Review of Recent Research into Cellulosic Whiskers, Their properties and their application in nanocomposites field

Review of Recent Research into Cellulosic Whiskers, Their properties and their...

(Parte 1 de 5)


Review of Recent Research into Cellulosic Whiskers, Their Properties and Their Application in Nanocomposite Field

My Ahmed Said Azizi Samir,†,‡,⊥ Fannie Alloin,‡ and Alain Dufresne*,§

Centre de Recherches sur les Macromolecules Vegetales (CERMAV-CNRS), Universite Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France, Laboratoire d’Electrochimie et de Physico-chimie des Materiaux et des Interfaces, (LEPMI-INPG), BP 75, F38402 St Martin d’Heres Cedex, France, and Ecole Francüaise de Papeterie et des Industries Graphiques (EFPG-INPG), BP 65, F38402 St Martin d’Heres Cedex, France

Received October 6, 2004; Revised Manuscript Received December 6, 2004

There are numerous examples where animals or plants synthesize extracellular high-performance skeletal biocompositesconsistingof a matrix reinforcedby fibrous biopolymers.Cellulose,the world’s most abundant natural, renewable, biodegradable polymer, is a classical example of these reinforcing elements, which occur as whiskerlike microfibrils that are biosynthesized and deposited in a continuous fashion. In many cases, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free, with the consequence of axial physical properties approaching those of perfect crystals. This quite “primitive” polymer can be used to create high performance nanocomposites presenting outstanding properties. This reinforcing capability results from the intrinsicchemical nature of celluloseand from its hierarchicalstructure.Aqueous suspensions of cellulose crystallites can be prepared by acid hydrolysis of cellulose. The object of this treatment is to dissolve away regions of low lateral order so that the water-insoluble, highly crystalline residue may be converted into a stable suspension by subsequent vigorous mechanical shearing action. During the past decade,many works have been devotedto mimic biocompositesby blendingcellulosewhiskersfrom different sources with polymer matrixes.

1. Introduction

Natural fibers are pervasive throughout the world in plants such as grasses, reeds, stalks, and woody vegetation. They are also referred to as cellulosic fibers, related to the main chemical component cellulose, or as lignocellulosic fibers, since the fibers usually often also contain a natural polyphenolic polymer, lignin, in their structure. Results suggest that these agro-based fibers are a viable alternative to inorganic/ mineral based reinforcing fibers in commodity fiberthermoplastic composite materials as long as the right processing conditions are used and for applications where higher water absorption may be not so critical. The use of lignocellulosic fibers derived from annually renewable resources as a reinforcing phase in polymeric matrix composites provides positive environmental benefits with respect to ultimate disposability and raw material use.1 Compared to inorganic fillers, the main advantages of lignocellulosics are listed below:

¥ renewable nature ¥ wide variety of fillers available throughout the world

¥ nonfood agricultural based economy

¥ low energy consumption

¥ low cost

¥ low density

¥ high specific strength and modulus

¥ high sound attenuation of lignocellulosic based composites

¥ comparatively easy processability due to their nonabrasive nature, which allows high filling levels, resulting in significant cost savings

¥ relativelyreactivesurface,which can be used for grafting specific groups.

In addition,the recyclingby combustionof lignocellulosics filled composites is easier in comparison with inorganic fillers systems. Therefore, the possibility of using lignocellulosic fillers in the plastic industryhas receivedconsiderable interest. Automotive applications display strong promise for naturalfiber reinforcements.2-5 Potentialapplicationsof agrofiber based composites in railways, aircraft, irrigation systems, furniture industries, and sports and leisure items are currently being researched.6

Despite these attractive properties, lignocellulosic fillers are used only to a limited extent in industrial practice due

* To whomcorrespondenceshouldbe addressed.E-mail: Alain.Dufresne@ Fax: 3 (4) 76 82 69 3. Tel: 3 (4) 76 82 69 95. † CERMAV-CNRS. ‡ LEPMI-INPG. § EFPG-INPG. ⊥ Present address: Division of Lightweight Structures, Department of

Aeronautical and Vehicle Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden.

10.1021/bm0493685 C: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005 to difficulties associated with surface interactions. The inherent polar and hydrophilic nature of polysaccharidesand the nonpolar characteristics of most of the thermoplastics result in difficulties in compounding the filler and the matrix and, therefore, in achieving acceptable dispersion levels, which results in inefficient composites. This hydrogen bonding is best exemplified in paper where these secondary interactions provide the basis of its mechanical strength.

