3.1 Amount of APTES grafted onto MCC surface
According to the TG curves (Fig. 2a), the initial mass loss observed in the 30–200°C temperature range could be attributed to the evaporation of physically adsorbed water from the surface. The values found for MCC, MCC-Si (1:3) and MCC-Si (1:4) were 5.75 wt%. For higher MCC content, (1:5) and (1:10) samples, the values were 4.5 and 3.7 wt%, respectively. This is indicative of the relatively hydrophobic nature of the silanized MCC.
Usually, cellulose shows a one-step degradation from 300 to 400°C (Fig. 2b) due to the depolymerization of cellulose chains (Ornaghi et al., 2020). For MCC, a shoulder around 310°C was observed associated with the dehydration of cellulose to dehydrocellulose as reported by Das et al., 2010). Additionally, the chemical modification of MCC decreased the Tmax of all samples. Some literature has associated this reduction in thermal stability to the reducing ends (RE) of the cellulose chain, whose C-OH bonds can form aldehyde groups that are susceptible to oxidation and reduction reactions (Puls et al., 2011; Agustin et al., 2016).
Since the silanization process occurred in an acidic medium (pH: 4–5), some breaking of the β1–4 glycosidic bonds may happen, producing more REs. The reduced MCC chains would tend to thermally degrade more easily than the original cellulose chain. This behavior was more pronounced for MCC-Si (1:5). A similar trend was reported by (Agustin et al., 2016) who studied the thermal characteristics of nanocrystals of cellulose, (Matsuoka et al., 2011) who studied MCC thermal glycosylation and degradation reactions, and (Leboucher et al., 2020) who studied hydrolyzation of cellulose nanocrystals. In all cases, the thermal stability was related to the chain ends rather than to crystallinity.
The results for grafted aminosilane amount onto MCC surface from TG curves are displayed in Fig. 2c. It is possible to observe that increasing the APTES content in the sample, increase the silane grafting amount. In other words, by increasing APTES in the solution, the quantity of aminosilanes able to bond with MCC slightly increases. These results corroborate with (Piscitelli et al., 2010) finds when they treated sodium montmorillonite clay with this silane.
The content of carbon (C), nitrogen (N) and hydrogen (H) elements in MCC-Si can be found in Table 1. The MCC treated with higher content of APTES showed a slightly higher nitrogen content, being the highest for MCC-Si (1:5). According to Hassanpour et al. (2017), higher N content can be related to a successful grafting of the aminosilane onto the cellulose surface. Saini, Belgacem, Salon, & Bras, (2016) also found that the initial silane concentration significantly affected the grafting efficiency. The content of APTES (MAPTES) in MCC calculated from the N amount, is also shown in Table 1.
Table 1
Elemental analyses and the calculated APTES content in the MCC-Si samples.
Sample
|
C
(%)
|
H
(%)
|
N
(%)
|
MAPTESa
(mmol Si/g sample)
|
MCC-Si (1:3)
|
39.219
|
7.237
|
0.448
|
0.0320
|
MCC-Si (1:4)
|
39.565
|
7.266
|
0.441
|
0.0315
|
MCC-Si (1:5)
|
39.669
|
7.269
|
0.563
|
0.0402
|
MCC-Si (1:10)
|
39.553
|
7.302
|
0.508
|
0.0363
|
a Determined using Eq. (3).
A Pearson correlation plot was done to identify possible correlations among the variables (Fig. 3). The red and blue ellipses refer to positive and negative correlations, respectively, and the smaller the ellipse, the stronger the correlation between the variables (also indicated by the p-value). The results indicate a better correlation between the (MAPTES) (Table 1) and the N content than between the silane grafted amount and the residual TG (Fig. 2c).
3.2 Solid-state 13C and 29Si NMR results
In a previous publication (Neves et al., 2020), NMR spectra of solid-state 13C from 130 to 30 ppm and NMR spectra of solid-state 29Si from − 20 to -100 ppm were reported, from which cellulose carbons C1 to C6, characteristic of cellulose samples, and three Si structures of: T1 (dimer), T2 (linear) T3 (three-dimensional structures), of the siloxane structure were reported. In the current paper, NMR measurements we performed at higher exposure times (total time of the analysis: 60h), extending the ppm axis.
