Gas-Phase Surface Esterification of Cellulose Microfibrils and

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Gas-Phase Surface Esterification of Cellulose Microfibrils and Whiskers Sophie Berlioz, Sonia Molina-Boisseau, Yoshiharu Nishiyama, and Laurent Heux* Centre de Recherches sur les Macromolećules Ve´ge´tales (CERMAV-CNRS; affiliated with the Universite´ Joseph Fourier and member of the Institut de Chimie Molećulaire de Grenoble), BP 53, F-38041 Grenoble Cedex 9, France Received March 19, 2009; Revised Manuscript Received May 12, 2009

A new and highly efficient synthetic method has been developed for the surface esterification of model cellulosic substrates of high crystallinity and accessibility, namely, freeze-dried tunicin whiskers and bacterial cellulose microfibrils dried by the critical point method. The reaction, which is based on the gas-phase action of palmitoyl chloride, was monitored by solid-state CP-MAS 13C NMR. It was found that the grafting density not only depended on the experimental conditions, but also on the nature and conditioning of the cellulose samples. The structural and morphological modifications of the substrates at various degrees of grafting were revealed by scanning electron microscopy and X-ray diffraction analysis. These characterizations indicated that the esterification proceeded from the surface of the substrate to their crystalline core. Hence, for moderate degree of substitution, the surface was fully grafted whereas the cellulose core remained unmodified and the original fibrous morphology maintained. An almost total esterification could be achieved under certain conditions, leading to highly substituted cellulose esters, presenting characteristic X-ray diffraction patterns.

Introduction Cellulose is the most common organic polymer and is considered as an almost inexhaustible source of raw material for the increasing demand of environmentally friendly and biocompatible products. In addition to its worldwide availability, cellulose displays interesting physical properties such as a low density together with high mechanical characteristics.1,2 However, due to a high Tg close to its decomposition temperature, cellulose cannot be melted and processed as a thermoplastic. In addition, its high hydrophilic character hampers its use as reinforcing filler in most common plastic composites.3 The modification of cellulose is therefore of great interest and has initiated a large amount of research programs for both surface modification and homogeneous or heterogeneous substitution of the cellulosic backbone. Cellulose fibers display a hierarchical organization consisting of slender microfibrils with diameter ranging from 2 to 20 nm in which cellulose chains are organized in crystalline order.4 These microfibrils can be either tightly packed as in wood or cotton fibers or loosely entangled in a web-like structure in parenchymal cell walls or bacterial cellulose pellicles. Different processes have been developed over the past decades to individualize these microfibrils. For instance, this can be achieved by mechanical disintegration for wood pulp5 or for sugar beet pulp,6 with possibly an additional enzymatic treatment.7 When subjected to acid hydrolysis, the individualized microfibrils become cut longitudinally, yielding “cellulose whiskers” of high aspect ratio,8,9 presenting mechanical properties close to those of the theoretical values of cellulose.10-12 The morphology of these whiskers, combined with their low density and renewable character, has initiated a flurry of works since the demonstration of their reinforcing properties as fillers * To whom correspondence should be addressed. E-mail: heux@ cermav.cnrs.fr.

in polymeric lattices.13 Such reinforcement has now been shown in many different systems.14,15 As aforementioned, the hydrophilic character of cellulose is the major drawback of its use as common filler for nanocomposite materials. The challenge is thus to confer a hydrophobic character to the cellulose surface only, while keeping the integrity of its crystalline core. To avoid a complex chemical modification procedure, surfactants can be used to obtain cellulose whiskers with hydrophobic surfaces16 that can be incorporated as reinforcing fillers into common thermoplastics.17,18 A recent development has described an elegant modification of cellulosic fibers by the adsorption of modified xyloglucan functionalized with moieties bearing reactive groups.19 Despite their efficiency, both methods rely on the physical adsorption of specific molecules, and thus, these adducts are potentially more labile than if they had been covalently bonded. Several examples have described the covalent derivatization of cellulose surface at a nanometric level. They include acetylation,20 silylation of either cellulose whiskers21 or microfibrils,22 cationic functionalization with epoxyproyl trimethylammonium chloride,23 coupling with N-alkyl isocyanate,24 and derivatization with alkylenyl succinic anhydride followed by heat annealing.25 Most of these techniques require tedious solvent exchanges that strongly diminish the environmental benefit of the use of cellulose. Thus, a solvent-free derivatization system appears as an important goal to get cellulose products with hydrophobic surfaces with a minimum environment impact. In line with the solvent-free cellulose derivatization concept, this paper deals with the surface heterogeneous esterification of model cellulose substrates. Such vapor-phase technique was recently reported for the surface acetylation of cellulose with the help of gaseous trifluoroacetic acid mixed with acetylating agents.26 In the present work, the focus is on the gas-phase esterification with palmitoyl chloride. In a 1978 patent such process was already proposed for introducing water repellency

