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

Abstract

Starting from solutions of unsubstituted cellulose (Avicel PH101, Mw = 30.1 kg/mol and Mw/Mn = 3 or Solucell 500, Mw = 230 kg/mol, Mw/Mn = 2.8) in either Ni‐tren (0.8 M aqueous solution of the dihydroxotris(2‐aminoethly)amine nickel(II) complex) or in a mixed solvent DMAc+LiCl (consisting of N,N‐dimethylacetamide plus lithium chloride) it was investigated whether the segregation of a second phase caused by the addition of suitable precipitants leads to polymer fractionation. With Ni‐tren the long chains accumulate in the precipitate formed upon the addition of sulfuric acid; as the pH falls below ≈ 9, the solution is free of cellulose. Nevertheless this route option for fractionation must be ruled out because of a pronounced chain scission taking place in that solvent. For (DMAc + LiCl) the best low molecular weight precipitant that could be found was acetone; it allows the fractionation of the lower molecular weight Avicel but fails for Solucell as a result of the high viscosities of its solutions in the presence of acetone. It was therefore investigated whether that deficiency could be overcome by the use of high molecular precipitants. In these experiments poly‐(methyl methacrylate), incompatible with cellulose, was used to cause phase separation. The results demonstrate the suitability of this system for discontinuous experiments even in the case of the higher molecular weight Solucell.

Fractional dissolution of “solid” unsubstituted cellulose

“…Another method that shows significant potential, described in the preceding article, [1] is based on the liquid/liquid phase separation caused by polymer incompatibility. Presently it is unclear which of the options is more efficient.…”

“…The other, simpler method avoids the use of Nitren (which is associated with cellulose degradation). [1] In this case the solutions in DMAc/LiCl are continuously pressed into cold water through a spinning nozzle as thin threads. Tests for inclusion of chloride ions along the lines described above were negative.…”

“…Many attempts have therefore been made in the past to fractionate cellulose, either from solutions in special solvents [1] or from solutions of readily soluble derivatives for which the substituents can be removed quantitatively after fractionation. [2] Since both routes, based on liquid/liquid equilibria, have turned out to be rather difficult and laborious, we have tried to break new ground by means of liquid/solid equilibria.…”

Membranes Directly Prepared from Solutions of Unsubstituted Cellulose

“…The preparation of the necessary homogeneous solutions of cellulose (either 3 or 5 wt.-%) in DMAc þ LiCl took place according to a procedure described in the literature. [13] In order to study the effects of the presence of a further polymer on the properties of the membranes we added either poly(methyl methacrylate) [PMMA] or cellulose acetate [CA] to the casting solution. Mixtures of these polymers remain homogeneously in the presence of sufficiently high concentrations of DMAc þ LiCl but are phase separated in the absence of the solvent.…”

Solutions of cellulose in DMAc�+�LiCl: migration of the solute in an electrical field

“…We have therefore studied whether a second liquid phase can be produced from solution of cellulose in DMAc + LiCl by adding a suitable precipitant, like acetone (Loske et al 2000). This procedure is, however, associated with considerable problems because of the high viscosities of the cellulose solutions, which impede the fractionation of higher molecular weight cellulose.…”