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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 Macromolécules Végétales (CERMAV-CNRS), Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France, Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et des Interfaces, (LEPMI-INPG), BP 75, F38402 St Martin d’Hères Cedex, France, and Ecole Française
de Papeterie et des Industries Graphiques (EFPG-INPG), BP 65, F38402 St Martin d’Hères 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 biocomposites consisting of a matrix reinforced by 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 intrinsic chemical nature of cellulose and from its hierarchical structure. 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 devoted to mimic biocomposites by blending cellulose whiskers from different sources with polymer matrixes.
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 polyphe- nolic polymer, lignin, in their structure. Results suggest that these agro-based fibers are a viable alternative to inorganic/ mineral based reinforcing fibers in commodity fiber- thermoplastic 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 com-
posites • comparatively easy processability due to their nonabra-
sive nature, which allows high filling levels, resulting in significant cost savings
• relatively reactive surface, which can be used for grafting specific groups.
In addition, the recycling by combustion of lignocellulosics filled composites is easier in comparison with inorganic fillers systems. Therefore, the possibility of using lignocel- lulosic fillers in the plastic industry has received considerable interest. Automotive applications display strong promise for natural fiber reinforcements.2-5 Potential applications of agro- fiber 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 whom correspondence should be addressed. E-mail: Alain.Dufresne@ efpg.inpg.fr. Fax: 33 (4) 76 82 69 33. Tel: 33 (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.
612 Biomacromolecules 2005, 6, 612-626
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to difficulties associated with surface interactions. The inherent polar and hydrophilic nature of polysaccharides and 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 polysac- charide fillers to commodity plastics such as polyethylene, polypropylene, poly(vinyl chloride), and polystyrene. How- ever, 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 corre- sponding 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 crys- tallites. 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 dimen- sions of 1-1000 nm. Because of the nanometric size effect, these composites have some unique outstanding properties with respect to their conventional microcomposite counter- parts. The first study was published in 1990 by researchers at Toyota (Japan) and dealt with montmorillonite clay reinforced polyamide 6.7 Thermoplastic pellets were obtained by polymerizing caprolactam with clay.
As most of the present-day polymers used for preparing nanocomposites are synthetic materials, their processability, biocompatibility, and biodegradability are much more limited than those of natural polymers. Compared to the studies in the field of conventional microcomposites and nanocom- posites based on synthetic nonbiodegradable materials, only limited work has been reported in the area of bionanocom- posites. Another advantage of the natural nanofillers is their availability and their resulting lower cost in comparison to synthetic nanofillers.
As reported above, one of the restrictions in lignocellulosic based systems is the difficulty in achieving acceptable dispersion levels of the filler within the polymeric matrix. An alternative way to palliate this restriction consists of 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 inorganic stabilized suspensions. 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 controlled conditions that lead to the formation of 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 signifi- cant changes in electrical, optical, magnetic, ferromagnetic, dielectric, conductive, and even superconductive properties. The tensile strength properties of whiskers are far above those of the current high-volume content reinforcements and 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 disper- sions consist of a hydrolyzed level-off degree of polymer- ization (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 pre- treatment to destroy the molecular bonds whereby micro- crystals are hinged together in a network structure and (ii) an appropriate use of mechanical energy to disperse a sufficient amount of the unhinged microcrystals in the aqueous phase to produce the characteristic rheological behavior and the smooth fat-like spreadability of 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.
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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 incorporating cellulose whiskers in polymer composites since 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 reinforcement material. 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 condensation such that the sugar rings are joined by glycosidic oxygen bridges. In nature, cellulose chains have a DP of approximately 10 000 glucopyranose units in wood cellulose and 15 000 in native cellulose cotton.15 However, chain lengths of such large, insoluble molecules are difficult to measure, due to enzymatic and mechanical degradation which may occur during analy- sis. 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, II, IIII, IIIII, IVI, 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 containing two antiparallel chains as a model for the cellulose crystal. Different models were proposed latter because of the Meyer-Misch model insufficiency to explain experi- mental data obtained by other teams.18-20 The most important factors that can explain the historic controversy in the cellulose crystal structure are the dependence of the cellulose structure on the origin of investigated cellulose and the influence of the experimental investigation conditions.21,22
From cross polarization/magic angle spinning 13C nuclear magnetic resonance (13C CP/MAS NMR) 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 native cellulose samples depend on the origin of the cellulose. The IR phase predominates for 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). Details of the crystalline structure of 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,32,33 and synchrotron- radiated 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 semicrys- talline 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
Cellulose chains are biosynthesized by 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 microfibril by amorphous domains and having a modulus close to that of the perfect crystal of native cellulose (estimated to be around 150 GPa41) and a strength that 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-sectional structure of a cellulose microfibril composed of six elementary fibrils 40 × 40 Å in width.
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et al.43 reported the parallel alignment of cellulose mi- crofibrils 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 properties such as density and glass transition temperature were estimated. However, im- portant 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 condi- tions, this transformation consists of the disruption of amorphous regions surrounding and embedded within cel- lulose microfibrils while leaving the microcrystalline seg- ments 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 physiologically inert. This material (a well-known commercial name is Avicel) presents a high potential for application in pharmaceutical (tablet binder), food (rheology- control 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.54,55
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 µm 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 cross- sectional shape of these rigid whiskers was rectangular with a calculated value close to 88 × 182 Å.
Dong et al.57 studied the effect of preparation conditions (time, temperature, and ultrasound treatment) on the resulting cellulose microcrystals structure from sulfuric acid 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.66
The aqueous suspensions stability of these surface charged whiskers has an electrostatic origin. Dispersion of MCC in
Figure 3. Transmission electron micrograph from a dilute suspension of hydrolyzed (a) cotton, (b) sugar-beet pulp and (c) tunicin.
Figure 4. Photograph of an aqueous suspension of tunicin whiskers observed between cross nicols, showing the formation of birefringent domains.