10.1021/bm900319k CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

Esterification of Cellulose Microfibrils

to cellulose sheets, pads, or cloths.27 More recently,28 the water repellency of filter paper obtained by surface derivatization with fatty acyl chloride was also demonstrated. The present work goes one step further by trying to understand the fundamental aspects of such gas-phase esterification by following the progress of the reaction at the surface level of aerogels of either cellulose whiskers or microfibrils. For this, we have prepared cellulose substrates of high crystallinity and accessibility and subjected them to palmitoyl chloride vapor at various temperatures and reaction times. The esterification of these samples as well as their structural modification was followed by solid-state NMR spectroscopy, X-ray crystallography, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) to propose a mechanism for such esterification.

Experimental Section Materials. Two different sources of cellulose were used. Tunicin whiskers originating from the cellulosic mantle of the sea animal Halocynthia roretzi were purified by alternating treatments with KOH and NaClO2 followed by a sulfuric acid hydrolysis.29 Cubes of bacterial cellulose from nata de coco, a food-grade commercial cellulose (Fujicco Co., Kobe, Japan), were used throughout. The cubes were extensively washed and then disintegrated into microfibrillar aqueous suspensions following the procedure described by Boisset et al.30 Palmitoyl chloride was purchased from Aldrich Chemical Co. and used without any further purification. Sample Preparation. Aerogels of tunicin whiskers were prepared by freeze-drying aqueous suspensions of concentration approximately 0.2% w/v. Suspensions of bacterial cellulose microfibrils were dehydrated with CO2 under super critical conditions (scCO2) to preserve their accessibility.31 The whole procedure comprises successive solvent exchange between water and ethanol at volumetric fraction of water of 90/10, 80/20, 60/40, 40/60, 20/80, 10/90, and 0/100, followed by washing with scCO2. For both the tunicin whiskers and the bacterial cellulose microfibrils, a soft and white solid foam-like material was obtained, which was kept in an oven at 60 °C until the beginning of the reaction to avoid humidity uptake. Experimental Setup. The reaction between solid-state cellulosic substrates and palmitoyl chloride was carried out in a 1 L open vessel. Approximately 150 mg of dried cellulose was placed on a grid 1 cm above typically 2 mL of the reagent to avoid direct contact. The vessel was positioned in a vacuum oven at controlled temperature and pressure during given periods of time. As the temperature was raised, a fraction of the reagent, which was in large excess, evaporated and diffused into the cellulosic substrate. During its diffusion, the acid chloride reacted with the hydroxyl groups of the glucosyl units to form hydrophobic ester bonds, while liberating hydrogen chloride. The pressure was set at 100 mbar and a continuous slow nitrogen stream was provided to eliminate the hydrogen chloride formed. The time and temperature conditions were varied in the different runs. Purification Procedure. In addition to leftover traces of palmitoyl chloride, palmitic acid was also detected in the samples as a result of interaction of palmitoyl chloride with the minute amount of residual water left in the sample. These contaminants were detected by DSC from peaks at, respectively, 4 and 45 °C. For further purification, the samples were extracted for 48 h in a Soxhlet operated with acetone, followed by drying at 60 °C for 12 h. The efficiency of this purification step was followed by DSC, by looking at the decrease of the melting peaks of the impurities. Characterization of the Esterified Cellulose. Solid-State 13C NMR Measurements. NMR experiments were performed with a Bruker Avance DSX 400 MHz spectrometer operating at 100.6 MHz for 13C, using the combination of cross-polarization, high-power proton decoupling and magic angle spinning (CP/MAS) methods. The spinning speed was set at 12000 Hz. The 1H radio frequency field strength was set to give a 90° pulse duration at 2.5 µs. The 13C radio frequency field

