For comparison purposes, CP/MAS 13C-NMR spectra were recorded on the water swollen samples, Fig. 3. Estimates of the average lateral fibril dimension (LFD) and the average lateral fibril aggregate dimension (LFAD) from CP/MAS 13C-NMR spectra, based on the method by Larsson and Wickholm (Larsson et al. 1997, Wickholm et al 1998) are given in Table 2.
Table 2
The average lateral fibril dimensions (LFD) and the average lateral fibril aggregate dimensions (LFAD) determined from CP/MAS 13C-NMR spectra recorded on dry and water swollen samples. Values within parenthesis are standard errors. LFAD cannot be measured in dry samples by the used CP/MAS 13C-NMR method.
Sample ID
|
LFD (nm)
|
LFAD (nm)
|
SWP
|
4.7 (0.1)
|
32 (1)
|
HWP
|
4.4 (0.1)
|
27 (1)
|
Lint
|
7.0 (0.2)
|
37 (2)
|
Small angle X-ray data was recorded on water swollen and dry samples and the recorded SAXS data was modelled using a simulation model described in detail in the Supplementary Information (SI). Here only a brief account of the main features of the simulation model are given.
The simulation of SAXS intensity profiles was based on a conceptual representation of the distribution of cellulose fibrils and fibril aggregates in the fibre wall, which is illustrated in Fig. 4. The model is conceptual in the sense that it was only used as a route to design the algorithm used for generating electron density paths.
Once a sufficient number of electron density paths was simulated, discrete versions of the spatial correlation function (SCF) and the pair distance distribution function (PDDF) was calculated and subsequently simulated SAXS intensity profiles, Ik(q) in Equation [1], was generated. The modelling of experimental SAXS data, IEXP(q), was performed by superposition of simulated SAXS intensity profiles Ik(q) weighted by Wk, with the addition of a modelling parameter describing any instrument background intensity b:
![](data:image/png;base64,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)
The advantage of the simulation method was that it made it possible to associate each simulated intensity profile with an apparent average particle size (AAPS) and an apparent average cavity size (AACS), connecting each intensity profile to a length scale characteristic of the structures in the sample material. Here, the concept of cavity was used to describe interstitial spaces between solid particles whether filled with water or evacuated. For all samples used in this study, three intensity components (six adjustable model parameters plus one background intensity) were used to model the experimental SAXS data.
Figure 5 shows the results from modelling SAXS data recorded on dry samples; Figure 6 shows the results from modelling SAXS data recorded on the corresponding water swollen samples. Modelling parameters are shown in Table 3 (dry samples) and Table 4 (water swollen samples).
Table 3. Summary of the modelling parameters used for the dry samples. Experimental data was modelled by use of three intensity components, I1, I2, I3 (Equation [1]) for all dry samples. Modelling was performed by adjusting the component weights, wk, and the real space step size, Dxk, for each intensity component. The “larger than” arrow in the table indicates modelling of structures too large to be unambiguously assigned using the q-range of the experimental data. The sum of squared relative residuals (SSRR) is given as an indicator of the quality of the fit. See Supplementary Information for details.
SWP
|
|
wk (%)
|
Dxk (pm)
|
AAPS (nm)
|
AACS (nm)
|
b
|
SSRR
|
I1
|
76 (3)
|
150 (10)
|
> 322 (110)
|
> 36 (12)
|
0.26 (0.01)
|
1.98
|
I2
|
5 (1)
|
13 (2)
|
28 (10)
|
3.1 (1.1)
|
|
|
I3
|
19 (1)
|
4.6 (0.2)
|
9.9 (3.4)
|
1.1 (0.4)
|
|
|
HWP
|
I1
|
78 (4)
|
150 (10)
|
> 330 (113)
|
> 37 (13)
|
0.62 (0.03)
|
1.00
|
I2
|
3 (1)
|
17 (5)
|
36 (12)
|
4.0 (1.4)
|
|
|
I3
|
20 (1)
|
4.3 (0.1)
|
9.1 (3.1)
|
1.0 (0.3)
|
|
|
Lint
|
I1
|
64 (2)
|
210 (5)
|
> 460 (157)
|
> 51 (17)
|
0.3 (0.02)
|
1.66
|
I2
|
23 (1)
|
11.0 (0.2)
|
24 (8)
|
2.6 (0.9)
|
|
|
I3
|
14 (1)
|
3.7 (0.3)
|
7.9 (2.7)
|
0.9 (0.3)
|
|
|
Table 4. Summary of the modelling parameters used for the water swollen samples. Experimental data was modelled by use of three intensity components, I1, I2, I3 (Equation [1]) for all water swollen samples. Modelling was performed by adjusting the component weight, wk, and the real space step size, Dxk, for each intensity component. The “larger than” arrow in the table indicates modelling of structures too large to be unambiguously assigned using the q-range of the experimental data. The sum of squared relative residuals (SSRR) is given as an indicator of the quality of the fit. See Supplementary Information for details.
