Development and Properties of Nanocrystalline Cellulose - ACS

Chapter 12

Development and Properties of Nanocrystalline Cellulose

Downloaded by CORNELL UNIV on June 16, 2012 | http://pubs.acs.org Publication Date (Web): July 11, 2011 | doi: 10.1021/bk-2011-1067.ch012

Wadood Y. Hamad*, Principal Scientist FPInnovations, 3800 Wesbrook Mall, Vancouver, BC, Canada V6S 2L9 *[email protected]

Nanocrystalline cellulose (NCC) is extracted as a colloidal suspension by acid hydrolysis of cellulosic materials, such as bacteria, cotton, and wood pulp. NCC has unique properties which make it an interesting starting point for the development of new materials. It is constituted of cellulose, a linear polymer of β(1→4) linked D-glucose units, the chains of which arrange themselves to form crystalline and amorphous domains. Colloidal suspensions of cellulose nanocrystals form chiral nematic structures upon reaching a critical concentration. The cholesteric structure consists of stacked planes of molecules aligned along a director (n), with the orientation of each director rotated about the perpendicular axis from one plane to the next. This structure forms spontaneously in solutions of rigid, rod-like molecules, and when the particles involved are optically active, chiral nematic structures may be formed.

1. Introduction In order to reinvigorate the forestry sector and pulp manufacture for the 21st century, we must venture into evolutionary developmental processes aiming to re-engineer and custom design the industry’s primary raw material, the lignocellulosic fibers, and profit from their unique properties. The competitiveness of forestry materials rests on associating product development with the concepts of fiber engineering and selective design, by using new technical tools to manipulate and re-structure fibers (and their constituents) at a scale as small as scientifically possible in order to add functionality. Functional materials based on lignocellulosics have the potential to compete with other established ones, e.g. © 2011 American Chemical Society In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


plastics, as well as metals and metal alloys, not only on performance, but also on merits of recyclability, biodiversity, biodegradability, sustainability and being a renewable resource. Nanoscience, where physics, chemistry, biology and materials science converge, deals with the manipulation and characterization of matter at molecular to micron scale. Nanotechnology is the emerging engineering discipline that applies nanoscience to create products. Because of their size, nanomaterials have the ability to impart novel and/or significantly improved physical (strength, stiffness, abrasion, thermal), chemical (catalytic, ion-exchange, membranes), biological (anti-microbial, compatibility) and electronic (optical, electrical, magnetic) properties. While the chemistry and physics of simple atoms and molecules is fairly well understood, predictable and no longer considered overly complex, serious attempts to bridge across the length scales from nano to macro remain a major challenge, and will occupy researchers and scientists for years ahead. The term nanotechnology embraces a broad range of science and technology working at a length scale of approximately 1 to 100 nanometers to manipulate atoms and molecules in order to create useful structures. The many successes that are currently attributed to nanotechnology have in fact been the result of years of research into conventional fields like materials or colloids sciences, and some of today’s nanotechnology products are even mundane, when compared to (unchallenged) popular perceptions: stain-resistant trousers, better sun cream and tennis rackets reinforced with carbon nanotubes. Lignocellulosic fibres are built from nanofibrils comprising a number of chain molecules in close alignment and lie parallel to one another (see Figure 1) with cross-sectional dimensions in the nanometer range (1). These nanofibrils comprise crystallites linked by amorphous areas (Figure 2a). A cellulose crystallite is essentially an aggregation of cellulose molecular chains of relatively low flexibility which tend to aggregate in parallel bundles with the desirable consequence of their axial physical properties approaching those of perfect crystals (2). The size, shape and orderliness of the crystallite regions play a predominant role as far as such fibre properties as tensile strength, density, rigidity, swelling and heat-sensitivity are concerned. The less ordered areas and the ratio of such amorphous areas to the crystalline regions are more influential in controlling extensibility. One of the most important properties of cellulose, a linear, long-chain polymer of 1,4-linked, β-D-glucose, is the tendency to form fibrils and then fibres. The biosynthesis of cellulose in wood and plants proceeds by the progressive inclusion of glucose units into long, slender, monocrystalline fibrillar structures, within which the cellulose molecules are arranged parallel to one another with a twofold screw symmetry along their length (3). These fibrils consist of cellulose chains stabilized laterally by hydrogen bonds between the hydroxyl groups and oxygen of adjacent molecules (4). Depending on their origin, the diameter of the fibrils ranges from about 2 to 20 nm, and their lengths several tens of microns. As they are devoid of chain folding (5) and contain only a smaller number of defects (6), fibrils have a high elastic modulus close to that of a perfect cellulose crystal – estimated to be 150 GPa (7) – and a strength that should be on the order 302 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


of 10 GPa. The structural defects that do exist allow the transverse cleavage of the fibrils into short nanocrystals, or nanofibrils, under acid hydrolysis.

