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Publication Date (Web): October 23, 2017 | doi: 10.1021/bk-2017-1251.ch003
Chapter 3
Towards an Understanding of
Cellulose Microfibril Dimensions from
TEMPO-Oxidized Pulp Fiber
Zehan Li,1 Noppadon Sathitsuksanoh,2 Wei Zhang,1
Barry Goodell,3 and Scott Renneckar*,4
1Department of Sustainable Biomaterials, Virginia Tech,
230 Cheatham Hall, Blacksburg Virginia 24061, United States
2Department of Chemical Engineering, Ernst Hall, Room 216,
University of Louisville, Louisville, Kentucky 40292, United States
3Department of Microbiology, University of Massachusetts,
639 North Pleasant Street, Amherst, Amherst, Maine 01003, United States
4Department of Wood Science, University of British Columbia,
2424 Main Mall, Vancouver, BC, Canada V6T 1Z4
*E-mail: [email protected]
A unique molecularly thin nanocellulose (MT nanocellulose)
structure –obtained by 2,2,6,6-tetramethylpiperidin- 1 oxyl
(TEMPO)-oxidation and sonication– was examined by TEM
and solid-state 13C NMR to advance the current understanding
on the supramolecular structure of cellulose I microfibrils.
The width distribution of the microfibril was determined from
TEM images, and a holistic view of the microfibril cross
section was developed by integrating the height distribution
result from previous work using atomic force microscopy
imaging. Systematic changes of NMR spectra upon oxidation
and sonication treatments were observed and attributed
to the corresponding changes to crystallinity, glycosidic
linkage torsion angles, as well as C6 primary hydroxyl
conformations. Lastly, current microfibril cross-section models
were collectively reviewed and the 24-chain diamond model
was identified as the most credible representation for the
experimental data and the known constraints.
© 2017 American Chemical Society
Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications
ACS Symposium Series; American Chemical Society: Washington, DC, 2017.
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Publication Date (Web): October 23, 2017 | doi: 10.1021/bk-2017-1251.ch003
Introduction
Nanocellulose has emerged as a field of research interest, not only because
of the extensive availability and sustainability of its precursor cellulose (1),
but also due to the broad chemical modification range, excellent physical and
mechanical properties, as well as the enormous potential applications related to
nanotechnology (2, 3). However, the accurate knowledge of the geometry of
nanocellulose is still poorly resolved in the literature. Lacking exact knowledge
of the geometry can misrepresent modeling efforts when nanocellulose is used
in composite materials. Moreover, cellulose source and isolation methods
impact nanocellulose dimensions creating ambiguity around cellulose microfibril
geometry in the native state along with the isolated counterpart.
In 1954, Frey-Wyssling suggested a near rectangular cross-sectional
arrangement of cellulose molecules to describe the organization of plant derived
cellulose microfibrils, based on microscopy and XRD evidence (4). Today,
wood-based cellulose microfibrils have been proposed having a cross section
shape approximate to either a hexagon (5), a rectangle (5, 6), or an ellipse (7),
based on direct microscopy observation or indirect evidence from its biosynthesis
from hexameric terminal complex of particle “rosettes” (8, 9). Since the rosettes
impart a six-fold symmetry (10, 11), to the cellulose microfibril, hexagonal and
elliptical cross sectional arrangements would seem to best reflect synthesis via this
process. Cellulose chain numbers contained in the microfibril are believed to be
constant based on the assumption that one terminal complex of 6-subunit rosette
extrudes one microfibril (5). Some authors have suggested a 36-chain packing
scheme (12, 13), while others have raised doubt over whether this packing scheme
is too large to fit experimental observations and they have suggested a 24-chain
packing scheme instead (5). Current understanding of cellulose microfibril
cross-sectional structure has been deduced from a combination of plant cellulose
biosynthetic origins, crystal lattice dimensional measurements, microfibril
dimensions, as well as assumptions about cross section shapes (5, 13–16).
However, due to the uncertainties in both the data and assumptions, agreement on
the arrangement of the microfibril cross section arrangement has yet to be reached.