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low polarity solvents was made possible recently thanks to coating or chemical modification of the MCC surface. Coating of cotton and tunicin whiskers by a surfactant such as Beycostat NA (BNA) was found to lead to stable suspensions in toluene and cyclohexane.67,68 Goussé et al.69
stabilized tunicin microcrystals in tetrahydrofuran (THF) by a partial silylation of their surface. Araki et al.64 prepared original sterically stable aqueous and nonaqueous MCC suspensions by grafting monoamino oligooxyethylene mono- methyl ether at the MCC surface using a carboxylation- amidation procedure. In a recent publication,70 we have shown that it is possible to obtain stable whiskers suspensions in dimethylformamide (DMF) without either addition of a surfactant or any chemical modification. This should allow avoiding the use of water as the solvent for composites polymer electrolytes processing.
In 1959, Marchessault et al.50 reported the birefringent character of acid-treated cellulose and chitin crystallites. The birefringence of MCC aqueous suspensions can be observed through a pair of cross-nicols. This birefringence results from two origins: (i) a structural form anisotropy of cellulose (∆n ≈ 0.05) and (ii) a flow anisotropy resulting from alignment of MCC under flow generally operated before observation. Shear induced alignment of cotton whiskers in colloidal aqueous suspension was studied by Ebeling et al.71
using SAXS experiments. They demonstrated that this alignment is shear rate-dependent, and it is completely reversible. They suggest the presence of planar domains of randomly oriented microcrystals which align at low shear rates and are broken up at higher shear rates enabling alignment of the individual whiskers. The whiskers appeared to be randomly aligned prior to shearing and are horizontally aligned along the shear direction when the shear rate exceeds 5 s-1. Ordering of diluted suspensions of cellulose whiskers was evidenced using static and dynamic light scattering by de Souza Lima and Borsali.72 These authors observed numerous peaks in the scattered intensity, which were interpreted in terms of electrostatic repulsive interactions between whiskers. These interactions are of long-range order, and the arrangement of whiskers was supposed to be in cylindrical/hexagonal packing. Addition of salt to the suspension induces destruction of this order and leads to whiskers flocculation, demonstrating the role of the electro- static interactions between whiskers to control both stability and order.
As predict by Onsager,73 the cellulose whiskers can undergo an orientational disorder-order phase transition from a disordered isotropic phase to an orientationally ordered phase. Whiskers point with equal probability in every direction in the isotropic phase. However, they cluster around a preferred direction in the anisotropic phase. Suspensions divided in isotropic and anisotropic phases when a critical concentration was reached.74 The phase transition depends on the geometrical axial ratio of whiskers,73 their surface charge,74,75 and their length polydispersity.76 Dong et al.77
reported an increase in the critical concentration at the phase transition in the presence of counterions. The latter influence also the stability of the cellulose whisker suspensions and modify the temperature dependence of the phase separation.
Cellulose whiskers are interesting model systems for the study of shear dynamics of rigid rods in aqueous suspension. Bercea and Navard78 reported the rheological comportment of tunicin whiskers aqueous suspensions. They distinguished two different behaviors according to whiskers concentration. In the isotropic phase (c < 0.8 wt %) where whiskers are randomly oriented, the decrease of the viscosity in relation to shear rate increase can be explained by whiskers align- ment. In the anisotropic phases (c > 0.8 wt %), the behavior is similar to that of liquid crystal polymers with a weak shear thinning region surrounded by two others shear thinning regions.
Araki et al.63 studying H2SO4 and HCl-prepared cellulose whiskers demonstrated that the charge of the surface is one of the main parameter which control the interwhisker interactions and so the rheological behavior of their suspen- sions. The suspensions of charged whiskers showed no time dependence in viscosity. However, the uncharged HCl- prepared whisker suspensions were thixotropic at concentra- tions above 0.5 wt % and antithixotropic below 0.3 wt %.
One of the most interesting characteristic of MCC aqueous suspensions consists of their self-organization into stable chiral nematic phases. Whiskers align in the same direction to form nematic planes. The later are stacked such that the angle of the director in each subsequent plan is offset incrementally. It was suggested that the chiral ordering origins from geometrical twists in the microcrystals and/or helical distribution of their surface charge.53,79,80 Evidence of the role of the twisted whiskers’ geometry in chirality was reported when the chiral nematic order was obtained for uncharged whiskers sterically stabilizated.64,68 Revol et al.53 observed this chiral ordering of MCC aqueous suspen- sions for various cellulose origins such as wood, cotton, ramie, and bacterial cellulose. They also reported the conservation of this structure after water evaporation. Sugi- yama et al.81 reported the possibility to align cellulosic whiskers and form a highly ordered cellulosic film. It was achieved by drying diluted aqueous suspensions of tunicin whiskers at room temperature under a strong homogeneous 7T magnetic field. Resulting films were examined using polarized optical microscopy and TEM. The microcrystal axis orientation perpendicular to the magnetic field direction was confirmed by electron and X-ray diffraction. The anisotropic magnetic susceptibility of the individual C-C, C-O, C-H, and O-H bonds and their relative orientation in the crystal were suggested to originate from the magnetic orientation. However, a negligible role in alignment was attributed to the ester groups at the whiskers surface that result from the sulfuric acid hydrolysis.
Using this process to orient cellulose microcrystals, Revol et al.82,83 created an important new material with original optical properties. Solidified liquid crystals of cellulose with the capability to reflect colored light were prepared from wood pulp microcrystals suspensions. Optical properties of these materials depend on both the cellulose origin and the preparation conditions. The wavelength of reflected light can be controlled by adjusting the ionic strength of the suspen- sion. These new materials have a high application potential like those for security papers.
