Cellulose Degradation in Pulp Fibers Studied As Changes in Molar Mass Distributions

Cellulose degradation in pulp fibers studied as changes in molar mass distributions

Rickard Berggren

Doctoral Thesis

Royal Institute of Technology Department of Fibre and Polymer Technology Division of Wood Chemistry and Pulp Technology Stockholm 2003

Cellulose degradation in pulp fibers studied as changes in molar mass distributions

Rickard Berggren 2003

Doctoral Thesis

Supervisor: Assoc. Prof. Mikael Lindström, Swedish Pulp and Paper Research Institute

This work has been performed at the Swedish Pulp and Paper Research Institute

Royal Institute of Technology Swedish Pulp and Paper Research Institute Department of Fibre and Polymer Technology Division of Wood Chemistry and Pulp Technology

Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknologie doktorsexamen fredagen den 25 april 2003 klockan 14.00 i STFI-salen, Drottning Kristinas väg 61, Stockholm. Avhandlingen försvaras på svenska.

 Rickard Berggren Stockholm 2003

En sjöman ber inte om medvind, han lär sig segla. G. Lindborg

If you're out there all alone and you don't know where to go to, come and take a trip with me to Future World K. Hansen

Cellulose degradation in pulp fibers studied as changes in molar mass distributions Rickard Berggren, Royal Institute of Technology, Department of Fibre and Polymer Technology, Stockholm, Sweden

Abstract In this thesis, size-exclusion chromatography (SEC) of wood polymers dissolved in lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) has been used to characterize the molar mass distributions (MMD) of wood polymers in pulp fibers after chemical degradation. Characterization of birch kraft pulps subjected to ozone degradation and acid hydrolysis, respectively, rendered different changes in the MMD. Ozone degradation resulted in large redistributions of the original MMD, observed as the development of a distinct fraction of cellulose with intermediate molar mass. Acid hydrolysis resulted in minor changes of the original MMD compared to ozonation. Fibers subjected to acid hydrolysis were considerably weaker than ozonated fibers. These results indicated that there are differences in how the two chemicals degrade the fiber. The solubility of softwood kraft pulp fibers was enhanced by derivatization of the fiber polymers with ethyl-isocyanate during simultaneous dissolution in LiCl/DMAc. The derivatization made it possible to achieve reliable estimations of the MMD, and hence molar masses, of softwood kraft pulps. The derivatization procedure made it possible to dissolve 90 % of softwood kraft pulps with kappa numbers over 50. Severe alkaline degradation of birch and Norway spruce wood chips was studied both by varying the pulping time and by varying the initial alkali concentration. Differences were found in the MMD of the two fiber types, and the alkaline degradation was found to affect polymers in the entire MMD. Multi-angular laser light scattering (MALLS) was used as a detection technique with SEC on cellulosic samples. The MMD and average molar masses obtained through direct- standard calibration with commercial standards were compared with MMD and molar masses as obtained by MALLS-detection. Large discrepancies were found, and two methods of correcting for these discrepancies were developed. Theoretical simulations of polymer degradation were performed. Random, or homogeneous degradation was used as a model for alkaline cellulose chain scission, and a resemblance with experimental data was observed. End-wise depolymerization of cellulose was also simulated and the results are discussed in the light of experimentally observed MMD. Keywords: cellulose, kraft pulp, birch, spruce, ozonation, acid hydrolysis, degradation, MMD, size- exclusion chromatography, light scattering, molar mass, chain scission

Sammanfattning I denna avhandling har storlekskromatografi, size-exclusion chromatography (SEC), av vedpolymerer upplösta i litiumklorid/N,N-dimetylacetamid (LiCl/DMAc) använts för att karakterisera molekylviktsfördelningarna av vedpolymerer efter kemisk nedbrytning. Karakterisering av björksulfatmassa nedbruten genom ozonbehandling eller sur hydrolys resulterade i olika förändringar i molekylviktsfördelningar. Ozonbehandling resulterade i stora förändringar av den ursprungliga molekylviktsfördelningen, observerat som bildandet av en tydlig cellulosafördelning med intermediär molekylvikt. Sur hydrolys resulterade i mindre förändringar av den ursprungliga fördelningen, jämfört med ozonbehandlingen. Surt hydrolyserade fibrer var mycket svagare än de ozonbehandlade fibrerna och resultaten visar att det finns skillnader i hur de två kemikalierna bryter ner fibern. Lösligheten av barrsulfatmassa ökades genom derivatisering av polymererna med etyl- isocyanat under samtidig upplösning i 8 % LiCl/DMAc. Derivatiseringen gjorde det möjligt att göra tillförlitliga uppskattningar på molekylviktsfördelningarna, och därmed molekylvikterna, av polymererna i barrsulfatmassa. Derivatiseringsförfarandet gjorde det också möjligt att lösa upp 90 % av barrsulfatmassa med kappa-tal över 50. Omfattande alkalisk nedbrytning av björkflis och granflis studerades genom en variation av antingen koktid eller initial alkalihalt. Skillnader observerades i molekylvikts- fördelningarna hos de två fibertyperna, och den alkaliska nedbrytningen påverkade hela molekylviktsfördelningarna. Ljusspridning (MALLS) användes som en detektionsteknik tillsammans med SEC av vedpolymerer. Molekylviktsfördelningarna och medelmolekylvikterna, mätta antingen med hjälp av kalibrering med standarder eller med hjälp av MALLS, jämfördes. Stora skillnader i dessa resultat observerades, och två metoder att korrigera för dessa skillnader utvecklades. Teoretiska simuleringar av nedbrytning av vedpolymererna genomfördes. Slumpmässig eller homogen nedbrytning användes som modell för alkalisk hydrolys av polymerkedjorna, och likheter med experimentella data observerades. Nedbrytning av enbart ändgrupperna (peeling) simulerades också, och resultaten diskuteras i förhållande till experimentella molekylviktsfördelningar.

List of publications This thesis is a summary of the following papers, which are referred to in the text by their Roman numerals:

I. Fiber strength in relation to molecular mass distribution of hardwood kraft pulp. Degradation by ozone and acid hydrolysis. Rickard Berggren, Fredrik Berthold, Elisabeth Sjöholm, Mikael Lindström Nord. Pulp. Pap. Res. J., 16, 4, 333-338, 2001. II. Dissolution of softwood kraft pulps by direct derivatisation in LiCl/DMAc Fredrik Berthold, Kristina Gustafsson, Rickard Berggren, Elisabeth Sjöholm, Mikael Lindström Submitted to Journal of Applied Polymer Science.

III. Alkaline degradation of birch and spruce: influence of degradation conditions on molecular mass distributions and fibre strength Rickard Berggren, Ulrika Molin, Fredrik Berthold, Helena Lennholm, Mikael Lindström Carbohydr. Polym., 51, 3, 255-264, 2003. IV. Improved methods to evaluate the molar mass distributions of cellulose in kraft pulp Rickard Berggren, Fredrik Berthold, Elisabeth Sjöholm, Mikael Lindström J. Appl. Polym. Sci., 88, 5, 1170-1179, 2003. V. Comparison between theoretical simulations and observed degradation patterns of cellulose in fiber cell walls: Alkaline degradation Rickard Berggren, Fredrik Berthold, Mikael Lindström Manuscript.

