Baixe celulose Pevinil acetate nanocomposites e outras Notas de estudo em PDF para Engenharia de Produção, somente na Docsity! Cellulose Poly(Ethylene-co-vinyl Acetate) Nanocomposites Studied by Molecular Modeling and Mechanical Spectroscopy Gregory Chauve, Laurent Heux, Rachid Arouini, and Karim Mazeau* Centre de Recherches sur les Macromolécules Végétales, CERMAV-CNRS, BP 53, 38041 Grenoble Cedex 9, France Received February 16, 2005; Revised Manuscript Received April 28, 2005 Structure property relationships have been established at two different scales to examine reinforcing effects of nanocomposites made of cellulose whiskers and polyethylene-vinyl acetate (EVA) matrixes with different vinyl acetate contents. The role of the polymer structure on the work of adhesion as predicted by molecular modeling at the atomic scale and on the mechanical performance of nanocomposites observed by dynamic mechanical analysis at the macroscopic level is reported. Concordant results were obtained by the two approaches; both demonstrated a reinforcing effect that increases with the acetate content of the polymer. However, a leveling of this effect was observed at high acetate contents. A detailed picture of the interactions at the interface between the two species accessed by modeling gives a reasonable explanation of this unexpected phenomenon. Introduction Cellulose fibers have long served as raw material in the textile and paper industries or in composite material as filler.1 Recently, research focuses on applications with high added value, such as nanocomposites including cellulose nano- crystallites, also called whiskers.2,3 Among their interesting characteristics, cellulose whiskers are abundant and renew- able. They exhibit also a low density as compared to mineral fillers (around 1.5 g cm-3), a high form factor of about 70 together with a high specific area of 150 m2 g. They have been shown to lead to remarkable reinforcing properties in such different matrixes as styrene-acrylate latex,4 starch,5 polyhydroxybutyrate octanoate,6 or poly(ethylene oxide).7 In most cases, the reinforcing effects came from the percolating network of cellulose whiskers together with good interfacial compatibility between the matrix and the fillers. However, the relative role of these two contributions is not easy to assess. In particular, the failure of our understanding of the interfacial phenomena comes from the absence of a precise description of the interactions between the filler and the matrix at the atomistic scale. So far, such a description relies on qualitative considerations, and the choice of the matrix is based mostly on trial and error. In contrast, the use of molecular models of the filler in contact with the polymer matrix can give new insights in terms of geometry and energies of the interaction.8-11 In this context, our recently developed models of the complex surface of cellulose were successfully used to study the adsorption process of aromatic species of low molecular weight.12,13 Such models are suitable to study the interface between cellulose and an amorphous polymer solid. The present study reports complementary approaches that address the question of the rational choice of synthetic poly- mer matrixes for nanocomposites reinforced with cellulose whiskers. To this end, we use molecular modeling and dynamic mechanical analysis experiments to establish a reliable structure-property relationship that in turn allows for the design of an optimal polymer matrix that shows good adhesion properties on native cellulose. As vinyl acetate copolymers are used widely as wood and paper adhesives, their interactions with cellulose are of great industrial importance. In addition, the acetate content of ethylene- vinyl acetate copolymers can be controlled easily during the synthesis giving the polymer matrixes that differ in their macroscopic polarity and surface energy. Such copolymers were then selected in this study as suitable for establishing a structure-property relationship. Cellulose whiskers were selected as fillers because the surface of such nanocrystals is well-defined at the atomic scale as shown by AFM.14-16 Such fillers allow realistic modeling of their surface proper- ties, in contrast to common cellulose fibers. Experimental Procedures Polymer Matrixes. Different polymer matrixes (see Figure 1) were considered for the experimental and modeling studies: the homopolymers polyethylene (EVA0) and poly- vinyl acetate (EVA100) and random copolymers of poly- ethylene-co-vinyl acetate with 12, 25, 40, and 75% w/w vinyl acetate. The acetate content and the random character of the distribution of the monomers within the copolymers were checked by 13C liquid-state NMR according to Sung and Noggle.25 Corresponding molar ratios and details of some physical properties are reported in Table 1. Molecular Modeling. The calculations were carried out using the all-atoms model using the Cerius2 