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4. Cellulosic Whiskers in Nanocomposite Applications
Since the first announcement of using cellulose whiskers as a reinforcing phase by Favier et al.,84 new nanocomposite materials with original properties were obtained by physical incorporation of cellulose whiskers into a polymeric matrix. Like for any multiphase materials, the properties of these cellulosic nanocomposites depend on that of the two constituents, namely whiskers and matrix in addition to morphological aspects and their interfacial comportments (whiskers/matrix interactions). Both natural and synthetic polymers were explored as the matrix. Natural polymers such as poly (â-hydroxyoctanoate) (PHO),85-87 starch,88 silk fibroin,89 and cellulose acetate butyrate (CAB)90 reinforced with cellulose whiskers were reported in the literature. Poly- (styrene-co-butyl acrylate) (poly(S-co-BuA)),91,92 poly(vinyl chloride) (PVC),93 polypropylene,94 waterborne epoxy,95 and poly(oxyethylene)96 were used as synthetic matrixes.
Processing techniques have an important incidence on the final properties of the composites. These techniques are conditioned by both intrinsic properties of whiskers and polymeric matrix (solubility, dispersibility, and degradation) and the desired final properties such as geometrical shape. Water is the preferred processing medium because of the high stability of aqueous cellulose whisker dispersions and the expected high level of dispersion of the filler within the host matrix in the resulting composite film. Therefore, this restricts the choice of the matrix to hydrosoluble polymers. The use of aqueous dispersed polymers, i.e., latexes, is a first alternative, which allows the use of hydrophobic polymers as a matrix and ensures a good level of dispersion of the filler indispensable for homogeneous composites processing. A second alternative consists of dispersing cellulose whiskers in an adequate (with regard to matrix) organic medium. This was made possible by the coating whiskers surface with a surfactant67 or by chemically modifying their surface.64,69,97 The use of a surfactant is the simplest method. However, the high amount of surfactant required to coat this high specific surface filler (four times higher than that of whiskers68) prohibits the use of this technique in composite applications. The surface chemical modification of cellulose whiskers is another way to disperse cellulose whiskers in organic solvents. It generally involves reactive hydroxyl groups from the surface of the whis- kers.59,64,69 However, the mechanical performances of the resulting composites were found to strongly decrease after chemical modification as reported for chitin whiskers from crab shell.98 Recently, it was shown70 that tunicin whiskers could be dispersed in DMF without additives or any surface modifications. This result opens the way for using some hydrophobic polymers as a matrix in addition to allowing additional possible chemical modifications of whiskers especially those incompatible with the presence of water.
The first step in composites processing consists of mixing the aqueous whiskers suspension and the dispersed or solubilized polymer matrix. Homogeneous suspensions were obtained by classical magnetic stirring at room temperature or by using an autoclave reactor for mixing at high temperatures. The suspensions were generally degassed under vacuum to remove air. Three different techniques were used
to prepare composites films: (i) by casting on Teflon or propylene dishes followed by water evaporation at moderate temperatures, (ii) by freeze-drying and hot-pressing, or (iii) by freeze-drying, extruding, and hot-pressing the mixture. Composite films were also obtained by cross-linking in situ an unsaturated matrix after solvent casting using a thermal or photo cross-linking agent.96,99
Experimental Investigation of Nanocomposite Films. Morphological Characterizations. Simple naked eye exami- nation of the film’s surface can prove the existence of remaining air bubbles or the inhomogeneous aspect. This is generally the case when either the degassing was not efficient or the evaporation temperature was too high. The problem can be complicated when the matrix is in the latex form. Great intention must be made for the choice of both time and temperature conditions during water evaporation with respect to the latex particles coalescence process.
Scanning electron microscopy (SEM) is generally em- ployed for the more extensive morphological inspection. It consists of the observation of fractured surface films at liquid nitrogen temperature. This technique allows for conclusions about the homogeneity of the composite, presence of voids, dispersion level of the whiskers within the continuous matrix, presence of aggregates, sedimentation, and possible orienta- tion of whiskers. By comparing the micrographs showing the surface of fracture of the unfilled matrix and of composites, cellulose whiskers can be easily identified. In fact, they appear like white dots. Their concentration is a direct function of the cellulose composition in the composite. These shiny dots correspond to the transversal sections of the cellulose whiskers. Their diameter determined by SEM microscopy is much higher than the whiskers diameter. This results from a charge concentration effect due to the emergence of cellulose whiskers from the observed surface.
The topological dispersion of whiskers in the nanocom- posite film strongly depends on the processing technique and conditions. A SEM comparison between either cast and evaporated or freeze-dried and subsequently hot-pressed composites based on a copolymer of styrene and butyl acrylate (poly(S-co-BuA)) matrix filled with wheat straw whiskers92 demonstrated that the former were less homoge- neous and displayed a gradient of whiskers concentration between the upper and lower faces of the composite film. This sedimentation phenomenon was confirmed using WAXS by comparing the diffracted X-ray beams by the two faces. It was suggested that this observation was most probably induced by the processing technique itself. Favier100 reported a large exploration of processing effects on the morphology of tunicin whiskers filled poly(S-co-BuA) composites. She used different techniques such as SEM, WAXS, MET, and polarized light microscopy. The latter technique showed homogeneity of the birefringent character of cast/evaporated films and the existence of different colored domains in hot- pressed and extruded/hot-pressed composites. From the different experimental techniques, it was concluded that the casting/evaporation technique results in the more homoge- neous films, where the whiskers have a tendency to orient randomly into horizontal plans. Heterogeneous composites with numerous locally oriented domains of whiskers were
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obtained by the other techniques. Small angle neutron scattering (SANS) is another way to check the dispersion of the whiskers into the matrix. This technique was used by Chazeau101 to conclude that there was an isotropic dispersion of tunicate whiskers in plasticized poly(vinyl chloride) and a lack of any aggregates.
