Civil Engineering Reference
In-Depth Information
15.4
Cellulose Nanocrystals
In terms of structure, cellulose i ber comprises bundles of microi brils, which are
comprised of elementary i brils having nanocrystalline domain linked by amorphous
region. On applying certain process conditions (acid-hydrolysis), breaking of cellulose
i bers dissolves the amorphous region and produces "rod-like" or "needle-like" nano-
crystals called CNCs (as shown in Figure 15.4). h e dimensions of nanocrystals mostly
depend on origin of cellulose i ber and employed process conditions. h e CNCs show
signii cant changes in dif erent properties such as high elastic modulus, optical, electri-
cal and magnetic properties as compared to native cellulose i bers [2]. Nanoscale i bers
or crystals isolated from natural i bers show much higher mechanical properties in
comparison to the original natural i ber source. Taking this property into account, great
emphasis is being given to the production of nanoi bers or nanocrystals using dif er-
ent processes and the combination of these nanoscale i brils with polymers to fabricate
nanocomposites, expecting improvement in strength and stif ness as compared to arti-
i cial i bers-reinforced composites [10].
In the 1950s, Ranby and Ribi were the i rst to synthesize stable colloidal suspensions
of cellulose crystals by controlled sulfuric acid-catalyzed degradation of wood and cot-
ton cellulose i bers inspired by the work of Nickerson and Habrle [47], and the dimen-
sions of CNCs were found to be approximately 50-60 nm in length and 5-10 nm in
width [48, 49]. h e i rst transmission electron microscopy (TEM) images of dried
CNCs suspensions themselves were investigated in 1953 [50] and revealed the presence
of aggregation of needle-shaped particles, while further analysis with electron dif rac-
tion showed that these rods had the same crystalline structure as the original cellulose
i bers [50, 51]. Along with the above studies, Battista [52, 53] developed the hydro-
chloric acid-assisted degradation of high-quality wood cellulose followed by sonica-
tion treatment, leading to the commercialization of microcrystalline cellulose (MCC),
which of ered a signii cant opportunity for multiple uses such as texturizing agent and
fat replacer in food applications, tablet binder in pharmaceutical applications, and as an
additive in paper and composite applications. Further, Marchessault and coworkers in
Elementary Fibril
(
a
)
Crystalline Region
Amorphous Region
Acid-Hydrolysis
Cellulose Nanocrystals
(
b
)
Figure 15.4
Illustration of (a) typical cellulose i ber with crystalline and amorphous regions, and
(b) nanocrystalline cellulose at er hydrolysis purii cation [10] .
