On the Specific Behaviour of Native Cellulose Fibers upon Dissolution

Chapter 7

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On the Specific Behaviour of Native Cellulose Fibers upon Dissolution N. Le Moigne and P. Navard* Mines ParisTech, CEMEF, CNRS UMR 7635, BP 207, 1 rue Claude Daunesse, F-06904 Sophia Antipolis Cedex, France *E-mail: [email protected] Member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu.

This chapter deals with a description of the mechanisms of swelling and dissolution of native cellulose fibers at the different scale of the fiber structure, i.e. from the walls to the macromolecular chains. New observation by scanning electron microscopy of the cellulose fibers after regeneration upon dissolution suggest that the structural transition between primary wall and S1 wall is very sharp. The distribution of precipitated cellulose around swelling fibers shows that cellulose chains are escaping the fiber at an early stage of the dissolution process.

Introduction Cellulose is a major source of materials for paper, films and textile industry as well as food, paints, cosmetics or pharmaceuticals industry. The difficulties of cellulose processing have attracted the attention of numerous scientists over the last century and the swelling and dissolution of cellulose fibers have been studied in a wide range of conditions. The most spectacular effect of the swelling and dissolution of natural cellulose fibers is the ballooning phenomenon. The swelling can take place in some selected zones along the fibers. This heterogeneous swelling gives the impression of having “balloons” growing. The ballooning phenomenon has been observed long ago, first by Nägeli in 1864 (1), then by © 2010 American Chemical Society In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Pennetier in 1883 (2), Fleming and Thaysen in 1919 and 1921 (3, 4), Marsh et al. in 1941 (5), Hock in 1950 (6) or Rollins and Tripp in 1952 (7, 8) (Numerous other studies are also reported in reference (9)). In 1954, an explanation of this phenomenon was proposed (10). It must be noticed that similar explanations were suggested in all the preceding references. For a wide range of cellulose fibers, the microfibrils of the secondary wall are aligned in a helical manner towards the long axis of the fiber. It was deduced that swelling must be greater transversely than lengthwise (as it is generally observed for fibers where the orientation of the cellulose chains is mainly in the fiber direction). Consequently, the author proposed: “when raw cotton fibers are placed in certain swelling agent, the radial expansion of the cellulose in the secondary wall causes the primary wall to burst. As the expanding swollen cellulose pushes its way through the tears in the primary wall, the latter rolls up in such a way as to form collars, rings or spirals which restrict the uniform expansion of the fiber” and forms balloons. All authors assume that the ballooning phenomenon has structural origins, i.e. linked to morphological variations within the cellulose fibers. Cuissinat and Navard (11, 12) described the main regions implied in the ballooning in details. As shown on Figure 1, three zones have been identified: the membrane, the inside of the balloons and the unswollen sections. However, the exact origin of these three zones was not reported. Stawitz and Kage (13) reported earlier a heterogeneous swelling and dissolution for carboxymethylcellulose fibers. Fibers were shown to swell by ballooning with a helical structure around the balloons. Then, the breakage of the helical structure and the unswollen sections between the balloons leads to a high homogeneous swelling. Finally, the highly swollen sections are then tear into thin sections and finally into fragments. Chanzy et al. (14) investigated the swelling and dissolution of various cellulose fibers, both native and regenerated, in N-methylmorpholine-N-oxide (NMMO) with different amounts of water. Four domains of water concentration were found to be important. When the amount of water was low (NMMO- 16% water), cellulose fibers, such as ramie, cotton and wood were dissolving rapidly by fragmentation without significant swelling. At higher water concentration (e.g., NMMO- 18-20% water), ramie fibers exhibited a heterogeneous swelling: a ballooning phenomenon was observed in localized places. In the case of wood and cotton fibers, the ballooning was well defined and the difference of swelling with ramie fiber was attributed to a difference of organization of the cellulose microfibrils within the various species. After the removal of the swelling agent, they observed that the initial ramie fibers were converted into an unoriented cellulose II crystalline structure. Chanzy et al. called this region “irreversible swelling”. With more water (e.g., NMMO- 20% and more), ramie fibers were sometimes having both cellulose I and II after the removal of the swelling agent but most of the time the cellulose I structure of the cellulose crystals was preserved. When water amounts were above 28%, no visible change was observed which thus corresponds to a region of non-activity. The most important result in this study is that by changing the water amount from 16% to 20 % w/w, an important transition from a dissolution by fragmentation of cellulose to irreversible swelling by ballooning was observed. In addition, pronounced 138 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Identification of the three zones of a wood fiber swollen by ballooning in NMMO- 22% water. differences were found in the crystalline structures of the fibers as a function of the amount of water in the solvent mixture. Based on the preceding study, Cuissinat and Navard (11, 12) performed observations by optical microscopy of free-floating fibers between two glass plates for a wide range of solvent quality. They identified four main dissolution modes for wood and cotton fibers as a function of the quality of the solvent, in NMMO: • Mode 1: Fast dissolution by fragmentation in good quality solvent (below 17% water w/w). • Mode 2: Swelling by ballooning and full dissolution in moderate quality solvent (from 18 to 24 % water w/w). • Mode 3: Swelling by ballooning and no complete dissolution in bad quality solvent (from 25 to 30 % water w/w). • Mode 4: Low homogeneous swelling and no dissolution in non solvent (above 35 % water w/w). These different dissolution mechanisms are summarized in Figure 2. These mechanisms have been also observed with ionic liquids solvent (15) and for a wide range of plant fibers (16) and some cellulose derivatives if the derivatization occurred without dissolution (17). These different studies point out the strong influence of the morphological structure in the dissolution mechanisms. If the original wall structure of the native fiber is preserved, the dissolution mechanisms are similar for wood, cotton, other plant fibers and some cellulose derivatives, the solvent quality driving the type of mechanism that will occur for a given fiber type. In the studies cited above, the roles of the different levels of the cellulose structure in the swelling and dissolutions mechanisms were not well established. By controlling the quality of the solvent, the dissolution conditions and the fiber sources, it is possible to reveal the characteristic mechanisms of the swelling and dissolution at the different length scales of the cellulose structure from the cell walls to the macromolecular chains. This will be discussed in the next paragraphs.