Another approach is to look at the deconstruction of the microfibrils, which
has been done through acid hydrolysis of wood pulp, which degrades cellulose
microfibrils into fragments. Another method to isolate microfibrils is through the
select oxidation of cellulose structure using 2,2,6,6-tetramethylpiperidin- 1 oxyl
(TEMPO).
Isogai and coworkers developed a facile way of producing nanocellulose
via TEMPO-oxidation and mechanical agitation to deconstruct pulp fibers (6).
Several efforts focused on the isolation, characterization, of the fine structure
of the oxidized cellulose microfibrils derived from native cellulose sources, as
well as applications for these materials (6). Utilizing the method to understand
the fundamental structure of cellulose, Okita et al. demonstrated that the degree
of surface oxidation corresponded with the X-ray data for cellulose microfibril
dimensions (17). This surface only microfibril oxidation stemmed from the
limitations of the reactants to penetrate the surface of the microfibril, along with
the lack of solubility of the partially oxidized cellulose chains to peel away
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from the surface. Unfortunately, direct visualization of native fibril dimensions
cannot be determined as mechanical agitation to isolate individual nanofibrils
influences the dimensions of the resulting isolated nanocellulose. This result was
exemplified through extended sonication of TEMPO-oxidization of softwood kraft
pulp fibers. Li and Renneckar isolated cellulose mono- and bi-layer molecular
sheets (i.e.: molecularly thin nanocellulose, or “MT nanocellulose”) where the
thickness profile of the sheets were determined by atomic force microscopy
(AFM) on atomically smooth surfaces (18). Further investigation with X-ray
diffraction, Raman, and FTIR indicated that delamination occurred along the
(200) plane in the cellulose Iβ crystalline structure (19). Following this method
Su et al. used synchrotron X-ray diffraction and small angle X-ray scattering
(SAXS) confirming thickness of mono and bi-layer sheets (20). An advantage
of the SAXS study was that nanocellulose was measured in the aqueous state,
limiting self-assembly, revealing sub nanometer dimensions of sonicated TEMPO
cellulose. In contrast to Li and Renneckar, Su et al. showed X-ray data that
highlighted changes in the (110) plane, which suggested breakage of the extensive
hydrogen bonding network of the fibril (within sheet) instead of intersheet bond
breakage. In both cases, molecular thin cellulose particles were isolated, however
the fracture process would have been characteristically different.
To date, height (thickness) and length profiles of MT nanocellulose have been
examined using atomic force microscopy. The width profile was not examined
using this method, because of the inherent tip convolution effect when using
AFM. Hence, development of a holistic (3D) structure for cellulose molecular
sheets has not been possible. Transmission electron microscopy (TEM) has
been applied in prior research to characterize the nanocellulose width and length
profiles because TEM permits accurate horizontal resolution (21–28). Several
methods have been used to prepare nanocellulose materials for observation by
TEM including hydrolysis (21), TEMPO-oxidation combined with sonication
(22, 27), using different raw material sources (wood pulp (22, 25, 27), cotton,
tunicin, Avicel (21)), with negative staining techniques typically used to enhance
contrast (21, 27). A better understanding of the supramolecular structure of MT
nanocellulose, and the multiple MT nanocellulose layers that are assembled into
the cellulose microfibril, could potentially be obtained through the use of TEM.
Another powerful method to study cellulose microfibrils is cross polarized
magic angle spinning solid-state 13C nuclear magnetic resonance (CP-MAS SS
NMR) spectroscopy (29–31). The NMR chemical shifts of cellulose material
can be obtained readily by conventional solid state 13C NMR to evaluate
conformation, hydrogen bonding, and molecular packing (32). There is a clear
association between hydrogen bonding and chain packing, and conformational
shifts for the C6 on glucopyranose ring (Figure 1A) in the cellulose structure
(33). It has also been reported that the glycosidic bond conformation, which is
defined by two glycosidic linkage torsion angles (34) Φ (O5′–C1′–O4–C4) and
Ψ (C1′–O4–C4–C5) (Figure 1B), affects the cellulose supramolecular structure
significantly and is related to C1 and C4 chemical shifts (35). This torsion angle
impacts the spacing of the cellobiose repeat unit and would be heavily influenced
by the constraints of the microfibril.