Table of contents 1. INTRODUCTION...... 1

2. WOOD: STRUCTURE AND CONSTITUENTS...... 3

2.1 TRACHEID FIBERS ...... 3 2.2 ULTRASTRUCTURAL ARRANGEMENT OF WOOD POLYMERS ...... 3 2.3 WOOD FIBER CONSTITUENTS...... 4 2.3.1. Cellulose...... 4 2.3.2. Hemicellulose...... 5 2.3.3. Lignin ...... 6 3. PULPING AND WOOD POLYMER DEGRADATION...... 7

3.1 DELIGNIFICATION ...... 7 3.2 ALKALINE DEGRADATION OF POLYSACCHARIDES ...... 7 3.3 ACID HYDROLYSIS ...... 8 4. MOLAR MASSES OF POLYMERS ...... 11

4.1 INTRINSIC VISCOSITY ...... 12 4.2 LIGHT SCATTERING ...... 12 4.3 SIZE-EXCLUSION CHROMATOGRAPHY ...... 13 5. SIZE-EXCLUSION CHROMATOGRAPHY OF CELLULOSE ...... 15

5.1 CELLULOSE SOLVENTS...... 15 5.1.1. The LiCl/DMAc solvent system ...... 16 5.1.2. Interpretation of a molar mass distribution ...... 16 5.2 CALIBRATION PRINCIPLES...... 19 6. OBJECTIVE AND APPROACHES...... 21

7. MATERIALS AND METHODS...... 23

7.1 PRODUCTION OF WOOD POLYMER BEADS ...... 23 7.2 DEGRADATION...... 23 7.2.1. Ozonation (paper I)...... 23 7.2.2. Acid hydrolysis (papers I and IV)...... 23 7.2.3. Alkaline degradation (paper III) ...... 24 7.2.4. Soda anthraquinone degradation of WPB (paper IV) ...... 24 7.2.5. Alkaline degradation of cotton linters (paper V)...... 25

7.2.6. Alkaline pulping (paper V)...... 25 7.3 SIZE-EXCLUSION CHROMATOGRAPHY ...... 25 7.3.1. Dissolution of underivatized samples...... 25 7.3.2. Dissolution of derivatized samples...... 26 7.3.3. Chromatography ...... 26 7.3.4. Multi-Angular Laser Light Scattering...... 27 7.3.5. Additional analyses ...... 27 7.4 THEORETICAL SIMULATION OF POLYMER DEGRADATION ...... 28 7.4.1. Description of the simulation model...... 28 8. RESULTS AND DISCUSSION...... 35

8.1 DEGRADATION PATTERNS OBSERVED DURING ACID HYDROLYSIS AND OZONATION (PAPER I)...... 35 8.2 IMPROVING THE SOLUBILITY OF SOFTWOOD KRAFT PULPS IN LICL/DMAC (PAPER II) ...... 39 8.2.1. Derivatization of cellulose ...... 39 8.2.2. Influence of derivatization on the MMD and molar mass ...... 41 8.2.3. Reproducibility, degree of dissolution and substitution ...... 43 8.3 DEGRADATION PATTERNS OBSERVED DURING ALKALINE DEGRADATION OF SOFTWOOD AND HARDWOOD (PAPER III) ...... 44 8.4 IMPROVING THE ACCURACY IN THE DETERMINATION OF MOLAR MASS OF CELLULOSE IN KRAFT PULPS (PAPER IV) ...... 49 8.4.1. Method I: Correction of the molar mass of cellulose...... 51 8.4.2. Method II: Cellulose-equivalent molar masses of pullulan standards ...... 52 8.4.3. The applicability of the improved evaluation methods...... 54 8.5 SIMULATION OF CELLULOSE DEGRADATION (PAPER V)...... 56 8.5.1. Validation of the simulation model...... 56 8.5.2. Random degradation...... 57 8.5.3. End-wise depolymerization ...... 58 8.5.4. Linear combinations of chain scission cases...... 59 8.5.5. Peeling of cotton linters ...... 62 8.5.6. Limitations of the simulation study ...... 64 9. CONCLUSIONS ...... 65

10. ACKNOWLEDGEMENTS...... 67

11. REFERENCES...... 69

APPENDIX I...... 79

APPENDIX II...... 81

1. Introduction Kraft pulping and subsequent bleaching is one of the most versatile processes for producing pulp to be used in paper and board products. The resulting kraft pulp fibers have superior strength properties to fibers produced by using other pulping techniques, such as mechanical pulping or sulfite pulping. It is central to measure the effects of the chemical reactions occurring during the pulping and bleaching of wood fibers in trying to obtain the optimal product. It is also central for an understanding of the underlying reasons for the deterioration of the strength properties of the fibers.

The fiber consists mainly of polymeric molecules of varying type and size. One of the fundamental properties of a polymer is its molar mass, which affects the strength properties of the polymer and the fiber. The decrease in molar mass of the wood polymers during chemical degradation is normally estimated by measuring the intrinsic viscosity of dissolved pulp fibers. The intrinsic viscosity is however a rather crude measurement, as it only provides one average value of the degree of polymerization of the wood polymer mixture. It does not give any information about which molecules have been degraded and to what extent. A more suitable technique to better describe the degradation of the wood polymers is size- exclusion chromatography (SEC), which provides the entire molar mass distribution of the wood polymers: lignin, hemicellulose and cellulose. The technique also provides the molar masses of the wood polymers. This thesis deals with the use of SEC of wood polymers in the solvent lithium chloride/N,N-dimethylacetamide as a tool to measure and understand the depolymerization of cellulose in pulp fibers during chemical degradation.

2. Wood: structure and constituents 2.1 Tracheid fibers Wood consists of different fiber types. Softwood consists mainly of tracheids and ray cells, while hardwoods consist of vessel elements, fiber tracheids or libriform cells and ray cells (Core et al. 1976; Sjöström 1993d). Softwood tracheids are normally the subject of interest in any model of the fiber cell wall. The length of Scandinavian softwood tracheids is about 3 mm and the width is about 30 µm. Hardwood (birch) tracheids are shorter, about 1 mm, and with similar widths as the softwood tracheids. The width of the tracheids and the thickness of the cell walls vary with growth season. A number of theoretical models have been proposed for these fiber types, and several models have been presented to describe the detailed structure of the wood fiber (Fengel 1970; Scallan 1974; Fengel and Wegener 1984c; Sell and Zimmermann 1993; Brändström 2002). The different models agree that the cell wall consists of different layers, all with different chemical compositions, thicknesses and functions in the wood, see figure 1. The layers of the tracheids are called, going from the outside to the inside: the primary wall (P), the secondary wall (S) and finally the warty layer, which is located on the inside of the cell. The middle lamella surrounds the tracheids and is enriched in lignin. The primary cell wall contains cellulose, hemicellulose, pectin, protein and lignin. The secondary wall is divided into three sub-layers numbered from the outside, S1, S2 and S3, all with different structural arrangements of cellulose in the layers. The S2-layer is the largest part of the cell wall, and it is suggested to have the greatest impact on the chemical and physical properties of the fiber. The main components of the S2-layer are cellulose, hemicellulose and lignin. The composition of the warty layer is complex and varies with species (Sjöström 1993e).

2.2 Ultrastructural arrangement of wood polymers The wood polymers form complex structures in the living tree, and this affects the reactivity of the polymers during delignification. The matrix polymers, lignin and hemicellulose, surround the cellulose (Fengel 1971), which is in turn arranged in more or less crystalline regions, called fibrils, elementary fibrils, or microfibrils. The arrangement of these fibrils in the S2-layer of the wood has been proposed to be either concentric (Kerr and Goring 1975) or radial (Sell and Zimmermann 1993), and the topic is still under debate.

Figure 1. Cell wall model of a Norway spruce tracheid from mature latewood. Reproduced with the kind permission from Brändström (2002).

The cellulose fibrils are assumed to be largely crystalline, and a decreasing crystallinity is proposed as a result of various types of surface imperfection (Rowland and Roberts 1972). Recent studies using solid-state NMR have increased the knowledge regarding the crystalline character of cellulose, and it has been shown that the cellulose fibrils form aggregates in the native wood (Wickholm 2001). These aggregates have been shown to increase in size with increasing temperature during kraft pulping (Hult 2001; Fahlén 2002), with a possible effect on the strength properties of the fibers (Molin 2002).

2.3 Wood fiber constituents 2.3.1. Cellulose Cellulose is the most important constituent of the wood cell wall, as it is the main component in both the native wood fiber and the processed pulp. The cellulose fraction of native wood is about 40 % (on dry weight), depending on the species. Cellulose is a homopolysaccharide on the molecular level, composed solely of β-D-glucopyranose units, joined together by (1→4)- glycosidic bonds (Sjöström 1993a) to form cellobiose units, the smallest repeating unit in cellulose (figure 2). The degree of polymerization (DP) of cellulose present in the living tree is unknown, but the size of the native molecule is often stated to be 5000-10000 glucopyranose units (Fengel and Wegener 1984d).

