Absorption of Water in Thermally Modified Pine Wood As Studied by

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Absorption of Water in Thermally Modified Pine Wood As Studied by Nuclear Magnetic Resonance Paï vi M. Kekkonen, Aapo Ylisassi, and Ville-Veikko Telkki* Department of Physics, University of Oulu, P. O. Box 3000, FI-90014 Oulu, Finland ABSTRACT: Thermal modification is an environmentally friendly method to increase the lifetime and improve the properties of timber. In this work, we investigate absorption of moisture in thermally modified pine wood (Pinus sylvestris) immersed in water using various nuclear magnetic resonance (NMR) methods. Magnetic resonance images (MRI) visualize the spatial distribution of absorbed free water. Spin−echo spectra measured both below and above 0 °C reveal that thermal modification partially blocks the access of water to cell walls; even modification at 180 °C slightly reduces the amount of bound water, and the amount decreases about 80% in the case of the sample modified at 240 °C. The spectra and MRI show that, above the modification temperature of 200 °C, the amount of free water decreases, indicating that high modification temperature tends to close the pits connecting the wood cells. T2 relaxation time distributions measured using the Carr−PurcellMeiboom−Gill sequence show four components, two associated with bound water and two with free water. NMR cryoporometry measurements indicate that the bound water sites are mostly below 2.5 nm in size. A unique combined NMR cryoporometry and relaxometry analysis showed that the size of cell wall micropores is between 1.5 and 4.5 nm, and thermal modification significantly hinders the access of water to the pores. walls contain micropores because of incomplete filling of an intermicrofibrillar region. The maximum size of the micropores is around 2−4 nm.3 When wood is dried, the micropores collapse. Blocking of micropores may be one mechanism in explaining some of the properties of modified wood (e.g., decay resistance).3 NMR spectroscopy has proven to be an efficient tool in studying the relationship between wood and water. 1H NMR signal of water in wood can be distinguished from the solid wood signal by their different relaxation times,5 and the intensity of the water signal is proportional to the moisture content (MC) of wood.6,7 Hartley et al.8 showed that the absolute MC below the fiber saturation point can be determined by comparing the signals of the solid wood and moisture. Merela et al.,9 in turn, proposed a single point NMR method for instantaneous determination of MC of wood. Here, the MC of a wood sample is determined on the basis of the mass of the moist wood sample and the amplitude of the freeinduction decay (FID) signal. It has also been shown that portable NMR provides a powerful tool for the determination of local MC of wood in situ.10−13 The method can be used in a variety of wood constructions and for both coated and noncoated wood. Anisotropic line shapes can also be associated with the structure of wood.14 Measurement of self-diffusion of absorbed fluids by pulsed-field-gradient spin−echo (PGSE) NMR15 gives information about the porous structure of

1. INTRODUCTION Wood is a hygroscopic material which makes it sensitive to prevailing moisture conditions. Dimensional instability, namely, shrinking and swelling due to changes in moisture content, is caused by water accessing the wood cell walls.1 Exposure to water enhances wearing and decay as well as growth of mold and fungi. Thermal modification lessens significantly the harmful effects of exposure of wood to water. The ThermoWood process2 is a three-stage process in which intensive modification takes place at temperatures of around 200 °C for 2−3 h. The process is environmentally friendly and does not involve the use of chemicals. Water vapor is used as a protective gas in the process, and it also accelerates the desired chemical changes in the wood. The process improves the dimensional stability, ability to withstand weather, biological durability, and thermal insulation properties of wood.3 Thermally modified wood is used, e.g., in exterior cladding and decking, internal wall and ceiling panels, and internal flooring. ThermoWood sales production has increased from 20,000 to 120,000 m3 from 2003 to 2012. The ThermoWood process is mostly used for the modification of softwoods, especially pine and spruce. Microscopic structure of softwoods consists of axial tracheid cells that transport water and act as a support structure for the tree and parenchyma cells that form rays and resin canals along with tracheids. Longitudinal tracheid cells constitute over 90% of the wood volume.4 The cell walls of wood are composed of microfibrils that form several layers. Each of the layers is also composed of many thinner laminas, and hence the whole wall structure can be thought of as a laminate of laminates. Cell © 2013 American Chemical Society

