Cellulose 7: 35–55, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

A comparative CP/MAS 13C-NMR study of cellulosestructure in spruce wood and kraft pulp

EVA-LENA HULT, PER TOMAS LARSSON and TOMMY IVERSEN∗Swedish Pulp and Paper Research Institute, STFI, Box 5604, SE-114 86, Stockholm, Sweden

Received 26 October 1999; accepted 29 February 2000

Abstract. CP/MAS 13C-NMR spectroscopy in combination with spectral fitting was usedto study the supermolecular structure of the cellulose fibril in spruce wood and spruce kraftpulp. During pulping, structures contributing to inaccessible surfaces in the wood celluloseare converted to the cellulose Iβ allomorph, that is, the degree of order is increased. Thisincrease is also accompanied by a conversion of cellulose Iα to cellulose Iβ. Cellulose fromwood composed of different cell types, that is, compression wood, juvenile wood, earlywood,latewood and normal wood exhibited a similar supermolecular structure. Assignments weremade for signals from hemicellulose which contribute significantly to the spectral C-4 region(80–86 ppm) in kraft pulp spectra but substantially less to the corresponding region in woodspectra.

Key words: cellulose, hemicellulose, kraft pulp, NMR, spruce wood

Introduction

Our knowledge of the structure of cellulose in native wood and of how thestructure is affected by various types of industrial processing is still limited.Any view of the factors influencing the functional properties of wood-basedproducts such as paper pulps requires an in-depth understanding of the struc-tural characteristics of the cellulose fibrils in native and processed wood. Thechanges occurring in the state of order of cellulose during kraft and otheralkaline pulping processes have been studied by several investigators (Page,1983a; Page, 1983b; Hattula, 1986; Shashilovet al., 1986; Isogaiet al., 1991;Newmanet al., 1993; Lennholmet al., 1995; Evanset al., 1995). However,the composite structure and the heterogeneous chemical composition of woodand pulp fibres, which are intimate mixtures of cellulose and other poly-mers such as hemicelluloses, pectins and lignin, make it difficult to identifyconclusively the changes in cellulose structure induced by the pulping.

∗ Author for correspondence.

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In previous papers (Larssonet al., 1995; Larssonet al., 1997; Wickholmet al., 1998), we reported a method for quantifying the states of order foundwithin cellulose I fibrils based on CP/MAS13C-NMR (Cross PolarisationMagic Angle Spinning Carbon-13 Nuclear Magnetic Resonance) spectro-scopy in combination with spectral fitting. The most informative region inthe NMR spectra of cellulose I samples is a signal cluster with a distribu-tion between 80 and 92 ppm (Atallaet al., 1980; Earl and Vanderhart, 1980;Teeääret al., 1987; Larssonet al., 1995; Wormlandet al., 1996; Larssonet al.,1997; Wickholmet al., 1998). This region contains fairly sharp signals (86–92 ppm) corresponding to C-4 carbons situated in crystalline Iα and Iβ do-mains together with para-crystalline cellulose. The signals from C-4 carbonsin more disordered regions are distributed in a broad band ranging from 80 to86 ppm. These signals have been assigned to cellulose at (solvent-) accessiblefibril surfaces, to cellulose at inaccessible fibril surfaces (interior or exterior)and to C-4 in hemicellulose (e.g.O-acetyl-4-O-methylglucuronoxylan) if it ispresent in the sample.

We now report a comparative study of the states of order of the cellu-loses found in samples of wood composed of different cell types and in kraftpulp fibres prepared from Norway spruce (Picea abies). Samples subjectedto acid hydrolysis were included in order to make it possible to identifyhemicellulose signals in the spectra.

Experimental

Samples

Compression wood, juvenile wood, earlywood and latewood samples (PiceaAbies) were kindly supplied by Stora Enso AB in Säffle. The pulpwood andthe spruce kraft pulp were kindly supplied by Södra Teknik AB. The sprucekraft pulp was laboratory cooked to a kappa number of 28 from the pulpwood(e.g. sorted normal wood) with a cellulose yield of 90% according to thecalculation method of Jansson (1974).

Treatment of wood and kraft pulp

The wood samples were Wiley milled with a 20 mesh grid in order to al-low homogeneous packing in NMR rotors. The compression wood, juvenilewood, latewood, earlywood, pulpwood and spruce kraft pulp were subjectedto a mild chlorite delignification with NaClO2 (1.5 g/g sample) under acidicconditions at room temperature followed by treatment with 0.1 M NaOHovernight. Between the NaClO2 and NaOH stages, the samples were rinsed

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with deionised water to pH 4–5. The overall procedure was repeated twice.The samples were then hydrolysed for 4 h in 2.5 M HCl at 100◦C. For thehydrolysis series, the pulpwood and the spruce kraft pulp were subjectedto the chlorite delignification method described above and hydrolysed with2.5 M HCl at 100◦C for different times between 30 min and 17 h.

Preparation of holocellulose

The pulpwood chips were subjected to a cyclic treatment with NaClO2(0.3 g/gsample) under acidic conditions (pH 4–5) at 70◦C until the pulpwood chipswere completely delignified (Klason lignin content 3× 106), kraft pulp,holocellulose and the corresponding pure celluloses. The cellulose, cellu-lose/hemicellulose and polyethylene contents were determined in the samplesgravimetrically, by sugar analysis and by integration of the spectra.