OH O OH HO HO HO O OH OH O OH

Figure 2. Chemical structure of cellobiose. Cellulose is the load-bearing element of the pulp fiber, and chemical degradation of the cellulose results in a fiber with inferior strength properties (Gurnagul et al. 1992; Page 1994).

2.3.2. Hemicellulose A number of different molecules present in wood are called hemicelluloses. The word hemicellulose first appeared in 1891 (Schulze, 1891), and its origin is based on the assumption that these polysaccharides were precursors to cellulose (Fengel and Wegener 1984a), but the assumption has proven to be wrong (Sjöström 1993b). The compositions of the hemicelluloses in hardwoods and softwoods differ slightly. In hardwood, such as birch, the main hemicellulose is O-acetyl-4-O-methylglucuronoxylan, called glucuronoxylan or simply xylan. The content of xylan in hardwood wood fibers is about 15-30%, depending on the species (Sjöström 1993c). In softwood, the main hemicelluloses are O-acetyl-galactoglucomannan and arabino-(4-O- methylglucurono)xylan, simply called glucomannan and xylan, respectively.

R O OH O OH R O O HO O HO O HO O O O O O OH O OH HO OH MeO HO O OH HO2C

Figure 3. Principal composition of arabinoglucuronoxylan. R denotes extension of polymeric chain.

5 The content of glucomannan and xylan in softwood is 20% and 10%, respectively. The hemicellulose polymers in wood are much smaller in size than the cellulose, as the hemicelluloses have DPs of 100-200 (Sjöström 1993f).

2.3.3. Lignin Lignin is a complex three-dimensional molecule that gives the wood fiber rigidity. It enhances the tree’s resistance towards microorganisms while it acts as a chemical adhesive joining the fibers together in the stem. Softwood lignin is composed of the precursor trans-coniferyl alcohol, hardwood lignin is composed of both trans-coniferyl and trans-sinapyl alcohol, and grass lignin is composed of trans-p-coumaryl alcohol. These precursors are joined together to form a polymeric macromolecule (Sarkanen and Ludwig 1971), which is assumed to have an infinite molar mass in the living tree. An example of a proposed softwood lignin structure is shown in figure 4. OH HO OH Lignin-O Lignin OMe OMe

O HO O O

OMe O HO MeO OH O HO HO OH OH HO O OH O OMe MeO

O O

HO OMe OH O

OH HO OH O OMe

OMe OH O

CHO

Figure 4. Model of softwood lignin, adopted from Brunow et al. (1998).

6 3. Pulping and wood polymer degradation 3.1 Delignification The main objective of the delignification of wood chips is to separate the pulp fibers. This separation may be performed by several different methods, either mechanically or chemically. Chemical delignification may be performed by adding a number of reagents such as alkali, sulfite, anthraquinone, polysulfide and hydrogen sulfide to the aqueous pulping liquor. The results presented in this thesis are mainly related to the properties of pulp fibers produced by the kraft pulping process and subsequent chemical treatments. Kraft pulping is the common name for the delignification of wood chips by the action of a solution of hydroxide and hydrogen sulfide on the wood chips at an elevated temperature. The process leads to pulps with better strength properties than the sulfite process (Rydholm 1965) and it is the most common process to produce chemical pulp in Sweden today (Skogsindustrierna, 2003). The chemical reactions occurring in the wood matrix during delignification are complicated: hemicellulose is partly degraded and dissolved, extractives are removed to a large extent, and the lignin is extensively degraded and dissolved (Gierer 1980). The kinetics of the delignification is normally divided into three different phases, all having different effects on the carbohydrates and the lignin (Gierer 1980). The process suffers from selectivity problems due to the high temperature, but the selectivity has been improved by implementation of the principles of modified kraft pulping (Teder and Olm 1981).

3.2 Alkaline degradation of polysaccharides The alkaline conditions under kraft pulping degrade the carbohydrates in the fibers. The most important reactions of the carbohydrates are end-wise depolymerization (peeling) and alkaline hydrolysis of the wood polymer chains. The peeling reaction removes one unit at a time from the reducing end of the cellulose chain, while a competitive stopping reaction, via a β-hydroxycarbonyl elimination that transforms the end-group of the cellulose chain to a stable D-glucometasaccharinic acid, hinders further peeling to take place (Fengel and Wegener 1984b). The activation energies for the peeling and stopping reactions have been estimated to be 103 kJ/mol and 135 kJ/mol, respectively (Haas et al. 1967), which implies that the importance of the stopping reaction increases with temperature. Franzon and Samuelsson studied un-mercerized cotton and concluded that in average 65 glucosidic units were peeled off before a stopping

7 reaction occurred (Franzon and Samuelsson 1957), and this was later confirmed (Lai and Sarkanen 1967). The corresponding number in mercerized cotton was 40 units. The peeling reaction is thus assumed to affect the yield of the alkaline pulping process.

The effect of the peeling reactions on the molar mass of cellulose is however small, compared to the effect of alkaline hydrolysis that decreases the molar mass of the cellulose by chain scission along the entire chain (Samuelsson et al. 1953; Corbett and Richards 1957). Nevertheless, although the molar mass of cellulose is rapidly reduced, the effect on the yield of the pure alkaline chain scission is low. The alkaline hydrolysis is more important at higher temperatures; the activation energy for the reaction has been determined to be 150 kJ/mol for cotton cellulose (Lai and Sarkanen 1967) and 179 kJ/mol in the soda-anthraquinone pulping of spruce (Kubes et al. 1983). The ultrastructure of the cellulosic fibers may also affect the rate of alkaline degradation. Peeling, stopping and random chain scission by alkaline hydrolysis of fibrous and amorphous cellulose substrates were studied by Gentile et al. (1987). Both the peeling and stopping reactions were found to occur more rapidly in the amorphous substrate than in the fibrous material. It was suggested that alkaline hydrolysis occurs only in the amorphous substrate at the temperatures studied (60°C and 80°C). The hemicelluloses are partly degraded during kraft pulping, and the loss of hemicelluloses is responsible for most of the total yield loss observed during kraft pulping. The hemicelluloses are involved in precipitation (Yllner and Enström 1956; Yllner and Enström 1957), the formation of hexenuronic acid residues (Teleman et al. 1995), saponification, swelling, dissolution, peeling and alkaline cleavage reactions (Lai 2001).

3.3 Acid hydrolysis Acid hydrolysis of cellulose is an interesting reaction as it provides information about the structure and morphological factors that affect the rate and extent of chemical reactions in cellulosic samples (Krässig 1993a). Dissolving pulp grades are usually produced under acidic conditions (Wilkes 2001). Cellulose may be degraded by acid hydrolysis either heterogeneously or homogeneously. In this case, homogeneity implies that the entire structure is susceptible for degradation. Heterogeneous acid hydrolysis of cellulose is performed at low concentration of acids, and leaves a residue of cellulose not susceptible for degradation. The degradation reaction proceeds according to at least two kinetically different phases. The first phase is rapid and is associated with the degradation of easily accessible regions in the cellulose structure. The reaction rate declines during the second stage of the reaction, until the degree of

8 polymerization (DP) of the cellulosic residue approaches the leveling-off-degree of polymerization (LODP). The LODP is defined as the DP at which no further acid hydrolysis takes place, and is around of 100-300 units (Battista et al. 1956). The rates of degradation in the different phases have been found to differ by two orders of magnitude (Durawalla and Shet 1962). It has been suggested that further degradation of the cellulosic residue with a size close to the LODP occurs according to an end-attack model (Sharples 1957; Wood et al. 1989), which has been expanded to apply only to cellulose of the crystalline form cellulose II (Lin et al. 1993).

Homogenous degradation is performed at a high acid concentration, which is required to disrupt and degrade the crystalline part of cellulose. Celluloses from different sources have been showed to react under homogeneous conditions at different rates depending on the source; wood cellulose degrades twice as quickly as cotton cellulose. This was explained as being due to the existence of oxidized groups in the cellulose chain of the wood cellulose (Marchessault and Rånby 1959). The study of the homogeneous degradation of cellulose in phosphoric acid has led to the development of the Ekenstam-expression, which is commonly used in kinetic studies of cellulose depolymerization (Ekenstam 1936). Much work has been devoted to the acid hydrolysis of cellulose and reviews discuss the details of the reactions (Nevell 1985; Krässig 1993a; Lai 2001).