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strength was obtained by matching the Hartman-Hahn conditions at 60 kHz. Recording at least 2000 transients with contact time and recycle delay, respectively, of 2 ms and 2 s represented standard conditions. The acquisition time was set at 30 ms and the sweep width at 29400 Hz. As the amplitude of the 13C NMR signal in a CP experiment is dependent on the contact time, cross-polarization kinetics were investigated with different contact time ranging from 0.3 to 6 ms. The different signal intensities were plotted against the contact time, tcp and fit to the following equation:

M ) M0(1 - exp(-tcp/TCH)) exp(-tcp/T1FH) where M is the measured intensity, M0 is the initial intensity of the magnetization, and TCH and T1FH are, respectively, the cross-polarization (CP) time constant and the relaxation time constant of protons in the rotating frame. The integral for each peak was corrected by the scaling factor M(2 ms)/M0 obtained from the CP kinetics. The degree of substitution was then calculated as the ratio of the corrected integrals using the following equation:

DSRMN )

6 × ICO IC

ICO is the integral of the peak corresponding to the carbonyl of the ester bond, IC is the sum of the corrected areas of all the carbons of the cellulose, and 6 is the number of carbon atoms in one anhydroglucose unit of cellulose. Scanning Electron Microscopy (SEM). The samples were coated with Au/Pd and observed in secondary electron mode with a JEOL JSM6100 scanning electron microscope operated at 8 kV. Wide-Angle X-ray Scattering (WAXS). WAXS data were collected at room temperature using a Wharus flat film vacuum camera mounted on a Philips PW3830 generator working with a Ni-filtered Cu KR radiation (λ ) 1.54178 Å) and operated at 30 kV and 20 mA. X-ray diffraction patterns were recorded after purification and drying of the specimens. Some spectra were recorded with the X-ray beam perpendicular to the sample mat surfaces, whereas others were recorded with the X-ray beam parallel. Diffraction patterns were recorded during 2 h exposures on Fujifilm imaging plates and read using a Fujifilm BAS 1800 II Phospho-imager. Calcite powder, with its characteristic ring at a d-spacing of 0.3035 nm was used for calibration purpose. Differential Scanning Calorimetry (DSC). DSC measurements were performed with a TA Instruments Q200 calorimeter using heating/ cooling scan rates of 10 °C/min from -20 to 250 °C. The analyses were performed on the purified and dried esterified samples. The temperature scale of the instrument was checked and effects due to thermal lag in the system were corrected by regular calibration using indium and zinc standards. An empty aluminum pan was used as reference. The amount of sample used in each case was 15 mg. The melting temperatures were determined automatically by the DSC software (Universal Analysis).

Results The list of samples that were used in this work, the conditions of their derivatization and the resulting degree of substitution (DS) are presented in Table 1. Solid State NMR. Figure 1 shows the CP-MAS 13C NMR spectra obtained from freeze-dried tunicin whiskers exposed to palmitoyl chloride vapors for different times and temperatures. The spectrum obtained from unmodified tunicin whiskers (Figure 1a) exhibited a pattern typical of highly crystalline cellulose with almost pure Iβ phase, with clear doublets for the resonances at the C1, C4, and C6 positions, as described in

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Table 1. Reaction Conditions and Degree of Substitution (DS) of Esterified Tunicin Whiskers and Bacterial Cellulose reaction conditions

