Fundamental Science and Applications for Biomaterials 49
the lignin. Today, the reactivity of cellulose is a topic of grave importance because of
its ability to supply an ever burgeoning bio-fuels and biomaterials community [13, 14].
As demonstrated earlier, the reactivity of cellulose is a function of accessibility, which is
severely hampered by its compact structure. This compact structure is a function of the
presence of a very strong hydrogen bonding network that gives rise to highly ordered
region.
Critical factors that have been suggested by a number of experts include the
number and size of pores in the cellulose structure, the molecular size and type of
chemical that is added to the cellulose, the internal surface as controlled by the size of
fibrils/aggregates, and the morphology of the cellulose macromolecules. These critical
factors can only be addressed by (i) ensuring that the cellulosic pores are sufficiently
open to accommodate the chemicals/reagents added, (ii) deconstructing fibrillar
aggregates, and (iii) deconstructing the ordered regions to no longer adopt a stiff,
compact network structure, that is, the hydrogen bonding network must be disrupted.
An optimal chemical interaction, therefore, must consider these latter three criteria;
recently, chemical, mechanical, and biological treatments have been tested, with a great
push toward the environmentally benign biological treatments.
Chemical treatments of cellulose [15] are employed to enhance the swelling of the
cellulose fibers. This swelling not only facilitates the passage of chemical agents, but
its primary purpose is to disrupt the hydrogen bonds because of the high osmotic
pressure induced by the swelling phenomenon. Thus, the hydrogen-bonded network
becomes disrupted, and as a result the compact structure is no longer available leading
to a more accessible structure. A pictorial representation of this phenomenon is shown
in Figure 2.9.
The scheme illustrates the inclusion of water molecules that interfere with the
hydrogen bonding between the C 6 ,andtheC 3 and C 2 hydroxyls of chain neighbors
and act to therefore swell the cellulosic substrate. As a result of this inclusion of water
molecules, the swelled structure loses its ordered nature and in some cases, there is
a complete loss of crystallinity, which consequently leads to an increase in the active
surface area or exposed hydroxyl groups (which were heretofore buried within the
tightly packed cellulosic crystallite). There have been a number of studies within this
Figure 2.9Shown is a simple cartoon
that illustrates the effect of introducing
water molecules within the H-bonded
network structure of cellulose.
Reprinted with permission from
Westermark,S.“UseofMercury
Porosimetry and Nitrogen Adsorption
in Characterisation of the Pore
Structure of Mannitol and
Microcrystalline Cellulose Powders,
Granules and Tablets.” Academic
Dissertation, November 2000,
University of Helsinki, Helsinki, Finland.