4. Molar masses of polymers Molecules of various chain lengths build up polymeric materials, i.e. the material has a molar mass distribution (MMD). This is especially true for natural polymers such as cellulose. The MMD of a polymer may be illustrated by calculating molar mass averages of the distribution. The commonly used averages are defined as Number average molar mass

n M = ∑ i i M n (1) ∑ ni Weight average molar mass

n M 2 = ∑ i i M w (2) ∑ ni M i z-average molar mass

n M 3 (3) = ∑ i i M z 2 ∑ ni M i where ni is the number of molecules having the molar mass Mi. The width of the MMD is called the polydispersity (PD), and is defined as

= M w PD (4) M n

The values of the molar mass averages are always in the order Mz>Mw>Mn, except in the case of a monodisperse distribution, where PD=1. Pure monodisperse distributions are seldom observed for polymers other than proteins. Several methods may be applied to determine the molar mass of a polymer, and brief descriptions of some of the techniques applicable to cellulose are given below.

11 4.1 Intrinsic viscosity The intrinsic viscosity of a dilute solution of a polymer is related to the molar mass of the polymer (Staudinger and Heuer 1930). The relation is described by the empirical Mark- Houwink equation: η = KM a (5) where η is the intrinsic viscosity of a sample dissolved in a specified solvent and M is the molar mass. K and a are empirical parameters, where a is a measure of the extension of the molecule. The value of a lies between 0.5 and 1.0 for random coils, and is over 1 for rigid rod molecules. The parameters are determined by fractionating the MMD into narrow fractions, followed by measurements of the molar mass by an absolute method and determinations of the intrinsic viscosity of the fractions. If the Mark-Houwink parameters are known for the polymer in the specified solvent, it is possible to calculate the viscosity average molar mass, Mv, from the molar mass distribution. Mv is defined as

1/ a  n M 1+a  M = ∑ i i v   (6)  ∑ ni M i  Normally, viscosity measurements of a pulp sample dissolved in cupriethylenediamine (CED) are used to estimate the carbohydrate degradation occurring during pulping and bleaching. The relationship between the intrinsic viscosity (η) and the degree of polymerization (DPν) of cellulose samples has been formulated as (Evans and Wallis 1989): 0.9 = η (7) DPv 1.65

4.2 Light scattering Light scattering is an absolute detection technique that provides the weight average molecular mass (Mw) of the sample without calibration with external standards according to the following equation

K ∗c 1 = + 2A c (8) ()θ ()θ 2 R M w P

12 where the constant K* is defined as

2  dn  4π 2   n 2 dc 0 (9) K * =   λ4 N A 0 c is the polymer concentration, R(θ) describes the excess of scattered light from the polymer at angle θ, P(θ) is a form factor, A2 is the second virial coefficient (a measurement of solvent-solute interaction), dn/dc the specific refractive index increment of the polymer in solution, no the refractive index of the solvent, λ0 the wavelength of the incident light in vacuum and NA the Avogadro number (Wyatt 1993). The measurements can be performed in batch mode, i.e. by direct measurement of the scattered light at a number of specified concentrations, from which a Zimm-plot is constructed (Zimm 1948). Measurements can also be performed by attaching the light scattering detector to a SEC-column, followed by a concentration-sensitive detector, such as a refractive index or an ultraviolet detector. The batch mode provides the Mw, the second virial coefficient and radius (root-mean-square radius) of the sample in solution, while the online mode provides mass and radius distributions, from which mass averages (Mn, Mw and Mz) are calculated.

4.3 Size-exclusion chromatography Size-exclusion chromatography (SEC) is a separation technology used to separate molecules according to their size in solution, and the origin of the technology is attributed to Porath and Flodin (1959). The sample is dissolved in a solvent and introduced into a separating column, which is filled with a porous packing material. The size of the pores in the packing material determines the molecular range over which the column is active. A constant flow of solvent is applied to the column, and separation takes place by trapping of the sample molecules within the pores. Smaller molecules are retained for a longer time in the column, as they are able to penetrate the porous packing material to a greater extent than larger molecules. Molecules larger than the pores are not retained in the column at all, and they are eluted first. They are followed by the molecules that are small enough to penetrate the pores, and finally the smaller molecules are eluted (Yau et al. 1979; Skoog and Leary 1992). The technique is used to monitor the molar mass distribution of polymeric samples, and the molar mass averages Mn, Mw and Mz can be calculated from MMD provided that the system has been correctly calibrated. The Mv-value may also be calculated from the MMD, provided that the correct Mark-Houwink parameters of the system are known.

5. Size-exclusion chromatography of cellulose 5.1 Cellulose solvents Size-exclusion chromatography (SEC) of cellulose requires that the sample is dissolved in a solvent. Unfortunately, high molecular mass cellulose is insoluble in most common solvents. This is due to the crystallinity, the crystallite size, the crystallite size distribution and the hydrogen bonds holding the cellulose structure together (Krässig 1993b). Attempts have therefore been made to dissolve derivatized cellulose and the MMD of derivatized cellulose has hitherto commonly been determined on cellulose carbamates obtained through derivatization with phenyl isocyanate (Schroeder and Haigh 1979; Sundquist and Rantanen 1983; Evans et al. 1989). The sample is derivatized in pyridine or dimethylsulfoxide (DMSO), precipitated and re-dissolved in tetrahydrofuran (THF), which is used as the mobile phase in the subsequent SEC-characterization. The precipitation step can lead to a loss of low molecular mass cellulose (Wood et al. 1986) and it is necessary to remove the lignin using e.g. chlorite treatment prior to the dissolution. Carbohydrate degradation is assumed not to occur during this treatment, but it cannot be excluded. The removal of lignin in combination with the strong UV-absorption of the phenyl isocyanate group also leads to a loss of information regarding possible lignin-carbohydrate complexes (LCC) present in the sample. A solvent, which was earlier used for the measurement of the intrinsic viscosity of underivatized cellulose, is cupraammonium hydroxide (Cuam). The solvent decomposes during long storage times, and it may, due to its alkalinity, degrade the dissolved cellulose. Cupriethylenediamine (CED) has now replaced Cuam as a solvent for instrinsic viscosity measurements, and the CED-solution is more stable than the Cuam-solution (Johnson 1985). Cadmium tris(ethylenediamine) (Cadoxen) may also be used to dissolve cellulose. Cadoxen is colorless, which facilitates SEC, and cellulose solutions are stable. However, the solution is time-consuming to prepare, and it contains the toxic compound cadmium (Brown 1967; Johnson 1985). A cellulose solvent used on an industrial scale is N-methyl morpholine oxide (NMMO) (Johnson 1969), which is used to dissolve fibers during the production of Lyocell-fibers. The solvent has a complex ternary phase diagram, and this means that careful control of the water content is required to achieve economical solutions of cellulose (Woodings 2001).

15 5.1.1. The LiCl/DMAc solvent system A solvent system frequently used to dissolve cellulosic samples is a mixture of lithium chloride/N,N-dimethylacetamide (LiCl/DMAc). The solvent/sample-system was first described two decades ago (McCormick and Lichatowich 1979; Turbak et al. 1981). The solvent/sample-system is colorless and it is thus also suitable for liquid chromatography coupled to light absorbing detectors. The UV-cutoff is 270 nm, which makes it possible to study the lignin distribution in kraft pulps. The system is fairly stable, as little or no degradation of the samples has been reported (Strlic et al. 1998). No simple explanation of the mechanism of dissolution of cellulose in LiCl/DMAc has been proposed, although several studies have been performed (Morgenstern and Kammer 1996). Cotton, sulfite and birch kraft pulps can be dissolved at high degrees of dissolution without derivatization (Kennedy et al. 1990; Westermark and Gustafsson 1994; Sjöholm et al. 1997; Striegel 1997). However, problems have been reported with underivatized softwood kraft pulps, probably due to gelation of the glucomannan fraction (Hortling et al. 1994; Sjöholm et al. 1997). LiCl/DMAc has also been found to be a good solvent for a number of cellulose derivatives (McCormick and Lichatowich 1979; McCormick and Callais 1987). The solvent/sample-system has other drawbacks. One example, in addition to the poor solubility of softwood kraft pulps, is the high salt concentration, which both increases the viscosity of the solution and complicates preparative SEC. Another is the corrosive nature of the solvent, which limits the choice of material in the chromatographic system. Although there are some drawbacks in the solvent/sample system, LiCl/DMAc is nevertheless one of the most suitable solvent systems available for cellulosic materials. It is compatible with common column materials, it is stable and it dissolves birch kraft pulp and cotton to a satisfactory extent without derivatization.