NMR spectroscopy

All spectra were recorded on wet samples (water content 40–60% by weight).The CP/MAS13C-NMR spectra were recorded on a Bruker AMX-300 in-strument (at ambient temperature) operating at 7.04T. A zirconium oxiderotor was used. The MAS rate was 4–5 kHz. Acquisition was performed witha CP pulse sequence using a 3.5µs proton 90◦ pulse, 800µs contact pulseand a 2.5 s delay between repetitions. Glycine was used for the Hartman–Hahn matching procedure and as external standard for the calibration of thechemical shift scale relative to tetramethylsilane ((CH3)4Si). The data pointof maximum intensity in the glycine carbonyl line was assigned a chemicalshift of 176.03 ppm.

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Chemical composition

The relative proportions of the neutral polysaccharide constituents were de-termined by sugar analysis according to Theander and Westerlund (1986).Klason lignin was determined according to the TAPPI T249 method.

Spectral fitting

The model and method given by Larssonet al. (1997) was used to performthe spectral fitting. Binary spectra are transported from the spectrometer to aPC by means of a LAN and FTP. The binary spectra are then phased and con-verted to ASCII and the real parts of the spectra are imported into a MicrosoftExcel 97 workbook. The software used, based on the Levenberg–Marquardtmethod (Presset al., 1988), was implemented at STFI as an OLE automa-tion server written in Borland Delphi 4. The server is used in conjunctionwith Microsoft Excel 97 under Windows9X on Intel X86 platforms to fit thespectra.

Results and discussion

Spectral fitting

For the spectral fitting we have applied a recently reported mathematicalmethod for quantifying the states of order found within cellulose I fibrilsby use of the C-4 (and C-1) region in13C CP/MAS spectra (Larssonet al.,1997). This method is based on a model for the spectral C4-region consist-ing of seven distinct lines. This model enables the C-4 region in cellulosefrom cotton linters and birch kraft pulp to be consistently interpreted. Themodel used for the fitting procedure includes the use of Lorentzian linesfor the three signals from the crystalline cellulose I allomorphs, celluloseIα (89.5 ppm), I(α + β) (88.7 ppm), and Iβ (87.9 ppm), and Gaussian linesfor the remaining four signals attributed to non-crystalline cellulose forms,para-crystalline cellulose (88.4 ppm), accessible fibril surfaces (84.1 and83.2 ppm) and inaccessible fibril surfaces (84.9 ppm) (Wickholmet al., 1998).Our interpretation of cellulose spectra thus requires the involvement of oneGaussian line at 88.4 ppm in the same chemical shift region as the crystallineallomorphs. The exact properties of the cellulose form which we name para-crystalline cellulose fitted by the Gaussian line at 88.4 ppm are not known.However, the13C spin-lattice relaxation time, experiments with solvent ex-change and the agreement between lateral fibril dimensions determined fromNMR spectra and those determined by microscopy support the view that

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the para-crystalline cellulose is a well-ordered ‘in-core’ structure (Larssonet al., 1997). The possible presence of a cellulose form of high order but notidentical to cellulose Iα or Iβ allomorphs has previously been indicated by1H-NMR (Paciet al., 1995), powder X-ray (Kulshrehtha and Dweltz, 1973),X-ray and electron diffraction studies (Chanzyet al., 1978; Chanzyet al.,1979). The description of the cellulose form given by Chanzyet al. (1978)was in terms of native cellulose I crystals having good longitudinal coherencebut with poor lateral organisation of the network of inter chain hydrogenbonds.

The spectral fitting of the C-4 region is performed by starting with thedominating signals for para-crystalline cellulose and cellulose at inaccessiblefibril surfaces. After convergence at this level, additional lines are added todescribe the crystalline cellulose allomorphs and cellulose at accessible fibrilsurfaces. After final convergence, the fitted result is evaluated with respect totheχ2-value, agreement of chemical shifts and line-widths with an appropri-ate reference spectrum, for example, a spectrum of cotton linters cellulose.If applicable, agreement between the obtained and the expected signal in-tensity ratios for cellulose Iα and cellulose Iβ is a criterion for acceptance(Yamamoto and Horii, 1993).

Further, results obtained by other methods, for example, lateral fibril di-mensions determined by microscopy and those calculated from spectral fit-ting, can be used for comparison when available. The above criteria shouldbe met to a reasonable level before the fitting result is accepted. For thehemicellulose-containing samples, the corresponding pure cellulose is usedas reference spectrum. To describe the composite hemicellulose-cellulosespectrum a minimum number of additional lines are added to the C4-regionof the spectrum to obtain an acceptable fitting of the cellulose part of thespectrum and an acceptableχ2-description of the overall spectrum.

Sample preparation

In order to avoid structural changes in the fibre cell wall due to drying (horn-ification) (Newman and Hemmingson, 1997), a never-dried kraft pulp wasused as starting material and the wood and the pulp samples were kept eitherin water suspension or water-saturated throughout the preparation sequences.For the same reason and to obtain spectra of sufficient resolution for spec-tral fitting, spectra were recorded on wet samples (Lennholm, 1994). Thewater contents in the samples after spectral acquisition where typically inthe range of 50±10% by weight, in fair agreement with the anticipatedwater-saturation points of these types of materials (Scallan and Carles, 1972).