molecular * To whom correspondence should be addressed. Phone: 33(0)4 76 03 76 39; fax: 33(0)4 76 54 72 03; e-mail: karim.mazeau@cermav.cnrs.fr. 2025Biomacromolecules 2005, 6, 2025-2031 10.1021/bm0501205 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/21/2005 modeling package.18 The default parameters were used except those explicitly mentioned. The pcff-300-1.0119 second generation force-field was employed. The bonded terms of the potential energy function exhibit quadratic terms to describe bond length and valence angles and also a three- term Fourier transform for torsional angles. For the non- bonded contributions, the van der Waals interactions were calculated using a 9-12 Lennard-Jones function. Electro- static interactions were accounted for through a classical Coulomb potential; the charge equilibration method was used to calculate charges for each atom.20 The cut-in and cutoff distances were fixed at 10 and 11 Å, respectively. The minimum image convention was imposed in order not to duplicate nonbonded calculations. The minimization uses the conjugate gradient procedure with the convergence criterion of the root-mean-square of the atomic derivatives of 0.05 kcal mol-1 Å-1. Molecular dynamic calculations were based on the canonical NVT ensemble (constant number of particles, volume, and tem- perature). The equations of motion were solved using the Verlet algorithm,21 with a time step of 1 fs. The system is coupled to a bath at T ) 400 K using Nose’s algorithm.22 Independently of the experimental data, this temperature was chosen because it allows a proper equilibration of the system within a reasonable simulation time. Cellulose. The initial two chain monoclinic unit cell of the Iâ allomorph23 was duplicated in all three dimensions, and a new triclinic supercell was redefined. This system was then equilibrated by a molecular dynamics run of 1 ns in the isothermal-isobaric NPT ensemble24 (constant number of particles, pressure, and temperature) and then optimized to reach its equilibrium geometry. As this superlattice possesses the (110) and (010) surfaces parallel to the faces of the periodic cube, it was redefined to expose the relevant (110) and (11h0) faces. The dimensions a and c of the final supercell are 28.33 and 21.20 Å, and the parameter, b, normal to the (11h0) surface is enlarged to provide a sufficient volume above the cellulose to insert the thin polymer film. The supercell includes five layers of cellulose chains, each composed of four glucose residues. All the chains are covalently connected to their respective periodic images to model infinite chain length. The inner layer, referenced as layer 3, was kept constrained during all the calculations to mimic the well-organized crystalline interior of the cellulose whisker. By contrast, the atoms of the cellulose chains belonging to the other four layers are unconstrained; this allowed the surface chains together with the first inner layer to adjust their geometry to account for the presence of the different polymer chains at the interface. The periodic boundary conditions defined an infinite surface of a cellulose whisker, exposing only its (11h0) surface. Polymer. Each polymer single chain was generated according to the monomer sequences reported in the literature from liquid-state NMR data.25 Unperturbed conformations were first generated following a Monte Carlo protocol coupled with an energy minimization procedure. The degree of polymerization (DP) was carefully chosen for each matrix (EVA0, EVA75, and EVA100), depending on the expected final film thickness (a 30-40 Å thickness has been shown to be reasonable according to preliminary computations) and on the experimental bulk density of the matrix (see Table 1). In fact, the height parameter (dimension b) of the final system should be large enough that a significant portion of the polymer is not perturbed by the interfaces with cellulose and, in addition, long-range interactions between the cellulose chains on both sides of the simulated cube should be minimal if not null. Also, the height parameter should be small enough for the whole system has to be computationally tractable. By contrast to the adjustable b parameter of the supercell, dimensions a and c, which depend on the cellulose crystalline organization, are strictly imposed. The selected chain lengths were DP ) 150, 295, and 385 for EVA100, EVA75, and EVA0, respectively, corresponding roughly to equivalent polymer volumes. A thin polymer solid film was then created by inserting the generated polymer chain into a periodic cell having the same a and c parameters as the crystalline cellulose system and a b parameter large enough to include the whole polymer chain in that direction and an impenetrable wall parallel to the a0c plane. Then, several compression cycles were applied; each consists of an initial energy minimization followed by molecular dynamics runs in the NVT ensemble. Then, the