Received: November 14, 2013 Revised: December 23, 2013 Published: December 31, 2013 2146

dx.doi.org/10.1021/jp411199r | J. Phys. Chem. C 2014, 118, 2146−2153

The Journal of Physical Chemistry C

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wood.16−19 Magnetic resonance imaging (MRI), in turn, reveals the spatial distribution of moisture.20−28 NMR relaxometry provides a more detailed insight into moisture in wood. T1 and T2 relaxation times of water molecules depend on their mobility and local environment, and hence relaxation time distributions have been used to obtain information about various moisture components present in both softwoods5,7,29−34 and hardwoods.32,35,36 The assigned moisture components encompass several structures of wood including bound water in the cell wall, free water in the vessel, fiber and parenchyma elements of hardwood, free water in lumens, and rays of softwood as well as heartwood, sapwood, juvenile wood, and compression wood. Bound water molecules may be hydrogen bonded to the hydroxyl groups of primarily the cellulose and hemicelluloses, and to a lesser extent also to the hydroxyl groups of lignin.1 They may also be in the void spaces between the cellulose chains,37 or in the cell wall micropores.3 NMR cryoporometry is a method for the determination of the pore size distributions of porous materials, based on the detection of the lowered solid−liquid phase transition temperature of a substance confined to pores.38,39 According to the Gibbs−Thomson equation, the melting point depression, ΔTm, is inversely proportional to the radius, r, of the pore: ΔTm = T0 − Tm(r ) =

2σclT0 k = r ΔHf ρc r

modified at three different temperatures (180, 200, and 240 °C) indicated that the dimensions of lumens decrease in all three orthogonal directions due to the modification, and the cell structure begins to decompose when the modification temperature is increased from 200 to 240 °C. As described above, the absorption properties are extremely important factors defining the critical properties, such as dimensional stability and decay resistance, of timber, and NMR techniques provide detailed information about the moisture components, which are not available with other methods. In this work, we investigate with the aid of various NMR techniques how thermal modification changes the water absorption properties of pine wood (P. sylvestris). The wood samples under investigation are immersed in water, and the NMR experiments are carried out after two different immersion periods (4 and 12 weeks). The spatial distribution of absorbed free water in wood is visualized by MRI. The relative amounts of bound water present in cell walls and free water in the larger cavities of wood structure, such as lumens of tracheid cells, rays, and resin canals, are determined by comparing the integrals of liquid water peaks in a spin−echo experiment measured both below and above the melting point of bulk water (0 °C). We have shown earlier that, in the former case, only bound water contributes to the NMR signal, because free water is frozen, whereas, in the latter case, the signal is proportional to the total amount of bound and free water.34 NMR relaxometry is used for revealing the nonfrozen moisture components as a function of temperature and combined NMR cryoporometry and relaxometry for the determination of micropore size distribution.

(1)

Here, T0 is the melting temperature of bulk liquid, Tm is the melting temperature of a cylindrical crystal with a radius r, and σcl, ΔHf, and ρc are the crystal−liquid interfacial energy, the bulk enthalpy of fusion, and the density of the frozen liquid, respectively.40 The T2 relaxation time of ice is much shorter than that of liquid water, and thus it is possible to exclusively measure the signal of the unfrozen liquid component by adding a small delay after the excitation pulse or by using a spin−echo pulse sequence. The melting point distribution is determined by measuring the amplitude of the liquid component as a function of temperature. The distribution can be converted to pore size distribution by means of eq 1. NMR cryoporometry has been used to study various porous materials including wood, pulp, and building materials.38,41−43 Viel et al.41 investigated water absorbed in ancient wood samples and determined the average pore diameter to be approximately 2 nm. Webber compared the NMR cryoporometry pore size distributions of softwood and hardwood samples saturated with cyclohexane.38 Ö stlund et al.42 used NMR cryoporometry to study the porosity changes of bleached wood pulp upon drying. Combined NMR cryoporometry and relaxometry provides interesting information about the temperature dependence of relaxation time components. It has been used, e.g., for the investigation of a range of building materials, including mortar, clay, and fired-clay brick.43 Chemical changes in wood at the molecular level caused by thermal modification have been studied using solid state NMR.44−47 We have investigated the changes in the connectivity of wood cells of Pinus sylvestris (P. sylvestris) pine wood by remote detection MRI.48 In addition, we have shown that dimensions of wood cell structures in pine can be determined using PGSTE (pulsed-field-gradient stimulated echo) NMR,19 and we used the method to measure the change in the dimensions of lumens of tracheid cells of pine wood due to thermal modification.49 Investigations of samples