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Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications
ACS Symposium Series; American Chemical Society: Washington, DC, 2017.
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Figure 1. Schematic drawings represent (A) three possible conformations
at glucopyranose C6 position, (B) cellulose torsion angles Φ, Ψ, and Χ
(O6′–C6′–C5′–C4′).
In the present study, TEMPO-oxidized kraft pulp was ultrasonicated at
different time intervals to produce MT nanocellulose. TEM and NMR were
utilized to examine the MT nanocellulose to obtain additional information on its
unique structure, including: width profile distribution, crystallinity, glycosidic
linkage torsion angles, C6 primary hydroxyl group conformations, as well as
how changes in these features occurred during sonication. The structural features
of MT nanocellulose were then assessed with regard to prior knowledge of
microfibril structure to describe the microfibril dimensions.
Experimental Section
Materials
Never-dried kraft pulp from the southeastern USA of southern yellow pine
softwood species (obtained from Weyerhaeuser Co., Ltd. with reported 88%
brightness and DP ranging from 1600-1694), was used as the starting material.
NaClO, NaBr, and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) were obtained
from Sigma Aldrich. Ultrapure water used all experiments was generated by
Millipore Systems (Direct-Q 3UV), with a conductivity of 0.30 μs/cm and purity
< 5 ppb (18, 19).
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TEMPO-Mediated Oxidation
The kraft pulp was oxidized following previously published techniques with
NaClO at 5 mmol of NaClO per gram of dry fiber used to convert accessible
primary alcohol groups to carboxyl groups (18). The final degree of oxidation,
determined by conductometric titration (36), was 1.43 mmol/g of fiber, equivalent
to a DS of 0.23 (23% of the total AGUs had their C6 primary hydroxyl group
converted to a carboxyl group).
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Sonication
Both kraft pulp and oxidized pulp were sonicated to generate nanocellulose
fibrils. Sonication was conducted at five time intervals (5, 30, 60, 120, and 240
min), at 0.1% (w/w) concentration of fiber slurry, in a temperature controlled bath
at 4°C. A 19 mm diameter medium intensity horn was used to sonicate the fibril
suspension at 20 kHz (VC700, Sonics and Materials). The sonicated suspension
was centrifuged at 4500 rcf for 15 min and the decanted transparent supernatant
was stored for later processing and analysis by TEM and NMR (lyophilized).
The complete sample set for TEM and NMR analysis included: kraft wood pulp
(WP), 120 min sonicated kraft wood pulp (WP120), TEMPO-oxidized wood pulp
(WT), TEMPO-oxidized wood pulp that undergone 5, 30, 60, 120, and 240 min
sonication (WT 5, WT30, WT60, WT120, and WT240).
TEM Analysis
Cellulose suspensions of ca. 5x10-3% (w/w) concentration were first
deposited onto Formvar TEM grids (400-mesh); incubating for 5 min before
the suspensions were blotted with filter paper. The cellulose samples were then
immersed for 2 minutes in a 2% uranyl acetate solution for negative staining
before blotting with filter paper. The stained samples were then immediately
observed under a ZEISS 10CA TEM, operating at 60 kV. TEM images were
analyzed with NIS-Elements BR software for fibril width measurement and
statistical analysis. Four hundred measurements were made from 10-15 images
for each sonication time level, and the images were magnified with the assistance
of a “zoom” tool in the software package to ensure accurate measurements to the
nearest resolved pixel. In order to avoid errors induced from incidental twisting
(abrupt narrow parts with color changes) of the fibrils, portions of the fibrils that
appeared to narrow abruptly or that had abrupt color change were avoided, when
taking measurements.