height parameter was diminished slightly before another compression cycle was performed. Such cycles were repeated until the polymer density reached the desired experimental value of the bulk, reported in Table 1. A final 500 ps dynamic run (using the NVT ensemble) was performed to correctly equilibrate the polymer film. Creation of the Interface. Having created and equilibrated both the amorphous polymer films and the crystalline cellulose, whose a and c dimensions are compatible, forma- tion of the interface was straightforward by inserting the film into the cellulose supercell above the cellulose chains. To avoid steric overlap, the distance between the polymer film and the cellulose chains was carefully selected as that of lowest energy. Finally, another 500 ps dynamic run was performed on the polymer/cellulose system; a view of a Figure 1. Schematic representation of EVA polymers. Table 1. Characteristics of the EVA Polymers Considered in the Present Study XEa XVA polarity γ (293 K) Tm Tg density EVA0 100 0 0 36 411 243 0.925 EVA12 94 4 Nd Nd 368b 240c 0.933b EVA25 90 10 Nd Nd 348b 240c 0.948b EVA40 82 18 0.108 37 383/393b 240c 0.925b EVA75 50 50 Nd Nd Nd Nd 0.955 EVA100 0 100 0.329 54 278** 1.191* a XE and XVA: molar ratio of ethylene and vinyl acetate monomers, respectively; γ: surface tension (mJ m-2); Tm: melting temperature (K); Tg: glass transition temperature (K); density (g cm-3). Data from Hand- book.17 b According to the supplier, Aldrich Chemical Co. c According to our experiments. 2026 Biomacromolecules, Vol. 6, No. 4, 2005 Chauve et al. typically a surface phenomenon confined to a limited depth of a few angstroms. Similar results are obtained experimentally by IR spec- troscopy, which detects a preferential orientation of the carbonyl groups located close to the interface of an EVA polymer adsorbed onto aluminum mirrors whose surfaces are covered by hydroxyl groups.28 Inspection of molecular models shows that hydrogen bonds, detected from standard geometric criteria, occur effectively between the carbonyl groups of the polymer and the hydroxyl groups of cellulose at the interface. A typical example is shown in Figure 6. Energetics. It is possible to evaluate numerically the interaction energy by considering the work of adhesion W12 necessary to separate two interacting surfaces. In our models, it is calculated as the difference between the energy of the total system with interacting surfaces (Esystem) and those of the two separated polymers (Efilm and Ecellulose) without interacting surfaces. The adhesion work at a molecular level should then correspond to this energy difference divided by two times the surface area, considering two interacting surfaces. Results are given in Figure 7. The work of adhesion values shows a clear difference between the ethylene homopolymer (EVA0) and the poly- mers with acetate groups (EVA75 and EVA100). As expected, a larger energy is needed to separate polar polymers from cellulose than that of the nonpolar polymer; adhesion work is 3 times lower for EVA0 than for EVA75 and EVA100. Surprisingly, no further increase in the adhesion work is obtained by increasing the molar content of acetate groups from 50 (EVA75) to 100 (EVA100), suggesting a leveling in the interaction energy. Furthermore, it is possible to split the interaction energy into its bonded and nonbonded contributions. All the sys- tems possess a comparable van der Waals energy, whichever the considered polymer, indicating that similar contacts between the different surfaces occurs. The electrostatic contribution, however, strongly differs from one polymer to the other. Electrostatic energy is positive and weak for EVA0, while it is negative and large for the polar EVAs. This confirms the role of hydrogen bonds previously illustrated in Figure 5. The leveling of the work of adhesion with the molar content of polar groups may have its origin in the confor- mational freedom of each polymer. As mentioned earlier, hindrance by the acetate groups can affect the adaptation of the polymer to the cellulose surface, and the number of possible hydrogen bond interactions may be a compromise between the total number of acetate groups and their ability to be engaged with hydroxyl partners at the cellulose surface. From a predictive point of view, estimated values for EVA75 and EVA100 systems are larger than the experi- mental one (96.5 mJ m-2) for vinyl acetate (EVA100) interactions with cellulose29 derived from surface energy measurements. This quantitative difference can be reasonably explained as follows. In the models, the contact between cellulose and polymer occurs between two idealized surfaces. By contrast, the cellulose surfaces used for the contact angle measurement are far from perfect. The contribution of defects and roughness to the surface energy is probably very high, rendering a more pronounced hydrophobic character to the cellulose surface.30 Similarly, the