Fractional dissolution of “solid” unsubstituted cellulose

Abstract

Activated cellulose (Solucell, DPw = 1 400) was extracted stepwise at room temperatures by means of mixed solvents consisting of N,N-dimethylacetamide (DMAc) and LiCl, starting with a salt concentration of 1 wt.-% and increasing it in increments of 1 wt.-% up to 7 wt.-%. Upon the regeneration of the thus obtained cellulose fractions by pouring the solutions dropwise into a large 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 pronounced polymer degradation. For that reason we have used a spinning nozzle to press the extracts in a highly dispersed form into water. Using this procedure the samples were no longer contaminated by inclusions. The intrinsic viscosities of the fractions (in an alkaline aqueous solution of ferric tartaric acid complex at 25°C) and their GPC diagrams (solvent DMAc + LiCl) demonstrate that the shortest chains ([η] = 208 mL/g) become soluble first and the longest chains ([η] = 680 mL/g) last. The present data lead to the following Kuhn-Mark-Houwink relation [η] = 4.13 DPw0.68. The current results indicate a promising route to obtain larger quantities of unsubstituted cellulose with narrow molecular weight distribution by means of suitable extraction strategies.