5.1.2. Interpretation of a molar mass distribution A SEC-characterization of unbleached birch kraft pulp generally gives a MMD having two distinctive distributions (figure 5). A larger, high molecular mass peak represents the MMD of the cellulose fraction and a smaller, low molecular mass peak represents the hemicellulose and lignin fractions (Sjöholm et al. 2000a)

1.6 RI 1.2 UV

0.8 dw/dlogM 0.4

0 34567 log M

Figure 5. Differential molar mass distribution of a birch kraft pulp. RI and UV denote refractive index and ultraviolet detection, respectively. The area under a MMD is always normalized to unity in order to make it possible to compare the MMDs from injections with different sample concentrations. The SEC columns may be attached to an ultraviolet detector (UV) and a differential refractive index detector (RI), connected in series. The results from the UV-detector describe the distribution of UV-absorbing molecules in the sample, which are related to the lignin (Westermark and Gustafsson 1994). The RI-detector describes the total mass distribution of the cell wall polymers in the sample. The combined information from the UV and the RI-detector can be used to evaluate possible lignin-carbohydrate complexes (LCC) in kraft pulps (Karlsson and Westermark 1996). The present thesis is mainly concerned with the MMD as obtained by RI-detection, which corresponds mainly to the carbohydrate fraction of the pulp fibers. The result of a SEC characterization of a kraft pulp is normally a plot of the differential mass fraction plotted versus the logarithm of the molar mass. The ordinate describes the mass fraction eluting between log M and log M+dlog M. This mass fraction describes the mass of the polymer, i.e. the number of molecules multiplied by its molar mass. The significance of the dw/d(logM)-term may be more easily understood if the cumulative weight fraction of the polymer is plotted versus the molar mass, figure 6. The dw/d(logM)- term describes the absolute value of the derivate of the cumulative weight fraction with respect to the logarithmic molar mass at each point of the cumulative distribution. If that value is plotted versus log M, figure 5 is obtained which is the normal way of representing a MMD of a cellulosic material.

100 RI 80 UV 60

20 Weight fraction (%) 0 34567 log M

Figure 6. Cumulative plot of the MMD in figure 5. The arrow denotes order of elution (larger molecules elutes before smaller molecules). The MMD may be represented in different ways, depending on the purpose of the characterization, and it is necessary to be familiar with the relations between these representations (see section 7.4). One way is to describe the differential mass fraction eluting between M and M+dM as shown in figure 7. However, this plot is difficult to interpret in the case of birch kraft pulp polymers.

0.003 RI UV 0.002

dw/dM 0.001

0 0 250000 500000 750000 1000000 M

Figure 7. Mass fraction plot of the MMD in figure 5. Another way is to plot the number fraction of the polymer eluting between M and M+dM (data not shown). Thus, it is possible to represent the same sample in different ways, all with very different appearances, but with the same content. The molar masses may be calculated from any of the distributions using the equations 1-4.

5.2 Calibration principles The signal from a detector in a SEC-system has to be calibrated. The output from a detector is only a record of the sample concentration versus elution time, and calibration is required so that the elution time can be related to the molar mass of the eluting sample.

The calibration of the system may be achieved by universal calibration (Grubisic et al. 1967; Yau et al. 1979; Striegel and Timpa 1996), provided that a viscosity detector is available. The principle behind the calibration is that for any polymer the product of the limiting viscosity, [η], and the molar mass, M, is proportional to the hydrodynamic volume of an equivalent sphere, Vh, i.e. η ∝ [ ]M Vh (10) In the ideal case, the molecules are separated in the columns according to their hydrodynamic volume, which is proportional to the radius of gyration (Rg) raised to the power of three. The same retention time will therefore be obtained for any two polymers having the same value of [η]M, Vh or Rg. Two molecules with the same hydrodynamic volume will have the same retention time. This fact is used when the system is calibrated with standards of known molar mass, known as direct-standard calibration. In this case, a set of standards, not necessarily of the same polymer as the sample, is used to relate the elution time to the molar mass. It is assumed that the samples and the standards have the same relation between molar mass and hydrodynamic volume, and thus between molar mass and elution time. It is therefore important to choose standards that resemble the samples as much as possible with respect to chemical structure and conformation, in order to achieve a correct calibration. For cellulose, the commercially available pullulan standard is often used (Westermark and Gustafsson 1994; Strlic et al. 1998; Sjöholm et al. 2000a; Sjöholm et al. 2000c), but the applicability of the pullulan has recently been questioned (Bikova and Treimanis 2002). If a light scattering detector is available, the molecular mass of the eluting sample is obtained in every slice of the chromatogram, and no external calibration is needed, provided that the dn/dc-value of the solvent/sample system is known (Wyatt 1993). Recent studies have illustrated the capability of characterizing dissolving pulps in LiCl/DMAc using light scattering detection (Schelosky et al. 1999; Schult et al. 2002).

6. Objective and approaches The objective of this thesis is to study the effect of chemical treatments on strength properties of pulp fibers and how these effects relate to changes in molar mass distributions of the pulp polymers. In order to do this, a new and improved method of dissolving the samples prior to the characterization by size-exclusion chromatography was developed. The use of light scattering as detection technique was studied, and a simulation model was developed in order to elucidate the experimentally observed degradation.

7. Materials and Methods A number of different pulps, both industrial and laboratory-made, were used. Cotton linters were also used as raw material. See appendix I for descriptive data of all these samples.

7.1 Production of wood polymer beads Regenerated cellulose spheres, as described by Lindström et al (1999), were produced as a model system for degradation studies. For a detailed description of the production of these wood polymer beads (WPB), see Östlund (2001). The principle behind the production of the beads is: fibers are dissolved in 8% LiCl/DMAc, followed by re-precipitation of the solute by dripping the solution into a mixture of ethanol and water, which results in sphere- shaped cellulosic beads. The degree of crystallinity is low in the beads, and the chemical composition is similar to that of the original pulp (Lindström et al. 1999).

7.2 Degradation Cellulosic samples were degraded by several different chemicals under different conditions during the course of this work.

7.2.1. Ozonation (paper I) Pulp samples were degraded by ozonation at high consistency (40%) at ambient temperature in a rotating glass vessel connected to a Fischer 5000 ozone generator. Prior to ozonation, the pH of the samples was adjusted to 3 with sulfuric acid followed by fluffing. Eight pulp samples were subjected to ozone dosages ranging from 0.1 to 3% (w/w). After washing with water, the samples were buffered to pH 7.3 with a 0.02 M phosphate buffer.

7.2.2. Acid hydrolysis (papers I and IV) Acid degradation was performed by adding 25 ml 2 M HCl per gram pulp in sealed polyethylene bags, which gave a pulp consistency of 3%. The temperature during hydrolysis was 41°C and the treatment was performed for 3, 6, 10, 14, 17, 25 and 40 h. After washing, the acid-treated samples were buffered to pH 7.3 with a 0.02 M phosphate buffer.

23 Wood polymer beads made from an unbleached pulp were also degraded by acid hydrolysis (paper IV). A sample of 15 mg was degraded with 5 ml 2M hydrochloric acid at ambient temperature for degradation times of 1h, 3h, and 23h. Degraded beads were washed repeatedly with deionized water to neutral pH before dissolution.