SWP
|
|
wk (%)
|
Dxk (pm)
|
AAPS (nm)
|
AACS (nm)
|
b
|
SSRR
|
I1
|
3.9 (0.3)
|
100 (4)
|
> 72 (20)
|
> 42 (12)
|
0.16 (0.01)
|
4.71
|
I2
|
5.8 (0.3)
|
20 (4)
|
14 (4)
|
8.4 (2.3)
|
|
|
I3
|
90 (6)
|
6.6 (0.1)
|
4.7 (1.3)
|
2.8 (0.8)
|
|
|
HWP
|
I1
|
4.9 (0.3)
|
100 (10)
|
> 48 (13)
|
> 48 (13)
|
0.31 (0.05)
|
7.74
|
I2
|
10 (1)
|
14.5 (0.3)
|
6.9 (1.8)
|
6.9 (1.8)
|
|
|
I3
|
85 (6)
|
5.6 (0.1)
|
2.7 (0.7)
|
2.7 (0.7)
|
|
|
Lint
|
I1
|
5.0 (0.2)
|
70 (2)
|
> 82 (24)
|
> 26 (8)
|
0.27 (0.02)
|
7.77
|
I2
|
23 (1)
|
14 (1)
|
16 (5)
|
5.2 (1.5)
|
|
|
I3
|
73 (3)
|
7.8 (0.2)
|
9.1 (2.7)
|
2.9 (0.9)
|
|
|
The values of the weights for the three simulated superposition components (I1, I2, and I3 in Table 3 and Table 4) used to model the recorded SAXS data are charted in Fig. 7. Although the length-scales (Δxk in Table 3 and Table 4) are not identical between samples, the three superposition components are coarsely viewed as representing the abundance of larger (I1), intermediate (I2) and smaller (I3) structural features in the samples.
The FSP values and consequently the volumetric fill factors were determined for the water swollen samples. For the dry samples, the volumetric fill factor cannot be determined by FSP measurements since it implies swelling of the sample. For this reason, the volumetric fill factor was set to a value of 0.9, corresponding to an FSP value of about 0.07. Cellulose-rich fibres conditioned in ambient conditions (23 ⁰C, 50 % RH) typically contains between 5 % to 10 % water. The same volumetric fill factor was applied to length scales, since only one FSP value was available for each sample.
Literature values for the density of cellulose rich fibres are available, for cotton fibres density values of 1540–1570 kg/m3 has been reported (Temming 1973). However, this is a density value close to that of cellulose I, implying a complete lack of cavities in a dry cotton fibre wall. This is contradictory to SAXS data (e.g. the experimental data recorded on the dry Lint sample in Fig. 5) where a signal is visible in the q-range 0.6 nm− 1 to 0.8 nm− 1, indicating a significant presence of non-uniformity in the electron density, consistent with the presence of cavities.
Using SAXS and CP/MAS 13C-NMR for nano-structural characterization of cellulose rich pulps, the differences in the principles of operation of SAXS and CP/MAS 13C-NMR opens the possible to obtain complementary information when applying the two techniques the same sample. The interpretation for the samples in this study was that the modelled I3 components related to size-ranges associated with lateral fibrils dimensions, corresponding to the LFD measures obtained from CP/MAS 13C-NMR. Similarly, the interpretation of the I2 components was that they were related to the lateral dimensions associated with fibril aggregates, corresponding to the LFAD measures obtained from CP/MAS 13C-NMR. The modelled I1 components, corresponded to structural features too large to be unambiguously assigned by either the SAXS technique or the CP/MAS 13C-NMR technique, the way they were used in this work.
