Impact of Drying on Wood Ultrastructure Observed by Deuterium

Dec 21, 2009 - The pulp, initially delignified by kraft pulping process and then bleached, was obtained from paper mill in Sunila, (Finland). The pulp...
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Biomacromolecules 2010, 11, 515–520

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Impact of Drying on Wood Ultrastructure Observed by Deuterium Exchange and Photoacoustic FT-IR Spectroscopy Miro Suchy, Jenni Virtanen, Eero Kontturi,* and Tapani Vuorinen Department of Forest Products Technology, Helsinki University of Technology, P.O. Box 6300, FIN-02150 TKK, Finland Received November 9, 2009; Revised Manuscript Received December 5, 2009

The impact of drying on the ultrastructure of fresh wood was studied by deuterium exchange coupled with FT-IR analysis. This fundamental investigation demonstrated that water removal leads to irreversible alterations of the wood structure, namely, supramolecular rearrangements between wood polymers. The deuteration of fresh wood was shown to be fully reversible by a subsequent exposure of the deuterated sample to water (reprotonation). Therefore, the presence of any OD groups in deuterated and then dried wood samples after reprotonation is a clear indicator of reduced accessibility. The extent of changes was affected by drying temperature and relative humidity. Application of this methodology for the evaluation of chemical pulp sample (reference material) resulted in similar response, only more pronounced. Two hypothetical alternatives were proposed for accessibility reduction in dried wood: (i) irreversible aggregation of cellulose microfibrils and (ii) irreversible stiffening of the hemicellulose/lignin matrix that extensively swells when exposed to water.

1. Introduction Wood is a native composite of polymers: semicrystalline cellulose microfibrils are embedded in a matrix of hemicellulose and lignin to form the cell wall of a wood fiber.1 The material potential of wood is enormous and it has been effectively utilized by humans for thousands of years. Traditional applications include papermaking and building materials among many others. On the other hand, modern materials science looks onto wood as a raw material for biofuels, a variety of specialty chemicals and endemic, functionally versatile nano-objects.2-6 In view of both the traditional and the modern applications, fundamental understanding of basic properties of the wood polymer composite is of great importance. One of the fundamental issues that has received surprisingly little attention is the alteration of the wood cell wall ultrastructure upon drying of wood. Trees grow in water-swollen conditions, and once a tree is felled, the wood quickly undergoes clearly perceptible changes in its texture, which are mostly associated with water escape from wood structure. A number of studies have previously demonstrated that this loss of water has a direct impact on mechanical properties of wood.7-11 On the other hand, several processes are determined to use wood in its “green” form (prior to its drying) because of easier pliability and higher accessibility in chemical reactions. As an example, alterations of wood cell wall ultrastructure and characteristics by water removal can have an impact on wood bioconversion processes. It has been shown that in addition to chemical components and their interactions, physical features of the wood ultrastructure can affect the efficiency of enzymatic hydrolysis of wood carbohydrates.12-14 In this paper, we intend to demonstrate that drying at moderate temperatures induces fundamental, irreversible changes in the properties of the wood polymers and wood ultrastructure. One wood-derived material, chemical pulp, has received considerable attention with respect to its behavior during drying. * To whom correspondence should be addressed. Tel.: +358 9 451 4250. Fax: +358 9 4514 259. E-mail: [email protected].