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Green P. sylvestris wood plank (cross-section, 50 mm × 175 mm) kiln-dried at the temperature of 70 °C was split into two pieces. One piece was thermally modified using the ThermoWood process2 while the other was left unmodified as a reference piece. Wood pieces modified at four different temperatures (180, 200, 230, and 240 °C) were produced. Cylindrical samples (axis along the longitudinal direction) with a diameter of 6 mm and length of 2.5 cm were cut from the thermally modified and corresponding unmodified reference pieces from sites close to each other in the original wood plank to ensure the comparability of the results. Each sample piece contained several annual rings and approximately 500 000 tracheid cells in the measurement coil volume. Consequently, the results represent an average of the properties of wood and the early and late wood areas are proportionally weighted. In the following text MODX refers to the thermally modified sample, where X is the modification temperature in Celsius degrees, and REFX refers to its unmodified reference sample. Prior to the NMR measurements, the samples were immersed in distilled water for 4 or 12 weeks. After the immersion, excess water on the sample surface was removed, and the sample was inserted into a 10 mm o.d. NMR tube. Teflon spacers were used to position the sample piece correctly inside the sample tube and to prevent it from moving during the experiments. The tube was closed with a plastic cap. 2.2. MRI. 1H MRI measurements were carried out on a Bruker Avance DSX300 spectrometer (field strength, 7.05 T; 1 H resonance frequency, 300 MHz) using a Bruker Micro 2.5 imaging probe head with a 10 mm birdcage coil and x, y, and z gradients. Two-dimensional (2D) axial spin−echo images were 2147



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Figure 1. Axial MR images of free water absorbed in the samples modified at 180, 200, and 240 °C as well as their reference samples, measured after the immersion times of 4 and 12 weeks. One-dimensional profiles along tangential (red) and radial (green) directions are shown on the right side of each image.

64 × 64 was used giving an in-plane spatial resolution of 125 μm. The images were processed by MATLAB. 2.3. NMR. 1H NMR measurements were carried out on the Bruker Avance DSX300 spectrometer using a high resolution 10 mm BBO probe. Spectra were measured at variable

measured using Bruker Paravision software (version 2.1) at room temperature. The echo and repetition times (TE and TR) were 10.6 and 500 ms, respectively, and the number of accumulated scans was 8. The slice thickness was 4.0 mm, and an isotropic field of view of 8 mm × 8 mm with a matrix size of 2148



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wood in the center of the samples. After full water saturation (12 weeks), the signal of free water in the early wood becomes larger than that in the latewood, because the lumen volume in the early wood is significantly larger than in the late wood.4 The images indicate that, at the lowest modification temperature (180 °C), the modification has practically no effect on the free water absorption, because the images of the modified and reference samples are similar. Modification at 200 °C decreases the water absorption rate slightly, because, after 12 weeks immersion, the MOD200 sample is not yet fully saturated while REF200 is (this is most clearly visible in the 1D profiles). Water absorption in MOD240 is very slow, because there is only a small amount of water absorbed into the sample even after 12 weeks immersion. Contrary to the other reference samples, the REF240 sample is not fully saturated after 12 weeks immersion, probably due to natural variations between wood samples. Nevertheless, the water absorption is significantly faster into REF240 than into MOD240, implying that the modification at the highest temperature considerably decreases the water absorption rate. 3.2. Amounts of Bound and Free Water. The echo time in the spin−echo NMR experiments (245 μs) was much longer than the T2 relaxation time of solid wood and ice, but much shorter than T2 of bound and free water (see T2 distributions in section 3.3). Hence, the integral of the signal is proportional to the amount of unfrozen water in the wood samples. In the experiments carried out at 265 K, free water is frozen, and the signal is proportional to the amount of bound water only, while, at 281 K, it is proportional to the overall amounts of free and bound water. Consequently, the relative amounts of free and bound water in the samples can be determined by comparing amplitudes corrected for Curie’s law at 265 and 281 K.34 Furthermore, because the sizes of the samples were equal and the experiments were conducted in the same spectrometer with the same parameters, the integrals allowed the comparison of the amounts of absorbed water in different samples. The relative amounts of free and bound water as a function of the immersion time determined by the spin−echo method are shown in Figure 2a,b, respectively. Both the amounts of free and bound water increase significantly from 4 to 12 weeks, confirming that neither of the sites is water saturated after 4 weeks. Parts c and d of Figure 2 show the amounts of free and bound water as a function of modification temperature, measured after two different immersion times. Whereas after a particular immersion time the amounts of free and bound water are equal for all the reference samples, the amounts of both free and bound water in the modified samples decrease at high modification temperature. At the two lowest modification temperatures (180 and 200 °C), the amounts are similar, but above 200 °C they begin to decrease significantly. Parts e and f of Figure 2 show how the amounts of free and bound water in thermally modified samples differ from those in their reference samples. Hence, the figures reflect the effect of thermal modification on the amount of absorbed water components. Interestingly, the results obtained after 4 and 12 weeks immersion are almost identical, although the absolute amount of absorbed water increases significantly from 4 to 12 weeks. The amount of free water seems to decrease only above 200 °C, and it is about 50% at the highest modification temperature (240 °C). On the other hand, the amount of bound water is slightly decreased even at the lowest modification temperature (180 °C), and it is only about 20% at 240 °C. Consequently, the modification has larger effect on