139 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Dissolution Mechanisms of Native Cellulose Fibers


Dissolution of a Solid Polymer Common synthetic polymers are dissolving in several steps (18, 19) as illustrated in Figure 3. When the polymer is placed in contact with the solvent (A), the solvent swells the solid phase that goes above the glass transition Tg (Tg decreases with increasing concentration of solvent) (B). The swelling increases up to the point of disentanglement (C). Finally, the polymer chains can move out of the swollen phase to the solvent phase (D) and the solubilization is completed (E). The dissolution indeed occurs from the outside to the inside of the solid polymer. Mechanisms are somewhat similar for amorphous and crystalline zones and the main differences are seen in terms of kinetics.

Dissolution of Native Cellulose Fibers Influence of the Cell Wall Structure We demonstrated recently (20) that the mechanism is inversed for native cellulose fibers. In fact, by studying the dissolution of raw cotton fibers, Gossypium barbadense, at different growth stages in varying solvent quality, we showed that the primary wall, the secondary S1 wall and S2 wall that are arranged concentrically could behave very differently depending on the solvent strength. When the solvent is very good, i.e. NMMO- 18 % water (w/w) and below, the whole fiber fragmentates and dissolves fast. When placed in a moderate quality solvent (NMMO- 20% water w/w), the cotton fibers are dissolving from the inside to the outside following the sequence illustrated in Figure 4, thus revealing a gradient of dissolution capacity. Same mechanisms have been observed for wood pulp fibers. • (A) The solvent penetrates inside the fiber • (B) The S2 wall dissolves by fragmentation • (C) The S1 wall swells under the pressure of the dissolving S2 wall • (D) The primary wall breaks to form collars (called unswollen sections in (11, 12, 15–17)) and eventually helices

Influence of the Chemical Environment of the Cellulose Chains The gradient in dissolution capacity of successively deposited cell wall layers described above has to be related with the specific composite structure of cellulose fibers. The primary wall is composed of many components. Cellulose and hemicelluloses are present as well as pectins and proteins (21) while the secondary S2 wall contains almost only cellulose and hemicelluloses. These components were shown by dynamic FTIR spectroscopy experiments to be strongly linked together (22). As described by Klemm et al. (23), the cellulose microfibrils are also differently arranged and compacted within the structure 140 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. The four different swelling and dissolution mechanisms of wood pulp? and cotton fibers as a function of the solvent quality.

Figure 3. Dissolution steps of a solid synthetic polymer in a solvent.