Thermal Properties. The glass-rubber transition temper- ature, Tg, of cellulose whisker filled polymer composites is an important parameter, which controls different properties of the resulting composite such as its mechanical behavior, matrix chains dynamics, and swelling behavior. Differential scanning calorimetry (DSC) and dynamic mechanical analy- sis (DMA) can be used to evaluate Tg of polymers and composites. In DSC experiments, Tg is generally taken as the inflection point of the specific heat increment at the glass-rubber transition. From DMA tests, not a transition but a relaxation process is evidenced in this temperature range. The temperature position of this relaxation process (TR) depends on both Tg and the frequency of the measure- ment. Its value can be taken as the temperature at the maximum peak of the internal friction factor (tan δ ) E′′/E′) or the loss modulus (E′′), where E′ corresponds to the storage tensile modulus. In shear solicitation, E′ and E′′ are replaced by the storage shear modulus, G′, and the loss shear modulus, G′′, respectively.
For tunicin whisker filled composites, no modification of Tg values was reported when increasing the amount of whiskers, regardless the nature of the polymeric ma- trix.84,88,93,100,102 Similar results were reported for bacterial90
and wheat straw92 cellulose whisker based composites. Hajji et al.102 demonstrated that in addition to their independence on the filler content, Tg’s of tunicin whiskers/poly(S-co-BuA) composites are independent of the processing conditions. In glycerol plasticized starch (amylopectin) based composites, Anglès and Dufresne88 reported peculiar effects of tunicin whiskers on the Tg of the amylopectin-rich fraction depending on moisture conditions. For a low loading level (up to 3.2 wt %), a classical plasticization effect of water was reported. However, an antiplasticization phenomenon was observed for higher whiskers content (6.2 wt % and up). These observations were discussed according to the possible interactions between hydroxyl groups on the cellulosic surface and amylopectin, the selective partitioning of glycerol and water in the bulk amylopectin matrix or at the whiskers surface, and the restriction of amorphous amylopectin chains mobility in the vicinity of the amylopectin crystallite coated filler surface. For tunicin whiskers/sorbitol plasticized starch,103
Tg’s were found to increase slightly up to about 15 wt % whiskers and to decrease for higher whiskers loading. Crystallization of amylopectin chains upon whiskers addition and migration of sorbitol molecules to the amorphous domains were proposed to explain the observed modifica- tions.
In semicrystalline matrix based nanocomposites, the melt- ing temperature (Tm) and heat of fusion (∆Hm) of the thermoplastic matrix were determined from DSC measure- ments. X-ray diffraction was also used as a technique to elucidate the eventual modifications in the crystalline structure of the matrix after the addition of the whiskers.
It was reported that Tm values were nearly independent of the filler content in plasticized starch88,103 and in poly- (oxyethylene) (POE)104 filled with tunicin whiskers. The same observation was reported for CAB reinforced with native bacterial cellulose whiskers.90 For the CAB matrix, Tm values were found to increase when the amount of trimethylsilylated whiskers arises. The authors ascribed this difference to the stronger filler-matrix interaction in the case of chemically modified whiskers.
A significant increase in crystallinity of sorbitol plasticized starch103 was reported when the tunicin whiskers content was increased. This phenomenon was ascribed to an anchoring effect of the cellulosic filler, probably acting as a nucleating agent. The same effect was reported for POE based com- posites104 and isotactic polypropylene (iPP) filled with cellulosic whiskers coated with a surfactant.94 In these systems, the nucleating effect was supported by optical polarized light observations of composites during their isothermal crystallization.
When whiskers are coated with surfactant, they are very good nucleating agents for iPP. On the contrary, untreated whiskers do not modify the crystallization of iPP. When whiskers are grafted by maleated iPP, cellulose acts like an antinucleating agent.94
It seems that the nucleating effect of cellulosic whiskers is mainly governed by surface chemical considerations. Indeed, Bonini94 did not observe any nucleating effect of the iPP matrix when using either untreated or grafted with anhydride maleic iPP tunicin whiskers. Grunert and Winter90
assumed from DSC measurements that native bacterial fillers impede the crystallization of the CAB matrix, whereas silylated ones help to nucleate the crystallization.
A transcrystallization phenomenon was reported for tunicin whiskers filled semicrystalline matrixes such as PHO86 and glycerol plasticized starch.88 It consists on a preferential crystallization of the amorphous polymeric matrix chains during cooling at the surface of the whiskers. For glycerol plasticized starch based systems, the formation of the transcrystalline zone around the whiskers was assumed to be due to the accumulation of plasticizer in the cellulose/ amylopectin interfacial zones improving the ability of amylopectin chains to crystallize. These specific crystalliza- tion conditions were evidenced at high moisture content and high whiskers content (>16.7 wt %) by DSC and WAXS. It was displayed through a shoulder on the low-temperature side of the melting endotherm and the observation of a new peak in the X-ray diffraction pattern. This transcrystalline zone could originate from a glycerol-starch V structure. In addition, the inherent restricted mobility of amylopectin chains was put forward to explain the lower water uptake of cellulose/starch composites for increasing filler content.
Noishiki et al.89 studied silk fibroin/tunicin whisker composites using IR spectroscopy. The silk fibroin was prepared from silk cocoon dissolved in LiSCN and com- posites were obtained by solution casting. The authors reported a conformational change of fibroin chains from a random coil to an ordered structure. This change seems to be related to the highly ordered surface of cellulosic whiskers.
618 Biomacromolecules, Vol. 6, No. 2, 2005 Azizi Samir et al.
Mechanical Properties. In the past 10 years, a great interest was focused on investigating the use of cellulose micro- crystals as a reinforcing phase in a polymeric matrix, evaluating the mechanical properties of the resulting com- posites, and elucidating the origin of the mechanical rein- forcing effect. These investigations were performed in the nonlinear range (classical tensile tests) and mainly in the linear range (DMA). The latter allows the determination of the mechanical behavior of the materials in a broad temper- ature/frequency range and is strongly sensitive to the morphology of the composite.
