Handbook of Size Exclusion Chromatography and Related ...

2 CHEMICAL, MACROMOLECULAR AND
MORPHOLOGICAL STRUCTURES
The molecular level of cellulose, that is, the chemical constitution, the steric
conformation,themolecular mass,thethreefunctional hydroxylgroups,andtheir
molecular interactions through hydrogen bonding influence the supermolecular
levelofthecellulosepolymeraswellasthemorphologyofcellulosefibers.These
factorsarethusimportant toconsiderwhencellulosicfibers, cellulosederivatives,
or cellulose in itself are to be studied and/or characterized by SEC.
Celluloseisalinear polymercomposedofb-D-(1!4) glucopyranoseunits
having a chair conformation with the hydroxyl groups in the equatorial
conformation (Fig. 1). The elemental composition of cellulose, from which the
empirical formula C6H10O5 of cellulose could be established, was determined by
Payen in 1838 (11), and the connecting b-(1!4) glycosidic linkages and the
linkages within the glucose molecule were established by Haworth. It was
Staudinger, however, who proved the polymer nature of cellulose. Due to the blink,thepyranoseringofeverysecondglucoseunitinthepolymerchainisturned
around about 1808 along the longitudinal axis. Because of this, cellobiose can
strictly be regarded as the smallest entity of cellulose. One of the terminal groups
ofthecellulosemoleculeiscalledthereducingendsincethehydroxylgroupatC1
of the cyclic hemiacetal is in equilibrium with the open-chain aldehyde form and
thushasareducingactivity.Theotherendiscalledthenonreducingendduetoits
alcoholic hydroxyl group at C4.
Themainfunctionalentitiesthatareavailableforderivatizationarethethree
hydroxyls at C2, C3, and C6 in each glucose unit. These hydroxyl groups also
formintra-andintermolecularhydrogenbondswithsuitablypositionedhydroxyls
within the molecule and with adjacent cellulose molecules, respectively. The
intermolecular hydrogen bonds are responsible for the stiffness of the cellulose
molecule, which is reflected in its high viscosity in solution, its tendency to
crystallize, and its ability to form fibrils. In its native state, the cellulose fibrils,
sometimes called microfibrils, are assembled to fibril aggregates, which are the
smallest morphological structure of the fiber. There is general agreement that
Figure 1 Chemical structure of cellulose. The bold figures denote the positions of the
derivatizable hydroxyl groups, that is, carbon number 2, 3, and 6 within the cellulose chain,
and carbon number 1 at the reducing end and carbon number 4 at the nonreducing end.
© 2004 by Marcel Dekker, Inc.