Handbook of Size Exclusion Chromatography and Related ...

cellulose contains both ordered and less ordered regions although the exact
arrangement in the microfibrils is still under debate.
Accordingtox-raydiffractionanalysis,theorderedcellulosemayexistinfour
crystallineforms,thatis,polymorphs:celluloseI,II,III,andIV(12).CelluloseIisa
compositeoftwocrystallineforms,IaandIb,givingrisetodifferentchemicalshifts
andsignalpatternsinsolidstate(CP/MAS) 13 C-NMRspectroscopy.TheratioofIa
and Ib content varies depending on origin and treatment. The dominantpolymorph
in higher plants such as cotton and wood is cellulose Ib (13,14), whereas algal and
bacterial cellulose are rich in cellulose Ia. It has been reported that cellulose Ia is
moresusceptibletoenzymaticdegradationandacetylationthancelluloseIb(15,16).
Cellulose Ican be transformed into cellulose II, during, for example, swelling in
strong alkali (mercerization) but cellulose II can also be synthesized by certain
bacteria(17)andalgae(18).CelluloseIIIcanbeformedbytreatingcelluloseIorII
with liquid ammonia, and cellulose IV can be obtained by treating regenerated
cellulose fibers in ahot bath under stretching (19).
The fiber walls of higher plants are built up by several layers differing from
each other both in chemical composition and in the direction of the cellulose
fibrils. The noncellulosic components of the plants are of importance to consider
when choosing the most appropriate method for purification and isolation of
cellulose. Molecules such as waxes, fats, pectins, and proteins present in, for
example, cotton and ramie can be removed by dilute alkali or organic solvents.
Other molecules,especiallyhemicellulosesandligninsthatsurroundthecellulose
fibrils in many plants like the wood tissue in trees, are more difficult to remove
without concomitant degradation and loss of cellulose. The hemicelluloses
are heteropolysaccharides and the lignins are amorphous polymers of phenylpropane
units. Toisolate cellulose from wood, harsh conditions are required and the
isolatedcellulosesamplesoftenremainmoreorlessimpure.Thewoodpulpsused
for papermakingareproducedbychemical(alkalineand/oracidic)ormechanical
treatment or by combining these types of treatments in order to liberate the fibers
and partially or completely remove the lignin. The amount and state of the
cellulose in the different processes differ widely; for more details the reader is
referred to Sjö strö m(20). The kraft-pulping process, which is alkaline, produces
about76%ofthewoodpulpintheworld(1).Theconditionisadjusteddepending
onthefinaluseofthepulpandtoavoidseveredegradationofcellulose.Thefibers
to be used in the paper industry still contain fairly large amounts of both
hemicellulosesandlignin(Table2).Thelattercanberemovedbyacidicbleaching
sequences. Thus, the pulp fibers are far from pure with respect to cellulose, which
further complicates the dissolution and the chromatographic characterization.
The molecular mass is of interest when dissolving cellulose samples. Like
most other natural polymers, cellulose is polydisperse, that is, it is a mixture of
molecules of varying chain length. The chain length is often expressed as the
number of glucose units, commonly known as the degree of polymerization (DP).
© 2004 by Marcel Dekker, Inc.