Favier et al.84,100 reported the first demonstration of the reinforcing effect of cellulose whiskers. The authors mea- sured by DMA in the shear mode a spectacular improvement in the storage modulus after adding tunicin whiskers into a poly(S-co-BuA) matrix. This increase was especially sig- nificant above the glass-rubber transition temperature. Figure 5 shows the isochronal evolution of the logarithm of the relative storage shear modulus (log G′T/G′200, where G′200 corresponds to the experimental value measured at 200 K) at 1 Hz as a function of temperature for such composites prepared by water evaporation. In the rubbery state of the thermoplastic matrix, the modulus of the composite with a loading level as low as 6 wt % is more than 2 orders of magnitude higher than the one of the unfilled matrix. Moreover, the introduction of 3 wt % or more cellulosic whiskers provides an outstanding thermal stability of the matrix modulus up to the temperature at which cellulose starts to degrade (500 K).
The reinforcing effect of tunicin whiskers was also evidenced from tensile tests. Incorporation of these cellulosic whiskers results in an improvement of both the elastic modulus and the tensile strength of the composites.102,105,106
The macroscopic behavior of cellulosic whisker based composites depends as for any heterogeneous materials, on the specific behavior of each phase, the composition (volume fraction of each phase), the morphology (spatial arrangement of the phases), and the interfacial properties. Three main parameters were reported to affect the mechanical properties of such materials:
(i) The geometrical aspect ratio (L/d, L being the length and d the diameter of the rodlike particle): this factor is linked
to the origin of the cellulose used and whisker preparation conditions. Fillers with a high aspect ratio give the best reinforcing effect. Indeed, Favier100 showed that the highest modulus increase in the rubbery state of the poly(S-co-BuA) matrix and thermal stability were obtained with tunicin whiskers (L/d ∼ 67) in comparison with bacterial (L/d ∼ 60) or Avicel whiskers (L/d ∼ 10). It is worth noting that, for these three different origins, the crystallinity of the raw material is directly linked to the aspect ratio of the nano- crystals. This demonstrates that the origin of the starting raw lignocellulosic material governs the resulting reinforcing effect via its degree of crystallinity.
Sakurada and Nukushina41 estimated the modulus of the native cellulose perfect crystal to 150 GPa and Eichhorn and Young107 reported a decrease of cellulosic whiskers’s Young modulus when their crystallinity decreases. Dubief et al.85
reported a different mechanical behavior of composites based on amorphous PHO when reinforced with tunicin or starch microcrystals. More than 10 wt % of starch microcrystals (with an aspect ratio close to unity) was needed to achieve a reinforcing effect comparable to that provided by only 1 wt % of tunicin whiskers.
(ii) The processing method: as described above, morpho- logical characteristics of cellulosic whisker based composites depend on the processing technique. Water evaporation seems to give the highest mechanical performance materials compared to freeze-drying/molding and freeze-drying/extrud- ing/molding techniques.100 This difference was suggested to be due to the predominance of the whisker/whisker interac- tions and their contribution to the overall reinforcing effect in the evaporated films in relation to homogeneous fillers dispersion into the matrix and the sedimentation phenom- enon.108
A similar effect of processing techniques was reported for chitin whisker filled poly(caprolactone)109 and natural rub- ber.110 Hajji et al.102 studied the tensile behavior of poly(S- co-BuA)/tunicin whisker composites prepared from different manners. The authors classified processing methods in ascending order of their reinforcement efficiency (tensile modulus and strength): extrusion < hot pressing < evapora- tion. This evolution was associated to probable breakage and/ or orientation of whiskers during processing.
(iii) The matrix structure and the resulting competition between matrix/filler and filler/filler interactions. Classical composite science tends to privilege the former as a condition for optimal performance. In cellulose whisker based com- posite materials, the opposite trend is observed. The higher the affinity between the cellulosic filler and the host matrix is, the lower the mechanical performances are. For instance, the transcrystallization phenomenon reported for semicrys- talline PHO on cellulose whiskers resulted in a disastrous decrease of the mechanical properties (especially above the melting temperature of the matrix) when compared to that obtained for fully amorphous PHO.86 Similar results were reported for plasticized starch reinforced with cellulose whiskers.105 This strong loss of performance demonstrates the event of outstanding importance of the filler/filler interactions to ensure the mechanical stiffness and thermal stability of these composites. This idea was supported by
Figure 5. Logarithm of the normalized storage shear modulus (log G′T/G′200, where G′200 corresponds to the experimental value measured at 200 K) vs temperature at 1 Hz for tunicin whiskers reinforced poly(S-co-BuA) nanocomposite films obtained by water evaporation and filled with 0 (b), 1 (O), 3 (2), 6 (4), and 14 wt % ([) of cellulose whiskers.
Recent Research into Cellulosic Whiskers Biomacromolecules, Vol. 6, No. 2, 2005 619
the decrease of the tensile storage modulus of composite materials filled with bacterial cellulosic whiskers with a silycilated surface.90 A similar loss of mechanical properties was reported for a natural rubber matrix filled with chemi- cally modified and unmodified chitin whisker.98
Mechanical Modeling. To understand the unusual reinforc- ing effect observed for cellulose whisker based composites at T > Tg at such low filler concentrations, comparisons between experimental results and different mechanical ap- proaches have been made. It is worth noting that at a temperature either much lower or much higher than Tg of the host matrix the tensile or shear behavior, as a first approximation, can be considered as fully elastic. In fact, the loss component of the tensile or shear modulus is more than 10 times lower than the elastic one in these temperature ranges. For this reason, we will only consider the elastic behavior in the following.
