Characterization of Jack Pine Early- and Latewood Fibers in Thermomechanical Pulping

Published: October 10, 2011

r 2011 American Chemical Society 13396 dx.doi.org/10.1021/ie2019992 | Ind. Eng. Chem. Res. 2011, 50, 13396–13402

ARTICLE

pubs.acs.org/IECR

Characterization of Jack Pine Early- and Latewood Fibers inThermomechanical PulpingFang Huang,*,† Robert Lanouette, and Kwei-Nam Law

Centre de recherche sur les mat�eriaux lignocellulosiques, Universit�e du Qu�ebec �a Trois-Rivi�eres, Qu�ebec QC G9A 5H7, Canada

ABSTRACT: The principal objective of this research was to study the morphological changes of Jack pine (Pinus banksiana)earlywood (EW) and latewood (LW) in thermomechanical pulping (TMP). The results indicate that EW fibers tend to separate atthe P/S1 interface, whereas LW fibers commonly fail in the regions between the primary wall and the transition layer or outer layer ofthe secondary wall (P/S1) and between the outer and central layers of the secondary wall (S1/S2). LW fibers exhibit mostly intrawallfailure and lower curl and kink indices, whereas EW fibers tend to fail in transwall mode (splitting) and suffer fiber cuttings. Inaddition, the thin-walled EW fibers show higher collapsibility and conformability than their thick-walled LW counterparts.Moreover, EW fines have higher surface lignin coverage, whereas LW fines have a higher specific volume (SV).

’ INTRODUCTION

In the temperate region, when the atmospheric temperaturerises in the spring, tree growth activities begin with the division ofcells in the cambium. This activity is believed to be regulated bythe growth hormone auxin. The early growth is fast and givesrises to fibers or tracheids in conifers (softwood) with large dia-meters and relatively thin cell walls. This wood tissue is calledspringwood or earlywood (EW). As the late summer approachesand the temperature falls, tree growth gradually slows, producingfibers with smaller cell lumens and thicker cell walls. This zone ofthick-walled fibers is called summerwood or latewood (LW). Thetree growth season ends in the fall as trees begin to shed leaves orneedles. The most visible differences between EW and LW fibersare that the former tend to have larger diameters, thinner cellwalls, and larger radial widths than the latter.1,2 These differencesaffect not only fiber properties, such as density and mechanicalstrength,3,4 but also chemical5,6 and mechanical pulping7,8 andpaper properties.8

Because of their differences in morphology, EW and LW fibersbehave differently in thermomechanical pulping. Studies1,9�11

have indicated that splitting of the cell walls occurs principally inEW fibers, particularly in the first stage of refining, and thatreduction of the wall thickness takes place more often in thethick-walled LW fibers.Moreover, EW fibers tend to change formeasily because of their greater compressibility and flexibility.Conversely, LW fibers are more resistant to the refining actionincluding flexion and kneading, which makes changing theircross-sectional shape difficult. It was reported that EW absorbsenergy easily and requires more energy to reach the samefreeness than LW.8 EW tends to break into fragments of irregularforms and sizes (shives), whereas LW disintegrates into slenderbundles or individual fibers.2

Despite these findings, it remains unclear how these two typesof wood tissue are transformed from solid wood into individualfibrous elements in refining, especially thermomechanical refin-ing. This study systematically characterized the changes in EWand LW during the thermomechanical pulping (TMP) pulpingprocess, including those in cell-wall thickness, fiber length,

coarseness, curl and kink indices, specific volume (SV) of fines,and water retention value (WRV). Light microscopy and scan-ning electronic microscopy were used to qualify and quantifyfiber fibrillation and cell-wall damage in refining. Electron spectro-scopy for chemical analysis (ESCA) was used to analyze the fibersurface coverage of lignin. The results obtained from this researchprovide a better understanding of the breakdown mechanism ofthese two types of wood tissue and are expected to help improverefining efficiency.