Figure 4 shows the 13C NMR spectra of MCC and MCC-Si samples, and the chemical shift assignments of the 13C are summarized in Table 2. It was possible to observe all the chemical shifts characteristics of cellulose: C1, C2, C3, C4, C5 and C6 (Maunu et al., Sundquist, 2000) and the location at the anhydroglucose unit (AGU) (Haslinger, Hietala, Hummel, Maunu, & Sixta, 2019; Idström et al., 2016). In addition, three peaks from 45 to 5 ppm were obtained for MCC-Si samples, assigned as γ-C (-CH2-NH2), β-C (-CH2-) and α-C (-Si- CH2-). According to Brochier et al. (2005) and Fernandes et al. (2013), these peaks are resonances from the aminopropyl groups of the APTES, which are grafted onto the MCC, confirming the silane coupling onto the MCC surface.
Table 2
Isotropic 13C chemical shift assignments (in ppm) for MCC and MCC-Si and its representations.
|
MCC
|
MCC-Si (1:3)
|
MCC-Si (1:4)
|
MCC-Si (1:5)
|
MCC-Si (1:10)
|
C1
|
104
|
104
|
104
|
104
|
104
|
C2
|
73
|
72
|
72
|
72
|
72
|
C3
|
73
|
72
|
72
|
72
|
72
|
C5
|
73
|
72
|
72
|
72
|
72
|
C4
|
84
|
85
|
85
|
84
|
84
|
C6
|
63
|
62
|
63
|
62
|
62
|
γ-C
|
-
|
42
|
42
|
43
|
41
|
β-C
|
-
|
22
|
22
|
24
|
23
|
α-C
|
-
|
10
|
10
|
10
|
11
|
Figure 5 shows the 29Si NMR spectra of MCC and MCC-Si samples, from − 5 to -35 ppm and the chemical shift assignments of the 29Si are summarized in Table 3. All the presented chemical shifts are corroborated with the literature (Cabrera, Berlioz, Fahs, Louarn, & Carriere, 2020; Rachini, Troedec, Peyratout, & Smith, 2011; Robles, Csóka, & Labidi, 2018).
Table 3
Isotropic 29Si chemical shift assignments (in ppm) for MCC-Si samples.
|
MCC-Si (1:3)
|
MCC-Si (1:4)
|
MCC-Si (1:5)
|
MCC-Si (1:10)
|
D1
|
-11
|
-9 to -13
|
-10 to -13
|
-11
|
D2
|
-19
|
-17 to -20
|
-18 to -23
|
-18 to -24
|
T1
|
-45 to -50
|
-46 to -50
|
-46 to -50
|
-46 to -51
|
T2
|
-57 to -62
|
-56 to − 63
|
-52 to -62
|
-53 to -61
|
T3
|
-63 to -68
|
-64 to -68
|
-64 to -76
|
-63 to -73
|
Q2
|
-85 to -90
|
-82 to -89
|
-85 to -90
|
-84 to -89
|
Q3
|
-91 to -97
|
-90 to -94
|
-90 to -97
|
-90 to -94
|
Q4
|
-105 to -108
|
-105 to -109
|
-104 to -110
|
-104 to -114
|
The type of structure formed is given by the self-condensation of the silanol groups, i.e. “T” structures. However, in the current study, two other structures appeared, “D” and “Q”, due to the longer exposure times in the 29Si NMR analysis. The notations “D”, “T”, and “Q” represent the different types of silicate structures commonly encountered in 29Si NMR studies, with two, three and four Si–O– bonds, respectively (see Fig. 5b). More information can be found in (Glaser & Wilkes, 1989, Brochier et al., 2005). Based on Fig. 5, “T” structures were found to be dominant, and more pronounced for higher contents of APTES (MCC-Si (1:5) and (1:10)). In all samples, “D” and “Q” structures also appeared. The chemical modification of cellulose is applied, in general, to add functional groups that will interact with some polymers, producing a stronger cellulose/polymer interface in a composite. Since the chemical condensation and grafting of APTES onto cellulose happens through Si-O-C bonds (Abdelmouleh et al., 2004), “T” and “Q” structures are interesting for having more oxygen to bond with cellulose.
3.3. Chemical and Morphological Characterizations
Figure 6 shows SEM micrographs and EDX spectra of MCC-Si samples. All samples presented the same pattern, already discussed in (Samat et al., 2013; Neves et al., 2020). All modified samples with APTES presented the Si characteristic peak at ~ 1.75 kEv, confirming the silanization process.