160 °C/4 h

170 °C/4 h

170 °C/6 h

170°/13 h

tunicin whiskers DS bacterial cellulose DS

0.15

0.25 1.47

0.61 2.7

1.17

literature.32 As the reaction proceeded, several new resonances appeared on the spectra, even at a low degree of substitution (Figure 1b,c). A signal arising at 172 ppm can be unambiguously assigned to the resonance of the carbonyl peak of the ester.33 A peak at 14.3 ppm arises from the terminal methyl carbons of the fatty acid chains. Peaks resonating between 18 and 37 ppm are typical of carbons of fatty aliphatic chains. It has to be noted that the resonance signals of the carbonyl function of palmitic acid is located around 180 ppm, whereas palmitoyl chloride exhibits a signal at 43 ppm assigned to the methylene next to the carbonyl. The absence of these two resonances confirms that the sample is free of the two potential contaminants after the purification treatment in accordance with the DSC data. This is a further evidence of the occurrence of the esterification of cellulose hydroxyls. A careful observation of both series of esterified spectra also revealed the appearance of small signals situated at 53 and 153 ppm, which are, respectively, assigned to spinning side bands of the main signals corresponding to carbonyl function (172 ppm) and CH2 (33 ppm). At higher degree of substitution (Figure 1d,e), the extent of the esterification reaction is accompanied by the appearance of new resonances at 101.2, 84.3, and 62.1 ppm referred to as C1′, C4′, and C6′, respectively. The existence of these new contributions has already been observed by 13C CP-MAS on fibrous long chain fatty esters of cellulose obtained from the heterogeneous substitution of bleached sulfite pulp with fatty acids34 and can be unambiguously assigned to substituted cellulosic backbone. A small shoulder is also observed at 77.6 ppm for the highest DS that apparently grew at the expense of the C4′ signal. As expected, the intensity of these resonances increased with the extent of the esterification reaction at the expense of

180 °C/4 h

190 °C/2 h

1.8

0.32 2.01

the corresponding C1, C4, and C6 signals of native cellulose. No precise information can be derived from the inspection of the C2-, C3-, C5 resonance region due to the overlap of the signals. One particularly interesting point is the clear superimposition of two contributions from, respectively, pure cellulose and pure cellulose ester contributions. This means that the domain of transition with intermediate degree of esterification is very small and the product is always composed of almost fully esterified domain and intact domain, with the latter growing at the expense of the former at increased harsher esterification conditions. The same kind of reactive conditions have been applied to scCO2 bacterial cellulose. The spectra obtained at different times and temperatures are shown in Figure 2. Details of the spectral region where glucosyl resonances usually appear are displayed in Figure 3. The overall evolution of the spectra is qualitatively similar to the one previously described for lyophilized cellulose whiskers, with the resonance of the ester linkage located at 172.4 ppm and the contributions of the alkyl chains between 10 and 40 ppm. The original sample exhibits mixture of IR and Iβ phase typical of bacterial cellulose, as can be evidenced in the spectral features of C1 and C4 signals between 104 and 107 ppm and between 88 and 91 ppm, respectively. During the course of esterification, the IR/Iβ ratio is preserved. However, under given experimental conditions, the DS calculated from the NMR data can be more than 6 times higher in the case of scCO2 bacterial cellulose than in the corresponding tunicin whisker samples (see Table 1). The higher reactivity of scCO2 toward the gaseous palmitoyl chloride is most probably related to its higher surface area and, hence, accessibility of this substrate. The interest of

Figure 1. CP/MAS 13C solid-state NMR (100 MHz) spectra of tunicin whiskers: (a) original sample, (b) 4 h/170 °C esterified whiskers (DS ) 0.25), (c) 2 h/190 °C esterified whiskers (DS ) 0.32), (d) 170 °C/6 h esterified whiskers (DS ) 0.61), (e) 170 °C/13 h esterified whiskers (DS ) 1.17). The spectral region corresponding to the aliphatic carbons (0-50 ppm) has been divided by a factor 5. Asterisks (*) and (**) indicate residual spinning sidebands from the CdO and CH2 signals, respectively.