(CP/MAS) Solid-State 13C NMR Experiment
Cellulose lyophilized samples were ground (Wiley® Mini Mill mesh #60, 250
μm) and powdered samples were packed into rotors for NMR scanning. NMR
spectra were obtained on a Bruker AVANCE DPX 300 instrument, operating at 75
MHz carbon frequency, with a 4 mm (o.d.) rotor sample spinning at 6.5 kHz, total
contact time 1ms, 3 s relaxation delay, and at ambient temperature. The chemical
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shift scale was calibrated relative to tetramethylsilane (TMS), with the CH highfield peak set at 29.5 ppm.
NMR Crystallinity Index Evaluation
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Crystalline (cr) and non-crystalline (non-cr) contributions were determined by
peak integration. For the non-crystalline region was delimited in the 86.5 - 80.6
ppm range, and the crystalline was delimited between 93 - 86.5 ppm (29). The
Crystalline Index (CI) was then calculated using equation (1)
Results and Discussion
TEM Evaluation of Nanocellulose
The nanocellulose in the TEM exists in long flat fibril form; the width of
individual fibrils was relatively uniform with periodic regions that narrow abruptly,
indicating possible twists in the structure (Figure 2A-C). The nanocellulose width
distribution was obtained by plotting 400 width data points for each sonication
time level (Figure 2D). For short sonication times, width distributions were
between 2 and 14 nm. Compared to this short sonication time, the average
width decreased and leveled off to approximately 4 nm after 60 min sonication,
which suggests that 60 min sonication would be sufficient in isolating majority
of the individual microfibrils. This result also reconfirms and extends the depth
of Johnson’s and Saito’s investigations related to sonication (27, 37). Tests of
statistical significance indicate that the width difference between the 5 min and 30
min sonication levels, and 30 min and 60 min levels are significant; whereas the
difference between 60 min and 120 min levels, and 120 min and 240 min levels
are statistically insignificant. The overall average width for extended sonication
groups “60MinPlus” (60, 120, and 240 min combined) was 3.93 nm.
Even though shorter sonication time resulted in longer distribution tails at
upper end, the minimum widths all have a cutoff value around 2 nm regardless
of sonication time, indicating that an approximate 3-chain sheet in the 200 plane
of the unit cell (one chain is 0.82 nm across based on unit cell values) could be
the smallest MT nanocellulose structure, which is consistent with the microfibril
cross section model suggested in our previous study (19). As sonication time
increased, the majority of the measurements, become clustered with the upper
box plot value near 5nm at the higher end of the measurements. This result
suggested the maximum width of individual sheet is around 5 nm, corresponding
to approximate 6 cellulose chains connecting each other side-by-side (4.92 nm).
The larger width values (5 nm and above) could be the manifestation of the
“non-individualized” fiber bundles and/or sheets connected side-by-side via
inter-chain hydrogen bonds (13, 38). However, these values may provide insight
to the way the microfibrils aggregate within the cell wall. For extended sonication
levels (60, 120, 240 min, collectively noted as “60MinPlus” group in Figure 2),
the majority (75% percentile) of the width measurements are below 5 nm. This
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Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications
ACS Symposium Series; American Chemical Society: Washington, DC, 2017.
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Publication Date (Web): October 23, 2017 | doi: 10.1021/bk-2017-1251.ch003
data clearly illustrates that sonication energy can induce fragmentation of the
larger aggregates into smaller sections, which correspond in lateral dimension to
chain lengths of approximately 6 cellulose chains across in the 200 plane.
Figure 2. TEM images of sonicated nanocellulose (A: 5 min; B: 60 min; C:
120 min) scale bar 100 nm, (representative twisting features were indicated
with arrows) and (D) nanocellulose width distribution from TEM Note 25%,
50%, and 75% percentiles are denoted as the long horizontal bars, 1% and
99% percentiles are denoted as “*”, the mean is denoted as “□”, and the full
range of the distribution is in between the short horizontal bars at both ends of
the box-whisker plot.
Integrating the TEM measurements with the previous AFM results on
microfibril thickness (18), the isolated cellulose molecular sheets can be visualized
as a long flat ribbon with a thickness of approximately 1 nm (18) or less with
width ranges from 2-5 nm (TEM data), and a length scale ranging from hundreds
of nanometers to several micrometers (18, 22). The periodic twist structure of
cellulose microfibrils reviewed previously (5, 21, 39), was observed as a twisting
feature within the TEM images. The structure appears as width variations along
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individual microfibrils (39), and is similar to that which has been observed in
other cellulose fibril structures (21).
