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.

Fractionation of unsubstituted cellulose from solutions in either Ni-tren or (N,N-dimethylacetamide + LiCl)

“…This procedure, which also shows great potential, is discussed in the subsequent communication. [4] Further work is required to find out which of the two options is suited best for the large-scale fractionation of cellulose.…”

“…[4] The samples were swollen in a large surplus of water for one day, the water was then removed by filtration and an amount of DMAc comparable to that of water was added to the still wet cellulose. After that the solvent was distilled off in high vacuum (0.3 -0.6 mbar) at room temperature to avoid damages of cellulose by heating.…”

“…In the subsequent paper [4] we examine whether fractional dissolution of unsubstituted cellulose can be used for that purpose.…”

“…During the investigation of liquid/liquid phase equilibria that problem could not be resolved satisfactorily. However, in the course of these extraction experiments [4] we have found a way to avoid this contamination by pressing the solutions through a spinning nozzle into a precipitation bath.…”

Membranes Directly Prepared from Solutions of Unsubstituted Cellulose

“…The other sample was a prehydrolysis kraft pulp and has the manufacturer ID: FEZ146/01; its intrinsic viscosity in EWN (ferric tartaric acid complex in alkaline aqueous solution as supplied by Fluka) amounts to 400 mL/g at 25 8C leading to a M n ¼ 135 kg Á mol À1 (using the Kuhn-Mark-Houwink parameters K ¼ 0.13 a ¼ 0.68 reported in ref. [11] ). For the determination of the phase diagram shown in Figure 1 we have used the highest molecular weight sample (M w ¼ 320 kg Á mol À1 and M n ¼ 48 kg Á mol À1 , according to the producer); this material, called Stockstadt, is prepared from beech trees.…”