’EXPERIMENTAL SECTION

Materials and Preparation. Logs of freshly cut Jack pine(Pinas banksiana Lamb.) were used in this study. The Jack pinetrees were taken from a 30-year old plantation in the St. Mauriceregion of Quebec, Canada. EW and LW chips were prepared bymeans of a chisel, as discussed in previous studies.10

In this study, the basic density (oven-dry weight/volume) ofEW/LW chips was measured following TAPPI method T-258om-02. The cross-sectional features of EW and LW, such as cell-wall thickness and lumen area were measured based on TAPPImethod T-263 sp-06 and the ImageJ algorithm.12 Some majorchemical components, namely, Klason lignin, holocellulose, α-cellulose, dichloromethane (DCM) extractives, and ash weredetermined by TAPPI methods, as listed in Table 1.Chemical Maceration of EW and LW Fibers. To study the

fiber characteristics of the starting EW and LW, such as fiberlength and cell-wall thickness, it was necessary to liberate thefibers from the wood matrix by means of chemical maceration.13,14

Thermomechanical Pulping. A Sunds Defibrator 300 CDpilot plant (Metso Paper) was used for refining of the wood chips.Its refining capacity is 2 t/day. The models of the refiner rotor andstator plate employed are R3809BG and R3803, respectively. EW

Received: September 2, 2011Accepted: October 10, 2011Revised: October 6, 2011

13397 dx.doi.org/10.1021/ie2019992 |Ind. Eng. Chem. Res. 2011, 50, 13396–13402

Industrial & Engineering Chemistry Research ARTICLE

and LW chips were pulped separately. In the process, the chipswere presteamed at atmospheric pressure for 10 min and thenscrew-fed into a digester using a 2:1 compression ratio.The refining was carried out in two stages. The first stage was

under pressure at 160 �C, and the pulps were produced with afreeness of about 500 mL. The Canadian standard freeness(CSF) was measured following TAPPI method T-227 om-04.The primary pulps were refined at atmospheric pressure to afreeness range of 50�200 mL. The specific energies were re-corded for the pulps of each freeness level. The first-stage refiningconsistency was about 20�24%, whereas that at the second stagewas around 10�14%. After being refined, all the pulp sampleswere disintegrated with 90 �C hot water to remove latency priorto further analysis.15,16

Fractionation of Pulps. The second-stage pulps were fractio-nated in a Bauer�McNett classifier to obtain six fractionsdenoted as R14, R28, R48, R100, R200, and P200 (fines).The Bauer�McNett fiber classification is a commonly used

method to characterize the fiber-length distribution of mechan-ical pulps. The fibers in different Bauer�McNett fraction are

morphologically different and have different effects on paperproperties. For example, the R14 fraction contains long and stifffibers that have poor bonding characteristics. The fines (P200)fraction comprises flakelike particles and fibrils that strengthenthe fiber network. For these reasons, it was necessary to fractionate

Table 1. Test Methods for Chemical Analysis

component test method

Klason lignin TAPPI T-222 om-98

Holocellulose TAPPI T-9 wd-75

α-cellulose TAPPI T-203 om-99

dichloromethane (DCM) extractives TAPPI T-204 cm-97

ash TAPPI T-211 om-02

Table 2. Basic Property of Jack Pine EW and LW Fibers

basic

density

(g/cm3)

fiber

length

(mm)

cell-wall

thickness

(μm)

outer

perimeter

(μm)

lumen

area

(μm2)

cell-wall area

(μm2)

EW 0.30 3.34 2.12 130 400 240

LW 0.49 3.55 4.75 105 260 350

Table 3. Major Chemical Components of EW and LW Fibers

Klason lignin

(%)

holocellulose

(%)

α-cellulose

(%)

DCM extractives

(%)

ash

(%)

EW 28.30 68.68 42.34 1.95 0.17

LW 27.09 71.01 44.55 1.62 0.17

Figure 1. Freeness as a function of specific refining energy.

Table 4. Weight Distribution of Bauer�McNett Fractions ofEW and LW from Pulp with a CSF of 150 mL

fraction R14 R28 R48 R100 R200 P200 (fines)

EW 0.7 34.0 19.2 10.0 7.5 28.6

LW 7.6 36.8 16.4 6.9 4.5 27.8

Figure 2. Fiber length reduction of the R28 fraction as a function ofspecific refining energy.

Figure 3. Cell-wall reduction of the R28 fraction as a function of specificrefining energy.