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NMR Results: Crystallinity, Molecular Conformation, Chain Conformation
Figure 3 contains the (CP/MAS) 13C NMR spectra across all treatment levels,
showing the variations between the kraft pulp, oxidized pulp, and oxidized and
sonicated pulp. A carboxylate peak emerges in the spectrum ca. 175 ppm for the
oxidized samples (Figure 3) (6). For 13C NMR spectra of cellulose, the relative
intensities of the peaks correspond to the proportion of the specific carbons giving
rise to them (31), hence the carboxylate peak intensity agree with the degree of
oxidation. Since oxidation only occurs on the fibril surface and ~23% of the
C6 were converted to carboxylate group under our experimental conditions (see
experimental section for details), the peak intensity ratio between carboxylate and
C6 should be near 23%. From the spectra, ratios of (ICarboxylate : IC6) are: 19%,
22%, 19%, and 23% for WT, WT30, WT60, and WT120, respectively; which is in
agreement with the degree of oxidation determined previously by conductometric
titration.
Figure 3. (CP/MAS) 13C NMR spectra of WP, WT, WT30, WT60, and WT120.
(A) expansion of the carboxylate group chemical shift region, indicating that i)
carboxylate group peak emerges after oxidation, and ii) the carboxylate peak
shifts ca. 1 ppm towards upfield upon sonication. (B) 55-125 ppm region.
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With increased sonication levels the carboxylate peak has an upfield
displacement indicating a slightly different environment for these carboxylate
peaks. This change may relate to the increased gg conformation at C6 position
as shown by the ~1 ppm upfield shift in the carboxylate peak region upon the
sonication treatment. Since after sonication, surface chain proportion should
increase from 40-50% to 80%; accordingly more C6 is exposed to the surface and
converted to (possibly more preferred) gg conformation.
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NMR Crystallinity Determination
Different methods have been developed to evaluate the crystallinity index of
cellulose with (CP/MAS) 13C NMR: curve fitting (40), peak area separation (41),
and chemometric analysis (i.e. principle component analysis) (42). All of these
methods seem to have their own limitations: curve fitting methods are not very
precise in terms of chemical shifts assignments associated with the disordered
regions, plus the results are operator dependent and difficult to reproduce even
when conducted by the same operator (42). Peak area separation tends to
overestimate the narrower crystalline peaks (29); and chemometrical analysis is
model-independent providing more consistent results (42), but requires large scan
numbers and extended data collecting time. The peak area separation method was
applied in this study to compare the differences introduced by different treatment
levels.
NMR crystallinity index (CI) calculation results for all levels are presented
in Table 1, and Figure 4 demonstrates the calculation for WP level. The general
trend for CI variation across all levels is similar to the results derived from XRD
previously, and the major cause for a significant CI drop upon sonication is that
the delamination effect has destroyed a portion of the crystalline structure (19).
While this result is in contrast to Heux and Vignon when homogenizing sugar beet
pulp (43), this data was supported by crystal size measurement using the Scherrer
equation, where the (200) plane thickness was reduced by ~30% (19). With regard
to the difference between the CI absolute value determined by “XRD height” and
“NMR peak integration” (noted that NMR CI values are overall much smaller
than XRD CI), Park et al. (41) have provided a comprehensive account, essentially
stating that the XRD height method is a “time-saving empirical measure of relative
crystallinity” and is likely to overestimate the crystalline portion.
One striking result though is the increase in crystallinity after oxidation.
A plausible explanation is that part of the amorphous hemicellulose in the
kraft pulp (up to 25% (44)) has been removed during the oxidation and the
follow-up purifying process, leading to an increase in the relative proportion of
the crystalline cellulose, and hence the CI. Similar CI increases for cellulose I
also was observed by XRD analysis (45).