Chemical pulp is prepared by breaking the anisotropic network with individualization of fibers and removing the lignin from wood cell wall, resulting in excessively porous fibers. Upon drying, the swelling potential of chemical pulp fibers is greatly reduced due to irreversible pore closure. This phenomenon, termed hornification, directly affects the fiber properties, which cannot be restored by rewetting.15-18 Hornification has been confirmed and described for chemical pulps only, and many of its characterization methods, water retention value (WRV) measurement, in particular, are suited exclusively for chemical pulp fibers. Therefore, to investigate the fundamentals of dryinginduced phenomena in wood fibers, we have chosen to utilize a simple concept of deuteration coupled with photoacoustic Fourier transform infrared (PAS FT-IR) spectroscopy. The concept makes use of the exchange of accessible OH groups in cellulose to OD groups upon exposure to D2O.19-22 The deuterium present in wood can then be easily monitored by IR spectroscopy because the OD stretch signal is located in an area of spectrum with no interference from other signals, circumventing the common problem in interpreting the complex IR spectrum of wood. Deuteration in combination with IR spectroscopy has been exploited previously to study accessibility of cellulose and cellulose derivatives,22-26 including native cellulose in wood.27,28 However, the fundamental changes in wood ultrastructure that take place during drying have never been properly evaluated from this point of view. In this study, the accessible OH groups were converted to OD groups by deuteration and the samples were subsequently dried under controlled conditions (temperature and relative humidity). The conversion of accessible OD groups to inaccessible ones, those retained in the sample after flushing with an excess of H2O, was a clear indicator of alteration in the wood sample ultrastructure. In addition, experiments with chemical pulps, a substrate known to undergo irreversible changes in fiber cell wall ultrastructure (hornification), were carried out by using similar experimental conditions and the results were compared to each other.

10.1021/bm901268j  2010 American Chemical Society Published on Web 12/21/2009

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Scheme 1. Schematic of Wood Sample Preparationa

a

Dimensions are not to scale.

2. Experimental Section Materials. Freshly felled pine (Pinus sylVestris) and spruce (Picea abies) wood samples from Eastern Finland were supplied in the form of discs, 7-10 cm thick and 20-40 cm in diameter. The sample preparation is depicted in Scheme 1. The discs were cut with a saw to form a rectangle, from which slivers of ∼1 mm thickness were sliced with a chisel. Using a sharp knife, small squares of approximately 5 × 5 mm were cut from the slivers. The dimensions of the specimen were selected to agree with the size requirements of the FT-IR photoacoustic detection cell. For pulp experiments, never-dried lignin-free (bleached) pulp was used. The pulp, initially delignified by kraft pulping process and then bleached, was obtained from paper mill in Sunila, (Finland). The pulp was made from spruce and pine wood (approximately 60% spruce and 40% pine). Deuterium oxide (99.9 atom % D, Sigma-Aldrich) was used for deuteration and relative humidity control. The salts used for humidity control were NaCl (99.5%, J. T. Baker) and NaOH (p.a. Merck). The saturated solution of these salts placed in a closed environment created approximately 7 and 75% D2O relative humidity, respectively. Although the approximations are based on literature data for water,29 the D2O relative humidity values reported for saturated solutions of these salts in deuterium at 20 °C did not differ significantly from the values reported for water.30 Deuteration and Controlled Drying Experiments. The deuteration was carried out in 10 mL glass vials by immersing wood/pulp samples in an excess of D2O for 2 × 20 min (pulp) or 60 min (wood). After the treatment, the samples were dried under different conditions and then flushed with an excess of water for identical period of time as deuteration (2 × 20 and 60 min for pulp and wood, respectively). Both deuteration and flushing were carried out at room temperature. All samples were then dried in a convection oven at 40 °C prior to measurement with the FT-IR spectrometer. The drying at controlled D2O relative humidity was carried out in vacuum desiccators. A schematic of the experiments is shown in Scheme 2. The samples, put in perforated aluminum containers, were placed on the porcelain plate in the desiccators containing D2O saturated solutions at the bottom. The desiccators were then evacuated and placed into oven (25 and 80 °C) for conditioning (7 days). Pulp samples for water retention value measurements were dried in similar manner; except the relative humidity was achieved using saturated aqueous solutions instead of solutions of D2O.

Suchy et al. Scheme 2. Schematic of Wood Drying Experiments at Controlled Temperature and D2O Relative Humidity

