Composition of kraft pulp
Table 2 shows the chemical composition of the kraft pulp used as well as its initial carboxyl group content. The pulp mostly consists of α cellulose, a low amount of β cellulose (degraded cellulose), a considerable amount of γ cellulose (hemicellulose), very little lignin and ash content, and a very low carboxyl group content.
Table 2
α cellulose % | β cellulose % | γ cellulose % | Lignin % | Ash % | Carboxyl group mmol/g |
86.8 | 0.91 | 12.3 | 0.14 | 0.20 | 0.06 |
Optical microscopy
Figure 2 shows the optical microscopy of swollen tracheid subjected to 1.3 mmol/g carboxymethylation and partial dissolution in 5% sodium hydroxide. Therefore, some regions along the tracheid cell would swell and form balloons while not-swollen parts remain as collars between them (part a). According to Fig. 2 (b and c), where the primary wall is broken, the secondary wall is able to swell. The membrane of balloons is composed of S1 and the swollen interior component is S2 (part d). When cellulose microfibrils absorb water, they mostly swell perpendicular to their axis. The direction of arrows in Fig. 1 is the direction of expansion layers of a wood cell wall. Despite the fact that the primary wall layer is thin and the cellulose amount is very low, because the microfibrils in this wall are interwoven randomly, it cannot swell considerably and wraps around the SW unless it is broken apart either by a sufficient internal swelling force or by external mechanical action. S1, with 5–20 nm lignin-free voids (Kesari et al. 2021), is also very thin, however it has enough cellulose in the form of helical microfibrils (50–80° in Norway spruce) to render it highly flexible with an enormous expansion capability. Brändström et al. (2003) indicated that the S1 layer of Norway spruce is a homogeneous layer oriented approximately perpendicular to the tracheid axis without a cross fibrillar structure in alternate S (clockwise) and Z (anticlockwise) helices. The transition of microfibril orientation from S1 to S2 is abrupt.
S2 on the other hand, is very thick (represents 80% of total cell wall in spruce (Fengel 1973)) and is composed of several lamellae and a lot of cellulose microfibrils with a very low angle (5–30°); it accounts for most of the physical and mechanical properties of the cell and wood such as shrinkage and swelling. Generally, the longitudinal shrinkage of wood is very low (0.1–0.2%) while the radial and tangential shrinkage of wood are much higher (black spruce has 4.1 and 6.8% shrinkage respectively) which is due to the low S2 angle of microfibrils (Glass and Zelinka 2021). Balloons and more specifically S2 swell transversely for the same reason and the swelling may be so large to cause S1 to break (Fig. 3, d), leading to the dispersion of S2 microfibrils, or when the carboxyl charge is sufficiently high, to their dissolution.
S1 constitutes a small portion of cellulose and, when swollen, appears as a skin and sometimes it has one or more spiral bands of highly crystalline cellulose which are more difficult to dissolve than the rest of secondary layer (Fig. 2, f and g). Spiral bands are also nearly insoluble in Schweizer’s solution and do not dissolve in nitrate and acetate and appear in viscose solutions too (Hagglund 1951). S1 in black spruce has three parallel running threads of such spiral bands, connected by delicate membranes consisting of fibrils that are nearly perpendicular to the edge of spirals (Hagglund 1951). A double spiral band of S1 layer without opposite directions was observed in Norway spruce by Brandstrom et al. (Brändström et al. 2003).
The innermost layer of SW is S3 with the highest concentration of cellulose in the form of microfibrils with a more than 70° angle. When the thin S3 expands in the direction depicted in Fig. 1, it has not enough space and would bend and wrinkle in the lumen (Fig. 2, e and h).
Microfibrils of S2, along to the tracheid axis are barely visible in high magnification optical microscopy (Fig. 2, i), while transverse splitting of layers is shown clearly (Fig. 2, d and j). Nevertheless, parallel microfibrils of S2 are readily observed by FE-SEM (Fig. 3, a). Transverse splitting of microfibrils can create microspheres discussed in the following section.
FE-SEM and TEM microscopy
The scanning electron microscopy images of films made with 1.3 mmol/g CMF are shown in Fig. 3, a, b, and c). To make films, CMF was dissolved in an alkaline solution, casted in molds and regenerated in a 10% sulfuric acid bath, then washed and dried. More details about film making and measuring the undissolved portion of CMF were described in our previous study (Moradian et al. 2021). This CMF contained 6.5% of undissolved fiber particles; an image of a piece of undissolved tracheid cell wall shows S2 parallel microfibrils (Fig. 3, a).