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Table 1. Crystallinity index (CI) value calculated by peak integration method
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CI
WP
WT
WT30
WT60
WT120
39.67%
43.50%
31.33%
30.33%
30.00%
Figure 4. (CP/MAS) 13C NMR spectrum of kraft wood pulp. Inset demonstrates
the area based crystallinity index (CI) calculation: CI=C/(C+A) (41).
NMR Implication on Molecular and Chain Conformation Changes under
Sonication
C1 and C4 Chemical Shift and Torsion Angles Φ and Ψ
Figure 5 shows the peak shifts for C1 and C4 after oxidation and different
sonication treatment levels. It should be noted that hemicelluloses impact
the chemical shifts at C1 and C4 and this is noted in the figure for the WP
original pulp sample (46). It is also clear that after oxidation at the conditions
used there was a significant reduction of the hemicellulose contribution to
the spectra (Figure 5). Kuramae et al. report that most of the neutral sugars
are removed during oxidation, especially the mannan component with only a
minimum residual xylan (47). Hence, caution should be noted for differences in
chemical shifts between wood pulp and oxidized pulp arising from differences
in composition. However, oxidized pulp and sonicated oxidized pulp should
have similar compositions making the following noted differences arise from
changes of the local environment of the cellulose chains in the microfibril. C1
peak positions do not show any displacement after oxidation, but exhibit 0.5 ppm
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ACS Symposium Series; American Chemical Society: Washington, DC, 2017.
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displacement towards upfield upon sonication. Similarly, C4 downfield peaks
do not show any displacement after oxidation, but exhibit a shift of ~0.2 ppm
towards upfield upon sonication. C4 upfield broad peaks are sharpened, and
their relative intensities to the downfield sharp peaks increase upon sonication.
The following four implications can be derived from the changes seen in the
spectra: i) Surface chains have an elevated impact on NMR spectra upon
sonication due to an increase in the proportional surface area. Both oxidation
and sonication treatments affect the signal from the surface microfibril structure
(19, 25, 48) according to the relationship between crystallite width and surface
area chain proportion determined by Vietor et al. (35) Cellulose microfibrils from
kraft pulp should have approximately 40~50% of their chains on the surface.
Sonication will have a major delamination effect on these microfibrils with 60
min of sonication on oxidized fiber reducing the average fibril thickness to around
1 nm (19), which results in an overall proportion of surface chain increasing
to ~80%, if there is no aggregation during freeze drying. Because the NMR
spectra reflect the mean conformation of surface and core chains (35), the spectra
of the sonicated samples are likely to be more representative of surface chains
as opposed to the core chains. ii) Oxidation does not change torsion angles
of glycosidic linkages but sonication does. C1 and C4 chemical shifts are
dependent on Φ and Ψ (49, 50), although not in a linear fashion as occurs with
Χ (33). Both WP and WT have C1 chemical shifts at 105 ppm (Figure 5) while
the three sonication groups WT30, 60, and 120 all have their C1 chemical shifts
displaced 0.5 ppm towards upfield to 104.5 ppm. This indicates that oxidation
does not significantly affect the torsion angle Φ but sonication does. Unlike
C1, the C4 chemical shifts do not reveal very explicit displacements across the
five levels. The three sonication levels do have their downfield peaks blunted,
indicating a tendency for upfield displacement, or change in Ψ. Therefore, both
spectral changes at the C1 and C4 position indicate that oxidation does not
change the glycosidic linkage conformation but sonication does cause a change
in bond angles. This means the addition of carboxylate group on the surface has
little impact on glycosidic linkage torsion angles, as long as the microfibril is
intact; and once the microfibril is delaminated, glycosidic linkage will gain more
freedom and exhibit more arrangements. This change in arrangement is important
when evaluating the delamination mechanism of the microfibril, as changes to
intersheet intermolecular bonds vs. intrasheet molecular hydrogen bonds possibly
restrain the glycosidic bond in different fashion. iii) Glycosidic linkage changes
are more pronounced on C1 than C4 chemical shifts. Since sonication alters
torsion angles at the glycosidic bond, assuming the rotation of each linkage is
shared almost equally between Φ and Ψ (35), the unbalanced response from
C1 and C4 chemical shift displacements may well support Vietor’s theory that
glycosidic conformation affects C1 chemical shifts more than C4 shifts, either
through association with torsion angle or through hydrogen bonding at O3 (35).