Figure 1. The ultrastructure of lignocellulosic fibres (1).

Figure 2. Schematic presentation of the crystalline structure of cellulose (adapted from ref (2)).

303 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 3. FE-SEM (left) and STEM (right) images of NCC film and suspension, respectively. The left image clearly depicts the chiral nematic orientation of the NCC crystals within each layer of dried NCC film. The image on the right illustrates the typical dimensions of NCC crystals in dilute water suspension. Both stubs were coated with AuPd and examined in high vacuum mode. Extraction is central to further developing and processing cellulose nanocrystals into functional, high value-added materials, and as such several techniques are available. A reliable recipe for their production with uniform size, shape and charge distribution is necessary; however, it is challenging. The success of any approach would be measured by two essential factors: (1) The feasibility of cost-effective commercial scale-up, and (2) the manner in which yield of the cellulose nanocrystals may be maximized. Typically, two primary steps comprise the process of extracting cellulose nanofibrils from wood (8): (1) a controlled chemical pretreatment to destroy the molecular bonds whereby crystalline nanofibrils are hinged together in a network structure; and (2) appropriate use of mechanical energy to disperse substantial quantities of unhinged nanofibrils in the aqueous phase. Employing acid hydrolysis, typically using concentrated mineral acids, such as sulfuric or hydrochloric, individual crystallites are prepared from wood pulp (9, 10) or cotton (11, 12) – each having a different extent of cellulose purity: cotton-seed fluff is composed of ≤ 94% cellulose, and wood contains ≤ 55%. The preparation procedure begins with initial acid action to remove the polysaccharide material closely bonded to the microfibrillar surface, resulting in an overall decrease in amorphous material. Subsequent hydrolysis breaks down those portions of the long glucose chains in accessible, non-crystalline regions. A levelling-off degree of polymerization (90 ≤ DP ≤ 100) is achieved: this corresponds to the residual highly crystalline regions of the original cellulose fibre. When this level is reached, hydrolysis is terminated by rapid dilution of the acid. A combination of centrifugation and extensive dialysis is then employed to fully remove the acid, and a brief sonication completes the process to disperse the individual particles of cellulose and yield an aqueous suspension, as shown in Figure 3. The cellulose rods that remain after this treatment are almost entirely crystalline (crystallinity ≥ 85%) and as such are termed crystallites. 304 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

The precise physical dimensions of the crystallites depend on several factors, including the source of the cellulose, the exact hydrolysis conditions, and ionic strength. Besides, complications in size heterogeneity are inevitable owing to the diffusion-controlled nature of the acid hydrolysis. Typical figures for crystallites derived from different species vary: (3 – 5) x 180 ± 75 nm for bleached softwood Kraft pulp (13), 7 x (100 – 300) nm for cotton (12), and 20 x (100 – 2000) nm for Valonia (14). The high axial (length to width) ratio of the rods is important for the determination of anisotropic phase formation (9, 12).