temperatures spanning from 192 to 295 K with a 0.5 K step using the spin−echo sequence.50 At the beginning of the temperature series, the sample temperature was allowed to stabilize at the lowest temperature for 1 h, and then the heating rate was kept at a constant value of 7 K/h by applying a proper temperature stabilization delay before the experiments. The probe was tuned with an interval of 10 K. The relaxation delay and number of accumulated scans were 1 s and 8, respectively, and the experiment time was 12 s. The echo time was selected to be 245 μs to keep it shorter than the T2 of liquid water but longer than that of ice. Integrals of the signals determined as a function of temperature were multiplied by the factor T/T0 in order to eliminate the temperature dependence of thermal equilibrium magnetization given by Curie’s law. After the correction the integral is proportional to the amount of unfrozen water, providing that the effect of the temperature dependence of the T2 relaxation time is negligible. NMR cryoporometry pore size distributions were obtained by a leastsquares fit of a model function to the measured integrals as described by Aksnes et al.51 Constant k in eq 1 was taken to be 30 nm K, the value experimentally determined for water absorbed in controlled pore glasses.52 Hansen et al.53 have estimated that there exists a nonfreezing layer with a thickness of 0.3−0.8 nm, and hence the value of 0.6 nm was added to the NMR cryoporometry diameters in order to obtain the true pore sizes. In addition to the spin−echo experiments, CPMG (Carr− PurcellMeiboom−Gill)54,55 experiments were carried out for the samples modified at 200 and 230 °C and their reference samples, using the following modified sequence: (π /2)x − τ1 − πy − τ1 − echo − τ2 − πy − τ2 − echo − [τ3 − πy − τ3 − echo]n − 2

Delays τ1, τ2, and τ3 were adjusted so that the first and second echoes appeared at 0.25 and 1 ms after the π/2 excitation pulse, and the time between the subsequent echoes was 1 ms. One complex point was collected at each echo time instant, and the total number of echoes (and collected complex points) was 512. The last echo was produced at 511 ms after the π/2 excitation pulse. The relaxation delay and the number of accumulated scans were 1 s and 8, respectively, and the experiment time was 17 s. The Laplace inversion of the measured echo amplitudes using the CONTIN program56 resulted in the T2 relaxation time distribution of water in the samples.

3. RESULTS AND DISCUSSION 3.1. Spatial Distribution of Free Water. Axial MR images of the samples modified at 180, 200, and 240 °C as well as their reference samples, measured after the immersion times of 4 and 12 weeks, are shown in Figure 1. One-dimensional profiles along tangential (red) and radial (green) directions are seen on the right side of each image. The echo time (10.6 ms) used in MRI experiments is much longer than T2 relaxation times of solid wood and bound water, and hence the images reflect the spatial distribution of free water. After 4 weeks immersion water has mainly absorbed into the outer layer of the sample while after 12 weeks immersion most of the samples are fully saturated with water. Absorption into the late wood appears to be faster than into the early wood; after 4 weeks immersion, the signal of free water in the late wood is stronger than in the early 2149