iv) Oxidation increases crystallinity, while sonication reduces crystallinity
but increases the proportional area of fibril surface cellulose chains. Three
features are identified contrasting the C4 downfield sharp peaks with upfield
broad peaks: 1) relative intensity of the upfield peaks versus the downfield peaks
increased significantly after sonication, indicating decreased crystallinity of the
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sonicated groups; 2) the C4 upfield broad peaks sharpened at ca. 84 ppm upon
sonication according to Newman’s assignment (51), which suggests an increased
proportion of surface chains, and corroborates previous published results (18, 19);
and 3) the C4 upfield broad peaks exhibit a downfield displacement from 83 ppm
to 83.75 ppm while the WT C1 peak exhibits a receding slope from 100-102 ppm
after oxidation. This evidence indicated the removal of residual hemicellulose
during oxidation (47) which resulted in reduced hemicellulose peaks at ca. 81.9,
81.2 ppm and 101-102 ppm, according to previously published peak assignments
(5, 52).
Figure 5. (CP/MAS) 13C NMR spectra of the peak shifts for C1 and C4 under
oxidation and different sonication treatment levels C6 chemical shift and torsion
angle Χ.
The peak shifts for C6 following oxidation and increasing sonication
treatment (Figure 6) show that the relative intensities of the C6 downfield peak
were reduced, while the upfield peak tended to shift to a lower ppm (63→2.5 ppm).
These observations lead to the following three implications: i) gg conformation
proportion rises with an increased proportion of surface chains. The C6
chemical shift was confirmed to have a linear relationship with torsion angle Χ’s
three energy minimal positions (tg Χ=300°, gt Χ=180°, gg Χ=60°) (33, 53). A
reduction in downfield peak intensity (65 ppm, related to tg conformation) and
the upfield peak shift towards 62 ppm (related to gg conformation) (50, 53, 54)
both indicate more CH2(OH) side groups were converted to gg conformation
from the dominant tg and gt conformation (55, 56) when additional surface chains
were exposed during sonication. ii) Surface chains favor gg conformation.
During sonication, cellulose microfibrils become delaminated and the total
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surface area therefore is expected to increase dramatically (18, 19). Enriched gg
conformation therefore suggests the regioselectivity of the C6 primary hydroxyl
group on the fibril surface, which would require that either surface chains are
more energy favorable towards gg conformation than the inner chains, their
spatial arrangements have an increased probability for gg conformation, or
both. While Newman et al. have confirmed gg conformation for cellulose
I at the cellulose-water interface (54), our results suggest that in the solid
state too, the gg conformation might also be more favorable, and may even
dominate, at the fibril surface. iii) Carboxylate side groups may contribute
in adopting gg conformation. Because TEMPO-oxidation converts ~23%
of the C6 primary hydroxyl groups to carboxyl groups, the larger carboxyl
groups may also contribute to gg formation more as a stereochemically-preferred
conformation compared to either gt or tg conformations at the cellulose-air
interface. Interestingly, this trend is not apparent when contrasting WP and WT
spectra, as the residual hemicellulose in WP may contribute to the upfield broad
peak, which may then be large enough to disturb the trend.
Figure 6. (CP/MAS) 13C NMR spectra of the peak shifts for C6 under oxidation
and different sonication treatment levels.
Reflections on Cellulose Microfibril Supramolecular Structure
Many research efforts revolving around the cellulose Iβ microfibril structure
of wood have been devoted into the understanding of its internal chain structure
and size (5, 10, 11, 40, 57–61). Nevertheless, three key areas of understanding
must be resolved before a well-defined microfibril structure is clearly identified:
lateral dimensions, cross section shape, and chain packing numbers within
individual microfibrils (5, 13, 39, 62).
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Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications
ACS Symposium Series; American Chemical Society: Washington, DC, 2017.