Moreover, the processing temperature of composites is restricted to about 200 °C because lignocellulosic materials start to degrade near 230 °C. This limits the type of thermoplastics that can be used in association with polysaccharide fillers to commodity plastics such as polyethylene, polypropylene, poly(vinyl chloride), and polystyrene. However, it is worth noting that these lower-price plastics constitute about 70% of the total amount of thermoplastics consumed by the plastics industry.

Another drawback of lignocellulosic fillers is their high moisture absorption and the resulting swelling and decrease in mechanical properties. Moisture absorbance and corresponding dimensional changes can be largely prevented if the hydrophilic filler is thoroughly encapsulated in a hydrophobic polymer matrix and there is good adhesion between both components. However, if the adhesion level between the filler and the matrix is not good enough, a diffusion pathway can preexist or can be created under mechanical solicitation. The existence of such a pathway is also related to the filler connection and therefore to its percolation threshold.

Stable aqueous suspensions of cellulose monocrystals can be prepared by acid hydrolysis of the biomass. Throughout the paper, different descriptors of the cellulosic colloidal particles will be used, including whiskers, monocrystals, nanocrystals, microcrystalline cellulose, and cellulose crystallites. Depending on the source, these nanocrystals offer a wide variety of aspect ratios (L/d, L being the length and d the diameter), from almost particulate fillers (L/d ) 1) to about 100. These suspensions display a colloidal behavior. They can be used to process nanocomposite materials using a polymer as the matrix. Nanocomposites are a relatively new class of composites that exhibit ultrafine phase dimensions of 1-1000 nm. Because of the nanometric size effect, these composites have some unique outstanding properties with respect to their conventional microcomposite counterparts. The first study was published in 1990 by researchers at Toyota (Japan) and dealt with montmorillonite clay reinforcedpolyamide6.7 Thermoplasticpelletswere obtained by polymerizing caprolactam with clay.

As most of the present-day polymers used for preparing nanocomposites are synthetic materials, their processability, biocompatibility,and biodegradabilityare much more limited than those of natural polymers. Compared to the studies in the field of conventional microcomposites and nanocomposites based on synthetic nonbiodegradable materials, only limited work has been reported in the area of bionanocomposites. Another advantage of the natural nanofillers is their availability and their resulting lower cost in comparison to synthetic nanofillers.

As reportedabove, one of the restrictionsin lignocellulosic based systems is the difficulty in achieving acceptable dispersion levels of the filler within the polymeric matrix. An alternativeway to palliatethis restrictionconsistsof using a latex, i.e., an aqueous suspension of polymer particles, or a water soluble polymer to form the matrix. From the former, it is well-known that emulsion polymerization can lead to materials easily processed, either by film casting techniques (water evaporation for paint applications for example) or by freeze-drying (or more simply by flocculation) followed by classical extrusion process. As a matter of fact, different monomers can be statistically copolymerized to adjust the glass-rubber transition temperature at a given value. More generally, it is also possible to mix different types of water suspensions including some polymer lattices and organic or inorganicstabilizedsuspensions.We will see that, in addition to some practical applications, the study of these systems can help us to understand some physical properties as geometric and mechanical percolation effects.

Whiskers are fibers which have been grown under controlledconditionsthat lead to the formationof high-purity single crystals.8 They constitute a generic class of materials having mechanical strengths equivalent to the binding forces of adjacent atoms. The resultant highly ordered structure produces not only unusually high strengths but also significant changes in electrical, optical, magnetic, ferromagnetic, dielectric, conductive, and even superconductive properties. The tensilestrengthpropertiesof whiskersare far abovethose of the current high-volume content reinforcementsand allow the processing of the highest attainable composite strengths.

The first references to the existence of definite crystalline zones interposed in the amorphous structure of cellulose materials were made by Nageli and Schwendener,9 who in 1877 already confirmed the optical anisotropy of vegetable products both in cell walls and in fibers.

Commercial forms of microcrystalline cellulose as stable thixotropic-gel systems involving aqueous colloidal dispersions consist of a hydrolyzed level-off degree of polymerization (DP) at high-solids concentration. They were first described by Battista and Smith in a patent10 issued in 1961. This patent, along with a later publication11 and book,12 described a combination of two characteristic prerequisites for producing colloidal phenomena from a fibrous high polymer such as cellulose: (i) a controlled chemical pretreatment to destroy the molecular bonds whereby microcrystals are hinged together in a network structure and (i) an appropriate use of mechanical energy to disperse a sufficient amount of the unhinged microcrystals in the aqueous phase to produce the characteristic rheological behaviorand the smooth fat-likespreadabilityof the resulting colloidal microcrystalline cellulose gels. It was clearly demonstrated that stable gel systems were obtained only when the mechanical energy was introduced into an aqueous suspension of level-off DP cellulose in which the total solids concentration was of the order of 5% or more and only if the mechanical energy was severe enough to liberate a minimum number of monocrystals to make a stable gel possible.