2. Fundamentals of NCC Extraction The production of colloidal suspensions of cellulose by sulfuric acid hydrolysis of cellulose fibres was first proposed by Rånby (8) over half a century ago. Rånby’s work (15) suggested that cotton and wood cellulose were built up from micellar strings of uniform width (~7 nm), and could be set free by mechanical treatment such as ultrasonic waves. Hydrolysis was viewed as cutting the micellar strings into short fragments, or micelles, while retaining their width. Rånby proposed that, in cellulose synthesis, micelles rather than molecules may represent the primary structural elements. This theory was subsequently refined by Frey-Wyssling’s experimental observations (3), which provided evidence that elementary microfibrils, separated (and joined) by paracrystalline cellulose, represent the basic building units (16). Within the same decade, development of the acid degradation of cellulose by (17) led to the commercialization of microcrystalline cellulose (MCC). MCC is cellulose which has been degraded to a degree of polymerization where there is little further decrease (the levelled off DP). This degradation can be achieved by either mechanical disintegration or by the hydrolysis of purified cellulose after 15 minutes in 2.5 N HCl at 105 ± 1°C (18). Because of its useful characteristics, including zero toxicity, good hygroscopicity, chemical inactivity and reversible absorbency (19, 20), MCC, derived from high-quality wood pulp, has found wide use in pharmaceuticals as a tablet excipient and in food production (owing to its properties as a stabilizer, texturizing agent and fat-replacer). Hydrolysis with sulfuric acid can result in the introduction of sulfate esters at the surface of the cellulose crystallites, leading to added electrostatic stabilization of the suspensions; and, at sufficiently high concentrations of such suspensions, birefringent, ordered, liquid phases may be observed (21). Subsequent work on carefully sulfuric acid-hydrolyzed and purified samples demonstrated that a chiral, nematic-ordered phase formed above a critical concentration (9). The phase-forming ability of a cellulose crystallite suspension depends on the mineral acid chosen for the initial hydrolysis. Use of either sulfuric or phosphoric acid yields a chiral nematic phase, but hydrochloric acid will not because it cannot form the required ester at the crystallite surface. The viscous suspension from hydrochloric acid hydrolysis can form a birefringent, glassy phase after a “post-sulfation” treatment (22). Further, Nishiyama (23) showed that evaporation under shear of suspensions of cellulose crystallites (from the green alga Cladophora sp.) gave a film of highly oriented uniaxial structure. 305 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Using acid hydrolysis, suspensions of cellulose crystallites have been prepared from a variety of sources, including bacterial cellulose (24), microcrystalline cellulose (25), sugar beat primary wall cellulose (26, 27), cotton (12)), tunicate cellulose (28–38), softwood pulp (9, 10, 13), hardwood pulp (eucalyptus) (39), and wheat straw (40, 41). The precise physical dimensions of the crystallites depend on several factors, including the source of the cellulose, the exact hydrolysis conditions, and ionic strength of the suspension. Size heterogeneity is inevitable owing to the diffusion-controlled nature of the acid hydrolysis. Typical figures for crystallites derived from different species vary: (3 – 5) x 180 ± 75 nm for bleached softwood kraft pulp (13, 37), 7 x (100 – 300) nm for cotton (12, 42), and 20 x (100 – 2000) nm for Valonia (14), (5 – 10) nm x (100 nm – several μm) for bacterial cellulose (24), and (10 – 20) nm x (100 nm – several μm) for tunicate cellulose (32). The high axial (length to width) ratio of the crystallites is important for anisotropic phase formation (9, 23). 2.1. Effects of Hydrolysis Conditions and Sulfation on NCC Yield Sulfuric acid hydrolysis of kraft pulp fibres is a heterogeneous process involving the diffusion of the acid into the pulp fibres, the cleavage of the glycosidic bond in cellulose, the possible sulfation of cellulosic hydroxyl groups (i.e. conversion of cellulose-OH to cellulose-OSO3H), and other side-reactions such as dehydration or oxidation of the cellulose. Hemicelluloses, if present in the pulp fibres, will also undergo similar reactions but at a faster rate because of their higher reactivity. As the hydrolysis proceeds, the degree of polymerization (DP) of the cellulose molecules is expected to decrease while the crystallinity of the extracted, H2O-insoluble cellulose materials is expected to increase because of the accessibility and selective hydrolysis of the amorphous regions of the cellulose molecules. The yield of the extracted, H2O-insoluble cellulose materials is also known to decrease. However, how the yield, one key factor in determining the economics of the hydrolysis process, is related to the extent of sulfation and/or the DP of the cellulose has not been studied or discussed in the literature. Figure 4 shows the data of the percentage yield vs. DP of the extracted, H2O-insoluble cellulose materials from the commercial, softwood (60% western red cedar and 40% mix of spruce, pine and fir or SPF) kraft pulp under various hydrolysis conditions (Hamad and Hu, 2010). At 16 wt.% acid concentration, the DP decreased significantly from ~680 to 180 as the hydrolysis temperature was raised from 45 to 85 °C, but the yield decreased only slightly from 93.7 to 91.5%. This result indicates that, at this acid concentration and over the temperature range studied, the hydrolysis of the cellulose molecules which likely occurred selectively on the amorphous regions did not proceed to the extent where the hydrolyzed cellulose molecules had sufficiently low DP and/or sufficiently high degree of sulfation to become soluble in the hydrolysis medium. A spruce sulphite dissolving pulp with a DP of 160 could be made completely H2O-soluble at ≥0.20-0.25 sulfate subsitution (i.e., ≥ 20-25 sulfate groups per 100 anhydroglucose units) achieved via a 2-step silylation-sulfation procedure (43). The yield loss of ~6-8% for the hydrolysis with 16 wt.% acid was due likely to the preferential hydrolysis and dissolution of the hemicelluloses from the pulp. 306 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 4. Yield (%) vs. DP of the extracted, H2O-insoluble cellulose materials from the hydrolysis of the softwood kraft pulp (DP = 1178) at various sulfuric acid concentrations and temperatures for 25 min (after ref (10)). At 40 wt.% acid concentration, the DP also decreased significantly from 280 to ~120 and the yield dropped only slightly when the temperature was increased from 45 to 65 °C (Figure 4). However, when the temperature was raised from 65 to 85°C, there was a relatively small decrease in DP from ~120 to 90 accompanied by a significant drop in the yield from 87.4 to 69.2%. This result indicates that when the more-difficult-to-hydrolyze, H2O-insoluble, crystalline cellulose material reaches a DP of 90, the “easier-to-hydrolyze” amorphous cellulose material has a sufficiently low DP and/or high sulfation to dissolve. Because no sulfation was detected on the H2O-insoluble material (Table I), the yield drop from 87.4 to 69.2% was likely due to lowered DP of the amorphous cellulose material at 40 wt.% acid concentration and 85 °C. Interestingly, at 64 wt.% acid concentration, even though the DP values of the extracted cellulose materials (DP = 90-96) were similar to those of the extracted material from the hydrolysis at 40 wt.% acid concentration and 85 °C (DP = 90), the yields were much lower (20.9-32.9% vs. 69.2%). It is possible that more sulfate (-OSO3H) groups were introduced to (the surfaces of) both the crystalline and amorphous cellulose materials at 64 wt.% concentration than at 40 wt.% acid concentration. The greater the level of sulfation of the cellulose materials, the more H2O-soluble the materials will become at similar DPs and the lower the yields of the extracted, H2O-insoluble materials will be. Indeed, this was found to be the case by elemental analysis (Table I). Hydrolysis with 64 wt.% acid introduced 4.7-6.7 sulfate groups per 100 anhydroglucose units to the extracted, H2O-insoluble cellulose materials, while hydrolysis with 40 wt.% acid did not introduce any detectable sulfate 307 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