Signal overlap from the lignin and probably also distortions in the cellu-lose fibrils due to geometrical restrictions imposed by the cell wall structure

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lead to spectra with a comparably low resolution for the native spruce woodsamples. To facilitate spectral fitting of cellulose signals in wood samples,these and as reference the kraft pulp, were subjected to a mild chlorite deligni-fication and partial removal of hemicellulose. A comparison of the quantific-ations of kraft pulp before and after this treatment indicated that the treatmentled to a slight increase in crystallinity from 19% to 24%. The spectra of thewood samples were recorded on materials prepared from milled wood. Inthis study, a Wiley mill with a 20 mesh grid was used since it has been repor-ted that this does not lead to detectable distortions in the cellulose structure(Newman and Hemmingson, 1990). This was confirmed by recording a spec-trum on a sample prepared from pulpwood manually cut into pieces (∼1 mm3)with a knife.

Pure cellulose from the pulpwood was prepared by using a standard holo-cellulose preparation, and an extended acid hydrolysis was performed in orderto remove hemicellulose. Pure cellulose from kraft pulp was obtained bya mild chlorite delignification followed by an extended acid hydrolysis toremove the hemicellulose.

Signal quantitativity

Previous investigations have shown that it is possible to acquire spectra withquantitative signal intensities from both dry (Horiiet al., 1984; Teeääret al.,1987; Larssonet al., 1997) and water-saturated celluloses (Larssonet al.,1997). However, it is by no means evident that the cellulose and the hemi-celluloses which can exist in different states of order and have different mo-bilities exhibit the same rotating-frame relaxation behaviour during cross-polarization.

To investigate the quantitativity, cross-wise quantification of water-saturated samples containing polyethylene as internal standard was performedat several contact times (Larssonet al., 1997). The samples investigated werethe holocellulose prepared from the pulpwood, the kraft pulp and the corres-ponding pure celluloses. The results of the cross-wise quantifications of thesamples at several contact times are shown in Figure 1 and summarised inTable 1. The results show a good agreement between the ratios determinedby CP/MAS NMR and the gravimetric ratios for the holocellulose and thetwo cellulose samples. However, a slightly higher NMR signal intensity ratiois evident in the kraft pulp, which is not observed in the kraft pulp cellulose.This can be taken as an indication of changes occurring in the supermolecularstructure (state of order or mobility) of the hemicelluloses as a consequenceof the kraft pulping. In all cases, cross-wise quantification above 600µs wasindependent of the contact time.

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Figure 1. Cross-wise quantification of holocellulose (filled triangle), kraft pulp (star), purepulpwood cellulose (filled square), pure kraft pulp cellulose (cross) and polyethylene (internalstandard) at several contact times. The labels give the intensity ratios obtained by integrationof the polythylene and the cellulose spectral regions, the horizontal line gives the expectedintensity ratio determined gravimetrically (1.00).

Table 1. The result of the spectral integration of the holocellulose, kraft pulp andthe corresponding celluloses mixed with linear low-density polyethylene. TheCP/MAS NMR ratio is determined by dividing the integrated signal intensity inthe polyethylene region with the integrated signal intensity in the cellulose andcellulose/hemicellulose region. The values in parentheses are the standard errors.C/PE is the ratio of carbohydrates to polyethylene

Sample CP/MAS NMR ratio Gravimetric ratio

(C/PE, contact time 800µs) (C/PE)

Holocellulose 1.08 (0.05) 1.00

Pure pulpwood cellulose 1.05 (0.05) 1.00

Kraft pulp 1.20 (0.05) 1.00

Pure kraft pulp cellulose 1.06 (0.05) 1.00

Cellulose structure

The CP/MAS13C-NMR spectra of samples prepared from compression wood,pulpwood (e.g. sorted normal wood) and spruce kraft pulp are shown in Fig-ure 2. The samples have similar cellulose contents (about 92% glucose), asseen in Table 2. A visual comparison of the NMR spectra reveals that the pulpsample is richer in spectral detail. Although native compression wood and

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Figure 2. CP/MAs13C-NMR spectra of samples prepared from compression wood (bottom)pulpwood and kraft pulp (top).

Table 2. Monosaccharide composition according to sugar analysis given as mol%. Thevalues of arabinose, xylose, mannose, galactose and glucose are normalised to a totalpolysaccharide level of 100%. Klason lignin is given in percentage (absolute value)

Arabinose Xylose Mannose Galactose Glucose Klason

lignin (%)

Compression wood 1.7 10.7 13.5 15.9 58.2 35.0

Pulpwood 1.9 10.2 19.5 3.2 65.2 26.7

Kraft pulp 0.9 8.9 6.4 0.4 83.4 3.8

Compression wooda 0.2 2.2 6.7 0.8 90.1 7.9

Pulp wooda 0.1 1.5 5.9 0.2 92.4 14.1

Kraft pulpa 0.2 2.7 3.9 < 0.1 93.2 < 0.1

aSamples subjected to a mild chlorite delignification and partial removal of hemicellulose.

normal wood represent two extremes with respect to their chemical (Higuchi,1997) and morphological structures, only subtle differences in spectral detailare detected.