In Fig. 7, the three dry samples SWP, HWP and Lint all showed dominating large structures, as modelled by the I1 components, that gave a considerable signal intensity contribution in the observable q-range. These structural features were too large to be unambiguously assigned by the used experimental setup, this is indicated in Table 3 and Table 4 by a “larger than” symbol. In all the investigated dry samples the largest relative component weight (wk) was observed for the I1 components. This agreed with expectations, cellulose fibrils aggregate into larger structures as a consequence of drying, which contributes to hornification (Krässig 1993).
Less abundant smaller structures were observed when modelling the dry sample’s experimental SAXS intensities. The AAPS corresponding to the intensity component I2 were found to be in a size range similar to the size range of the LFAD measured by CP/MAS 13C-NMR recorded on water swollen samples, Table 2, though no direct correlation between samples was found.
The AAPS related to the intensity component I3 showed the presence of particles larger than the LFD measured by CP/MAS 13C-NMR on water swollen samples. One possible reason for this is illustrated in Fig. 8. If, during drying, intimate local aggregation of fibrils occur, this could lead to local removal of cavities (electron density contrast) between fibrils, yielding SAXS AAPS representing partial fibril aggregates. One interesting finding was that all modelling results of the SAXS data of dry samples contained an I3 component associated with AAPS’s significantly smaller than the LFAD (CP/MAS 13C-NMR, water swollen samples) and AACS’s in the range of 1 nanometre. This indicates the existence of porosity of the cellulose structure also in the dry state, consistent with the materials known ability to rapidly re-swell.
In cases where intimate proximity between fibrils reduces the electron density contrast to the point where a SAXS measurement may not distinguish the width of individual fibrils, distinct fibril surfaces polymer conformations may still be present, allowing for CP/MAS 13C-NMR to distinguish fibrils also in the dry samples. As shown in Fig. 9 the signal positions interpreted to originate from carbon-13 nuclei at the C4 and the C6 positions of the anhydroglucose units in fibril surface polymers occur at the same average positions in spectra recorded on dry and water swollen samples, though significantly broadened in the cases of dry samples.
Common to all water swollen samples, SWP, HWP and Lint, in Fig. 7 was an increased abundance of smaller structural features as a consequence of swelling, compared with their dry counterparts. This agrees with expectations, swelling increase the specific surface area and increases the abundance and size of cavities in cellulose rich fibres, accompanied by an increased electron density contrast at smaller length scales.
In the cases of water swollen samples, the SAXS modelling results showed a significantly smaller abundance of the intensities originating from structures too large to be unambiguously assigned (relative weight of component I1) compared with their dried counterparts (Fig. 7 and Table 3 and Table 4). For intensity components, I2 and I3 in Table 4, the ranking of the samples’ AAPS was in the order Lint > SWP > HWP, in agreement with the sample ranking based on the CP/MAS 13C-NMR LFD and LFAD measured on water swollen samples, Table 2.
For the water swollen samples, comparing the AAPS for the I2 intensity components (Table 4) with the CP/MAS 13C-NMR LFAD measures (Table 2), the modelling of SAXS results showed AAPS consistently smaller that the corresponding LFAD. One interpretation of this observation could be that the SAXS measurements could detect cavities within cellulose fibril aggregate, cavities that the CP/MAS 13C-NMR measurements could not distinguish. Some support for this interpretation can be found in previous results obtained by NMR relaxometry data (Larsson 2017), where probing the cellulose specific surface area (particle surface-to-volume ratio) by 2H-relaxation measurements showed results intermediate between the extremes obtainable from CP/MAS 13C-NMR results. Consistent with the hornification behaviour cellulose, discussed above, if part of the structures illustrated in the right panel of Fig. 8, remain after re-swelling in water this can result in a structure inside fibril aggregates that is not compact, Fig. 10.
The SWP and HWP samples were produced by two different isolation procedure, the SWP was produced by an alkaline pulping process, while the HWP was produced by a comparatively more acidic pulping process. This may be an explanation for the differences observed in the modelling results for the two samples.