Handbook of Size Exclusion Chromatography and Related ...

considered to be 3. After the DS has been determined, corrections are made to
achieve the original molecular mass of the underivatized cellulose sample. The
characterization is then assumed to reflect the molecular mass distribution
(MMD) of the original cellulose.
The validity of the data depends, however, on whether a molecular
dispersed solution is achieved, the cellulose has been degraded during the
reaction, or the low molecular mass partly lost, rendering a nonrepresentative
sample. Physical properties such as swelling and solubility are strongly affected
by the DS. It is difficult to get a complete substitution or an even distribution of
substituents in a cellulose molecule. This is partly due to the heterogeneous
nature of cellulose, that is, within ordered and between ordered and less ordered
regions. In heterogeneous derivatization systems the relative reactivity is
commonly C2OH . C6OH . C3OH (34), but is strongly dependent on the
derivatization conditions. Also, the reactivity between the different hydroxyl
groups differs. When all hydroxyl groups are equally accessible the usual order
of reactivity is C6OH .. C2OH . C3OH (35). Conventional derivatization
procedures are heterogeneous, although it has become more common to perform
derivatization in cellulose solvents. Examples of cellulose solvents used in
conjunction with derivatizations are N-methylmorpholine-N-oxide/dimethylsulfoxide
(MMNO/DMSO) (36), sulfur dioxide/diethylamine/dimethylsulfoxide
(SO2/DEA/DMSO) (37) but in particular lithium chloride/N,N-dimethylacetamide
(LiCl/DMAc) (38–46). By performing the derivatization in a
homogeneous system it is possible to achieve an even substitution throughout
the cellulose molecule (47), controlled DS (48), and to use milder reaction
conditions than for a corresponding heterogeneous derivatization. Another
advantage in doing the derivatization in LiCl/DMAc is that SEC may be
performed in the same solvents as used for derivatization.
3.2 Cellulose Solvents
Prior to performing SEC, the sample is dissolved in the same solvent as used as
mobile phase. The requirements for a solvent to be used in SEC are that it must
dissolve the sample completely, not degrade the sample, be stable, and be
compatible with the stationary phase. In addition the solution obtained should not
have too high viscosity. Although there are several solvents to dissolve cellulose,
only a few are suitable for use in SEC.
Cellulose solvents are generally divided into four main categories (49);
where (a) cellulose acts as a base, for example, concentrated acids or Lewis acids,
(b) cellulose acts as an acid, for example, amines, sodium hydroxide solutions,
(c) cellulose forms complexes, for example, with solvents such as cupriethylenediamine
(Cuen), [cadmium tris(ethylenediamine)] dihydroxide (cadoxen),
and (d) cellulose forms derivatives such as cellulose xanthate and methylol
© 2004 by Marcel Dekker, Inc.