfluidics Corp., Newton, MA, U.S.A.) at a rate of 135 mL/min for five times. After homogenization, the concentration of CNC I suspension was 1.0 wt % and was adjusted to 0.1 wt % by adding water. Meanwhile, for preparing the CNF I, the concentration of sulfuric acid was changed to 48 wt %, while the other conditions were kept unchanged. In addition, for preparing the CNC II and CNF II, bleached wood pulp was pretreated. Dry bleached wood pulp was mercerized using 20 wt % NaOH solutions for 3 h, and then was filtered and washed by distilled water until a pH neutral solution was obtained. This mercerized bleached wood pulp was the raw material for the production of CNC II and CNF II, and the preparation procedures were the same as those for the CNC I and CNF I. The obtained suspensions of these four nanocellulose samples were then quickly frozen at −75 °C for about 2 h and freeze-dried to form films. The final film was placed between two cover slips and then was sealed in plastic bags.