Biomedical Engineering Reference
In-Depth Information
interfacial area, the interactions between the particle and matrix phases become critically
important to composite performance. Typical methods to improve compatibility between
the polysaccharide filler and the thermoplastic matrix involve either creating dispersive
coatings around the particles or covalently modifying the particles with either hydropho-
bic molecules, or coupling agents that covalently link the two phases. The approach used
in this study involves the topochemical modification of cellulose and chitin nanoparti-
cles with different hydrophobic moieties to achieve better phase compatibility. The
mechanical effects that cellulose and chitin nanoparticles, and their derivatives, have
on different bio-based or biodegradable thermoplastic nanocomposites are also briefly
described.
8.3
Preparation and Microscopic Characterization of Cellulose
and Chitin Nanoparticles
Processes for the purification of cellulose and chitin are well established, and involve
the removal of the organic and/or inorganic materials naturally associated with them,
generally through enzymatic, acidic or basic treatments. When purified, both cellulose
and chitin are semicrystalline materials. Both materials are susceptible to degradation in
strongly acidic media. Acid hydrolysis of cellulose and chitin is essentially the same,
since the occurrence of an acetamido group at C(2) in the latter is the only primary struc-
ture difference between them. The first step in acid hydrolysis involves the protonation
of the acetal oxygen of the glycosidic linkage. An intermediate carbocation is formed at
the anomeric carbon through heterolysis, causing a destruction of the glycosidic bond (6).
The carbocation then reacts with water, forming a hydroxyl group, and a proton (6). The
reaction is first-order, with the speed of reaction highly dependent on both the cellu-
lose and acid concentrations (6). The amorphous regions of the cellulose and chitin are
digested first due to their higher accessibility relative to the crystalline regions. Hetero-
geneous acid hydrolysis was performed in this work, where the hydrolysis occurs first
in the amorphous regions, then decreases considerably when the amorphous cellulose or
chitin is digested. The acid hydrolysis conditions are controlled to digest the amorphous
cellulose and chitin segments connecting crystallites in the elementary and microfibrils,
to create smaller, highly crystalline segments, called nanocrystals or nanoparticles. The
nanocrystals are desirable for their retention of the native crystalline properties, and their
high stiffness and specific surface areas. A process of high-energy mechanical dispersion
via homogenization is often used as an additional route to the production of individ-
ualized nanocrystals. After processes of acid hydrolysis and homogenization, well-
dispersed aqueous suspensions of the nanoparticles are obtained.
As demonstrated through the preparation of cellulose nanocrystals from bagasse, a
change in the morphological structure of the whole cellulose fibers occurs upon acid
hydrolysis and can be observed using SEM. Figure 8.1 is an optical micrograph of the
cellulose fibers prior to acid hydrolysis.
The average particle length is approximately one to two millimeters. However, the
SEM micrographs of the fibers post hydrolysis indicate that the majority of the microfib-
rils are in the submicron range having high aspect ratios between 50 and 120. The larger
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