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Figure 2. CP/MAS 13C solid-state NMR (100 MHz) spectra of scCO2 bacterial cellulose: (a) original sample, (b) 4 h/170 °C esterified bacterial cellulose (DS ) 1.47), (c) 2 h/190 °C esterified bacterial cellulose (DS ) 2.01), (d) 170 °C/6 h esterified bacterial cellulose (DS ) 2.7).

Figure 3. Detailed spectra of scCO2 bacterial cellulose anhydroglucose carbons in cellulose esters: (a) original sample, (b) 4 h/170 °C esterified bacterial cellulose (DS ) 1.47), (c) 2 h/190 °C esterified bacterial cellulose (DS ) 2.01), (d) 170 °C/6 h esterified bacterial cellulose (DS ) 2.7).

the derivatization of scCO2 bacterial cellulose is to reach higher substitution degree with reasonable experimental conditions. For example, an almost fully esterified bacterial cellulose sample could be synthesized (Figure 2d) in which all signals arising from crystalline cellulose in its native state have disappeared. It is also worth noting that native cellulose contributions are still visible for DS as high as 2.0, revealing the highly heterogeneous character of the reaction. For this series of spectra, the appearance of new signals most probably related to the modified cellulose backbone can be followed as the degree of substitution increases (see Figure 3). At DS lower than 2.0, the esterification of cellulose is accompanied by the onset of new contributions at 101.2, 84.3, and 62.1 ppm previously assigned to C1′, C4′, and C6′ carbons, respectively, as for tunicin whiskers. The contribution of the C4′ peak at 84.3 ppm decreases for the highest degree of substitution (Figure 3c,d) and shifts to higher fields merging

into the undefined C2′, C3′, and C5′ cluster, as observed for tunicin whiskers, but much more clearly. For the higher DS, the cellulose samples have only a few remaining hydroxyl groups. In cellulosic structures, the network of hydrogen bonds is responsible for the constraints imposed to the glycosidic bonds. The removal of hydrogen bonds, especially O3H-O5, which is responsible for the 21 helical conformation of the chains, could explain the shift of C4 to resonance close to that occurring in liquid state. This effect has already been observed in the case of soda-cellulose complexes, where the suppression of the O3H-O5′ hydrogen bond is accompanied by a conformational transition of the cellulose backbone from a 21 to a 31 helical conformation, inducing a shift of the C4 chemical shift to lower values.35 Morphological Observation. SEM. Figure 4a,b shows scanning micrographs of the freeze-dried tunicin whiskers before and after esterification with gaseous palmitoyl chloride. The

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Figure 4. SEM of freeze-dried tunicin whiskers (a) before and (b) after esterification 6 h/170 °C (DS ) 0.61).

Figure 5. SEM of scCO2 bacterial cellulose (a) before and (b) after esterification 4 h/170 °C (DS ) 1.47).

initial structure displays macropores larger than 10 µm in diameter with walls made of intertwined whiskers that have kept their fibrillar morphology (Figure 4a). The presence of these macropores is obviously related to the formation of ice crystals within the whiskers suspension during the first freezing step. After esterification (Figure 4b), the macroporous structure remained untouched, whereas the isolated whiskers seemed to be embedded in a kind of smeared structure, even if the fibrillar morphology was still detectable. Figure 5a,b displays scanning micrographs of the scCO2 bacterial cellulose before and after esterification for a degree of substitution of 1.47. The original sample (Figure 5a) presents a web-like structure typical of bacterial cellulose that has been preserved by the scCO2 drying technique. After esterification (Figure 5b), the web-like structure is fully preserved, but the individual microfibrils appeared wider, as if an additional sheath, likely caused by the grafting of long fatty alkyl chains, covered them. Although the size of the microfibrils is very heterogeneous, the apparent diameter can be evaluated from an initial value of 80 ( 20 nm to a final value after esterification of 150 ( 35 nm. This spectacular lateral expansion can also be estimated from the degree of substitution and the density of the cellulose and fatty alkyl chains respectively. The weight gain corresponding to a DS of 1.47 is 215%. By considering a density of 1.6 g/cm3 for highly crystalline cellulose and 0.99 g/cm3 for cellulose tripalmitate,36 the volume increase corresponding to a weight gain of 215% would be 380%. Assuming that dimensional change in the chain direction is negligible and that the lateral thickening of individual microfibrils is isotropic, the corresponding increase in lateral dimension would be the square root of the volume increase, so approximately 200%. This factor of 2 is therefore in good agreement with the SEM

