Eckelt, J., Stryuk, S., Wolf, B. A.
"Fractionation of Cellulose"
Cellulose 2003, 105-10.
Abstract: Cellulose samples with molecular weight distributions that are considerably narrower than those of the natural products can be obtained by at least three fundamentally different routes. (i) Synthesis of easily soluble derivatives, fractionation by means of well-established methods and subsequent regeneration, (ii) selective extraction of short chains from activated cellulose, using solvents of suitable marginal quality,and (iii) partition of the homologs between two coexisting phases formed by the demixing of homogeneous solutions. All,three methods can be applied successfully. However, the efforts in terms of labor and required solvent differ considerably. Most of the experiments were performed with the following three cellulose samples: Avicel (M-w = 30 kg mol(-1), U = (M-w/M-n) - 1 = 2.0), Solucell (M-w = 230 kg mol(-1) , U = 1.8), and Stockstadt (M-w = 320 kg mol(-1), U = 5.7). Options (ii) and (iii) emerged most promising for large scale fractionation. The mixed solvent consisting of DMAc and LiCl turned out to be particularly versatile in both cases. In the pure state it can be used for incremental extraction (yielding quick access to orienting information on the width of the molecular weight distribution) as well as for one-step extraction. In combination with suitable precipitants (like acetone) it enables the realization of the coexistence of two liquid phases required with route (iii). One obstacle for fractionation that all methods share is the high viscosity of cellulose solutions. With the last method it is possible to mitigate this limitation considerably by the use of spinning nozzles for the mixing of feed and extracting agent.
preprint number: 239