Pulp Analysis. The water retention value (WRV) of the pulps was determined according to the standard ISO 23714:2007 with a Jouan GR 4 22 centrifuge. FT-IR Spectroscopy. The spectra were collected using a Bio-Rad FTS 6000 spectrometer (Cambridge, MA) with a MTEC 300 photoacoustic detector (Ames, IA) at a constant mirror velocity of 5 kHz, 1.2 kHz filter, and 8 cm-1 resolution. First, a background spectrum with standard carbon black was measured. After collecting the background spectrum, the wood or pulp sample was put into a detection cell that was placed into the detector. After flushing with helium gas for 5 min, the cell was sealed, and the actual spectrum of the sample was recorded. The background measurement was carried out at the beginning of each set of measurements. For each measurement a minimum of 400 scans per spectrum were collected and processed using the Win-IR Pro 3.4 software (Digilab, Randolph, MA). Each spectrum was normalized to have the same value at 1200 cm-1. The spectra presented throughout this article are averages of at least four measurements. Each treatment and drying scenario was carried out in triplicate. A minimum of two samples were measured (both sides), with an additional sample measured if a noticeable difference between the two measurement were observed.

3. Results and Discussion 3.1. Wood Deuteration and Drying at Different Temperatures. The studies of cellulose deuteration have demonstrated that the rate and extent of OH f OD exchange depend on several factors, including the type and crystallinity of the cellulose sample,31 temperature, and relative D2O vapor pressure,21,23 as well as mode of deuteration (liquid or vapor phase).22 Depending on the cellulose type and conditions, the deuterium exchange in accessible regions has been shown to be complete from less than or close to 1 h20,31 up to several hours.32 The deuteration of cellulose in wood samples showed the majority of OH f OD exchange completed at early stages of the process (less than 100 min), in all accessible regions.27,28 Therefore, 60 min deuteration was assumed sufficient for the purpose of this investigation. In addition to deuteration, it has been previously shown that the majority of the exchanged OD groups in cellulose samples can be readily reversed back to OH when exposed to water or water vapors. The published data were obtained for pure cellulose-viscose films22 and sheets made from wood pulp,26 and, thus, it was important for this investigation to establish the extent of reprotonation, or revers-

Drying of Wood Observed by D2O Exchange and FT-IR

Figure 1. Top: FT-IR spectra of deuterated spruce (lower) and pine (upper) wood samples dried under different conditions and flushed with water. Spectra in black indicate drying at 75% D2O RH; gray spectra indicate drying at 7% D2O RH. Below: Direct peak size comparison of samples dried at different temperature-enlargement of the spectrum segment of interest indicated by frames in original spectra.

ibility of deuteration, for the wood samples under the conditions used. Ideally, complete reversibility of the exchange would be desirable, and if achieved, the OD groups remained in the wood structure after reprotonation would be a clear indicator of the changes in wood ultrastructure. The detailed analysis of the reversibility studies are shown in the Supporting Information (Figure S1). The main outcome of the testing was that no residual OD groups were retained within wood samples after exposure to an excess of water immediately following deuteration. This demonstrates a complete reversal of the OH f OD exchange for wood samples, and therefore, the presence of OD groups in dried samples after flushing would indicate occurrence of irreversible changes in wood structure taking place between the deuteration and the flushing steps. The deuterated fresh wood samples were dried for seven days under controlled D2O relative humidity (7 and 75%) at 25 and 80 °C. The controlled D2O environment during drying and different relative humidity levels were designed to prevent the undesirable impact of water on deuteration reversal occurring before any possible structural changes, and to evaluate the impact of RH on the exchange. After drying, the samples were flushed with an excess of water and then dried in a convection oven prior to the FT-IR measurement. The measured FT-IR spectra for spruce and pine samples are shown in Figure 1. A detailed image of the spectral region of interest is included in the lower part of the figure.