Figure 4. Coarseness of the R28 fraction as a function of specificrefining energy.



the pulps before characterization. Because of space considera-tions, only the R28 and P200 (fines) fractions are discussed inthis article.FQA Analysis. The Fiber Quality Analyzer (FQA, OpTest

Equipment Inc.) employed in this study was an optical deviceused to measure the length-weighted mean fiber length (llw),fiber coarseness, and fiber curl and kink indices.17�19

Cell-Wall ThicknessMeasurements.Cell-wall thickness is animportant morphological characteristic that is related to a fiber’sstiffness or its ability to form interfiber bonds. In this study, thecell-wall thickness was measured by means of a MorFi cell-wallthickness analyzer (Techpap, St.-Martin-d'H�eres, France).20

Rejects Analysis. The reject contents were determined bymeans of a Pulmac shive analyzer equipped with a 0.1-mm screenplate as described in TAPPI test method T-274 sp-97. The rejectswere collected for microscopic analysis of fiber types and rupturemodes.Specific Volume (SV) Analysis of Fines by Sedimentation.

The quality of the fines is an important characteristic of pulp, andit can be qualified in terms of the specific volume (SV) of theseparticles. The specific volume of the fines was measured asfollows: First, 10 g of pulp was diluted to 0.2�0.3% consistencywith demineralized water. The pulp suspension was then passedthrough a 200-mesh wire into a 2-L dynamic draining jar (DDJ).The filtrate containing fines was condensed to a consistency of0.4�0.5% by centrifugation.In this work, 50 mL of fines suspension was first mixed with a

solution containing 120 mg/L Na2SO4 and 30 mg/L CaCl2(mixed in a volume ratio of 1:2) to equilibrate the ion content,and then the pHwas adjusted to 6�6.5 with 1 g/L NaOH or 1 g/LHCl.11 Next, the suspension was transferred into a 100 mLgraduated glass cylinder, and the surplus was discharged. Before

the settling test, the air in the suspension was removed with avacuum pump for 10 min. After air removal, the cylinder wassealed with parafilm and manually shaken well to disperse thesuspension. Then, the suspension was kept still for 24 h, after whichthe volume of the settled fines was read (accuracy of(1mL), andthe suspension was filtered to recover the fines. The mass of fineswas obtained after they had been dried at 105 �C; the weight ofthe recovered fines was used to determine the SV value. The ratioof the volume of the fines suspension to the oven-dry weight ofthe recovered fines is defined as the specific volume (SV) ofthe fines.Water Retention Value (WRV) Determination. The WRV

reflects the fiber cell-wall damage occurring during refining, suchas splitting and internal fibrillation (delamination). The WRVwas determined using TAPPI method UM 256.Microscopy Analysis. This study employed both light micro-

scopy and scanning electronic microscopy (SEM). Light micro-scopy and image analysis were used to quantitatively evaluatefiber cross-sectional characteristics in refining, including cell-walldamage21 and collapsibility.22 In this study, the proportion offibers damaged in the cell wall was assessed by counting the fiberspresent in the features of interest, namely, transwall and intrawallfailures. The form factor (FF) was used to evaluate the fibercollapsibility.23,24 It is defined as

FF ¼ 4πAt

P2

where At is the area of filled fibers (fiber wall area + lumen area)and P is the outer perimeter of the cell wall without fibrils.

Figure 6. Specific volume of fines as a function of specific refiningenergy.

Figure 5. Curl and kink indices of the R28 fraction as a function of specific refining energy.

Figure 7. Water retention value of the R28 fraction as a function ofspecific refining energy.



The samples for light microscopy analysis were prepared asfollows: First, the fibers were aligned,25 and then the alignedfibers were progressively dehydrated using water�ethanol mix-tures and impregnated with a resin medium.26 The resin-curedsamples were sectioned into 3-μm-thick sections using a sledgemicrotome. The thin sections were stained with Toluidine BlueO (T161, Fisher Scientific Co.) and then mounted on micro-scope slides for observation using a Zeiss photomicroscope.SEM (JEOL, JSM-500) was employed to qualitatively examine

fiber surface failures, particularly exposures of the outer layer ofthe secondary wall (S1), the central layer of the secondary wall(S2), and the interface between the outer and central layers of thesecondary wall (S1/S2). Fibers were air-dried and coated withcoal and gold prior to SEM analysis.Electron Spectroscopy for Chemical Analysis (ESCA).Dur-