Results of spectral fitting (Larssonet al., 1997; Wickholmet al., 1998)for the C-4 regions (80–92 ppm) of the pulpwood and kraft pulp samples areshown in Figure 3 and Table 3. It is apparent that there are differences insupermolecular structure between the two samples. In the spectrum of thepulpwood, the ordered region (86–92 ppm) is dominated by one wide lineat 88.9 ppm, previously assigned to para-crystalline cellulose (Larssonet al.,1997; Wickholmet al., 1998). This dominance makes it impossible to detectall the individual signals from the cellulose Iα and Iβ allomorphs. Only the

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Figure 3. The results from the spectral fitting of the C-4 region recorded on (a) the pulpwoodsample and (b) the kraft pulp sample. The broken lines represent the experimental spectra. Thefitted lines and their superposition are shown as solid lines. The results of the fitting and theassignments are presented in Table 3.

largest I(α+β) composite signal at 88.9 ppm is distinguishable, indicating acrystallinity of 8.8% (calculated as twice the signal intensity of I(α + β))(Table 3). On the other hand, both signals from the cellulose Iβ allomorphare observed in the kraft pulp spectrum and the crystallinity is substantiallyhigher (24%) (Shashilovet al., 1986; Newmanet al., 1993; Evanset al.,1995). To confirm that the low crystallinity in the pulpwood sample (8.8%)is not an artifact due to inappropriate sampling, samples prepared from woodcomposed of different cell types, compression wood, earlywood, latewoodand juvenile wood, were analysed. As seen in Table 4, all the wood samplesexhibited similar low crystallinities.

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In the spectral region typical of non-crystalline cellulose forms (80–86ppm), there were substantial differences between the pulpwood and the kraftpulp (Table 3). Both samples exhibited a broad signal at 83.8–84.1 ppm as-signed to inaccessible fibril surfaces and hemicellulose. The intensity of thissignal was much larger in the wood sample (49.4%) than in the kraft pulpsample (26.7%), although the hemicellulose contents of the samples (meas-ured as the sum of mannose and xylose) are similar, 7.4% and 6.6% respect-ively (Table 2). The combined intensities of the two signals at 84.3 and 83.3ppm previously assigned to accessible surfaces were the same (12%) in thetwo samples.

Since the pulpwood sample contained both hemicellulose and lignin andthe kraft pulp sample contained hemicellulose, spectral fitting was also per-formed on pure celluloses (glucose content 97%) isolated from the pulpwoodand the kraft pulp by chlorite delignification and acid hydrolysis to excludeany possibilities of signal overlap from non-cellulose components. Resultsof the spectral fitting (Larssonet al., 1997; Wickholmet al., 1998) for theC-4 region (80–92 ppm) of the pure pulpwood and kraft pulp celluloses areshown in Table 5. It is evident that the differences in the ordered region (86–92 ppm) between the pure pulpwood and kraft pulp celluloses are still clearlydiscernible, even after the severe conditions in the isolation procedure. In thewood cellulose, only the composite signal I(α+β) is distinguishable, indicat-ing a crystallinity of 12%, while both signals from the cellulose Iβ allomorphare observed in the kraft pulp cellulose corresponding to a crystallinity of20%. However, in the spectral region typical of non-crystalline cellulose, thedifference in signal intensity of the broad signal at 83.8–84.1 ppm assignedto inaccessible fibril surfaces is no longer as pronounced, pure pulpwoodcellulose 32% and pure kraft pulp cellulose 29%.

Hemicellulose structure

In the kraft pulp sample, two lines are visible which are absent in the spectrumof the pulpwood sample, a minor signal at 81.9 ppm (0.7%) and a relat-ively large at 81.2 ppm (3.8%), see Table 3. Despite the smallness of theminor signal, its narrowness makes it easily distinguishable by the spectralfitting procedure. The positions of these lines closely resembles an earlierassignment to a xylan C-4 signal at 81.7 ppm in a spectrum recorded onbirch kraft pulp (Wickholmet al., 1998). However, since the main hemi-celluloses of spruce wood areO-acetylgalactoglucomannan and arabino-4-O-methylglucuronoxylan while birch wood contains mainlyO-acetyl-4-O-methylglucuronoxylan, the comparison is not straightforward.

To further corroborate the presence of signals originating from hemicel-lulose in the spectral C-4 region, hydrolysis was carried out on the mildly

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Table 3. The results of the spectral fitting of the C-4 region in the CP/MAS13C NMR spectra of the pulpwood sample and of thekraft pulp sample

Assignment Kraft pulp sample Pulpwood sample

Chemical shift FWHHa (Hz) Relative Chemical shift FWHHa (Hz) Relative

(ppm) intensity (%) (ppm) intensity (%)

I(α + β) 88.8 (0.05)b 42 (4) 11.2 (0.8) 88.9 (0.04) 60 (16) 4.4 (0.8)Para-crystalline 88.6 (0.08) 166 (9) 32.4 (2.5) 88.8 (0.05) 177 (7) 34.5 (1.0)

Iβ 87.9 (0.05) 89 (7) 12.9 (1.4) – – –

Accessible fibril surface 84.3 (0.05) 67 (3) 6.8 (0.6) 84.4 (0.08) 86 (7) 6.3 (0.6)

Inaccessible fibril surfaces 84.1 (0.15) 324 (23) 26.7 (5.6) 83.8 (0.25) 589 (73) 49.4 (2.0)

and hemicellulose

Accessible fibril surface 83.3 (0.05) 55 (2) 5.5 (0.6) 83.3 (0.07) 68 (5) 5.4 (0.5)

Hemicellulose 81.9 (0.06) 48 (5) 0.7 (0.3) – – –

Hemicellulose 81.2 (0.11) 192 (23) 3.8 (3.3) – – –

aFWHH is the full width at half-height.bValues in parentheses are the standard errors.