7.2.3. Alkaline degradation (paper III)

Hand-sawn spruce chips with minimal damage and hand-sorted laboratory-chipped birch chips were used to perform extended alkaline degradation under conditions similar to those prevailing during kraft pulping. The pulps were digested in autoclaves which were heated in a glycol bath. The liquid:wood ratio was of 8:1 during the experiments. After the cooks, the pulps were washed with de-ionized water overnight and defibrated. The degradation conditions in the different cooks were varied by changing the pulping time or the initial alkali concentration. Chemical charges and reaction times in these experiments are given in appendix II. All the spruce chips were impregnated in the cooking liquor by increasing the temperature from 70οC to 170οC at a rate of 1oC/min. The temperature was kept at 170οC for the rest of the cooking time. All the birch chips were impregnated at 120οC for 30 minutes and then heated at maximum speed (10 minutes) to 170οC and kept there for the rest of the cooking time. The initial hydrogen sulfide ion concentration was 0.3 M for all the cooks. The residual alkali in the black liquor was determined by titration with HCl to pH 10.7. To achieve a correct value, the black liquor was first diluted eight times and the dissolved lignin and carbonate ions were precipitated with BaCl2. The hydrogen sulfide ion concentration was determined using a potentiometric titration with AgNO3 modified from the method of Chiu and Paszner (1975).

7.2.4. Soda anthraquinone degradation of WPB (paper IV) Wood polymer beads from a fully bleached pulp were degraded under soda-anthraquinone (soda-AQ) pulping conditions. For full details of the chemical conditions, see Östlund (2001). Two temperatures were used (130°C and 170°), the alkali concentrations were varied between 0.1 and 0.7 M and the AQ concentrations were varied between 0.1% and 9%. The liquid:wood ratio was kept constant at 200:1.

24 7.2.5. Alkaline degradation of cotton linters (paper V) Cotton linters were subjected to degradation in alkali. 5 grams of fibers were put in a steel vessel, and 80 ml 1 M NaOH solution was added. The vessel was heated in a glycol bath, thermostated to 130°C for 1, 2, 4, 5.5 and 7.5 hours. The samples were thereafter washed repeatedly with water until a washing solution with a neutral pH was obtained.

7.2.6. Alkaline pulping (paper V) Hand-sorted spruce chips (4g) were pulped in steel autoclaves. Initial alkali, hydrogen sulfide and sodium concentrations were 0.4 M, 0.22 M, and 1.7 M respectively. The liquor:wood ratio was 20:1. The pulping included a heating stage from 70°C to 145°C, after which the pulping liquor was removed and replaced with fresh liquor. Thereafter, the autoclaves were heated to the pulping temperature 170°C at which pulping took place for 70, 100, 130 and 200 minutes. Subsequent defibration and washing of the pulps were carried out according to normal laboratory procedures.

7.3 Size-exclusion chromatography 7.3.1. Dissolution of underivatized samples The underivatized birch kraft pulps and cotton linters were dissolved according to a procedure developed earlier at STFI (Sjöholm et al. 1997; Sjöholm et al. 2000a; Sjöholm et al. 2000b). Washed pulp samples (15 mg oven dry weight pulp) were activated in 15 ml deionized water at 4°C for one hour. The water was removed by vacuum filtration and 15 ml of methanol was added and removed by vacuum filtration after 30 minutes. The procedure was repeated twice with methanol and three times with degassed DMAc. A solution of 8% (w/v) LiCl in DMAc was added and gently stirred under a nitrogen atmosphere at 4°C for five days. The samples were then equilibrated at room temperature for 30 minutes and diluted with degassed DMAc to sample and LiCl concentrations of 0.05 and 0.5%, respectively. After 2 hours, the samples were de-aggregated (Sjöholm et al. 2000b) in a Retsch vibratory ball mill type MM-2 (intensity 70) for 30 minutes and filtered through a 0.45 µm PTFE filter before the chromatographic characterization. The dissolution procedure for WPB was similar, but performed with a lower amount of solvents (water, methanol and DMAc) since only 2-5 mg sample were used.

25 7.3.2. Dissolution of derivatized samples Kraft pulp samples were simultaneously derivatized and dissolved as follows: 15 mg fibers were activated for one hour in 15 ml deionized water at 4°C. Thereafter, the fibers were solvent-exchanged 3 times with 15 ml dry DMAc at ambient temperature and placed in a flat-bottomed glass cylinder. 1.9 ml 8% LiCl/DMAc and 3 mmol of the derivatizing reagent then added. The samples were left under argon at ambient temperature for 5 days with mild magnetic stirring. Finally, the samples were diluted to 0.5% LiCl by the addition of 27.4 ml DMAc and the excess reagent was quenched by adding 500 µl dry methanol. The samples were de-aggregated and filtered in same manner as the underivatized samples before the chromatographic separation. After dilution with DMAc, the samples for the determination of the degree of substitution were precipitated in methanol, washed twice with water:methanol (7:3) and twice with water. The samples were freeze-dried and the nitrogen content was determined at Mikrokemi AB, Uppsala, Sweden. Non-dissolved residues were determined gravimetrically after ultra-centrifugation and washing of the residue with water to remove salt and traces of DMAc.

7.3.3. Chromatography The chromatographic system consisted of a 2690 Separation Module (Waters Corp, Milford, MA, USA.) equipped with a guard column (Mixed-A 20 µm 7.5x50 mm, Polymer Laboratories, Shropshire, UK) followed by four columns (Mixed-A 20 µm, 7.5 x 300 mm, Polymer Laboratories) connected in series. The mobile phase was 0.5% (w/v) LiCl/DMAc, the flow rate was 1 ml/min and the injection volume was 200 µl. The mobile phase was filtered through a 0.2 µm PTFE inline filter. The separations were performed at 80°C. Four mixtures of narrow pullulan standards with nominal masses of 738 Da, 5.8 kDa, 12.2 kDa , 23.7 kDa, 48 kDa, 100 kDa, 186 kDa, 380 kDa, 853 kDa, and 1 660 kDa (Polymer Laboratories) were used for calibration in papers I, III and IV while narrow pullulan standards with molecular masses of 1660, 380, 48, 5.8, and 0.738 kDa (Polymer Laboratories) were used to calibrate the chromatographic system in papers II and V. Detection was performed online by a 2487 dual wavelength absorbance (UV) detector (Waters Corp.) at 295 nm, followed by a 410 differential refractive index (RI) detector (Waters Corp.) thermostated at 40 °C.. Data analysis were performed using the Millenium 3.05.01 software (Waters Corp.).

26 7.3.4. Multi-Angular Laser Light Scattering The configuration of the detectors used for the light scattering study (paper IV) was slightly different from the configuration used during the other studies. SEC- characterization was performed relative to standards using one detector configuration (SEC/RI, System I), and by light scattering (SEC/MALLS/RI, System II) using another detector configuration. Relative and absolute detection were also carried out simultaneously in series. The detectors for System I were a 2487 dual wavelength (UV) detector (Waters Corp.) operating at a wavelength of 295 nm followed by a 410 differential refractive index (RI) detector (Waters Corp.) thermostated at 40 °C. For the light scattering detection in System II, the configuration was a multi-angular laser light scattering detector (MALLS) (DAWN DSP), followed by an Optilab DSP RI detector (both from Wyatt Technology Corp, Santa Barbara, CA, USA). Both the MALLS and the Optilab DSP RI detectors were operating at 488 nm. The order of the detectors was MALLS-UV-Optilab DSP RI–Waters RI. Two separate column sets were used (both obtained from Polymer Laboratories), one for calibration and one to verify the calibration. The column sets consisted of a guard column (Mixed-A, 20 µm, 7.5 x 50 mm) and four Mixed-A (20 µm, 7.5 x 300 mm) columns connected in series, all thermostated to 80°C The Optilab RI used in system II was thermostated at 40°C. The output voltage from the detector was calibrated to known refractive indexes by injecting six known concentrations of sodium chloride dissolved in deionized water, and this gave an instrument-specific RI- calibration constant. The light scattering detector was calibrated with toluene and normalized by injecting 200 µl of a 30 kDa narrow polystyrene standard solution having a concentration of 1 mg/ml in 0.5% LiCl/DMAc. The detectors used in the DAWN instrument were numbers 4, 5, 6, 7, 8, 10, 12, 14 and 16. Narrow interference filters to eliminate the fluorescence from any lignin present in the samples were placed in front of all the detectors. Detectors at higher and lower angles were omitted due to the low signal-to-noise (S/N) ratio. Light scattering data were evaluated using the software ASTRA 4.73.04 (Wyatt Technology Corp).