ing refining, fiber cell walls are peeled off in different ways (e.g.,transwall and intrawall), which influences their surface chemicalcomposition, especially in terms of the lignin. ESCA was used tostudy the fiber surface coverage of lignin. ESCA measurementswere performed on an AXIS ULTRA electron spectrometerequipped with a monoenergetic Al X-ray source. The analyzedarea was 2� 2 mm, and the angle from the X-ray detector to thesample was 45�. Peak intensities were determined by peak areaintegration. The sensitivity factors used were 0.278 for C 1s and0.780 for O 1s. The matching of the C 1s peaks was carried outwith a Gaussian curve-fitting program. Sheets with a layergrammage of 60 g/m2 were made for the ESCA analysis. In theESCA technique, the chemical composition is normally evalu-ated using the C 1s and O 1s peaks by calculating the total O/Cratio. The surface coverage of lignin, ϕlignin, was calculated fromthe O/C atomic ratio as27

ϕlignin ¼ 100� O=Cðpulp sampleÞ �O=CðcarbohydrateÞO=CðligninÞ �O=CðcarbohydrateÞ

" #

where O/C(carbohydrate) = 0.833 and O/C(lignin) = 0.333.Statistical Analysis. For statistics reasons, at least 300 fibers

per sample were measured. The standard error for each analysiswas (5%.

’RESULTS AND DISCUSSION

Physical Properties. Table 2 indicates that EW fibers havethinner cell walls and larger lumens than their LW counterparts.The cell-wall thickness of LW (4.75 μm) is more than twice thatof EW (2.12 μm). EW fibers have greater outer perimeters andlumen areas than LW fibers. LW fibers have larger cell-wall areasbecause of their thicker cell walls and smaller lumens. As a result,the density of LW (0.49 g/cm3) is greater than that of EW (0.30g/cm3). In addition, LW fibers (3.55 mm) are generally longerthan EW fibers (3.34 mm). These findings are well in line withthose reported earlier.28

Chemical Composition. The experimental data on the chemi-cal components of EW and LW of Jack pine are presented inTable 3. Because of its thicker cell walls, LW has about 5.2%moreα-cellulose than EW. However, EW has a 4.5% higher lignincontent than LW, which is probably due to its relatively thickerlignin-rich compound middle lamella (CML), as explained byFengel.29 Interestingly, EW shows 20.4% higher dichloro-methane (DCM) extractives than LW. This difference in DCMextractives might be due to the fact that EW contain more resin-rich canals than LW.30 In contrast, there are no significant dif-ferences in the ash content between EW and LW.Refining Energy. Figure 1 clearly shows that, at a given

freeness value, refining EW was found to require more energythan refining LW. As discussed later, EWwas defibrated into pulpfibers with relatively little fibrillation compared to LW. As aresult, the EW pulp had higher freeness for a given energy con-sumption. This finding is in agreement with that reported byother researchers.8

Weight Distribution of Bauer�McNett Fractions. Theweight distributions of Bauer�McNett fractions of EW andLW fibers in pulp with a CSF of 150 mL are listed in Table 4.It can be seen that the LW fibers had a much higher content oflong fibers (R14 fraction) than the EW fibers. This might bebecause the original LW fibers were longer than the EW fibersand/or the thin-walled EW fibers suffered more fiber cuttingduring refining than the thick-walled LW fibers. In contrast, therewere no significant differences between EW and LW fines.Fiber Quality Analysis. Fiber Length. As can be seen in

Figure 2, as the specific energy increased, the fiber length decreasedfor all EW and LW fibers. The reduction in fiber length wasdetermined from a comparison with the initial fiber length in thewood. Because the thin-walled EW fibers are more flexible andcollapsible than the thick-walled LW fibers, they experienced agreater reduction in mean fiber length than the LW fibers.Cell-Wall Thickness. Cell-wall thickness is an important mor-

phological characteristic that affects fiber stiffness and, as a result,influences interfiber bonding. Data on cell-wall thickness provideinformation on how the fibers respond to the refining actions. Asindicated in Figure 3, the cell-wall thicknesses of fibers in the R28fractions of EW and LW fibers decreased with increasing refiningenergy.The reductions in cell-wall thickness shown in Figure 3 were

based on the initial cell-wall thickness in the wood samples. Theyindicate that the thick-walled LW fibers suffered a greater reduction

Figure 8. Cell-wall failure of EW and LW fibers in refining:(A) intrawall failure in an LW fiber, (B) transwall failure in an EW fiber.