perfection of the modeled interfaces cannot obviously be attained experimentally. Consequently, our calculations should correspond to an upper limit of the work of adhesion, which cannot be reached by experiment. Mechanical Tests. Influence of Whisker Content. The dynamic mechanical analysis (DMA) has been performed on a series of commercially available EVA matrixes with varying acetate content. Figure 8 shows the evolution of the tensile modulus with temperature for different whisker contents in the case of EVA25, which is representative of the general behavior of all the matrixes. For the neat polymer, the glassy region below the glass temperature (Tg ) 250 K) is followed by a rubbery plateau until the flowing of the Figure 6. Details of a molecular model that shows hydrogen bonds at the upper interface (EVA100). Figure 7. Work of adhesion of the different systems. W12 ) - (Esystem - Efilm - Ecellulose)/(2 × surface) Figure 8. Dynamic mechanical analysis of EVA25 copolymer with cellulose whiskers (w/w): 0% lozenges, 3% squares, and 6% triangles. Poly(Ethylene-co-vinyl Acetate) Nanocomposites Biomacromolecules, Vol. 6, No. 4, 2005 2029 polymer above its melting temperature. No significant change is observed at the glassy plateau after the addition of 3% whiskers. By contrast, it leads to a significant increase of the storage modulus E′ at the rubbery plateau by a factor of 2. However, the reinforcing effect is difficult to analyze as it proceeds from the combination of cellulose whiskers and polymer crystallites in the matrix.6 As the whiskers content is increased to 6% w/w, the modulus in the rubbery region is further increased, and a new phenomenon takes place. The presence of 6% w/w of whiskers prevents the flowing of the polymer, as already observed for latex including the same type of cellulose whiskers.4 Except for EVA0, this phenom- enon was observed for all the matrixes, whatever the whisker content. Influence of the Polarity of the Matrix. Figure 9 displays the storage modulus E′ obtained above the melting temper- ature for all the matrixes with a 6% w/w cellulose whiskers content. As already mentioned, the same reinforcing phe- nomenon is observed for all the matrixes but clearly depends on the polarity of the matrix. The more polar matrixes exhibit the stronger storage modulus. It also appears that a minimal number of acetate groups is needed to prevent flowing of the polymer and to observe a reinforcing phenomenon above the melting temperature. However, a leveling off of this effect appears for EVA40 and EVA100, for which the storage modulus E′ is approximately the same. This subtle effect was impossible to predict by consideration alone of macro- scopic properties such as the surface energy of the respective polymers but clearly corresponds to the leveling effect evidenced in the modeling study. Conclusion Both atomistic modeling and the macroscopic mechanical behavior on EVA nanocomposites reinforced by cellulose whiskers lead to the same conclusions. The calculated work of adhesion depends on the polarity of the matrix, here the acetate groups, but reaches a plateau with increasing global polarity of the matrix. Similarly, the reinforcing properties observed experimentally for the nanocomposites underline also the effect of the polarity on the mechanical properties and a leveling of this effect. As the nanocomposites are obtained by the same solvent-casting method, this excludes artifacts generated by differences arising from processing. These concordant results confirm the intuitive idea that the reinforcing effect of the filler is enhanced by the strongest interaction with the polymer. However, molecular modeling brings new insights to the details of this global behavior, especially the leveling of the reinforcing properties of the cellulose whiskers. Naively, the higher polarity of the matrix should lead to greater reinforc- ing effects. The calculated models clearly show that the polymer adapts its geometry at the filler surface to be able to interact with greater efficiency. By considering the ethylene-co-vinyl acetate series, it appears that the presence of ethylene groups gives to the polymer backbone the flexibility needed to maximize the interaction of the acetate groups, even if they do not interact efficiently with the cellulose surface. Indeed, the potential reinforcing effect of cellulose whiskers or other type of fillers does not depend only on the consideration of macroscopic properties, like surface energy and polarity. Similarly, consideration of the experimental results alone would not have been clear enough to give a detailed understanding of such a reinforcing effect. Atomistic details of the interaction have to be taken into account. To this end, the modeling approach can help greatly to evaluate the potential affinity of reinforcing properties of a given pair of filler and matrix. 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