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Microfibril lateral dimensions have been characterized in many plant species
using many techniques (SEM, TEM, AFM, XRD, SAXS, NMR) and it is
understood that the lateral dimensions are source dependent but in a range from
approximately 2-7 nm (7, 18). The cross sectional shape of plant cellulose
microfibrils has been suggested as either hexagonal (13), rectangular (63), or
approximate elliptical (7, 64), but recent crystallography data of microfibrils
from lignified wood cells indicates that the rectangular shape is more likely to
be the case (5). The chain packing numbers are determined by the number of
cellulose synthase units found within the rosette terminal complex (TC) in the cell
membrane of wood cells (11). The microfibril cross-sectional area and the chain
packing patterns are used together to derive the chain packing numbers (14).
The number of chains extruded from the TC are assumed to be constant under
normal conditions (although an exception has also been reported) (5), hence the
microfibril diameter (lateral dimensions) should also be consistent at least within
the same plant species. A 36-chain model has frequently been suggested after
Herth’s initial proposal based on estimation from electron microscopy (11, 12,
14, 15, 65), and this was deduced from the diameter of the rosette TC and the fact
that each rosette is composed of 6 subunits in non-lignified plants (66). However,
a recent study has challenged this model, arguing that the actual cross-sectional
area (assuming a circular shape) of a microfibril can only accommodate 22 chains
sufficiently. Thus, a 24-chain model has instead been proposed (5).
Despite the exposure to pulping treatment (but not drying), the TEMPO
oxidized fragmented fibril isolated in our current work should accurately represent
the fibril in the native state. We argue that the width of the microfibril measured
ranging from, 2-12 nm, with an average value of 3.93nm for the extended
sonication time, should therefore be relatively close to that found in woody
plant species. Using molecular dynamics analyses, Oheme et al. suggest that
microfibrils aggregate together at the 110 plane via hydrogen bonds (67). This
would provide justification for the widths we measured in the TEM converging
on a value of 3.93 nm at the lower end. In height data measured previously, we
reported less than 1% of our measurements were over 3.11 nm, with average
values ranging from 1.38 to 0.74 nm depending upon the sonication time (18).
Because of the fibril fragmentation, this data implies an upper bound of 3.11
nm. These data agrees with the 24-chain packing scheme recently proposed
by Fernandes et al (5), as their two models for a diamond shape fibril and
square shaped fibril are 3.2 x 3.9 nm and 3.2 x 3.1 nm, respectively. Based
on the distribution of widths, the data supports the diamond shape, as width
measurements were observed at 2 nm, which would account for delaminated
sheets of the fibrils near the top and bottom of the diamond geometry.
68
Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications
ACS Symposium Series; American Chemical Society: Washington, DC, 2017.
Conclusions
1.
2.
3.
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Publication Date (Web): October 23, 2017 | doi: 10.1021/bk-2017-1251.ch003
4.
TEM analysis of sonicated TEMPO oxidized cellulose converged at
width values of 3.93 nm for sonication times of 60 - 240 mins.
Solid state NMR was used to estimate the oxidation level of cellulose,
nearing the conductometric titration value which represents 23% of the
primary hydroxyl groups.
Crystallinity measurements and glycosidic torsion angles reveal
disruption of the microfibril surface with sonication, while there was a
minimum impact as a function of sonication time: 30, 60, 120 min.
Experimental data fits the 24 chain diamond shaped microfibril model.
Acknowledgments
This work was funded in part by the Institute of Critical Technology and
Applied Science of Virginia Tech, via the ICTAS Doctoral Scholar’s program
(support for Zehan Li, formerly Qingqing Li). Additional support is from
the Canada Research Chairs program for S. Renneckar’s Chair in Advanced
Renewable Materials. B. Goodell was supported by the National Institute of Food
and Agriculture, U.S. Department of Agriculture, the Center for Agriculture,
Food and the Environment, and the Microbiology department at University of
Massachusetts Amherst, under project number MAS00511. The contents are
solely the responsibility of the authors and do not necessarily represent the official
views of the Canada Research Chairs, USDA or NIFA.”
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