Table 5. Failure Percentages in Refining

intrawall failure (%) transwall failure (%)

EW 0.3 26.3

LW 15.3 0.6



than the thin-walled EW fibers, which means that the LW fibersexhibited more cell-wall peeling and external fibrillation than theEW fibers under the same refining conditions.Fiber Coarseness. Changes in fiber coarseness constitute

another feature of fiber development. As seen in Figure 4, thecoarseness of the EW and LW fibers decreased with increasingrefining energy, with the fiber wall being peeled off progressivelyas the refining action proceeded. Evidently, the LW fibers werecoarser than the EW fibers because of their initial thicker cellwalls. Greater coarseness means fewer fibers per gram of pulpand, consequently, fewer surfaces for fiber bonding.8

Curl and Kink Indices. Changes in the curl and kink indices ofthe EW and LW fibers in refining are shown in Figure 5. It can beobserved that the curl and kink indices increased with increasingspecific refining energy. Note also that the thin-walled EW fibershad higher curl and kink indices than the thick-walled LW fibers,indicating that the former are more curlable or more flexible thanthe latter. This finding implies that LW fibers are more resistantto the mechanical actions of refiner bars.Specific Volume (SV) of Fines.Measurements of the specific

volume (SV) of fines, which indicates the physical nature of thefines fraction, provide useful information on the mechanism ofthe breakdown of EW and LW during refining. Figure 6 showsthat the SV of fines increased with increasing refining energybecause more fibrils were generated at higher refining energies.The increase in fibril component in the fines led to an increase inSV. In addition, refining of the thick-walled LW fibers producedmore fibril elements than that of the thin-walled EW fibers. As aconsequence, the LW fines had a higher SV than the EW fines.Water Retention Value (WRV). The water retention value

(WRV) is a useful parameter for evaluating the water holdingcapacity or wetness of fibers.31 The WRV of pulp is related toboth the external fibrillation and the internal fibrillation or delami-nation of cell wall. Splitting and delamination of the cell wallunder refining action facilitate water absorption, increasing thewater holding capacity of fibers. Therefore, the WRV reflects therefining response of the fibers: external fibrillation, cell-wall split-ting, and internal fibrillation (delamination). All of these effectsincrease the swellability of the fibers and interfiber bonding.19

Figure 7 shows that the EW fibers had a higher WRV than theLW fibers. According to Law,32 internal fibrillation is mostlyinfluenced by compression forces in refining. Under such forces,the thin-walled and flexible EW fibers collapsed, their cell wallsfractured, and cell corners became separated. These changes

would improve the fibers’ capability of absorbing of water. On theother hand, the thick-walled LW fibers, being more rigid andresistant to compression forces, tended to retain their form byseparating in interfiber mode and exhibiting little internal fibril-lation.33 Consequently, it is understandable that the EW fibershad higherWRVs than the LW fibers and those inmixed samples.Microscopy Analysis. Cell-Wall Damage. As shown in Fig-

ure 8, fiber failure could be of two types: transwall and intrawall.In this study, the percentage of each type of failure was assessedusing a light microscope.As seen in Table 5, most of the transwall failures occurred in

EW fibers, whereas the intrawall failures took place uniquely inLW fibers. Under the shear and compression actions, the thin-walled and flexible EW fibers tended to collapse (as shown inFigure 9) and split, resulting in transwall failure.On the contrary, thethick-walled and rigid LW fibers, which are more resistant tomechanical forces, retained their form and exhibited intrawallfailures.Cross-Section Deformation. Great differences between EW

and LW fibers were observed in terms of cross-section character-istics. Knowledge on the cross-section deformations of fibers isuseful for assessing the overall quality of the fibers. Such infor-mation would helpful in determining whether special treatmentshould be given to some fibers to improve papermaking proper-ties and in predicting the effects of processing variables on thefibers. Cross-sectional parameters, such as the form factor (FF),reflect the fiber flexibility and collapsibility, which are essentialfor paper consolidation.34�36

Table 6 shows that the EW fibers had a lower FF than the LWfibers, which means that the former had a higher collapsibility.Under the compression forces of refining, the thin-walled EWfibers were readily deformed and collapsed. Because the thick-walled LW fibers were rigid and resistant to mechanical forces,they tended to retain their forms after refining.Fiber Surface Failure. After refining, the EW fibers were

completely collapsed, diminishing their lumen volume, as shown

Figure 9. Surface nature of EW fibers in refining.