7.3.5. Additional analyses The intrinsic viscosity and kappa number of the cellulosic samples were determined according to standard methods SCAN-C 15:99 and SCAN-C 1:77, respectively. The

27 carbohydrate compositions were determined using a gas chromatographic method (Theander and Westerlund 1986). The fiber length and fiber shape of the samples were determined by image analysis with the STFI-Fibermaster (Karlsson et al. 1999). Laboratory sheets for the measurement of strength properties were made according to SCAN-CM 26:99. The fiber strength was measured as dry and rewetted zero-span tensile index according to ISO 15361. Nitrogen contents of derivatized softwood pulp samples dissolved in LiCl/DMAc were determined at Mikrokemi AB, Uppsala, Sweden.

7.4 Theoretical simulation of polymer degradation A common way to monitor polymer degradation is by measuring the average molar mass, which is then used to calculate the rate constants for the degrading reactions. This was done in the 1930:s by Ekenstam, who proposed the following relation when studying the homogeneous degradation of cellulose in phosphoric acid (Ekenstam 1936).

=  1 − 1  kt ln  (11)  DPt DP0 

The k is the reaction rate constant, t the reaction time and DP0 and DPt the degrees of polymerization of the cellulose initially and at time t, respectively. Tanford (1961) describes other methods of using the average molar masses to estimate the kinetics of polymer degradation. It is stated by Tanford that cellulose is a special case since the structure of the reacting material also is affecting the rate of scission of the molecules. The use of molar mass averages does not reveal any information about the molar mass distribution of the actual polymer subjected to degradation. The purpose of the theoretical work in this study was to develop a model that could predict and relate the molar mass distribution of the wood polymers during the degradation to any type of scission pattern present in the fiber. The model is described below and the first attempts to use the model on wood polymers are discussed in the results section.

7.4.1. Description of the simulation model The development of the MMD during polymer degradation has been the subject of several studies (Montroll and Simha 1940; Guiata et al. 1990; Viebke et al. 1996). The two main approaches in the recent literature are Monte Carlo simulations and a deterministic equation solving technique. A Monte-Carlo approach has been thoroughly investigated and developed by Tobita for a variety of polymers and scission cases (Tobita 1995; Tobita

28 2001). An example using a Monte-Carlo approach has also been published modeling degradation of linear polymers (Emsley and Heywood 1995). In this study the deterministic equation solving technique developed by Ballauff and Wolf (1981) was adopted. The same approach has recently been used to study the degradation of nucleic acids (Tanigawa et al. 1996) and guar galactomannan (Tayal and Khan 2000). The main reason for choosing this approach was the relative ease with which the system of equations could be handled and solved. The following is a description of the approach that was used in this thesis. Normally, a MMD of a cellulosic polymer is depicted graphically as a plot of the differential mass fraction dw/d(logM) versus the logarithm of the molar mass, log M (figure 5). The dw/d(logM)-term describes the mass fraction of the polymer eluting between molar masses log M and log M+dlogM. This description is achieved by measuring the detector response W(t) at the elution time t and normalizing the response with respect to the sum of the detector response over the entire elution time of the distribution according to the equation:

()= W (t) (12) WN t ∞ ∫W (t)dt 0

The normalized detector response, WN(t), is then calibrated to the elution time via the calibration constant of the separation system according to:

dt dw / d(log M ) = −W (t) (13) N d(log M ) where dt/d(logM) is the inverse of the calibration constant for the system at the elution time t.

The model considers the rate constant, ki, for the degradation of a molecule of chain length i, and the rate constant of degradation into two new fragments of lengths j and i-j, described as ki,j and ki,i-j, respectively. Hence, summarizing all ki,x over all x for a molecule of length i gives the value of ki which describes the degradation rate for a molecule of length i.

29 The model assumes that the degradation is first order with respect to the number fraction of i-mers and the following mass balance for a polymer with degree of polymerization i holds in the entire system of polymers:

df  i−1  i = − k  f + (k + k ) f + ... + (k + k ) f (14) ∑ i, j  i i+1,1 i+1,i i+1 r,i r,r−1 r dt  j=1  where dfi/dt is the change in concentration of the molecule of length i with respect to simulation time, ki,j are the individual rate constants, fi is the number fraction of polymer of length i, and r is the highest degree of polymerization of the polymers in the system. The change in concentration of the molecules in the system with respect to simulation time is given by the equation 15 where the matrix A is described by equation 16.

dF (t) (15) = AF (t) dt

0 k + k k + k ... k + k   2,1 2,1 3,1 3,2 r,1 r,r−1  − + 0 k2,1 k3,2 k3,1 ......   2  (16) 0 0 − k ......  A = ∑ 3, j  j=1  0 0 0 .. ...   r−1  − 0 0 0 0 ∑ kr, j   j=1  This matrix contains the rate constants for chain scission satisfying equation 15 over the entire distribution of chain lengths and it is triangular since it is stipulated that no recombination of original or degraded fragments can occur. The vector F(t) in equation 15, is described by:

 f (1,t)     f (2,t) F(t) = (17)  ..       f (r,t) The F(t)- term thus describes the number fraction of the molecules within the DP-range from 1 to r in the time-dependent system of degrading molecules, where f(i,t) is the number fraction of polymers with a DP of i at the simulation or degradation time t .

30 The model handles the scissions of individual linkages between the subunits constituting the polymer, and a transformation of the differential mass fraction to provide the number fraction of the polymers is thus necessary. The mass fraction dw/d(logM) that is normally used to depict the MMD of a polymer, is used to express df/dM, which is the mass fraction of the polymer eluting between masses M and M+dM.

dw df d log M = dM (18) d(log M ) dM which yields

df = d(log M ) dw = 1 dw (19) dM dM d(log M ) M ln10 d(log M ) where M is the mass of the polymer of length i. The number ratio, f(i,t), of the polymer eluting between the degrees of polymerization, DP, and DP+dDP at a simulation time t is then obtained by:

= 1 dw f (i,t) 2 (20) moi ln10 d(logM ) where m0 is the mass of the monomer/subunit constituting the polymer. Equation 15 can now be solved. This can be done by linear algebra, but in our case it is solved using the built-in ordinary differential equation solver in the software Matlab (The Mathworks Inc., Natick, MA, USA) running on a desktop computer. The resulting vector F(t) from the simulation is transformed to the dw/d(log M) distribution according to equation 20. The dw/d(log M) distribution is finally plotted versus log M and the molar masses of the simulated distribution are calculated from this plot.

Chain scission cases Two basic cases of chain scission, both studied alone and as linear combinations of each other, have been considered. The purposes of the cases are to simulate either alkaline hydrolysis or peeling, and both cases describe the scission rate of a glucosidic linkage as a function of the position of the linkage within a polymer chain of length i. Several other cases may be studied (Ballauff and Wolf 1981; Tanigawa et al. 1996; Tayal and Khan 2000) but the following two are the most interesting for alkaline pulping.

Random scission In this mode, the rate constant for chain scission is equal over the entire chain length, irrespective of molar mass of the polymer, i.e. for any i or j,

= ki, j k (21)

In this case, the diagonal element in the matrix A is given by:

j−1 A = − i, j (22) Aj, j ∑ i=1 2

An interpretation of equation 22 is that the polymeric fragments of length i and j-i are retained within the system after scission, and that the number of molecules susceptible for degradation in the system increases with simulation time. The modeling of random scission was intended to simulate alkaline hydrolysis of cellulose.

Peeling or end-wise depolymerization The rate constants for end-wise depolymerization are given by: (23) = > ki,i−1 k , i 2 k = 0 otherwise where i is the length of the polymer. Thus, the simulated end-wise depolymerization reaction removes only one monomeric residue at a time from the reducing end of the polymer, as a first approximation to the experimental case. The diagonal element in matrix A is given by:

= A1,1 0 j −1 (24) = − = − Aj, j ∑ Ai, j k i=2 since the units peeled off cannot undergo further chain scission. Hence, the number of molecules in the system susceptible to degradation does not increase during simulation.