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bound water in the cell walls, since free water is frozen, and the ice signal is not observable in the CPMG experiments because of its short T2. Based on our previous observations,34 we interpret that the shorter T2 component with the large amplitude arises from water molecules hydrogen bonded to the hydroxyl groups and between the cellulose chains, whereas the longer T2 component with the small amplitude originates from water in cell wall micropores. Four components are observed above 273 K. The two shortest T2 components are interpreted to correspond to the bound water components observed below 273 K. The increase in the relaxation time of the longer T2 component may be a consequence of the fast exchange between the micropore water and some free water.57 The two longer T2 components arise from free water, and, based on the investigations of Menon et al.,20 the longest T2 component is assumed to arise from early wood tracheid lumens while the other component originates from smaller late wood tracheid lumens as well as ray lumens. 3.4. Size of Cell Wall Micropores. The amplitudes of variable temperature spin−echo experiments of the samples thermally modified at 180, 200, 230, and 240 °C as well as their reference samples are shown in Figure 4a. The pore size distributions resulting from NMR cryoporometry analysis of the data are seen in Figure 4b. The distributions imply that the size of bound water sites is 1.2−2.5 nm, and the distributions of the thermally modified samples are otherwise similar to their reference samples, but the amplitudes are smaller due to a smaller amount of bound water. However, it is questionable whether the pore size distributions are exactly true, because, as shown in Figure 3, T2 of bound water decreases rapidly with decreasing temperature below 230 K, and this may decrease the signal amplitude in the spin−echo experiment, leading to an artificial peak in the pore size distribution. Furthermore, the investigations of Morishige et al.58 imply that water confined in very small pores (diameter of 1.2 nm) freezes gradually over a wide temperature range (in pores with a diameter of over 1.75 nm the freezing transition was observed to be sharp). Hence, the pore size distributions in Figure 4b cannot be considered to be quantitative, but, nevertheless, they confirm that the bound water sites are mostly below 2.5 nm in size. Figure 4c shows the result of NMR cryoporometry analysis of the CPMG micropore component only (the second shortest T2 component in Figure 3) for the samples thermally modified at 200 and 230 °C and their reference samples. To the best of our knowledge, this is the first time relaxometry and NMR cryoporometry methods are combined in this manner to obtain a unique resolution of a certain moisture component. The distributions imply that the size of the micropores is between 1.5 and 4.5 nm. This is in good agreement with solute exclusion measurements that indicate the maximum size for the cell wall micropores to be 2−4 nm.3 The area of the micropore size distribution is significantly reduced due to modification at 230 °C, indicating that cell wall micropores are also blocked due to the modification at high temperature. We note that in this case the melting of water takes place at high enough temperature and the pore size of the micropores is so large that the validity of the results does not need to be questioned in the same manner as in the previous case. In addition, we note that the micropore size distributions in Figure 4c are also superimposed to the corresponding distributions in Figure 4b. However, as the amount of micropore water is very small as compared to other bound water, the latter dominates in the distributions shown in Figure 4b.

Figure 2. Amounts of bound water (BW) and free water (FW) determined by spin−echo NMR: (a) bound and (b) free water as a function of immersion time; (c) bound and (d) free water as a function of modification temperature after 4 and 12 weeks immersion time. Change in the amounts of (e) bound and (f) free water due to thermal modification. The data labeled by asterisk in panels e and f was collected in separate experiments, and hence the corresponding data is not shown in panels a−d, because absolute integrals are not comparable.

bound water content than free water, indicating that modification partially blocks the access of water to cell walls, and this is most likely the reason for improved dimensional stability of thermally modified wood.3 Modification at the highest temperature also partially blocks the absorption routes of free water. The cells in a conifer tree are of closed type, and free water is transported from cell to cell through the pits in the cell walls.4 The results indicate that a large amount of pits are closed in the thermal modification above 200 °C, which is in agreement with our previous time-of-flight remote detection MRI observations48 and other investigations that show that the pits are closed when the wood is heated.3 3.3. Environments of Bound and Free Water. T2 relaxation time distributions of the samples thermally modified at 200 and 230 °C and their reference samples, measured as a function of temperature using the CPMG sequence after 4 weeks immersion, are shown in Figure 3. Altogether four components are distinguished in the distributions, both in the case of thermally modified and unmodified samples. The two components observed below 273 K can be associated with 2150