Recent Research into Cellulosic Whiskers Biomacromolecules, Vol. 6, No. 2, 2005 613

The present paper is to provide a review of the literature on cellulose whiskers, their properties, and their possible use as a reinforcing phase in nanocomposite applications. Its scope is mainly limited to the mechanical effects of incorporatingcellulosewhiskersin polymercompositessince the phase separation phenomena and chiral nematic texture of cellulose whisker suspensions was recently reviewed.13

2. Structure and Polymorphism of Cellulose:

Cellulose, the most widespread biopolymer, is known to occur in a wide variety of living species from the worlds of plants, animals, and bacteria as well as some amoebas. In many of these, the main function of cellulose is to act as a reinforcementmaterial. This is, for instance, the case in plant cells where the osmotic pressure of the inner cell has to be counterbalanced by the tight winding of the cellulose within the cell wall. It has been estimated that globally between 1010 and 1011 tons of cellulose are synthesized and also destroyed each year.14

The structure and the morphology of cellulose have been the subject of a large amount of work. It is a polydisperse linear polymer of poly-â(1,4)-D-glucose residues (Figure 1). The monomers are linked together by condensationsuch that the sugar rings are joined by glycosidic oxygen bridges. In nature, cellulose chains have a DP of approximately 10 0 glucopyranose units in wood cellulose and 15 0 in native cellulose cotton.15 However, chain lengths of such large, insolublemoleculesare difficultto measure,due to enzymatic and mechanical degradation which may occur during analysis. A specific characteristic of lignocellulosic compounds is the high density of hydroxyl groups which provides the hydrophilic nature of these materials. Cellulose displays six different polymorphs, namely I, I,

I, II,I VI, and IVII with the possibility of conversion from one form to another.16 For a long time, native cellulose

(cellulose I) attracted the interest of a large scientific community in attempt to elucidate its crystal structure. In 1934, Meyer and Misch17 proposed a monoclinic unit cell containingtwo antiparallelchainsas a model for the cellulose crystal. Different models were proposed latter because of the Meyer-Misch model insufficiency to explain experimentaldata obtainedby other teams.18-20 The most important factors that can explain the historic controversy in the cellulose crystal structure are the dependenceof the cellulose structure on the origin of investigated cellulose and the influence of the experimental investigation conditions.21,2

From cross polarization/magic angle spinning 13C nuclear magneticresonance(13C CP/MASNMR)experiments,Attala and VanderHart23,24 proposed that native cellulose was a composite of two crystalline forms, namely a one-chain triclinic structure IR and a two-chain monoclinic structure Iâ. This model was supported by electronic diffraction study of native cellulose from algal cell walls25 and by computa- tional prediction.26 The fractions of IR and Iâ phases in any nativecellulosesamplesdependon the originof the cellulose. The IR phase predominatesfor example in Valonia cellulose, whereas the Iâ fraction prevails in cotton cellulose and it is close to unity in tunicin (the cellulose from tunicate,27 a sea animal).Detailsof the crystallinestructureof these two forms were reported by Kono et al.28 using 13C NMR technique and by Nishiyama et al.29,30 using synchrotron X-ray and neutron fiber diffraction. The estimation of the phase’s composition of native cellulose is possible using different techniques such as FT-IR,31 13C NMR,32,3 and synchrotronradiated X-ray diffraction.34 The IR phase is a meta-stable form which can be converted to the more stable Iâ form by annealing in a different medium.35,36

In addition to the crystalline phases, native cellulose contains disordered domains which can be considered like amorphous. Native cellulose may be assigned to a semicrystalline fibrillar material. The presence of disordered phases was supported by experimental results from 13C CP/MAS/ NMR,37 tensile tests of cellulose fibers,38 and wide-angle (WAXS)39 and small-angle X-ray scattering (SAXS).40

Cellulosechains are biosynthesizedby enzymes,deposited in a continuous fashion and aggregate to form microfibrils, long threadlike bundles of molecules stabilized laterally by hydrogen bonds between hydroxyl groups and oxygens of adjacent molecules. Thus, all of the chains in one microfibril would have to be elongated during biosynthesis at the same rate. This extended chain conformation and microfibrillar morphology result in a significant load-carrying capability. Depending on their origin, the microfibril diameters range from about 2 to 20 nm for lengths that can reach several tens of microns. As they are devoid of chain folding and contain only a small number of defects, each microfibril can be considered as a string of cellulose crystals, linked along the microfibrilby amorphousdomains and having a modulus close to that of the perfect crystal of native cellulose (estimatedto be around 150 GPa41) and a strengththat should be in the order of 10 GPa. This modulus value is similar to that of aramid fibers (Kevlar). Microfibrils aggregate further to form cellulose fibers (Figure 2).42 Therefore, natural fibers are themselves composite materials. They result from the assembling of microfibrils embedded in a matrix mainly composed of lignin.