Figure 6. X-ray patterns of original and esterified tunicin whiskers. (a) Original tunicin whiskers freeze-dried, (b) esterified whiskers with a DS of 0.61, (c) esterified whiskers with a DS of 1.17. For (b) the film was parallel to the beam and oriented vertically with respect to the pattern.

observations and the consistency between the morphological and the spectroscopic data confirms the extent of the reaction. X-ray Diffraction Analysis. The X-ray diffraction patterns of tunicin whiskers before and after esterification are presented in Figure 6. The unmodified sample (Figure 6a) displays a typical pattern of highly crystalline cellulose Iβ with the main sharp diffraction signals assigned to the diffraction planes (1-1 0), (110), (102), (200), and (004).37 As a result of the esterification reaction (Figure 6b, corresponding to a DS of 0.61, and Figure 6c corresponding to a DS of 1.17), one observes a substantial decrease of the intensity of the cellulose reflections,



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Figure 7. X-ray diffraction patterns of bacterial cellulose: starting material after freeze-drying (a) and esterified cellulose with a DS of 1.8 (b) and 2.7 (c), respectively.

but there is no line broadening effect. This absence indicates that even at a DS as high as 1.17, there are still cellulose crystals of lateral size close to that of the initial sample, but their proportion is reduced in the derivatized samples. In addition to the native cellulose diffraction signals, a series of new diffraction rings are observed in Figure 6b,c, corresponding to d-spacings 3.9, 1.2, 0.42, 0.36, and 0.29 nm. Whereas the rings at 0.36 and 0.29 nm are very sharp, this is not the case for the others, the one at 0.42 nm being particularly broad. A close examination of the diffraction pattern in Figure 6b reveals that some of these rings, namely those of the 1.2 and 0.42 nm as well as the (1-1 0), (110) of the initial cellulose, are not continuous, but are composed of arcs: this is due to the fact that the corresponding patterns were recorded with the X-ray beam parallel to the sample mat surface. Taking the orientation of the underivatized cellulose for a reference, the reflection at 1.2 nm appears to be equatorial, that is, corresponding to a periodicity parallel to the cellulose chain axis. Conversely, the broad reflection at 0.42 nm has a meridional characteristic, indicating a periodicity perpendicular to the chain axis. Figure 7 corresponds to the sample of bacterial cellulose, before (Figure 7a) and after two levels of esterification (Figure 7b,c). As opposed to the case of derivatized tunicin whiskers, the esterified bacterial cellulose samples do not show any hint of the initial cellulose. Figure 7b, which corresponds to a sample of DS of 1.8, displays a broad ring centered at 0.42 nm together with a sharper ring at 3.9 nm. In the most esterified sample (Figure 7c corresponding to a DS of 2.7) the pattern is similar to the one in Figure 7b, with the exception of the additional diffraction ring at 1.2 nm that was also present in the esterified tunicin samples (Figure 6b,c). Taken together, all the esterified samples have in common the broad 0.42 nm diffraction peak together with a somewhat sharper one at 3.9 nm, neither of these existing in the starting cellulose material. Regarding the peak at a d-spacing of 0.42 nm, it has been known for a long time that polymers having long n-alkyl side chains, systematically present an intense diffraction peak at a d-spacing of 0.42 nm.38,39 More recently, such an intense peak, which was also reported in LangmuirBlodgett films of cellulose palmitate prepared on indium-tin oxide, was assigned to the average distance between the palmitoyl side chains.40 In the present work, the occurrence of

Figure 8. Schematic drawing of the proposed structure of cellulose palmitate produced by solvent free esterification.