Figure 4. Dissolution of a raw cotton fiber, Gossypium barbadense, in NMMO - 20% water w/w (left); Description of the dissolution steps of a cellulose fiber in a moderate quality solvent (right).



depending on the considered wall. The higher amount of non-cellulosic materials and the structural organisation in older wall layers, i.e. the outside walls, may explain their lower dissolution capacity. Independently of solvent systems, this explanation pre-supposes that some of the non-cellulosic components as the hemicelluloses are still bound to cellulose after pulping and could impede the dissolution. This assumption is supported by recent results (24). We found by a selective separation by centrifugation of insoluble and soluble cellulose fractions in NaOHwater mixtures and further analyses by size exclusion chromatography a large superposition of their molar mass distributions meaning that a fraction of short cellulose chains do not dissolve while a fraction of longer ones do. It indicates that some cellulose chains are less accessible than others and embedded in regions difficult to dissolve. As shown on Figure 5, the carbohydrate composition analysis of the different fractions reveals that soluble fractions contains a lower amount of mannan than the insoluble ones. Beyond thermodynamic considerations, the dissolution capacity of cellulose chains is thus very dependent of their localization in the cell wall structure and the chemical environment around the cellulose chains, as the hemicelluloses matrix, should be considered as a key parameter for the dissolution efficiency.

Influence of the Chain Mobility As pointed out by Isogai and Attala (25), “the disruption of long range order present in solid cellulose samples may be viewed as the key” to improve cellulose dissolution. This statement was recently highlighted by dissolution experiments in moderate quality solvent where it was shown that a uniaxial elongational stress is able to prevent full dissolution or decrease the efficiency of a chemical derivatization (24, 26). Even if the solvent has a full access to the cellulose chains, the chains must perform conformational conversions to dissolve that are limited if the whole chains is blocked into a long range fixed H-bond network. The acetylation of Lyocell fibers without tension leads to degree of substitution (DS) values up to 1.3 while, under tension, DS values are in a much lower range of 0.2 to 0.5 (26).

Swollen Morphologies Observed by Scanning Electron Microscopy In the preceding paragraph, the mechanisms of swelling and dissolution were described in details from the morphological scale by means of optical microscopy to the macromolecular scale by means of molar mass measurements and carbohydrate composition analyses. However, optical microscopic observations do not show a complete visualization of the surface and volume of the swollen morphologies. Since, the more contrasted behaviour are seen when the cellulose



Figure 5. Comparison of the amount of mannan in the insoluble and soluble fractions of a steam exploded softwood (spruce) sulphite pulp and a microcrystalline cellulose after dissolution in NaOH-water mixtures. fibers are swelling by ballooning, we will investigate in these paragraph the possibility to observe ballooned fibers by scanning electron microscopy.

Experimental Protocol The swelling experiments were performed on bleached cotton fibers (Mn = 262,900 g/mol ; Mw = 606,300 g/mol determined by gel permeation chromatography). While observed by optical microscopy in transmission mode with a Metallux 3 (Leitz) equipped with a Linkam TMS 91 hot stage, fibers were swollen in NMMO- 22% water (w/w) at 90°C between two glass plates up to the formation of balloons and distilled water was then injected by capillarity to stop the dissolution process. Swollen fibers were then extracted from the two glass plates and deposited on a sample holder covered with a carbon tape. The sample holder was directly introduced on the stage of the scanning electron microscope (Philips FEI XL30 ESEM with LaB6 gun). Swollen fibers were observed in environmental mode. The environmental mode preserves humidity and prevents drying which could induce modifications of the fibers (pore or cavity closure, plastic deformation). Humidity level must be carefully monitored since too much humidity is obstructing the observation (only water layers are then observed). The best conditions were the following: pressure 5.5 mbar, acceleration voltage 15 keV and relative humidity 30%.

Results and Discussion In the dried state, some grooves can be seen at the surface of the bleached cotton (Figure 6 left). Upon swelling, the grooves previously observed are deeper (Figure 6 right) and the fiber looses its kidney shape (27, 28) to take a regular cylinder shape that exhibits swelling ratio of 1.5 (defined as the initial diameter over the swollen diameter). 143 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