Mean Field Approach. The theoretical mean field me- chanical model of Halpin-Kardos111 has been extensively used to predict the elastic modulus of short-fiber composites. In this approach, fibers are assumed to be embedded in the matrix to form a homogeneous continuum. The modulus, the mechanical anisotropy, and the geometry of the fibers are accounted for, but one assumes that there is no interaction between the fibers. In particular, the mean-field approach is based on the concept that a material made of short fibers, homogeneously dispersed in a continuous matrix, is me- chanically equivalent to the superposition of four plies. Within each ply, the fibers are parallel to one another, and the mutual orientation of the plies is 0, +45, +90, and -45°. The mechanical properties of each ply can be derived from the micromechanic equations of Halpin-Tsai (“self consis- tent” approach):112
where Eii is the stiffness in the fiber direction of a unidirectional ply, E22 is the stiffness estimate perpendicular to the fiber direction, G12 is the in-plane shear modulus estimate, and VR is the volume fraction of fibers. The subscripts m and f refer to the matrix and the filler, respectively. The geometry of the filler is involved through the êii parameters, where L, l, and e are the length, the width, and the thickness of the fibers, respectively. In our case, l ) e ) d, the diameter of the whiskers, and it follows:111
The engineering constants characteristics of the unidirectional plies are then given by
which leads to the following expressions for the invariant terms The tensile modulus of the “quasi-isotropic” laminate,
the behavior of which is assumed to be close to that of the short fiber composite, is then given by
The Poisson’s ratio ν12 is approximately given by a mixing rule
Predicted modulus values from this model agree generally very well with the experimental data in the glassy state but fail to describe the rubbery modulus.84,85,92,93,102,108 Above Tg of the matrix, the observed reinforcing effect in terms of the increase in the modulus is found to be much higher than possible for a composite only reinforced with discrete fibers. It was concluded that the assumption of “no filler/filler interaction” hypothesis seems to be wrong, despite the fact that this model displayed limits when the stiffness of the reinforcing phase is far above the continuous phase.113
An acceptable fit of experimental data could be obtained by modifying the geometrical (aspect ratio) and/or stiffness (modulus) properties of the cellulosic fillers.84,92 However, the former results from experimental microscopical observa- tions and the latter is not easily questionable. Therefore, the description of the mechanical behavior of cellulosic whisker based composites needs to take into account the topological arrangement of the fillers and their interactions. The high specific surface of whiskers and their numerous hydroxyl groups suggest a hydrogen bonding type for the filler/filler interactions and the formation of a possible fillers network within the host matrix. This led to the idea of a percolation type approach applied to cellulose whiskers reinforced polymeric matrixes.
Percolation Approach. The term percolation for the statistical-geometry model was first introduced in 1957 by Hammersley.114 It is a statistical theory, which can be applied to any system involving a great number of species likely to be connected. The aim of the statistical theory is to forecast the behavior of a noncompletely connected set of objects. By varying the number of connections, this approach allows the transition from a local to an infinite “communication” state to be described. The percolation threshold is defined as the critical volume fraction separating these two states.
Qii ) Eii
1 - ν12ν21 (i ) 1,2)
Q12 ) ν12Q22 ) ν21Q11
Q66 ) G12 (4)
U1 ) 1 8
(3Q11 + 3Q22 + 2Q12 + 4Q66)
U2 ) 1 8
(Q11 + Q22 - 2Q12 + 4Q66) (5)
EC ) 4U2(U1 - U2)
ν12 ) νmVm + νfVR ) Q11 Q22
) (1 + êii)Eiif + êii(1 - VR)Em (1 - VR)Eiif + (êii + VR)Em
(i ) 1,2) (1)
) (1 + VR)Gf + (1 - VR)Gm (1 - VR)Gf + (1 + VR)Gm
ê11 ) 2 L e
) 2L d
ê22 ) 2 l e
) 2 (3)
620 Biomacromolecules, Vol. 6, No. 2, 2005 Azizi Samir et al.
Various parameters, such as particle interactions,115 orienta- tion,116 or aspect ratio117 can modify the value of the percolation threshold.
The use of this approach to describe and predict the mechanical behavior of cellulosic whisker based composites suggests the formation of a rigid network of whiskers which should be responsible for the unusual reinforcing effect observed at high temperatures. The modeling consists of three important steps:
(i) First, the calculation of the percolation threshold (VRc) should be carried out. The volume fraction of whiskers required to achieve geometrical percolation was calculated using a statistical percolation theory for cylindrical shape particles according to their aspect ratio and the effective skeleton of whiskers was estimated.118 The latter corresponds to the infinite length branch of whiskers connecting the sample ends. Favier et al.118 used computer simulation and showed that about 0.75 vol % tunicin whiskers (assuming L/d ) 100) are needed to get a 3-D geometrical percolation. The authors calculated the effective skeleton by eliminating the finite length branches. The following relation was found between the percolation threshold (VRc) and aspect ratio of rodlike particles100
Flandin et al.119 performed DC electrical conductivity measurements to evaluate the tunicin whiskers percolation threshold. In this study, the surface of the cellulosic filler was covered with conductive polypyrrole before incorpora- tion in a latex matrix. The coated whiskers were characterized by SEM. The final sticks have an average length of about 2 µm and a diameter of 160 nm. The percolation threshold was determined to be about 3 vol % in agreement with the percolation theory and experimental data120 for an aspect ratio close to 15. The electrical percolation threshold is generally lower than the geometrical one, which provides the mechan- ical stiffness. The presence of the continuous path can be enough to make system conducting but is not necessary sufficient to ensure the rigidity of the whole system.
(ii) The second step is the estimation of the modulus of the percolating filler network. It is obviously different from the one of individual whiskers and depends on the origin of the cellulose, preparation procedure of the whiskers, and obviously the nature and strength of interwhisker interactions. This modulus can be assumed to be that of a paper sheet for which the hydrogen bonding forces provide the basis of its stiffness. Experimental tensile tests were performed on films prepared from the evaporation of a suspension of cellulose whiskers in a Teflon mold. For tunicin91,100 and wheat straw cellulose whiskers,92 the tensile modulus was around 15 and 6 GPa, respectively. The apparent tensile modulus of a cellulose whisker network can be calculated by a 3-D finite elements simulation.118,121 The linking elements were con- sidered as beams with adjustable stiffness. All of the calculated values were lower than 1 GPa. For link modulus values below 1 GPa, the network modulus was found to increase with increasing whisker concentration and seemed to increase linearly with the link modulus. For a higher
linking modulus, the modulus of the percolating network tends toward the value for totally rigid links.
