Cellulose and Chitin as Nanoscopic Biomaterials - The Nanoscience and Technology of Renewable Biomaterials - page 217

(a)

100 80

(ppm)
(b)

Figure8.9 ExpansionoftheC4region,80-100ppm,incellulose 13 CCP/MASNMRspectra,
for(a)intactcellulosefibersand(b)celluloseMFsobtainedafterhydrolysisanddispersion.

The shoulder in the C6 signal that was associated with the disordered regions also
decreased considerably. This provides further support for preferential degradation of
cellulose as a result of hydrolysis and mechanical dispersion.
13 C CP-MAS NMR spectroscopy was also used to estimate an average degree of
N-acetylation (DS acetyl ) for the chitin nanocrystals, and to assess changes in acetylation
after hydrolysis. The DS acetyl was determined by the ratio of the integration values of
the methyl carbon to the anomeric carbon signal. The methyl signal is preferred over
the carbonyl signal due to the attached protons, which allow for better magnetization
transfer in the cross-polarization experiment.
The DS acetyl from CP-MAS NMR was calculated to be 0.90 for the chitin nanocrystals
after hydrolysis and 0.89 prior to hydrolysis, indicating that the hydrolysis treatment had
little effect on the DS acetyl . The NMR spectrum for the shrimp shell chitin nanocrystals is
displayed in Figure 8.10. The signal assignments shown are based upon a paper published
by Tanner and Chanzy et al . (30). Furthermore, from the spectrum in Figure 8.10 we
can confirm that the chitin nanocrystals are essentially free from residual protein. The
solid-state NMR spectrum for a pure, unhydrolyzed α -chitin sample is not noticeably
different from that of the nanocrystals, and is not shown. Surface and amorphous
regions of chitin are not detectable by 13 C CP MAS NMR, thus an analysis similar to
that performed with cellulose, as described above, was not possible.
X-ray powder diffraction (XRD) was used to monitor changes in crystallinity and
morphology of the chitin nanocrystals upon acid hydrolysis, and to estimate crystallite
sizes in the chitin samples. Figure 8.11 shows the diffraction profiles of the chitin
nanocrystals, and the purified shrimp shell chitin prior to hydrolysis. The diffraction
patterns for both materials exhibit Bragg reflections typical of pure α -chitin, indicating
that the chitin crystal structure is maintained after hydrolysis.
Analysis of the XRD data from the native chitin and the chitin nanocrystal
samples indicated that the crystallinity of the material was observed to increase after