120 Introduction to Renewable Biomaterials95 90 85 80AmorphousCrystalline
AmorphousCrystalline4 4
3 21 OOO
O5566211 O1 O OO(^41)
3
3
2
2
5
6
5
6
3
4
OH
OH
OH
OH
OH OH
OH
HO
HO
HO
OH
4
3 2
1
O
O
5
5
6
6
3 2
4
OH
OH
OH
HO
HO
HO OH
120 100 80
(^13) C (ppm)
60 40
Avicel
C 4 C 6
C 1
C2,3,5
67 62 57
Figure 4.4Illustration of information obtained from CP/MAS^13 C NMR spectrum of microcrystalline
cellulose. C 4 and C 6 regions are typically used for Crl determination and evaluation of changes in
inter-/intramolecular hydrogen bonding.
bonding of polysaccharide chains [15, 16, 73]. This method uses radio electromagnetic
radiation to perturb spins of isotopes (with an odd number of spins) and the localized
environment relaxes the perturbed spins back to its normal state. For cellulose,
each carbon has a response to this perturbation. Figure 4.4 shows the spectrum of
microcrystalline cellulose (Avicel). The C 1 ,C 4 ,andC 6 carbons are differentiated in
the spectrum, while there is considerable overlap in the C2,3,5signals. C 4 region is
commonly used to determine CrI [67], and C 6 region is used to observe changes in
hydrogen bonding among cellulose chains. CrI can be determined by two methods:
(i) C 4 peak deconvolution and (ii) amorphous subtraction.
For polymers in a highly organized state, localized interactions among cellulose chains
will cause their response to differ. The core chains of cellulose microfibrils will be in
a distinct local environment relative to the chains on the surface of the microfibrils.
Hence, the relative amount of crystallinity in the sample can be determined using the
C 4 peak deconvolution (i.e.,X/(X+Y)inFigure4.4).
Similar to XRD, C 4 peak deconvolution of the NMR spectrum is subjective, and the
results are user dependent to a degree. However, the method is widely used to report
the relative crystallinity of cellulose as the signal intensities are directly related to the
amount of species present. Caution should be taken because hemicellulose components
also generate signals in the region that corresponds to the amorphous cellulose signals.
To avoid falsely assigning signals to amorphous cellulose, the signal can be further
deconvoluted for the individual contributions of the signal to the hemicellulose,