Table 6. Form Factors (FFs) of EW and LW Fibers inRefining

form factor

EW 0.48

LW 0.69



in Figure 9. In fact, the compression and shear forces left the EWfibers split and twisted. The majority of the failures took place atthe interface between the primary wall and the transition layer orouter layer of the secondary wall (P/S1), especially around thepits. Failures at the interface between the outer and central layersof the secondary wall (S1/S2) were also occasionally noticed.Fiber external fibrillations associated with shear forces were notevident in EW fibers; a few fibrils were occasionally observed.For LW fibers, the compression forces had limited effects on

the change in lumen dimensions because collapsed fibers wererarely observed (Figure 10). Most of the LW fibers showedexposed surfaces with various natures, such as the primary wall(P), the outer layer of the secondary wall (S1), and the centrallayer of the secondary wall (S2). However, S1/S2 separation wascommonly seen in LW fibers. Fibrils were frequently seen alongthe cell walls of LW fibers. Some long and ribbonlike layers weredetached from the cell walls.Surface Lignin Coverage. ESCA is a useful and efficient means

for characterizing the surface chemical nature of fibers. The principleof this analysis is based on the fact that the lignin concentrationdecreases gradually across the fiber wall, with the highest concentra-tion being in themiddle lamella. At the same time, the concentrationof cellulose, which is practically absent in the middle lamella, in-creases from the primary wall toward the inner secondary layer.29

In this study, ESCA was conducted on two types of pulpsamples including the whole pulp without fines (fines-free pulp)and the fines. Because the fines include flakelike particles andfibrils37,38 generated from the fiber surface and the secondary

layer, respectively, analysis of both the fines and fines-free pulppermit an understanding of the mode of fines formation.As shown in Figure 11, the surface lignin coverage of the fines was

always higher than that of the fines-free pulp for both EW and LW.This is not surprising because the flakelike fines are the materialsdetached from the outer layer of cell wall. Because the lignin-richmiddle lamella is the outermost layer of the cell wall, it is always thefirst to be peeled from the cell wall. As a result, the surface lignincoverage is higher in the fines than in the fines-free pulp.The surface lignin coverage of EW pulp was found to be higher

than that of LW pulp for both the fine-free pulp and the fines.This finding seems to support the conclusion that the thin-walledEW fibers break down more readily with less surface fibrillation.The reduced fibrillation means that the surface of the fibersmaintained more lignin than the LW fibers that suffered morefibrillation. On the other hand, EW fibers had greater surfaceperimeters and hence produced more lignin-rich flakes in termsof the surface. This might be another reason that the EW fineshad higher lignin contents than the LW fines.

’CONCLUSIONS

LW fibers of Jack pine have a cell-wall thickness of 4.75 μm,which is approximately twice that of EW fibers (2.12 μm), and acell diameter (radial width) one-half that of EW. Because of theirmorphological differences, EW and LW fibers behave differentlyin thermomechanical pulping (TMP). Studies based on the fiberfractions R28 and fine P200 revealed the following: The thin-walledEW fibers tend to separate in the P/S1 interface and show littleexternal fibrillation. In contrast, thick-walled LW fibers commonlyfail in the P/S1 and S1/S2 regions, generating considerable amountsof external fibrillation. LW fibers exhibit mostly intrawall failure andlower curl and kink indices, whereas EW fibers tend to fail intranswall mode (splitting) and show higher curl and kink indices. Asa result, LW yields higher fibers length, whereas EW suffers morefiber cutting. In addition, the thin-walled EW fibers show highercollapsibility and conformability than their thick-walled LW coun-terparts. Moreover, EW fines have higher surface lignin coverage,whereas LW fines have a higher specific volume (SV).

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

Figure 10. Surface nature of LW fibers in refining.

Figure 11. Surface lignin coverages of different samples.



Present Addresses†School of Chemistry and Biochemistry, Georgia Institute ofTechnology, Atlanta, Georgia 30332, United States.

’ACKNOWLEDGMENT

The authors gratefully appreciate the financial support fromthe Natural Science and Engineering Research Council of Canada(NSERC).

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Documents

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