Transverse splitting of microfibrils can create plenty of microspheres observed on the surface of the films (Fig. 4, b and c). After the etherification reaction, fibers can absorb lots of water, balloons form and microfibrils expand transversely while cross splits are created that can be observed by optical microscopy (Fig. 2, j). When the carboxyl charge density is low, some parts of fibers do not swell and appear as collars between balloons. More charge density results in less collars in swollen fibers and hence less undissolved fibers will remain in the suspension. At a charge density of 2.5 mmol/g, no undissolved particles and collars were observed in water dispersed CMF by optical microscopy, while microspheres could be observed by transmission electron microscopy (Fig. 4, d). Microspheres can be broken up in smaller pieces (nanoparticles) by applying a shear force and/or sonication, which can be weaker if the alkalinity is increased. Figure 4, e, shows the TEM image of a homogenized (3 min, 10,000 RPM) and sonicated (2 min, 50 Hz) 1% water solution of 2.5 mmol/g CMF. It can clearly be seen that the microspheres were broken up into nanorods.
FTIR and XRD analysis
The FTIR spectra of kraft pulp, CMFs, and the regenerated film are shown in Fig. 4 (top). For the original kraft pulp, both CMFs and film, the OH and CH absorption bands were appeared in the ranges around 3330 and 2900 cm− 1 respectively. However, the peaks at 1600 and 1730 cm− 1 represent carbonyl groups in sodium and proton forms on CMFs and the film respectively proving the successful carboxymethylation reactions. Figure 2 (bottom) shows the X-ray diffraction pattern of kraft pulp, CMFs with 1.3 and 2.5 mmol/g carboxyl groups and the regenerated film. The kraft pulp peaks appear at around 2θ = 15°, 16.5°, and 22.5° representing (11̅0), (110), and (200) crystallographic planes of celluloseⅠ, respectively (French 2014, 2020, Yang et al. 2012). After carboxymethylation reaction both CMFs and the regenerated film show cellulose II structure (Langan et al. 2001, Yang et al. 2012). The main peak of CMFs is at 2θ = 20.4° representing (110) crystallographic plane while peak at about 22.5° (020) is most likely too small to be seen. Generally, carboxymethylation would decrease the crystallinity of cellulose, thus 2.5 mmol/g CMF showed less intense peaks than 1.3 mmol/g CMF (Rachtanapun et al. 2012). Presumably the S2 layer (the main component of a fiber) inside the balloons is partially dissolved and when drying into a powder is regenerated into Cellulose II. The regenerated film made with 1.3 mmol/g CMF showed the highest intensity at about 11.5° representing (11̅0) crystallographic plane, similar to regenerated cellulose made by Yang et al. 2012.
Electroacoustic spectroscopy
The size distribution of cellulose nanocrystals with 2.5 mmol/g was measured by acoustic attenuation spectroscopy and it was found that 99% of the particles were in the range 18–208 nm, as shown in Fig. 5 and Table 3. The size distribution of cellulose nanocrystals with 1.3 mmol/g, after separation of undissolved fiber fragments by centrifugation (5 min, 1000 RPM), ranged from 27–613 nm (Fig. 5, Table 3).
Table 3
Cumulative particles size of 1% CMF suspensions with 1.3 and 2.5 mmol/g carboxyl groups
cumulative % | 1 | 3 | 5 | 10 | 50 | 75 | 90 | 95 | 99 |
cumulative size (nm) | 1.3 mmol/g CMF | 27 | 36 | 42 | 54 | 126 | 200 | 295 | 384 | 613 |
2.5 mmol/g CMF | 18 | 23 | 26 | 31 | 60 | 86 | 117 | 144 | 208 |
Microspheres
S2 is considerably thicker than other layers of a tracheid cell wall and consists of a large number of very thin concentric lamellae. The thickness of individual lamella when very much swollen is about 1µm. Mechanical, chemical treatment or a combination of both, would disintegrate lamellae into long fibrils or bundles of fibrils (Hagglund 1951). Dolmetsch et. al (1944) have reported that a transverse splitting of lamellae would happen when acid treatment of fibers is followed by sodium hydroxide treatment. The thickness of transverse cleavage was reported 1–2 µm depending on the wood species and type of cell (Fig. 6). When the cleaved microfibrils disperse in a solution, elliptical and round particles are observed. The diameter of round particles of spruce was reported 0.6 µm (Dolmetsch et al. 1944; Hagglund 1951). Microspheres are micro-units into which the S2 microfibrils can be disassembled.
In our study it was observed that etherification of spruce tracheid and dispersion in water similarly, splits the fibrils transversely and produces microspherical fragments (Fig. 4). Furthermore, microspheres of carboxylated cellulose contain bundles of nanorods that can be dispersed to cellulose nanocrystals by mechanical action and/or by mixing with a sodium hydroxide solution. The higher the carboxyl charge, the less shear force and/or sodium hydroxide is necessary for producing carboxylated CNC.