(iii) The description of the composite requires the use of a model involving three different phases, viz., the matrix, the filler percolating network, and the nonpercolating filler phase. The simplest model consists of two parallel phases, namely, the effective whiskers skeleton and the rest of the sample. Ouali et al.122 extended the classical phenomenologi- cal series-parallel model of Takayanagi123 and proposed a model in which the percolating filler network is set in parallel with a series part composed of the matrix and the nonper- colating filler phase (Figure 6).
In this approach, the elastic tensile modulus Ec of the composite is given by the following equation:
The subscripts S and R refer to the soft and rigid phase. The adjustable parameter, ψ, involved in the Takayanagi et al. model corresponds in the Ouali et al. prediction to the volume fraction of the percolating rigid phase. With b being the critical percolation exponent, ψ can be written as
where b ) 0.4124,125 for a 3D network. This percolation approach was found to fit satisfactory
the experimental data, especially for high filler load- ing.84-86,100,102,108 At high temperatures when the polymeric matrix could be assumed to have a negligible stiffness, the calculated stiffness of the composites is simply the product of that of the percolating fillers network by the volume fraction of percolating fillers phase
Figure 7 shows the comparison between experimental and predicted data from both the Halpin-Kardos model and the percolation approach. In this figure, the logarithm of the relative storage tensile modulus measured at Tg + 50 K (log E′CTg+50K/E′mTg+50K, where E′CTg+50K and E′mTg+50K
VRc ) 0.7 L/d
Figure 6. Schematic representation of the series-parallel model. R and S refer to the rigid (cellulosic filler) and soft (polymeric matrix) phases, respectively, and ψ is the volume fraction of the percolating rigid phase.
Ec ) (1 - 2ψ + ψVR)ESER + (1 - VR)ψER
(1 - VR)ER + (VR - ψ)ES (9)
ψ ) 0 for VR < VRc
ψ ) VR(VR - VRc1 - VRc ) b
for VR > VRc (10)
Ec ) ψER (11)
Recent Research into Cellulosic Whiskers Biomacromolecules, Vol. 6, No. 2, 2005 621
correspond to the value measured at Tg + 50 K for the composite and the matrix, respectively) is plotted as a function of tunicin whiskers content expressed in volume fraction. Experimental data were obtained for tunicin whis- kers reinforced poly(S-co-BuA),100 amorphous PHO,85 and glycerol plasticized starch conditioned in different moisture conditions.105 For poly(S-co-BuA) based systems, the shear modulus values from ref 100 were converted into tensile moduli assuming a mixing rule for the Poisson’s ratio. The various parameters used for plotting Figure 7 are collected in Table 1. For the glycerol plasticized starch based systems, it is worth noting that the matrix appeared as a complex heterogeneous system composed of glycerol-rich domains dispersed in an amylopectin-rich continuous phase. Each phase exhibited its own glass-rubber transition, for which the temperature decreases as the moisture content increases owing to the plasticizing effect of water. The values of Tg reported in Table 1 correspond to the amylopectin-rich domains. The calculated curves are similar whatever the nature of the matrix because the relative moduli are reported.
It is clearly observed in Figure 7 that the calculated curve based on the percolation approach allows accounting for the reinforcing effect in poly(S-co-BuA) based systems. The discrepancy observed between the calculated curve based and the experimental data at low filler content is probably due to the fact that the prediction does not account for the distribution of the whiskers lengths. Above this critical percentage, the calculated curve based on the percolation theory precisely fits the observed E′ values.
On the contrary, calculation underestimates the relaxed modulus of amorphous PHO based composites. The experi- mental data for the PHO based systems are 5 times higher than the predicted one. This discrepancy was ascribed to the latex particle size that could affect the whiskers network formation.85 The latex particle size was around 150 nm and 1 µm for poly(S-co-BuA) and PHO, respectively. Indeed, the particles act as impenetrable domains to whiskers during the film formation due to their high viscosity. Increasing particle size leads to an increase of the excluded volume and to a decrease of the percolation threshold.
For glycerol plasticized starch based systems, the reinforc- ing effect is very low and even lower than the predicted data from the mean field approach. This observation was ex- plained by competitive interactions between the components and by a plasticizer accumulation in the cellulose/amylopec- tin interfacial zone.105 This plasticizer accumulation phe- nomenon, enhanced in moist conditions, could most probably interfere with hydrogen-bonding forces that are likely to hold the percolating cellulose whiskers network within the matrix. The absence of whiskers network formation was also mentioned for PVC based composites.93,101,106 For semicrys- talline PHO reinforced with tunicin whiskers, the cellulosic network was assumed to originate from the formation of interwhisker links through transcrystalline layers grown on the cellulose surface.86 This resulted in a disastrous decrease of the mechanical properties of the composite material as soon as the melt temperature of the matrix was reached. However, a suitable in situ thermal treatment was found to allow restoring the cellulose whiskers network through hydrogen bonding.