Plasma Optical Emission spectrometry (ICP-OES) has been used to identify silicon (Si) (Grümping et al., 1998; Yang et al., 2018). In this paper, the ICP-OES was used as an indirect measurement of grafted Si and the results are reported in Fig. 7. The difference in Si amount before and after APTES condensation onto MCC surface was 230, 685, 814 and 515 mg/L for MCC-Si (1:3), (1:4), (1:5) and (1:10), respectively. These results evidence that APTES has been grafted onto the MCC surface, and the higher difference in Si amount before and after APTES condensation was found for MCC-Si (1:5) sample. These results corroborate the previous TG, EA, and 29Si NMR findings.
The FTIR study showed marked differences between the different samples (Fig. 8 and Table 4). In general, in the –OH region, the bands assigned to the weak intramolecular O(2)H···O(6), to the intermolecular O(6)H···O′(3) and to the intramolecular O(3)H···O(5) showed a displacement towards higher wavenumber values (Table 4) which is assigned to a constrained mobility of these –OH groups. The “weak” band is often assigned to the most accessible cellulose portions and it is strongly shifted after APTES treatment, which supports a grafting at the periphery of the crystalline portions. The main –CH band near 2900 cm− 1 is also affected by the treatment and it shows a slight wavenumber increase of 4 cm− 1 at the lowest relative concentration (MCC-Si 1:3), which progressively disappears as the concentration is increased. It is often thought that this band has little sensitivity to treatments, but this is in fact not true and it can be affected both by chemical and thermal treatments (Leboucher et al., 2020). The MCC-Si 1:10 treatment condition stands apart with respect to the intermolecular O(6)H···O′(3), to the intramolecular O(3)H···O(5) and to the –CH band (Table 4 and Fig. 8), and this supports the assumption of a different grafting architecture. The relative amount of free water in the 1800 − 650 cm− 1 region was also evaluated by measuring the relative area of the band near 1650 cm− 1. Unsurprisingly, the amount of free water is lower after silanization, which is consistent with the higher hydrophobicity provided by the treatment. Eventually, there are several distinct Si-O peaks at 1456, 1464 and 1473 cm− 1 (as seen in PCA, not reported here) that indicate successful silane grafting. The relative prominence of the 1456 cm− 1 band for 1:10 sample confirms a different grafting architecture also.
Table 4
– Band positions as a function of the band assignment, method of determination and treatment parameters. (*): as described by (Watanabe et al., 2006)
Band assignment
|
Intramolecular O(2)H···O(6)
“weak type” (*)
|
Intermolecular O(6)H···O′(3)
|
Intramolecular O(3)H···O(5)
|
Intramolecular O(2)H···O(6)
|
Max
–CH
|
Free -OH water
|
Method
|
From 2nd derivative
|
From max intensity
|
From 2nd derivative
|
From max intensity
|
Integral from 1800 to 1550 cm− 1
|
MCC
|
3480
|
3415
|
3340
|
3271
|
2899
|
8.7%
|
MCC-Si 1:3
|
3446
|
3409
|
3334
|
3276
|
2895
|
1.8%
|
MCC-Si 1:4
|
3445
|
3409
|
3333
|
3274
|
2896
|
3.5%
|
MCC-Si 1:5
|
3446
|
3410
|
3334
|
3275
|
2897
|
2.4%
|
MCC-Si 1:10
|
3446
|
3407
|
3332
|
3276
|
2899
|
2.5%
|
APTES
|
/
|
/
|
/
|
/
|
2973
|
/
|
3.4 Structural study
The X-ray diffraction spectra were analyzed using a bimodal size distribution model, shown. The results are shown Fig. 8 and Table 4. The crystallite thickness in the direction orthogonal to the (200) plane is ~ 2.Rx ± 7% and an increase of this value can be understood as a co-crystallization occurring on the more hydrophobic planes, as detailed in previous works (Duchemin, 2017; Leboucher et al., 2020). In contrast, the crystallite thickness in the direction orthogonal to the (010) plane is equal to 2.Ry and an increase of this value can be understood as a co-crystallization occurring on the more hydrophilic
planes. Several things can therefore be assessed from the X-ray fitting work. Regarding the MCC sample, the relative allomorphic content, the crystallite shape and the dimensions of the two allomorphs are very much in line with our previous work, despite differences in the MCC origin and measurement setup (Duchemin, 2017). The exposure to the APTES in the acidic medium had several effects on the initial MCC. Regarding the dimensions of the larger cellulose Iβ crystallites, their diameter 2.Rx is decreased by ~ 25% along the hydrophobic sides for all the treatment conditions except for the treatment with the highest APTES concentration (MCC-Si 1:10) for which 2.Rx remains the same (Table 4). More strikingly, the diameter 2.Ry is roughly halved by all of the treatment conditions, but this is also slightly less marked for the MCC-Si 1:10 sample. This means that, statistically, the silanization splits the larger Iβ crystallites in half and along the more reactive hydrophilic sides. The diameter of the smaller IVI crystallites is also largely reduced by the action of the treatment and this action is more pronounced as the silane concentration increases up to the 1:5 ratio, after which point this diameter is large again. The 8–10 Å size observed for the 1:4 and 1:5 treatment conditions is actually in line with the hypothesis of protofibrils ~ 2–3 chains in width. Interestingly, the allomorphic content is also showing two distinct groups: one in which small IVI crystallites are rather dominant (54% for MCC and 61% for MCC-Si 1:10), and one in which larger Iβ crystallites are prevalent (between 74 and 81% for MCC-Si 1:3 to 1:5) (Fig. 9).
