Fractional dissolution of "solid" unsubstituted cellulose

Activated cellulose (Solucell, DPw = 1400) was extracted stepwise at room t emperatures by means of mixed solvents consisting of N,N-dimethylacetamide (DMAc) and LiCl, starting with a salt concentration of 1 wt-% and increasin g it in increments of 1 wt.-% up to 7 wt.-%. Upon the regeneration of the t hus obtained cellulose fractions by pouring the solutions dropwise into a l arge surplus of water, part of the mixed solvent is occluded in the polymer . For that reason the cellulose samples were purified by redissolving them in Ni-tren and by a second precipitation. This process, however, leads to p ronounced polymer degradation. For that reason we have used a spinning nozz le to press the extracts in a highly dispersed form into water. Using this procedure the samples were Ilo longer contaminated by inclusions. The intri nsic viscosities of the fractions (in an alkaline aqueous solution of ferri c tartaric acid complex at 25 degreesC) and their GPC diagrams (solvent DMA c + LiCl) demonstrate that the shortest chains ([eta] = 208 mL/g) become so luble first and the longest chains ([eta] = 680 mL/g) last. The present dat a lead to the following Kuhn-Mark-Houwink relation [eta] = 4.13 DPw0.68. Th e current results indicate a promising route to obtain larger quantities of unsubstituted cellulose with narrow molecular weight distribution by means of suitable extraction strategies.