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Although the OD stretch peak was clearly visible in the spectra of all deuterated samples, a marked difference in the peak size was observed between samples dried at different temperatures. The samples dried at 25 °C exhibited noticeably smaller peaks than those observed for the samples dried at 80 °C. In addition, at 25 °C the relative humidity of D2O did not appear to have an effect on the amount of deuterium retained in the dried wood samples after flushing with water. This was indicated by the equal size of the OD peaks of the samples dried at low and high relative humidity. In contrast, during drying at 80 °C, the D2O relative humidity appeared to have an impact on the deuterium retention. The OD stretch peaks for samples dried at 75% RH were noticeably greater compared to the peaks of the samples dried at 7% RH. The chemical composition and ultrastructure of both spruce and pine wood are comparable and the IR spectra are similar. The response to drying represented by the size of the peaks measured for spruce and pine samples was practically identical. A direct comparison of both spruce and pine samples is shown in Supporting Information (Figure S2). 3.2. Deuterium Retention Stability. Photoacoustic (PAS) FT-IR measurement requires the tested materials to be dry, thus, all wood and pulp samples had to be dried prior to the measurement. To minimize the possible impact of this drying on wood samples, particularly since the effect of actual drying on the wood structure was investigated, the drying was carried out at rather moderate temperature of 40 °C. The samples had to be dried in the vicinity of the IR spectrometer and the available convection oven used for the drying was not equipped with sufficient humidity control. The humidity (5.5% RH) present in the oven during premeasurement drying had a significant impact on the deuterium retention in the wood control samples, as shown in the Supporting Information (Figure S1). Although the interaction of humidity with deuterated samples should preferably be prevented, no significant additional reprotonation was expected, assuming all accessible OD groups available after drying already reprotonated in the flushing stage. However, it was important to realize and potentially quantify the impact of slightly elevated temperature coupled with water vapor present in the drying prior to the FT-IR measurement on the deuterium retention, particularly considering the reductions observed for the control samples. The actual evaluation was carried out by conducting parallel experiments, drying of deuterated spruce and pine species at 25 and 80 °C, with 75% D2O relative humidity at both drying temperatures. After the subsequent flushing, the samples were dried before the FT-IR measurement at 40 °C. While one set was dried in the oven without RH control, the other set was dried at 0% RH, achieved by drying the samples in the same oven in a desiccator containing drying agent. The comparison of treated samples dried prior to the measurement at 0% RH and without humidity control is shown in Figure 2. Despite reprotonation of the deuterated and dried samples by flushing with an excess of water, the impact of water vapor on additional protonation is evident (Figure 2). The reduction in retained deuterium is mainly observed for the samples dried at 25 °C. Based on the OD peak area comparison, a decrease of 69 and 67% for spruce and pine samples, respectively, was observed. The overall reduction was much less for the samples dried at 80 °C, with the corresponding reductions measured at 34% for both spruce and pine samples. A complete comparison of the OD peak areas and relative retained deuterium values in percentage for spruce and pine samples is shown in Figure 3.

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Figure 2. Effect of water vapors present in drying before FT-IR measurement. Comparison of deuterated spruce (top) and pine (bottom) samples dried at 25 (left)/80 °C (right) and 75% D2O RH. Black lines (also indicated by 0% RH) represent peaks of samples dried in desiccator with a drying agent; gray lines represent the peaks of samples dried without humidity control.

Figure 3. Effect of water vapor present in drying before FT-IR measurement. OD peak area comparison of spectra shown in Figure 2. Darker (dotted) columns represent samples dried at 0% RH; empty columns indicate samples dried without RH control (black and gray peaks in Figure 2, respectively). Relative values (peak areas of the sample dried without RH control vs sample dried at 0% RH) in percentage are shown on the right.

It appeared that even after exposure to water during the flushing stage, further reprotonation occurred during the premeasurement drying. The majority of OD sites within the structure of the sample dried at 25 °C that were inaccessible to water during flushing became accessible to water vapor at slightly elevated temperature (40 °C). This could indicate that the structure alterations induced by drying at 25 °C can be almost completely reversed by applying water vapor at slightly elevated temperature. This is to a certain extent in agreement with the work of Hofstetter and co-workers who demonstrated

Suchy et al.

