Biomedical Engineering Reference
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increasing to 84% for the nanocrystals after acid hydrolysis and homogenization. This
signifies a reduction in the amorphous contribution to the diffraction data, and suggests
that some of the amorphous regions in the cellulose and chitin are digested during the
hydrolysis process. The intensity of the Bragg peaks in the nanocrystal sample is slightly
less than those in the pure chitin sample, indicating that some of crystalline domains
are mildly affected by the hydrolysis treatment. However, with the increasing percent
crystallinity upon hydrolysis, the losses due to the digestion of the amorphous material
are greater than the losses to the crystalline material.
Information regarding the average crystallite sizes for the chitin materials was also
obtained from the X-ray diffraction data. Curve deconvolution permitted measuring the
peak widths at half-maximum intensity so that crystallite size could be calculated using
the Scherrer relation (33):
D hkl = ( 0 . 9 )(λ CuK α )/( FWHM ) hkl ( cos θ) hkl
where FWHM is the full width at half maximum intensity, in radians, for a single Bragg
peak, λ CuK α is the wavelength of the X-ray radiation (0.15418 nm), and θ represents
the location of the maximum intensity of a particular Bragg reflection. The FWHM is
used to measure the line broadening, which arises primarily from the finite size of the
crystallites. Paracrystallinity and instrumental broadening factors can contribute to line
broadening as well but were not evaluated in this work.
Based on crystallographic studies of α -chitin from various sources it is apparent that
the molecular repeat axis and the microfibrillar axis are parallel, so that the 001 set
of planes correspond to the repeat period along the major axis of the microfibrils (31,
34-36). Also, it has been previously shown that the 100 set of planes correspond to the
growth plane for α -chitin (34). Given the orthorhombic geometry of the crystals, if the
normal to the 001 planes is parallel to the major axis of the microfibrils, then the 100 and
010 sets of planes, which are perpendicular to the 001 planes, represent periodicities on
the transverse axes of the microfibrils Line broadening from Bragg peaks for these planes
or multiplicities thereof can be used to measure crystallite widths. Crystallite dimensions
for the 020 and 110 reflections appearing in the data were used to derive the width along
the 100 set of planes, which is coincident with the crystallite width. Due to the high
crystallinity of the particles and the apparent small transverse dimensions of the particles
visualized from TEM, it is apparent that the crystallite width, in this system, measures
the width of the microfibrils. Further justification for this assumption comes from a
previous study on α -chitin from lobster tendons, where a strong resemblance between
α -chitin microfibrils and cellulose microfibrils is observed, in that with both materials,
the microfibrils are elongated single crystallites (34). From the line broadening data,
crystallite dimensions for the chitin nanoparticles were calculated at 8.33 nm for the 100
planes, and 6.65 nm for the 010 planes. This corresponds to crystallites approximately
6 . 65 nm
×
8 . 33 nm in cross-section.
8.5
Chemical Modification of Cellulose and Chitin Nanoparticles
A common drawback, which has impeded the use of cellulosic and chitin fibers as
reinforcing agents in thermoplastic composites, has been the lack of compatibility at
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