groups (Table I). These results illustrate that sulfation plays a significant role in determining the yield of the extracted, H2O-insoluble cellulose materials within the 20 to 70% yield range.


2.2. Hydrolysis Reproducibility and Hydrolysis Time Effects Table II lists the yields, elemental analysis data and DPs of the extracted cellulose materials for experiments on softwood kraft pulp using 64 wt.% sulfuric acid at 65 °C for 25, 15, and 5 minutes, respectively. Good reproducibility is evident for the yields of the extracted materials (c.v. = 1.8%), as well as elemental analysis (c.v. = 5.8%), for hydrolysis runs performed at 64 wt% acid and 65°C for 25 min. Decreasing the hydrolysis time from 25 to 15 min did not produce, statistically, any difference in the degree of sulfation or the yield. However, decreasing the hydrolysis time further to 5 min did give a slightly higher yield accompanied by a lower degree of sulfation (Table II). The coefficient of variation for all DP and intrinsic viscosity determinations are inherently higher than those for elemental analysis or yield but do not affect the interpretation of the results.

Table I. Elemental analysis data,a calculated sulfate group/anhydroglucose unitb, yield and degree of polymerization (DP) of the extracted cellulose materials from hydrolysis of softwood kraft pulp at various sulfuric acid concentrations and temperatures for 25 minutes (10) Acid (wt.%)

Temp. (°C)

C (%)

H (%)

S (%)

-OSO3H/ 100 glucose units (n)

16 16 16 40 40 40 64 64 64

45 65 85 45 65 85 45 65 85

42.16 41.97 42.44 42.33 42.31 40.78 37.35 43.37

5.83 5.77 5.98 6.09 6.13 5.78 5.95 5.63

Development and Properties of Nanocrystalline Cellulose - ACS

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