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Figure 3. (a−d) T2 relaxation time distributions of the samples thermally modified at 200 (c) and 230 °C (d) as well as their unmodified reference samples (a and b), measured as a function of temperature. Instead of amplitude A, the natural logarithm of value A + 1 [ln(A+ 1)] is plotted in order to enhance small amplitude components. (e and f) The corresponding distributions measured at 265 and 280 K, with normal, nonlogarithmic intensity scale.

when the sample is thermally modified above 200 °C, and, for the sample modified at 240 °C, the amount of free water is only about 50% of the amount in the reference sample. These results indicate that large amount of pits connecting wood cells are closed in the thermal modification above 200 °C. T2 relaxation time distributions show four components corresponding to four different environments of absorbed water both in the case of thermally modified and unmodified samples. Two of them are interpreted to arise from bound water, one from water hydrogen bonded to the hydroxyl groups and between the cellulose chains (shorter T2), and the other from cell wall micropores (longer T2). Two free water components, in turn, are interpreted to originate from early wood tracheid lumens (longer T2) and late wood tracheid lumens as well as ray lumens (shorter T2). NMR cryoporometry measurements indicate that the sizes of bound water sites are mostly below 2.5 nm both in thermally modified and unmodified samples. A unique NMR cryoporometry analysis of the micropore component only, in turn, shows that the size of micropores is

4. CONCLUSION In this work, absorption of water in thermally modified wood was investigated by applying various NMR methods. MRI showed that free water absorbs faster in late wood than in early wood, and thermal modification at 180 °C does not have detectable effect on the free water absorption, while modification at 200 °C slightly, and at 240 °C significantly, decreases the absorption rate. Comparison of the integrals of spin−echo signals measured below and above the melting point of bulk water reveals that even the lowest modification temperature (180 °C) reduces the amount of absorbed bound water, and the amount decreases quite linearly with increasing modification temperature, being only about 20% of the amount in the unmodified reference sample in the case of the highest modification temperature (240 °C). Consequently, the results show that thermal modification partially blocks the access of water to cell walls, and this is most likely the reason for improved dimensional stability of thermally modified wood. The amount of free water, in turn, decreases noticeably only 2151



