Biomedical Engineering Reference
In-Depth Information
Figure8.6
SEMmicrographofchitinfibersisolatedfromshrimpshells.Actualmagnification
shown is (
1200). Reprinted with permission from (23). Copyright (2007), American
ChemicalSociety.
×
domains, while darker regions are considered to be amorphous (21). Although the
banding is longer than that described in the original Hess model, it is consistent with
numerous reports of a leveling off degree of polymerization, LODP, for cellulose of
approximately 150-200 glucose residues (22). In a crystalline domain, each glucose
residue subtends 0.5 nm along the major axis so a LODP of 150-200 nm corresponds
to a crystallite length of 75-100 nm, as seen in Figure 8.5.
Purified chitin was produced after treatments with acid and base. This material was
studied by SEM and TEM and demonstrates a hierarchal breakdown, similar to that
observed with cellulose. Figure 8.6 shows an SEM micrograph of purified shrimp shell
chitin prior to hydrolysis and homogenization.
The particle dimensions for the isolated fibers are observed to be between 5
µ
mand
10
m in width, and several hundred microns in length. Figure 8.7 is a TEM micrograph
of chitin nanocrystals cast from a dilute aqueous suspension. From this micrograph it is
apparent that individual chitin nanoparticles range from 200 to 500 nm in length, and 8
to 12 nm in width.
µ
8.4
NMR Characterization of Cellulose and Chitin Nanoparticles
Cross Polarization/Magic Angle Spinning (CP/MAS) NMR studies proved to be a very
useful technique in monitoring the morphological changes taking place in cellulose dur-
ing the course of hydrolysis. The
13
C NMR spectra of (a) intact cellulose fibers and
(b) hydrolyzed microfibers (MFs) are illustrated in Figure 8.8a and b respectively.
The six carbon atoms that are assigned to the cellulose molecule dominate the spectrum
in both cases. The chemical shift values range from 105 ppm to 60 ppm. The anomeric
carbon (C1) appears furthest downfield at around 105 ppm. This is followed by the
signal from the C4 atom between 82 and 89 ppm, a range arising from the different
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