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Table 4. Quantifications made by spectral fitting of the C-4 region in the CP/MAS13C-NMR spectra of the investigated samples. Allvalues are relative intensity in percent

Cellulose source Crystalline Para-crystalline Accessible fibril Inaccessible fibril Hemicellulose Hemicellulose

cellulose cellulose surfaces surfaces and

hemicellulose

Compression wood 4 (1)a 26 (1) 12 (1) 57 (2)

Early wood 6 (3) 22 (2) 9 (1) 62 (4)

Late wood 5 (1) 28 (1) 9 (1) 58 (3)

Juvenile wood 9 (3) 23 (2) 10 (1) 59 (4)

Pulpwood sample 9 (2) 30 (1) 12 (1) 49 (2)

Pulp woodb 8 (2) 26 (1) 15 (1) 50 (3)

Kraft pulp 19 (2) 30 (1) 8 (1) 33 (8) 1 (0.3) 9 (5)

Kraft pulp sample 24 (2) 32(1) 12 (1) 27 (6) 1 (0.3) 4 (3)

aValues in parentheses are the standard errors.bSample was hydrolysed for 17 h.

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Table 5. Quantification made by spectral fitting of the C4-region in the CP/MAS13C NMRspectra of pure pulpwood cellulose and pure kraft pulp cellulose. All values are given as relativeintensity in percent

Cellulose source Crystalline Para-crystalline Accessible fibril Inaccessible fibril

cellulose cellulose surfaces surfaces

Pure pulpwood 12 (1)a 41 (1) 15 (1) 32 (1)

cellulose

Pure kraft pulp 20 (1) 38 (2) 13 (1) 29 (1)

cellulose

aValues in parentheses are the standard errors.

chlorite delignified pulpwood and kraft pulp. In Figure 4, the decreases in thepooled intensities of the signals at 83.8–84.1, 81.9 and 81.2 ppm are plottedagainst the decrease in the content of xylose and mannose as determinedby sugar analysis. The sum of xylose and mannose only is used, since theglucose units originating from the galactoglucomannan chains cannot be re-solved by sugar analysis. Hence the measured sum of the xylose and mannosecontents, as determined by sugar analysis, is a slight underestimation of theactual hemicellulose content. Taking this into consideration, it is evident fromFigure 3 that the C-4 carbons of hemicellulose contribute significantly to thenon-crystalline cellulose C-4 region of the kraft pulp spectrum but substan-tially less to the pulpwood spectrum. The hemicelluloses in the spruce kraftpulp spectrum thus behave in a manner similar to that observed earlier forxylan in birch kraft pulps (Wickholmet al., 1998). The C-4 carbons of thehemicellulose chain units in the pulpwood samples are either invisible in thespectra (e.g. due to mobility properties) or shifted out from this region (e.g.due to structural characteristics). The situation resembles the spectral beha-viour of (1–4)-β-D-mannans where the crystalline form mannan I has a sharpC-4 resonance line at 81.4 ppm and the mannan II form a somewhat broaderC-4 resonance line at 83.0 ppm (Marchessaultet al., 1990). In contrast, theC-4 signals of hydrated less ordered galactomannan gels are largely hiddenin the C-2, C-3, C-5 signal cluster at 70–80 ppm (Gidleyet al., 1991).

A tentative explanation of these differences in spectral behaviour betweenthe hemicelluloses in the pulpwood and kraft pulp samples may be that theyare related to the well-known process-induced changes in the molecular struc-ture of the hemicelluloses (Rydholm, 1965). Branch units and acetyl groupsare removed from the hemicelluloses during kraft pulping, and the lower de-gree of substitution could facilitate intermolecular aggregation (Baileyet al.,1991). Such an ‘aggregation’ could also explain the observed relative increase

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Figure 4. The decrease in pooled relative signal intensities obtained by spectral fitting of thesignals at 83.8–84.1, 81.9 and 81.2 ppm plotted against the decrease in contents of xylose andmannose as determined by sugar analysis during different stages of acid hydrolysis of (a) kraftpulp and (b) pulpwood.

in CP/MAS signal intensity of the kraft pulp. Whether the changes in spec-tral behaviour of the hemicellulose are solely induced by such alterationsin the molecular structure or whether a reorganisation of native cell wallstructures is required to give domains enriched in hemicelluloses cannot bedistinguished in the present investigation.