this diffraction peak in a meridional position indicates therefore that the palmitoyl side chains are stacked in layers roughly perpendicular to the cellulosic backbones. The diffraction ring at 3.9 nm may be tentatively assigned to the distance between the derivatized chains. In support of such hypothesis, it is known that crystalline saturated carboxylic acids of even numbers of carbons get organized in layers with the molecular axis inclined with respect to the n-alkyl layers. Typically, in the case of the C allomorph of palmitic acid, one of the allomorphs of this compound, the extended molecule is 2.1 nm long, but the layers, which contain two molecules headto-head, have a thickness of 3.56 nm indicating that, within the crystal, the molecules are inclined by 55° with respect to the layer plane.41 When, as in the present case, the carboxylic moiety is attached to the cellulose backbone, one expects therefore an increase in the layer thickness, and a thickness of 3.9 nm appears reasonable, assuming that the diameter of the cellulose backbone is about 0.35 nm. The scheme presented in Figure 8 summarizes the molecular model that we propose to tentatively explain these diffraction data. In this model, the C16 n-alkyl chains are separated by the classical 0.42 nm and are inclined with respect to the cellulosic backbone about an angle of 55 degrees. This model, is consistent with the description of the fully decanoated cellulose,42 which showed columns of cellulose chains surrounded by a shell of perpendicular alkyl chains. Regarding the other diffraction rings at d-spacings of 1.2, 0.36, and 0.29 nm, which occurred only in the derivatized tunicin but not clearly in the esterified bacterial cellulose samples, they may tentatively be attributed to some local crystallization phenomenon of the palmitate domains, but such occurrence could not be ascertained.

Discussion The above results, obtained with different experimental techniques bring complementary information on the ultrastructure of our esterified material and on the underlying mechanism leading to the observed morphologies. The 13C CP-MAS solidstate experiments clearly show the heterogeneous nature of the

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Figure 9. Scheme of the progress of the gas-phase esterification of cellulose whiskers or microfibrils with palmitoyl chloride with (a) DS ) 0, (b) DS ) 0.25, (c) DS ) 0.75, and (d) DS ) 1.5.

derivatization. The SEM micrographs further confirmed the conservation of the fibrillar structure of the cellulosic substrate, which seems to be maintained all the way to the total derivatization. Moreover, in the case of bacterial cellulose samples, the calculated increase of diameter from the degree of substitution due to the expanded molar volume of fatty acid esters, compared to that of native cellulose is in very good agreement with the experimentally observed increase of diameter. Finally, the use of X-ray diffraction clearly showed the heterogeneous nature of the derivatization. Throughout the reaction, fully substituted cellulose palmitate domains coexisted with unreactive native crystalline cellulose. A schematic model describing the progress of the gas-phase palmitoyl chloride esterification, applied to a cellulose whisker is drawn in Figure 9. In this model, which would also apply to individual crystalline microfibrils, the esterification proceeds essentially from the skin to the core of the whisker. Indeed, palmitoyl chloride, as well other acid acylating reagents, has no swelling power toward crystalline cellulose and therefore must act at the substrate surface. When the cellulose hydroxyl groups located at the surface are fully or partially esterified, a layer of cellulose palmitate is produced, more or less in a molten state, because the melting point of the fully derivatized cellulose is only of 105 °C,34 that is, way below the temperatures of esterification used in this work. In a subsequent step, further gaseous palmitoyl chloride can readily diffuse in the newly formed layer of cellulose palmitate and thus reach the underlying layer of unreacted chains, which in turn become esterified, leading to further esterification. The process is, therefore, able to proceed until the core of the whisker is fully esterified. Because the gas phase is equivalent to a nonsolvent for cellulose and cellulose palmitate, the cellulose tripalmitate, which is being produced, sticks to its parent cellulose skeleton and the net effect is an increase in the whisker diameter, thus doubling the diameter observed in Figure 5b for an overall DS 1.47. The above description of ultrastructural modifications occurring during the esterification of cellulose with palmitoyl chloride can be correlated with the well-described morpho-chemical evolution of cellulose subjected to acetylation. Indeed, it has been known for a long time that the acetylation of cellulose progressed from the skin to the core of the cellulose elements.43,44 One distinguishes two basic acetylation mechanisms, namely, the so-called “homogeneous” and “fibrous” process. In the homogeneous mechanism, the acetate, which is produced, is soluble in the medium and the acetylation is essentially a “stripping” process. Indeed, the chains located at the cellulose surface diffuse into the surrounding solvent, as soon as they are substantially derivatized, and prefer being in the solvent rather than remaining on the underlying cellulose microfibril.20 As a consequence, a direct decrease of the microfibril diameter with the progress of the acetylation was observed. In the