The comparison between predicted and experimental data demonstrates the major role of filler/filler interaction in the final mechanical behavior of the composites based on cellulosic whiskers. Moreover, the percolation approach allows accounting for the spectacular thermal stabilization of composite’s modulus observed at high temperatures. This cellulosic network was reported to have no specific effect on the modulus in the glassy state of the matrix.85,102 In the rubbery state of the matrix, nonlinear tensile tests performed on tunicin whisker reinforced poly(S-co-BuA) displayed the progressive destruction of the percolating network.102 Suc- cessive tensile test experiments carried out on nanocomposite materials based on chitin whiskers obtained from crab shell and natural rubber131 help us to understand the existence of a three-dimensional nanocrystals network formed as a result of hydrogen bonding forces. This test consists of stretching the sample up to a certain elongation, then releasing the force, and stretching again the material up to a higher elongation, etcetera. A clear decrease in modulus values of the samples
Figure 7. Logarithm of the relative storage tensile modulus measured at Tg + 50 K (log E′CTg+50K/E′mTg+50K, where E′CTg+50K and E′mTg+50K correspond to the value measured at Tg + 50 K for the composite and the matrix, respectively) vs volume fraction of tunicin whiskers: comparison between the experimental data for poly(S-co-BuA) (b), PHO (O), and glycerol plasticized starch based systems conditioned at 35 (2), 43 (4), 58 ([), and 75% RH (]) and predicted data from the Halpin-Kardos model (s) or from the percolation approach (- - -).
Table 1. Parameters Used for Plotting Figure 7
(g cm-3) νma Tg (K)
poly(S-co-BuA) 1.1 0.5 273 PHO 1.019 0.5 250 starch 35% RH 1.1 0.5 300 starch 43% RH 1.1 0.5 274 starch 58%RH 1.1 0.5 271 starch 75%RH 1.1 0.5 260
tunicin whiskers Ff
(g cm-3) νfb L/dc E11f
1.58 0.3 67 150 15 5
percolation approach vRc
(vol %)e b ER
1 0.4 15
a The matrix being in the rubbery state at Tg + 50 K. b Cellulose being in the crystalline state. c Estimated from TEM. d Average value from literature.41,126-130 e Calculated from eq 8.
622 Biomacromolecules, Vol. 6, No. 2, 2005 Azizi Samir et al.
during the first stretching cycles indicated the progressive damaging of the percolating network, which was initially present.
Interfacial Phenomena. DMA experiments can provide additional information on the microstructure of heterogeneous materials from the characteristics of the relaxation process associated to the modulus drop at the glass-rubber transition. It is displayed through the concomitant energy dissipation process giving rise to a maximum of the mechanical loss factor, tan δ, and loss modulus, E′′. In particular, in nanocomposite materials, interfacial phenomena are expected to be important owing to the high specific area of the involved fillers. For tunicin whiskers, the geometrical specific area is around 170 m2 g-1, and for a 6 wt % filled composite, they are of the order of 100 000 cm2 of surfaces per cm3 of material.87 Studying tunicin whiskers reinforced PVC com- posites, Chazeau et al.101,132 claimed the presence of an immobilized polymeric matrix layer in the vicinity of the filler. The changes in the shear loss modulus induced by the cellulosic filler were discussed using a mean field approach and assuming a three-phase system.
Noishiki et al.89 investigated nanocomposite films based on silk fibroin reinforced with tunicin whiskers. It was observed that the cellulose surface caused morphological modifications of silk fibroin chains. The highly ordered tunicin whiskers surface induced the silk crystallization at the filler-matrix interface.
For tunicin whiskers and PHO based composites, signifi- cant differences were observed depending on the nature of the matrix.133 When a semicrystalline PHO was used as the matrix, the molecular mobility of amorphous macromolecular chains was reported to be only slightly affected by the presence of cellulose whiskers, owing to a possible trans- crystallization phenomenon leading to a coating of the filler with the crystalline phase. In contrast, the flexibility of chains in the surface layer was lowered by the conformational restrictions imposed by the cellulose surface, when an amorphous PHO was used as the matrix. This phenomenon resulted in a broader interphase and in a broadening of the relaxation process associated to the glass-rubber transition. A physical model based on a percolation approach, suitable for the prediction of the reinforcing effect of cellulose whiskers, was used to predict the mechanical loss angle. The ratio of the experimental and predicted magnitude of the main relaxation process of the composite was used to calculate the thickness of the interphase.
Possible Applications. Cellulose crystallites in the form of microcrystalline cellulose are currently utilized widely industrially. Novel applications in a diverse range of fields were recently presented,134 from iridescent pigments to biomolecular NMR studies. In the nanocomposite field, up to now, cellulose whiskers were only used as geometrically and structurally well-defined model cellulosic filler and no practical industrial application was envisaged. This is mainly ascribed to the duration of the preparation technique. However, two important applications may be cited. The first one concerns the solidified liquid crystals for optical ap- plications like in security paper (refs 82 and 83). The second is related to the use of these whiskers as mechanical
reinforcing agents for low-thickness polymer electrolytes for lithium batteries application.96,99,135-137 For the latter, it is worth noting that the filler content is generally relatively low, below 10 wt%, avoiding significant decrease of the ionic conductivity.
The present contribution, reporting 10 years worth of work performed on the processing and behavior of new nanocom- posite materials of thermoplastic polymers reinforced by polysaccharide microcrystals, can be considered as an effort aimed at providing further knowledge to a research area presenting yet a variety of pending issues. It was shown that the use of high aspect ratio cellulose whiskers induces a mechanical percolation phenomenon leading to outstanding and unusual mechanical properties through the formation of a rigid filler network. In addition to some practical applica- tions, the study of such model systems can help to understand some physical properties as geometric and mechanical percolation effects.
Cellulose can also be used as a microfibrillar filler, which is more accessible in terms of available amounts and preparation. The mechanical behavior is then very sensitive to the cellulose purification level and cellulose microfibrils individualization state.
The subject generates substantial interests, and the variety of possible sources of cellulose for preparing nanocrystals is broad. Polysaccharide nanocrystals can also be obtained from other abundant renewable resources such as chitin and starch by acid hydrolysis. For the former, they appear as rodlike particles with an aspect ratio related to the origin of the chitin, whereas for the latter, the nanoparticles consist of platelets with nanometer scale dimensions. Practical applications of such fillers and transition into industrial technology require a favorable ratio between the expected performances of the composite material and its cost. There are still significant scientific and technological challenges to take up.
Acknowledgment. The authors thank the Région Rhône- Alpes for financial support.
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