After this first low regular swelling, ballooning occurs beginning by small excrescences scattered along the fibre length to finally give fully developed balloons with swelling ratio up to 5.3 (Figure 7). As can be seen, the longitudinal grooves are present on the cylindrical sections linked to the balloons (see insert in Figure 7), while they are not observable on the surface of the balloons. On the contrary, the surface of the balloons is very smooth (it can be surrounded by some remaining crystals of NMMO) and the outside wall with grooves is not present anymore. As previously described, the ballooning is thought to occur by (i) the breaking of the primary wall, i.e. the outside wall with grooves, and (ii) the swelling of the S1 wall that contains the dissolved S2 wall. It is very clear that the interface between primary wall and S1 layer, which is the surface of the balloon seen on figure 7 is very smooth and regular, without any remaining parts of the primary wall. The transition in terms of structure and composition from the primary wall to the S1 wall is very sharp, as seen with the neat splitting of these two walls. This shows that there should be a very clear change in biosynthesis at the primary wall-S1 transition, with a rather low adhesion between these two layers. The very smooth surface of balloons can be either an image of reality, i.e. a very smooth interface between primary wall and S1, or due to the high swelling. Based on these microscopic observations, the cohesion between the primary wall and the S1 wall can thus be considered as rather weak. It has to be emphasized that the distilled water injected to stop the dissolution process may have slightly modified the macroscopic morphology of the ballooned fibers due the cellulose I - cellulose II transition. After several minutes, the electron beam leads to a strong degradation of the sample that prevents any further observation.

Diffusion of the Cellulose Chains upon Dissolution As previously described, the dissolution of cellulose fibers in NMMO- 20% water occurs first by a ballooning stage. We know that during this process, cellulose is dissolved inside the balloon, but nothing is known about the possibility to have already cellulose chains escaping from the balloons. One way to investigate this point is to send a regenerating medium like distilled water during the dissolution and to look if it is possible to see the traces of precipitated cellulose outside the balloon.

Experimental Protocol We achieved the partial dissolution of bleached cotton fiber (Mn = 262,900 g/mol ; Mw = 606,300 g/mol determined by gel permeation chromatography) in NMMO- 20% water (w/w) at 90°C between two glass plates. No convection was applied to the system. Several droplets of water were then injected by capillarity during the dissolution after the beginning of the ballooning stage. The additional water dilutes the solvent and precipitates the dissolved cellulose chains that can 144 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 6. Bleached cotton fiber in the dried state (left) ; Bleached cotton fiber in the swollen state before that the ballooning occurs (right).

Figure 7. First excrescence along the fiber (top left) ; Fully developed balloons surrounded by some crystals of NMMO (top right and bottom). then be easily observed. The samples were investigated in transmission mode by optical microscopy with a Metallux 3 (Leitz) equipped with a Linkam TMS 91 hot stage and the data were recorded with a high resolution 3-CCD camera (1360*1024 pixels) JVC KY-F75U.



Figure 8. Precipitation of the dissolved cellulose during the dissolution of a bleached cotton fiber in NMMO - 20 % water. Tiny cellulose gel droplets can be observed around the partially dissolved fibers.

Results and Discussion The precipitation of dissolved cellulose in NMMO-water by the addition of distilled water generates tiny cellulose gel droplets of nearly spherical shape around the dissolving fibers with diameter ranging from 1 to 10 µm (see insert in Figure 8). It is not possible to firmly establish by which mechanism the phase separation occurred, either spinodal decomposition or nucleation and growth. The fact that neighbouring droplets are of the same size, aligned along strings and sometimes attached suggests that the mechanism is a spinodal decomposition. This is supported by several studies made on cellulose objects regenerated from NMMO solutions (29–31) or during regeneration by NMR (32). The possibility to visualize dissolved cellulose when precipitated enables to analyse its distribution around the dissolving cotton fiber. As expected, the 146 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


concentration of precipitated cellulose is higher in fully dissolved regions (zones 1 and 2). However, it is intriguing to observe the presence of smaller concentration of precipitated cellulose around ballooned regions as well as around unswollen regions (zones 3 and 4 respectively). Although dissolution begins by the inside of the fiber with the fragmentation of the S2 wall, these observations show that some dissolved cellulose chains are able to leave the dissolving cellulose fiber during the early stage of swelling and dissolution even before ballooning occurred. Nothing is known about the molar mass distribution of these chains. They may come from the surface of the fiber or most likely from the inside of the fiber meaning that the outside walls, i.e. the primary wall and the S1 wall, are porous enough to enable the diffusion of the dissolved cellulose chains even in the non-swollen state.

Conclusions Native cellulose fibers exhibit complex dissolution mechanisms that have to be related to their specific structure. Contrary to classical semi-crystalline polymers, which can be considered as binary system with an amorphous and a crystalline phase, native cellulose fibers are composed of several layers varying in terms of structure and composition. Consequently, the behaviours can be rather different depending on the zones of the structure considered. Despite advanced knowledge in the chemical modification and dissolution of cellulose fibers, the swelling and dissolution of cellulose still remain a complex scientific field and a breakthrough should be made by the understanding and the integration of all the biosynthesis processes.

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On the Specific Behaviour of Native Cellulose Fibers upon Dissolution

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