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(2) Viitaniemi, P.; Jämsä, S.; Ek, P.; Viitanen, H. Method for Improving Biodegradation Resistance and Dimensional Stability of Cellulosic Products U.S. Pat. US 5678324, Oct. 21, 1997. (3) Hill, C. A. S. Wood Modification: Chemical, Thermal and Other Processes; Wiley: Chichester, U.K., 2006. (4) Kettunen, P. O. WoodStructure and Properties; Trans Tech Publications: Durnten-Zurich, Switzerland, 2006. (5) Menon, R. S.; MacKay, A. L.; Hailey, J. R. T.; Bloom, M.; Burgess, A. E.; Swanson, J. S. An NMR Determination of the Physiological Water Distribution in Wood During Drying. J. Appl. Polym. Sci. 1987, 33, 1141−1155. (6) Sharp, A. R.; Riggin, M. T.; Kaiser, R.; Schneider, M. H. Determination of Moisture Content of Wood by Pulsed Nuclear Magnetic Resonance. Wood Fiber Sci. 1978, 10, 74−81. (7) Riggin, M. T.; Sharp, A. R.; Kaiser, R. Transverse NMR Relaxation of Water in Wood. J. Appl. Polym. Sci. 1979, 23, 3147− 3154. (8) Hartley, I. D.; Kamke, F. A.; Peemoeller, H. Absolute Moisture Content Determination of Aspen Wood Below the Fiber Saturation Point using Pulsed NMR. Holzforschung 1994, 48, 474−479. (9) Merela, M.; Oven, P.; Serša, I.; Mikac, U. A Single Point NMR Method for an Instantaneous Determination of the Moisture Content of Wood. Holzforschung 2009, 63, 348−351. (10) Casieri, C.; Senni, L.; Romagnoli, M.; Santamaria, U.; De Luca, F. Determination of Moisture Fraction in Wood by Mobile NMR Device. J. Magn. Reson. 2004, 171, 364−372. (11) Dvinskikh, S. V.; Furó, I.; Sandberg, D.; Söderström, O. Moisture Content Profiles and Uptake Kinetics in Wood Cladding Materials Evaluated by a Portable Nuclear Magnetic Resonance Spectrometer. Wood Mater. Sci. Eng. 2011, 6, 119−127. (12) Pourmand, P.; Wang, L.; Dvinskikh, S. V. Assessment of Moisture Protective Properties of Wood Coatings by a Portable NMR Sensor. J. Coat. Technol. Res. 2011, 8, 649−654. (13) Johansson, J.; Blom, Ǻ .; Dvinskikh, S. NMR-Measurements for Determination of Local Moisture Content of Coated Wood. J. Coat. Technol. Res. 2013, 10, 601−607. (14) Terenzi, C.; Dvinskikh, S. V.; Furo, I. Wood Microstructure Explored by Anisotropic 1H NMR Line Broadening: Experiments and Numerical Simulations. J. Phys. Chem. B 2013, 117, 8620−8632. (15) Stejskal, E. O.; Tanner, J. E. Spin Diffusion Measurements: Spin Echoes in the Presence of a Time-Dependent Field Gradient. J. Chem. Phys. 1965, 42, 288−292. (16) Wycoff, W.; Pickup, S.; Cutter, B.; Miller, W.; Wong, T. C. The Determination of the Cell Size in Wood by Nuclear Magnetic Resonance Diffusion Techniques. Wood Fiber Sci. 2000, 32, 72−80. (17) Meder, R.; Codd, S. L.; Franich, R. A.; Callaghan, P. T.; Pope, J. M. Observation of Anisotropic Water Movement in Pinus Radiata D. Don Sapwood Above Fiber Saturation Using Magnetic Resonance Micro-Imaging. Holz Roh- Werkst. 2003, 61, 251−256. (18) Johannessen, E. H.; Hansen, E. W.; Rosenholm, J. B. Fluid SelfDiffusion in Scots Pine Sapwood Tracheid Cells. J. Phys. Chem. B 2006, 110, 2427−2434. (19) Kekkonen, P.; Telkki, V.-V.; Jokisaari, J. Determining the Highly Anisotropic Cell Structures of Pinus sylvestris in Three Orthogonal Directions by PGSTE NMR of Absorbed Water and Methane. J. Phys. Chem. B 2009, 113, 1080−1084. (20) Menon, R. S.; MacKay, A. L.; Flibotte, S.; Hailey, J. R. T. Quantitative Separation of NMR Images of Water in Wood on the Basis of T2. J. Magn. Reson. 1989, 82, 205−210. (21) Quick, J. J.; Hailey, J. R. T.; MacKay, A. L. Radial Moisture Profiles of Cedar Sapwood During Drying: A Proton Magnetic Resonance Study. Wood Fiber Sci. 1990, 22, 404−412. (22) McMillan, M. B.; Schneider, M. H.; Sharp, A. R.; Balcom, B. J. Magnetic Resonance Imaging of Water Concentration in Low Moisture Content Wood. Wood Fiber Sci. 2002, 34, 276−286. (23) Rosenkilde, A.; Glover, P. High Resolution Measurement of the Surface Layer Moisture Content During Drying of Wood Using a Novel Magnetic Resonance Imaging Technique. Holzforschung 2002, 56, 312−317.

Figure 4. (a) Normalized integrals of the spin−echo measurements of the thermally modified samples as well as their reference samples as a function of temperature. The samples were immersed in water for 4 weeks before the experiments. (b) NMR cryoporometry pore size distributions of bound water sites resulting from the analysis of the data in panel a. (c) Micropore size distribution resulting from the analysis of the amplitudes of the micropore component in T2 distribution shown in Figure 3.

between 1.5 and 4.5 nm, and thermal modification at 230 °C significantly reduces access of water to the micropores.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ville-veikko.telkki@oulu.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Academy of Finland, the Alfred Kordelin Foundation, the Finnish Cultural Foundation, the Tauno Tönning Foundation, and the Finnish Science Foundation for Economics and Technology for financial support. We thank Professor Emeritus Jukka Jokisaari for his valuable comments on the manuscript.

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REFERENCES

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Absorption of Water in Thermally Modified Pine Wood As Studied by

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