Effects of temperature and molecular weight on dissolution of cellulose in NaOH/urea aqueous solution

Dissolution of cellulose having different viscosity-average molecular weight (M-eta) in 7 wt% NaOH/12 wt%urea aqueous solution at temperature from 60 to -12.6 degrees C was investigated with optical microscope, viscosity measurements and wide X-ray diffraction (WXRD). The solubility (S-a) of cellulose in NaOH/urea aqueous solution strongly depended on the temperature, and molecular weight. Their S-a values increased with a decrease in temperature, and cellulose having M-eta below 10.0 x 10(4) could be dissolved completely in NaOH/urea aqueous solution pre-cooled to -12.6 degrees C. The activation energy of dissolution (E-a,E-s) of the cellulose dissolution was a negative value, suggesting that the cellulose solution state had lower enthalpy than the solid cellulose. The cellulose concentration in this system increased with a decrease of M-eta to achieve about 8 wt% for M-eta of 3.1 x 10(4). Moreover, cellulose having 12.7 x 10(4) could be dissolved completely in the solvent pre-cooled to -12.6 degrees C as its crystallinity (chi(c)) decreased from 0.62 to 0.53. We could improve the solubility of cellulose in NaOH/urea aqueous system by changing M-eta, chi(c) and temperature. In addition, the zero-shear viscosity (eta(0)) at 0 degrees C for the 4 wt% cellulose solution increased rapidly with an increase of M-eta, as a result of the enhancement of the aggregation and entanglement for the relatively long chains.