Handbook of Size Exclusion Chromatography and Related ...

3 CELLULOSE STRUCTURE AND SEC
Tocharacterize cellulose by SEC, cellulose has to be purified or isolated from its
native source and/or dissolved. The most common way to purify cellulose is to
extract other molecules prior to dissolution of the cellulose. Holocellulose
(cellulose þhemicellulose) can be obtained by removing lignin from wood or
wood pulps by acid chlorite (24). Hemicelluloses in delignified fibers can be
consecutively extracted with potassium hydroxide and barium hydroxide (25).
Another way to reduce the hemicellulose content is by treating delignified fibers
with hemicellulose-degrading enzymes (26,27), although acomplete removal of
hemicellulose is difficult, if not impossible, to achieve. Since the isolation
proceduremaydegradecelluloseandthefinalsamplemayalsocontainimpurities,
it is important to report the applied isolation method when evaluating the
molecular characteristics of the cellulose fraction.
Tobe defined as cellulose, the polymer must have aDP of at least several
hundred (28). According to this definition, cellulose is not soluble in common
solvents. The low solubility is partly due to the degree of crystallinity, the
crystallite size, and crystallite size distribution (29). The strong interchain forces
that bind the cellulose together restrict the accessibility and prevent complete
penetration even of hydrophilic solvent systems. Tofacilitate direct dissolution or
derivatizationofcellulose,thehydrogenbondsintheorderedcelluloseregionsare
partly broken by an activation step prior to dissolution. The activation is
commonlyachievedbysolventexchangeusing,forexample,wateroramines(30).
Tobe able to chromatograph cellulose, solubility is generally achieved either by
forming derivatives that are soluble in common solvents, or by using certain
solventmixturescapableofdissolvingthecellulosedirectly.Themainobstaclefor
asuccessful characterization of cellulose by SEC is the difficulty to achieve
molecular dispersed solutions, both for cellulose and incompletely substituted
derivatives (31–33).
3.1 Cellulose Derivatives
Cellulose can be derivatized by introducing functionalities at the primary and at
the secondary hydroxyl groups of the glucose unit (Fig. 1). Five positions are
available for derivatization; C2, C3, and C6 within the chain, C1 at the reducing
end, and C4 at the nonreducing end of the chain, respectively. A variety of
soluble cellulose derivatives suitable for SEC can thus be obtained such as
esters, for example, cellulose nitrate, cellulose acetate, cellulose carbamate, and
ethers such as methyl cellulose, carboxymethyl cellulose, trimethyl cellulose.
The degree of substitution (DS) is defined as the average number of hydroxyl
groups substituted in a glucose entity. Owing to the high molecular mass of
cellulose, the substitution at C1 and C4 is disregarded and the maximum DS is
© 2004 by Marcel Dekker, Inc.