nearly complete reprotonation of OD groups in cellulose by exposure to water vapor.26 In those experiments, however, the testing was carried out using pure cellulose from wood pulp that was dried before deuteration. In contrast, the impact of water vapor on samples previously dried at 80 °C was lesser and the retained OD groups after flushing were markedly more resistant to reprotonation. It appeared that the alterations taking place in the wood structure during drying at 80 °C are for the most part irreversible. This decrease of the retained deuterium in the samples flushed with water indicated that the rate and extent of reprotonation may differ depending on whether it is carried out in liquid or gaseous phase. Although the slightly elevated temperature has to be considered, it appeared that the water vapor could reach regions inaccessible by liquid water. Preliminary testing done prior to this investigation showed that the extent of reprotonation of dried deuterated wood samples in liquid water is the same after 24 h exposure as that of 60 min flushing. The exposure to water vapor in excess of 24 h during the premeasurement drying was sufficient to reprotonate all accessible OD groups and thus the measured spectra indicate the final extent of reprotonation at the given conditions. Even though various aspects of cellulose deuteration have been studied previously, a systematic study on the impact of temperature on stability or accessibility of the exchanged OD groups has not been carried out. In conclusion, the impact of drying temperature on the extent of wood structure alterations is clearly evident; however it appears that the drying temperature may also have an effect on the reversibility of the changes. 3.3. Pulp Testing. The structure of the cell wall of chemical pulp fibers bears a similarity to the cell wall of the fibers present in native wood. The major distinction between the two is lignin, which is present in wood but is removed during the pulping process, ensuing the presence of larger pores within the matrix of the pulp fiber.33 This fundamental similarity coupled with the previous extensive studies on the impact of drying makes the chemical pulp fibers a suitable reference substrate for evaluation of the analytical concept. In addition, the correlation of the behavior of pulp fibers and wood during deuteration and drying can help better understand the alteration within the ultrastructure during initial drying of fresh wood. The pulp drying evaluation was carried out in a similar manner as on the wood samples. Deuterated pulp samples were dried at two different temperatures (25 and 80 °C), in a 7% D2O relative humidity environment. The lower RH level was selected to reflect the superior OH group accessibility and greater extent of ultrastructure alteration expected for pulp fibers compared to wood samples. An additional set of deuterated pulp samples was dried overnight at 105 °C. After drying, the samples were flushed with an excess of water. The measured spectra for the pulp samples are shown in Figure 4. Compared to the original pulp sample (control), the deuterated, dried, and flushed pulp samples exhibited a distinctive peak in the OD stretch region of the IR spectrum. Similarly, as observed in wood sample testing, the deuterated pulp samples dried for a shorter time at 105 °C (without conditioning) and flushed showed a distinct peak in the OD region, although visibly smaller compared to OD peaks of the conditioned samples. The lower amount of deuterium retained in the samples dried at 105 °C agrees with findings for the wood control samples in this investigation (Supporting Information, Figure S1). Although a reduction of retained deuterium during drying at this temperature was previously described for deuterated films of regenerated cellulose,21 the low amount of deuterium was

Drying of Wood Observed by D2O Exchange and FT-IR

Figure 4. FT-IR spectra of deuterated softwood bleached kraft pulp samples dried under different conditions and flushed with water. Table 1. Comparison of OD Peak Area and Water Retention Values (WRV) of Pulp Samples Dried at Different Temperaturesa sample control dried at 25 °C dried at 80 °C

OD peak area

WRV (%)

MC (%)

219 831

142 126 80

66.1 5.1 0.2

a The drying conditions were similar except the samples for OD peak area measurement were dried at 7% D2O relative humidity and the samples for WRV and moisture content (MC) were dried in 7% H2O relative humidity environment.

not indicative of the extent of hornification the pulp is expected to undergo when dried at this temperature. In contrast, the impact of temperature on retention of inaccessible OD groups resisting reprotonation was clearly evident for the samples dried at lower temperatures in D2O relative humidity environment. The OD peak size comparison indicated that the extent of changes in the pulp samples dried at 80 °C was significantly greater than the alterations in the samples dried at 25 °C. This extent of changes was confirmed by measurement of water retention values of the pulp samples under similar condition but water vapor environment. The comparison of the OD peak size and WRV of the dried samples is shown in Table 1. The impact of drying and of drying temperature on the decrease in WRV of dried samples is clearly evident. This suggests that the presence of the inaccessible OD groups after drying is an indicator of hornification. Although only two temperature data points were measured, it is interesting to note that the ratio of the WRV reduction at 25 and 80 °C (16 and 62%, respectively) is identical to the relative OD peak area increase, both calculated to be 1:3.8. 3.4. Wood and Pulp Testing Comparison. In the current study, chemical wood pulps behaved as expected: a decrease in WRV was accompanied by a comparable reduction in the accessibility of water as indicated by the irreversible retention of OD groups upon drying. The subsequent alterations in the pulp fiber properties have acquired the term hornification in the previous literature.15-18 Although the origins of hornification are still under debate, most recent studies agree that “irreversible aggregation of cellulose microfibrils” is the ostensible phenomenon leading to fiber stiffening.16,34-36 The aggregation hy-

