Polyvinyl alcohol, PVA 120-99, 1200 polymerization degree and 99% hydrolysis degree was used as matrix. Microcrystalline cellulose (MCC) with a mean particle size of 20 pm and an aspect ratio of 2 – 4, from Sigma-Aldrich, was used as raw material for the preparation of nanofibers and cellulose microfibers obtained from regenerated wood fibers (L) through mechanical treatment were donated by National Institute of Wood.
1.2 Nanofibers isolation
(a) By ultrasonic treatment
MCC with a mean particle size of 20 pm, as revealed by SEM image (Fig. 1), was dispersed under continuous stirring in distilled water (1/500) and sonicated at 80% power for 20 minutes using an ultrasonicator type Vibra Cell VC505 (500 W, 20 KHz). In order to control the process temperature, the beaker with the cellulose suspension was put in a water bath with controlled temperature. The appearance of turbidity in the supernatant was a definite indication of the presence of cellulose nanofibers. Cellulose fibers suspension was obtained by decanting the supernatant into other vessel. Cellulose fibers – sample U was characterized by X ray diffraction after drying 4 hours at 400C.
(b) By acid treatment
MCC was dispersed in distillated water (1/10). The resulted suspension was placed in an ice bath and stirred while concentrated sulfuric acid was added by drop until the desired acid concentration of 63% was reached. The suspension was heated at 400C while stirred for 5 h. The obtained mixture was washed with distillated water using repeated centrifuge cycles (20 min at 7000 rpm, Universal 320R Ultracentrifuge) until a pH of 4 was reached. Further, the obtained sample of cellulose fibers (H) was ultrasonicated (Elmasonic S40H, Elma) for five hours in an ice bath in order to avoid the sample overheating. Cellulose fibers – sample H was characterized by X ray diffraction after drying 4 hours at 400C.
(c) Cellulose fibers obtained through mechanical treatment (L) were used as received.
Fig. 1. SEM image of commercial microcrystalline cellulose (MCC)