However, discussion about the nature of the amorphous phases and their role in the final morphology of native cellulose is not yet closed. Using SAXS experiments, Muller

Figure 1. Chemical structure of cellulose.

Figure 2. Modified Frey-Wyssling model (Sarko and Marchessault, 1989)42 showing the cross-sectionalstructure of a cellulose microfibril composed of six elementary fibrils 40 40 Å in width.

614 Biomacromolecules, Vol. 6, No. 2, 2005 Azizi Samir et al.

et al.43 reported the parallel alignment of cellulose microfibrils to the fiber axis. Results from inelastic neutron scattering experiments44 suggest that the major part of the disordered chains can be attributed to surface molecules of the microfibrils. However, these disordered chains retain a preferential orientation parallel to the microfiber axis. A molecular model of amorphous cellulose phase was recently proposed,45 and some physical propertiessuch as density and glass transition temperature were estimated. However, important work is necessary for the future comprehension of properties of surface disordered phase in native cellulose.

3. Preparation of Cellulosic Whiskers Suspensions and Their Properties

The amorphous regions act as structural defects and are responsible for the transverse cleavage of the microfibrils into short monocrystals under acid hydrolysis.46,47 This procedure can be used to prepare highly crystalline particles called microcrystalline cellulose (MCC).48 The preparation of such colloidal aqueous suspensions of cellulosic whiskers is described in detail elsewhere.49,50 Under controlled conditions, this transformation consists of the disruption of amorphous regions surrounding and embedded within cellulose microfibrils while leaving the microcrystalline segments intact. It is ascribed to the faster hydrolysis kinetics of amorphous domains compared to crystalline ones. MCC is a naturally occurring substance, and it has proven to be stable and physiologicallyinert. This material (a well-known commercial name is Avicel) presents a high potential for applicationin pharmaceutical(tablet binder), food (rheologycontrol agent), paper, and composites manufacturing.

MCC consists generally of a stiff rodlike particle called whiskers. Geometrical characteristics of cellulose whiskers depend on the origin of cellulose microfibrils and acid hydrolysis process conditions such as time, temperature, and purity of materials. The most studied cellulose sources were: valonia,51 cotton,52 wood pulp,53 and sugar-beet pulp.54,5 Figure 3 shows transmission electron micrographs (TEM) obtained from dilute suspensions of cotton, sugar-beet pulp, and tunicin (the cellulose extracted from tunicate) whiskers. The length and lateral dimension are around 200 nm and 50 Å and 1 ím and 150 Å for cotton and tunicin whiskers, respectively. Terech et al.56 used small-angle (neutron and X-ray) scattering techniques to determine the precise shape of tunicin whiskers. They demonstrated that the crosssectional shape of these rigid whiskers was rectangular with a calculated value close to 8 182 Å.

Dong et al.57 studied the effect of preparation conditions (time, temperature,and ultrasoundtreatment)on the resulting cellulosemicrocrystalsstructurefrom sulfuricacid hydrolysis of cotton fiber. They reported a decrease in MCC length and an increase in their surface charge with prolonged hydrolysis time. Characterization of cellulose whiskers were performed using different techniques such as transmission electron microscopy (TEM),57-59 X-ray and neutron diffraction,60 NMR,54,28 and atomic force microscopy (AFM).61,62

MCC is insoluble in common solvents. However, it leads to the formation of colloidal suspensions when suspended in water (Figure 4). The stability of these suspensions depends on the dimensions of the dispersed particles, their size polydispersity and surface charge. The use of sulfuric acid for cellulose whiskers preparation leads to more stable whiskers aqueous suspension than that prepared using hydrochloric acid.63 Indeed, the H2SO4-prepared whiskers present a negatively charged surface, whereas the HCl- prepared whiskers are not charged. Another way to achieve charged whiskers consists of the oxidation of the whiskers surface64,65 or the post-sulfation of HCl-prepared MCC.6

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