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heterogeneous or “fibrous” process, the acetylation is achieved in a medium that is a nonsolvent for both native cellulose and the acetylation product. Thus, the acetylated chains remain at their production locus, but the X-ray patterns of the samples gradually change from cellulose I to cellulose triacetate I (CTA I) following the progress of the acetylation.45,46 In the fibrous acetylation there is a remarkable preservation of the cellulose morphology, even at the ultrastructural level, as fully or partially acetylated microfibrils look exactly the same as those of the starting material. In particular, there is no intermingling of the microfibrils, as each cellulose I microfibril gets converted to one CTA I microfibril.47 In mimic with the acetylation mechanisms, the present production of cellulose palmitate can be qualified of fibrous process, but at a nanometric level. Indeed, because the surrounding reaction medium is gaseous, the palmitate chains that are formed are forced to remain on the reactive substrate, thus leading to the observed skin-core morphology, with the crystalline core diminishing with the progress of the derivatization. As opposed to the case of fibrous acetylation, where the increase of the microfibril diameter was modest, there is here a dramatic increase because a doubling of the microfibril diameter is obtained when only half of the available hydroxyl groups of cellulose are esterified. By a close examination of the scanning micrographs shown in Figures 4b and 5b, it is interesting to notice that there is no coalescence of the microfibrillar morphology, even if the cellulose palmitate is produced above its melting point. This phenomenon must be due to a high viscosity and low surface tension of the cellulose palmitate that prevents it to flow and thus to aggregate the microfibrils together.

Conclusions In this paper, we have applied a new gas-phase treatment for the esterification of cellulose with palmitoyl chloride. This treatment was applied to two model substrates, namely tunicin whiskers and scCO2 bacterial cellulose, showing different reactivity, covering a wide range of degree of substitution. The esterification reaction was confirmed by solid-state CP-MAS 13 C NMR. We have shown that this new method, which was highly efficient, could be tuned to esterify cellulosic substrates to different extents with fatty acid chlorides in the absence of solvent. Several analytical techniques were used to monitor morphological and structural changes of the cellulosic substrates after the esterification. For the same reaction conditions, the extent of the reaction is dependent on the origin and the accessibility of the substrate and not surprisingly, strongly on time and temperature. These two parameters can be used to monitor the penetration depth of the chemical modification. The extent of the reaction is clearly limited by the diffusion of the reactant to the core of the whiskers or microfibrils. Work is also in progress to quantify the role of the accessibility of the cellulosic substrate. The substitution limited to the surface is of great interest on the one hand, for improving the compatibility of the cellulosic substrate with apolar matrix, and on the other, for preserving the crystalline heart of cellulose and its high mechanical properties. In principle, the reaction scheme presented here can be adapted to any reactive molecule, providing that the vapor pressure falls in the optimum range at the reaction temperature. Too low vapor pressure would slow down the kinetics, while a vapor pressure too high would require a pressure vessel. Fully substituted samples could also be obtained for bacterial cellulose due to the high accessibility of this substrate. So the tuning of


the conditions of the reaction allows the designing of cellulose substrates suitable for their future application. Acknowledgment. S.B. was a recipient of French Grant for her Ph.D. from the Ministe`re de l’Enseignement Supe´rieur et de la Recherche. The authors thank also H. Chanzy for his active participation to the discussion and writing of the paper. I. Paintrand and N. Montesanti are also acknowledged for their help for the SEM experiments and J.-L. Putaux for the processing of the X-ray diffraction images. K. Mazeau is acknowledged for the drawing of the molecular structure displayed in Figure 8.

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Gas-Phase Surface Esterification of Cellulose Microfibrils and

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