Alkali treatment of holocellulose

In order to study the effects on the cellulose structure during kraft cookingwithout simultaneous delignification and without having to perform a sub-sequent isolation, holocellulose isolated from pulpwood was subjected to analkali treatment at elevated temperatures. The experiments were carried outin 1 M aqueous sodium hydroxide at 170◦C to mimic a kraft cook and at alower temperature of 130◦C as comparison. Spectral fitting results for theC-4 region (80–92 ppm) of the three samples are shown in Figure 5 andTable 6. When an attempt was made to resolve signals from the crystallinecellulose allomorphs in the ordered C-4 region (86–92 ppm) of the pulpwoodholocellulose, only the two lines corresponding to the cellulose Iα allomorphcould be fitted, giving a crystallinity of 9%. In the sample heated to 130◦Conly the largest I(α + β) composite signal at 88.9 ppm (corresponding to acrystallinity of 10%) is distinguishable. In the spectrum of the sample heatedto 170◦C, the two lines corresponding to the cellulose Iβ allomorph could

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Figure 5. The results from the spectral fitting of the C4-region recorded on (a) the holocellu-lose (b) the holocellulose treated with 1 M NaOH heated to 130◦C (c) the holocellulose treatedwith 1 M NaOH heated to 170◦C. The broken lines represent the experimental spectra. Thefitted lines and their superposition are shown as solid lines. The results of the fitting and theassignments are summarised in Table 6.

be fitted and showed a crystallinity of 15%. The increase in crystallinity isaccompanied by a minor decrease in intensity from 51% to 48% of the signalat 83.8–84.1 ppm assigned as inaccessible fibril surfaces and hemicelluloses.The general features of the alkali-treated holocellulose sample agree withthose of the kraft pulping, that is, a high crystallinity where the dominating

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crystalline form is cellulose Iβ. However, due to the richer spectral detailof the holocellulose spectrum, it is apparent that the increase in crystallinity(Atalla, 1978) is accompanied by a conversion of cellulose Iα to cellulose Iβ(Lennholmet al., 1995; Lindgrenet al., 1995).

Lateral dimensions

If the ability of cellulose fibrils to form aggregates with coinciding fibril mainaxes is considered and if we assume that fibril–fibril contact surfaces con-stitute the main contribution to the signal intensity assigned to inaccessiblesurfaces (83.8–84.1 ppm), it is possible to calculate lateral fibril dimensionsfrom the NMR spectra (Wickholmet al., 1998; Haet al., 1998; Smithet al.,1998; Heuxet al., 1998). The geometrical arrangement of such cellulose fib-rils in the aggregates, or in the cell wall, has been debated, but it has still notbeen properly determined (Bass, 1982). Assuming that the aggregates have asquare cross-section, the lateral dimension of the fibril aggregate can also beestimated from NMR-spectra.

Table 7 shows the lateral fibril dimensions and the lateral aggregate di-mensions, for the pulpwood sample, pure pulpwood cellulose and pure kraftpulp cellulose. The samples were subjected to a prolonged acid hydrolysis(17 h) in order to remove interfering hemicellulose signals. To establish themorphological relevance of the samples hydrolysed for 17 h, their fibril di-mensions and fibril aggregate dimensions were compared with literature val-ues (Stoll and Fengel, 1977; Revolet al., 1987; Ioelovitch, 1992; Newman,1992; Jakobet al., 1995; Duchesne, 1999). The agreement was found to begood.

The small fibril dimensions estimated for the pulpwood sample (2.8 nm)may be related to the relatively large amount of lignin (Klason lignin con-tent 10%) present in the sample in contrast to the pure pulpwood and kraftpulp cellulose (Klason lignin content

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Table 6. Quantifications made by spectral fitting of the C4-region in the CP/MAS13C-NMR spectra of the holocellulose, holocellulosetreated with 1 M NaOH, heated to 130◦C and holocellulose treated with 1 M NaOH, heated to 170◦C. All values are given as relativeintensity in percent

Sample Cellulose Cellulose Cellulose Para-crystalline Accessible fibril Inaccessible fibril Hemicellulose

Iα I(α + β) Iβ cellulose surfaces surfacesHolocellulose 5 (1)a 4 (1) – 31 (1) 9 (1) 51 (1) –

Holocellulose, – 5 (1) – 37 (1) 8 (1) 50 (1) –

130◦CHolocellulose, – 8 (1) 7 (1) 30 (1) 7 (1) 48 (1) 1 (0.2)

170◦C

aValues in parentheses are the standard errors.

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Table 7. The lateral fibril dimensions and the lateral fibril aggregate dimensions for thepulpwood sample, pure pulpwood cellulose and pure kraft pulp cellulose. The calcula-tions were performed using a conversion factor of 0.55 nm per cellulose polymer (Krässig,1993)

Sample Lateral fibril Literature Lateral fibril Literature

dimension (nm) values (nm) aggregate values (nm)

dimension (nm)

Pulpwood sample 2.8 (0.2)a 2.5b 14.2(1.4) 14f

Pure pulpwood 4.0 (0.1) 3.5–4c 14.4 (0.6) 14f

cellulose

Pure kraft pulp 4.6 (0.2) 5.2d 16.3 (0.7) 16–20g

cellulose 4.0e

aValues in parentheses are the standard errors.bJakobet al., 1995.cStoll and Fengel, 1977.dIoelovitch, 1992.eRevolet al., 1987.fNewman, 1992.gDuchesneet al., 1999.

Effect of pulping

When the pulpwood sample is compared with the kraft pulp sample, it isseen that the major effect on the cellulose structure during kraft cooking is anincrease in the crystallinity (24% in the kraft pulp sample compared with 9%in the pulp wood sample). This increase correlates with a decrease in the rel-ative amount of cellulose at inaccessible fibril surfaces. The magnitude of thechanges and the cellulose yield during pulping (90%) rule out an explanationin terms of the preferential removal of small fibrils or disordered cellulose. Inthe kraft pulp sample, the dominating crystalline cellulose form is celluloseIβ. During kraft pulping a change in spectral behaviour of the hemicelluloseis also observed.