Table 5
Fitting parameters of the model
|
|
MCC
|
MCC-Si (1:3)
|
MCC-Si (1:4)
|
MCC-Si (1:5)
|
MCC-Si (1:10)
|
Iβ
|
Lattice parameter a (Å)
|
7.95
|
7.94
|
7.95
|
7.93
|
7.97
|
Lattice parameter b (Å)
|
8.29
|
8.00
|
7.93
|
7.96
|
8.15
|
Lattice parameter c (Å)
|
10.59
|
10.30
|
10.30
|
10.30
|
10.50
|
γ (°)
|
95.66
|
95.24
|
95.17
|
95.11
|
96.82
|
March-Dollase strength
|
1.51
|
0.80
|
0.73
|
0.77
|
0.94
|
Ellipsoid diameter 2.Rx along the b x c axis (Å)
|
62
|
48
|
46
|
46
|
60
|
Ellipsoid diameter 2.Ry along the b axis (Å)
|
56
|
26
|
24
|
24
|
30
|
Ellipsoid radial squareness n
|
1.85
|
0.10
|
0.10
|
0.10
|
0.10
|
IVI
|
Diameter (Å)
|
22
|
16
|
10
|
8
|
26
|
Relative area
|
Background
|
28%
|
22%
|
18%
|
18%
|
32%
|
%Ibeta
|
33%
|
57%
|
65%
|
66%
|
26%
|
%IV(1)
|
39%
|
20%
|
17%
|
16%
|
42%
|
Fitting
|
Correlation coefficient
|
0.999
|
0.998
|
0.998
|
0.998
|
0.998
|
R-pattern
|
0.027
|
0.043
|
0.043
|
0.042
|
0.036
|
These measurements can be interpreted in the light of the previous 29Si NMR and ATR-FTIR results. During the APTES treatment, reactive silanols are created and they condense with the hydroxyl groups of cellulose. Since these hydroxyl groups are found in priority along the hydrophilic surfaces, the condensation reaction should occur on these sites first. This is what is observed here since the 2.Ry values are halved after the APTES treatment, meaning that the crystallites were split apart along the
planes. Interestingly, the silane treatment is therefore able to act as a spacer between the hydrophilic sides of the cellulose crystallites. This transformation is correlated with the cross-sectional shape of the Iβ phase: a swap from a lateral diamond shape (MCC) to a square shape occurs at all treatment conditions (Fig. 9 and Table 5). It is also correlated with the content reduction of slender IVI crystallites with respect to larger cellulose Iβ, possibly due to dynamic reorganizations and perhaps recrystallization of the former phase.
When the highly reactive silanol structures are formed, they can also react to some extent with the less reactive “hydrophobic” cellulose sides, which explains the crystallite size change along a (Rx values). However, when the APTES concentration is increased in the acidic solution, the acidic hydrolysis of the APTES moieties evolves and certain forms become more frequent. For instance, the prevalence of the more reactive T3 structure seems to be at an optimum for the 1:5 concentration (Fig. 6a), resulting in the lowest Rx values observed (Table 5). However, when the APTES concentration is too high, self-condensation might occur and bulkier, less reactive silanols are created that are unable to react along with the accessible “hydrophobic” groups. This explains the similarities between the initial and 1:10 condition in terms of crystallite size (Rx remains the same), lattice parameters (less lattice distortions due to lower reactivity), and allomorphic content (the unchanged prevalence of the IVI form).