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pothesis has been strengthened by the studies, showing that an increased hemicellulose content in pulps clearly reduces the extent of hornification, which is a logical causality considering that the hemicellulose matrix between the cellulose microfibrils would hinder their inherent tendency to aggregate.37 Untreated wood, on the other hand, boasts an extensive matrix of both hemicellulose and lignin between the microfibrils and the microfibril aggregation, therefore, appears like a remote possibility. Nevertheless, in the light of the current results (Figures 1-3), it is indisputable that the accessibility of water is altered during drying also in the case of fresh wood samples. This must be indicative of supramolecular rearrangements between the wood polymers, something that is smaller in scale than the previously reported microcracks in the cell wall after drying.11,38 We propose two hypothetical alternatives for the ultrastructural rearrangements: (i) irreversible aggregation of cellulose microfibrils in a similar manner (but to a smaller extent) that occurs during hornification of pulp fibers and (ii) irreversible stiffening of the hemicellulose/lignin matrix that extensively swells when exposed to water. The first hypothetical alternative is partially backed up by literature accounts. Microfibrils tend to form lamellar bundles in wood and rigorous freeze-drying procedures in the sample preparation are required to visualize single microfibrils in high resolution microscopy images.39 Elazzouzi-Hafraoui et al. recently found aggregated microfibrils even after severe acid hydrolysis, which was intended to individualize cellulose crystallites, and they suggested the association of adjacent cellulose microfibrils may take place already during biosynthesis.40 Similarly, Pa¨a¨kko¨ et al. and Abe et al. ended up with larger units than individual microfibrils after mechanical disintegration of fibers into nanocellulosic objects.5,41 Only Saito et al. have managed to truly individualize native microfibrils by the so-called TEMPO-mediated oxidation, which is a process that was shown not to be affected by the drying history of the fibers.42 According to our hypothesis, this association of microfibrils could be an artifact of drying. It is very seldom that plant cell walls are subjected to, for example, hydrolysis, enzymatic treatment, or microscopic analysis before at least partial dehydration, which results from removing the plant from its native growth environment. The second hypothetical alternative of the stiffening hemicellulose matrix is a known phenomenon: it is bound to take place during water removal, which elevates the glass transition temperature of dry hemicellulose to over 200 °C.43 However, the possible irreversible character of stiffening has not been reported and its verification would require testing of material properties by, for example, differential scanning calorimetry or dynamic mechanic analyzer, which is outside the scope of this introductory article. We emphasize that these two hypothetical mechanisms are not mutually exclusive.

4. Conclusions The presence of inaccessible OD groups in the deuterated and then dried wood samples after reprotonation with water was detected by FT-IR. This change in accessibility indicated that supramolecular rearrangements between wood polymers are occurring within the cell wall. The observed changes in wood ultrastructure were somewhat similar to the well-reported reduction of swelling properties in chemical pulps upon drying. As with chemical pulp, the effect was enhanced with elevated temperature during drying. Two hypothetical alternatives were proposed for the accessibility decrease in wood upon drying: (i) irreversible aggregation of cellulose microfibrils in a similar

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manner (but to a smaller extent) that occurs during hornification of pulp fibers and (ii) irreversible stiffening of the hemicellulose/ lignin matrix that extensively swells when exposed to water. Acknowledgment. Prof. Mark Hughes and Lauri Rautkari (M.S.) are thanked for help with sampling and deuteration experiments. Rita Hatakka is acknowledged for assisting with FT-IR measurements and data transfer. The authors acknowledge the support by UPM-Kymmene corporation. Supporting Information Available. Deuteration reversibility testing of spruce wood samples (S1), comparison of pine and spruce wood samples (S2), reproducibility comparison (S3), and uniformity of deuteration throughout the tested wood specimen (S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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