Conclusion

Structures contributing to inaccessible surfaces in the interior of the fibril(lattice distortions) in wood cellulose are converted to cellulose Iβ duringkraft pulping, that is, the degree of order is increased. This increase is alsoaccompanied by a conversion of cellulose Iα to cellulose Iβ.

Cellulose from wood composed of different cell types and normal woodexhibited a similar supermolecular structure. The C-4 carbons of hemicel-

53

lulose chain units contribute significantly to the spectral region representingnon-crystalline cellulose (80–86 ppm) in kraft pulp spectra, but much less tothe corresponding region in wood spectra. Our interpretation of the spectralbehaviour of the hemicelluloses is that changes in intermolecular aggregationhave occurred as a consequence of the kraft pulping.

Acknowledgements

This work was carried out within the framework of the Wood UltrastructureResearch Centre (WURC), a NUTEK competence centre at the Swedish Uni-versity of Agricultural Sciences. The study has been carried out with financialsupport from the Commission of the European Communities, Agriculture andFisheries (FAIR) specific RTD program, CT96-1624.

References

Atalla, R. H. (1978) The influence of elevated temperatures on structure in the isolation ofnative cellulose.J. Polym. Sci. (Polymer Letters Edition)16, 601–605.

Atalla, R. H., Gast, J. C., Sindorf, O. W., Bartuska, V. J. and Maciel, G. E. (1980)13C NMRspectra of cellulose polymorphs.J. Am. Chem. Soc.102, 3251–3252.

Bailey, M. J., Puls, J. and Poutanen K. (1991) Purification and properties of 2 xylanases fromaspergillus-oryzae.Biotechnol. Appl. Biochem.13, 380–389.

Bass, P. (1982)New Perspectives in Wood Anatomy. The Hague, Martinus Nijhoff.Chanzy, H., Imada, K. and Vuong, R. (1978) Electron diffraction from the primary wall of

cotton fibers.Protoplasma94, 299–306.Chanzy, H., Imada, K., Mollard, A., Voung, R. and Barnoud, F. (1979) Crystallographic as-

pects of sub-elementary cellulose fibrils occuring in the wall of rose cells culturedin vitro.Protoplasma100, 303–316.

Duchesne, I. (1999)Surface Ultrastructure of Norway Spruce Kraft Pulp Fibres.LicentiateThesis. Swedish University of Agricultural Sciences, Uppsala, Sweden.

Earl, W. L. and VanderHart D. L. (1980) High-resolution, magic angle spinning C-13 NMRof solid cellulose-I.J. Am. Chem. Soc.102, 3251–3252.

Evans, R. E., Newman, R. H., Roick, U. C., Suckling, I. D. and Wallis, A. F. A. (1995)Changes in cellulose crystallinity during kraft pulping. An analysis by three methods.Holzforschung49, 498–504.

Gidley, M. J., McArthur, A. J. and Underwood, D. R. (1991)13C NMR characterization ofmolecular structures in powders, hydrates and gels of galactomannans and glucomannans.Food Hydrocoll.5, 129–140.

Ha, M.-A., Apperley, D. V., Evans, B. W., Huxham, M., Jardine, W. G., Remco, J. V., Reis, D.,Vian, B. and Jarvis, M. C. (1998) Fine structure in cellulose microfibrils: NMR evidencefrom onion and quince.The Plant J.16, 183–190.

Hattula, T. (1986) Effect of kraft cooking on the ultrastructure of wood cellulose.Paperi jaPuu12, 926–931.

54

Heux, L., Dinand, E. and Vignon, M. R. (1999) Structural aspects of ultrathin cellulosemicrofibrils followed by13C CP-MAS NMR.Carbohyd. Polym.40, 115–124.

Higuchi, T. (1997)Biochemistry and Molecular Biology of Wood. Germany, Springer Verlag.Horii, F., Yamamoto, H. and Kitmaru, R. (1984) CP/MAS carbon-13 NMR study of spin

relaxation phenomena of cellulose containing crystalline and noncrystalline components.J. Carbohyd. Chem. 4, 641–662

Ioelovitch, M. (1992) Zur übermolekularen Struktur von nativen und isolierten Cellulosen.Acta Polym.43, 110–113.

Isogai, A., Akishima, Y., Onabe, F. and Usuda, M. (1991) Structural changes of amorphouscellulose by alkaline pulping treatments.Nord. Pulp Pap. Res. J.4, 161–165.

Jakob, H. F., Fengel, D., Tschegg, S. E. and Fratzl, P. (1995) The elementary cellulose fib-ril in Picea abies: Comparison of transmission electron microscopy, small-angle X-rayscattering, and wide-angle X-ray scattering results.Macromolecules28, 8782–8787.

Janson, J. O. G. (1974) Analytik der Polysaccharide in Holz und Zellstoff.Faserforschungund Textiltechnik25, 379–380.

Krässig, H. A. (1993)Cellulose. Structure, Accessibility and Reactivity. Polymer Monographs.Vol.11, Swizerland: Gordon and Breach Science Publishers.

Kulshreshtha, A. K. and Dweltz, N. E. (1973) Paracrystalline lattice disorder in cellulose. IReappraisal of the application of the two-phase hypothesis to the analysis of powder X-raydiffractograms of native and hydrolysed cellulosic materials.J. Polym. Sci.11, 487–497.

Larsson, P. T., Westermark, U. and Iversen, T. (1995) Determination of the cellulose Iα al-lomorph content in a tunicate cellulose by CP/MAS13C-NMR spectroscopy.Carbohyd.Res.278, 339–343.

Larsson, P. T., Wickholm, K. and Iversen, T. (1997) A CP/MAS13C NMR investigation ofmolecular ordering in celluloses.Carbohyd. Res.302, 19–25.

Lennholm, H. (1994)Investigations of cellulose polymorphs by13C-CP/MAS-NMR spectro-scopy and chemometrics.Dissertation Thesis. Royal Institute of Technology, Stockholm,Sweden.

Lennholm, H., Wallbäcks, L. and Iversen, T. (1995) A13C-CP/MAS-NMR-spectroscopicstudy of the effect of laboratory kraft cooking on cellulose structure.Nord. Pulp Pap.Res. J.10, 46–50.

Lindgren, T., Edlund, U. and Iversen, T. (1995) A multivariate characterisation of crystaltransformations of cellulose.Cellulose2, 273–288.

Marchessault, R. H., Taylor, M. G. and Winter, W. T. (1990)13C CP/MAS NMR spectra ofpoly-β-D(1->4)mannose: mannan.Canadian J. Chem.68, 1192–1195.

Newman, R. H. and Hemmingson J. A. (1990) Determination of the degree of cellulose crys-tallinity in wood by carbon-13 nuclear magnetic resonance spectroscopy.Holzforschung44, 351–355.

Newman, R. H. (1992) Nuclear magnetic resonance study of spatial relationships betweenchemical components in wood cell walls.Holzforschung46, 205–210.

Newman, R. H., Hemmingson, J. A. and Suckling, I. D. (1993) Carbon-13 nuclear magneticresonance studies of kraft pulping.Holzforschung47, 234–238.

Newman, R. H. and Hemmingson, J. A. (1997) Cellulose cocrystallisation in hornification ofkraft pulp, ISWPC, Montreal, Canada, O1-1–O1-4.

Newman, R. H. (1998) Evidence for assignment of13C NMR signals to cellulose crystallitesurfaces in wood, pulp and isolated celluloses.Holzforschung52, 157–159.

Paci, M., Federici, D., Capitani, D., Perenze, N. and Segre, A. L. (1995) NMR study of paper.Carbohyd. Polym.26, 289–297.

55

Page, D. H. (1983a) The origin of the differences between sulphite and kraft pulps.J. PulpPap. Sci.9(1), TR 15–TR 20.

Page, D. H. (1983b) Changes in cellulose structure during pulping.Proceedings InternationalPaper Physics Conference, pp. 63.

Press, W. H., Flannery, B. P., Teukolsky, S. A. and Vetterling, W. T. (1988)Numerical Recipes,The Art of Scientific Computing, Cambridge, Cambridge University Press.

Revol, J. F., Dietrich, A. and Goring, D. A. I. (1987) Effect of mercerization on the crystallitesize and crystallinity index in cellulose from different sources.Canadian J. Chem.65,1724–1725.

Rydholm, S. A. (1965)Pulping processes. UK, Wiley.Scallan, A. M. and Carles, J. E. (1972) The correlation of the water retention value with the

fibre saturation point.Svensk Papperstidning17, 699–703.Shashilov, A. A., Evstigneev, E. I., Shalimova, T. V. and Zakharov, V. I. (1986) Structural

changes in spruce wood cellulose during soda and soda-anthraquinone pulping.Khim.Drev.7–11.

Smith, B. G., Harris, P. J., Melton, L. D. and Newman, R. H. (1998) Crystalline cellulosein hydrated primary cell walls of three monocotyledons and one dicotyledon.Plant CellPhysiol.39(7), 711–720.

Stoll, M. and Fengel, D. (1977) Studies on holocellulose and alpha-cellulose from sprucewood using cryo-ultramicrotomy. Part I: Structural changes of the fibre walls duringdelignification and alkali extraction.Wood Sci. Technol.11, 265–274.

Teeäär, R., Serimaa, R. and Paalkkari, T. (1987) Crystallinity of cellulose, as determined byCP/MAS NMR and XRD methods.Polym. Bull.17, 231–237.

Theander, O. and Westerlund, E. A. (1986) Studies on dietary fiber. 3. Improved procedure foranalysis of dietary fiber.J. Agric. Food Chem.34, 330–336.

Wickholm, K., Larsson, P. T. and Iversen, T. (1998) Assignment of non-crystalline forms incellulose I by CP/MAS13C NMR spectroscopy.Carbohyd. Res.312, 123–129.

Wormald, P., Wickholm, K., Larsson, P. T. and Iversen, T. (1996) Conversions between orderedand disordered cellulose. Effects of mechanical treatment followed by cyclic wetting anddrying.Cellulose3, 1–12.

Yamamoto, H. and Horii, F. (1993) CP/MAS13C NMR analysis of the transformation inducedfor Valoniacellulose by annealing at high temperatures.Macromolecules26, 1313–1317.

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