32 The turnover of linkages, X, was used as a measure of the number of degraded linkages during a simulation. The value of X was determined after each time interval of simulation according to:

r ∑[]f (i,t)⋅(i −1) = i=2 X r (25) ∑[]f (i,t = 0)⋅(i −1) i=2 since the number of glycosidic linkages in a polymer of length i is equal to i-1. The theoretical yield of the simulations, Y, was determined according to:

r []f (i,t) ⋅i ⋅ m ∑ 0 (26) = i=1 Y r []= ⋅ ⋅ ∑ f (i,t 0) i m0 i=1

During the simulations, no consideration was given to the kind of polysaccharide in which the actual bond was located. All the linkages between the chain units are treated equally in terms of reactivity even though there are degradation rate differences between cellulose and different hemicelluloses (Lai 2001).

8. Results and discussion 8.1 Degradation patterns observed during acid hydrolysis and ozonation (paper I) Ozone has been considered to be one of the bleaching chemicals to be used as and alternative to chlorine dioxide (van Lierop et al. 1996). Unfortunately, the delignification reactions taking place during bleaching are accompanied by a concomitant degradation of cellulose and hemicelluloses, which leads to both a decrease in yield and a deterioration in fiber properties, e.g. strength. This is also the case for ozone-delignification that also has been claimed to suffer from low selectivity towards lignin, compared to chlorine dioxide (van Lierop et al. 1996). The cellulose degradation during ozonation and acid hydrolysis was studied in paper I on an unbleached birch kraft pulp and on cotton linters, both subjected to aqueous-phase acid hydrolysis or ozonation at high consistency. However, this section deals only with the results from the degradation of the birch pulps. The viscosity and fiber strength of the samples decreased in different manners with increasing ozone dosage and with increasing hydrolysis time, as shown in figure 8.

200 Z-pulp 150

100 A-pulp (Nm/g) 50

Zero-span tensile index 0 400 600 800 1000 1200 Viscosity (ml/g)

Figure 8. Rewetted zero-span tensile index of pulp fibers subjected to acid hydrolysis (A-pulp, □) or ozonation (Z-pulp, ●) plotted versus intrinsic viscosity. The initial zero-span tensile index of the pulp was 183 Nm/g. Up to an ozone dosage of 1.6% (w/w), the loss in fiber strength was only 5% of the initial strength, while the intrinsic viscosity dropped from 1160 ml/g to 880 ml/g. Even at the highest ozone dosage, at an intrinsic viscosity of 510 ml/g, the loss in fiber strength was only 25%. Acid hydrolysis of the pulp was more detrimental to fiber strength than ozone treatment. The

35 most severely acid-degraded pulp fibers had an intrinsic viscosity of 810 ml/g and 70% of the initial fiber strength was lost. The intrinsic viscosity is clearly insufficient for evaluating different cellulose depolymerization reactions, and similar results have been reported from studies of different kinds of alkaline pulping (Kubes et al. 1981). The viscosity losses were similar in the acid- degraded and ozone-degraded pulp samples, but the fiber strength of the acid-treated fibers decreased more than that of the ozone-treated fibers. The assumption that acid degradation occurs at local deformations, weak points, in the fiber cell wall (Gurnagul et al. 1992) could explain our observations. The strengths and viscosity levels shown in figure 8 indicate that there are differences in the homogeneity of the degrading reactions. The interpretation of the results is that ozone degradation occurs more evenly over the entire cellulose fiber than acid degradation, resulting in retention of fiber strength during ozone treatment. If the ozone had preferentially attacked the weak points in the fiber, the fiber strength would immediately have decreased as was seen in the case acid-treated pulp samples. A shape factor of 100% corresponds to a straight fiber, and it has been suggested that a deformed fiber in general leads to a decrease in zero-span tensile index (Mohlin et al. 1996). Fiber deformation, determined as shape factor, is presented together with the zero- span tensile index in figure 9 for the degraded pulp samples. The shape factor changed irregularly with increasing ozone dosage. During acid hydrolysis, a continuously decreasing trend was more evident. Although the strength loss was greater in the acid- degraded pulp than in the ozone-degraded pulp, there were almost no differences in shape factor between the two treatments. Thus, the reason to the strength differences is not related to differences in shape factor.

36 200

150

100 Z-pulp A-pulp (Nm/g) 50

Zero-span tensile index 0 94.0 94.4 94.8 95.2 95.6 96.0 Shape factor (%)

Figure 9. Rewetted zero-span of pulp fibers subjected to acid hydrolysis (A-pulp, □) or ozonation (Z-pulp, ●) plotted versus shape factor. Figure 10 shows the MMDs for the ozone-degraded birch pulp samples. All the MMDs reported in this section are based on refractive index (RI)-detection, unless otherwise are stated. The larger, high molecular mass peak represents the MMD of the cellulose fraction and the smaller, low molecular mass peak represents the hemicellulose and lignin fractions (Sjöholm et al. 2000a). The cellulose fraction of the pulp was considerably degraded in the presence of ozone. From the original peak, with a log peak molecular mass (log Mp) = 6.2, a part of the pulp cellulose fraction was extensively degraded and a second cellulose peak having a value of log Mp ≈ 5 was observed. Part of the cellulose seems however to be only slightly degraded, as there is still a peak with log Mp ≈ 6 after the treatment with 3% ozone. The polydispersity, i.e. the ratio of the weight average to the number average molecular mass (Mw/Mn), of the cellulose fraction increased from 2.2 in the untreated pulp to 3.9 in the most severely degraded pulp.

dw/dlogM

3.0 4.0 5.0 6.0 7.0 log M (relative to pullulan)

Figure 10. MMD of birch kraft pulps subjected to ozonation. The arrow denotes increasing ozone dosage. dw/dlogM

3.0 4.0 5.0 6.0 7.0 log M (relative to pullulan)

Figure 11. MMD of birch kraft pulps subjected to acid hydrolysis. The arrow denotes increasing time of degradation. Figure 11 shows the MMDs of birch fibers subjected to acid hydrolysis. The degradation pattern is quite different from that of ozonation (figure 10). The shortest acid treatment time, i.e. three hours, shifted both the cellulose and hemicellulose distributions towards lower molecular mass. However, the rate of degradation decreased after this initial degradation, and only a slight drop in log Mp of the hemicellulose distribution and almost no shift in the log Mp of the cellulose distribution were observed upon prolonged treatment. A minor broadening of the cellulose peak, which may indicate the development of an intermediate peak, was detected as the polydispersity of the cellulose fraction increased from 2.2 to 2.8.

38 One of the reasons for the differences in degradation patterns may be the lignin content of the pulp, as lignin can promote the formation of secondary radicals in the course of ozone delignification (Eriksson and Reitberger 1995; Lind et al. 1997). Under kraft cooking conditions, lignin is enriched at the surface of fibers (Laine 1996), where it has been suggested that the ozone reactions take place (Secrist and Singh 1971). It has been suggested that radicals formed in ozone-lignin reactions contribute to the cellulose degradation only if the supply of ozone is large enough to propagate radical chain reactions (Johansson and Lind 1999). The dosage of ozone in the present study was sufficiently high to achieve significant cellulose degradation, which also is observed as a shift in the MMD in figure 10. It is thus suggested that the intermediate cellulose distribution in the MMD represents cellulose degraded by radicals formed in lignin-ozone reactions. As the reaction front proceeds inwards through the fiber wall according to a shrinking core model (Zhang et al. 1998), it is plausible to suggest that the degraded cellulose is located in the external regions of the pulp fiber. The formation of two distinct cellulose distributions during ozonation of a unbleached birch kraft pulp implies two modes of degradation, either at the ozone-front in the fiber or in the already ozonated region of the fiber, as discussed previously (Zhang et al. 1999). The same phenomenon has also been discussed in terms of the MMD of kraft hemlock pulp in the development of the above-mentioned shrinking core model (Griffin et al. 1998). The cellulose degradation and the bimodality observed in the MMD in the present study are more distinct than has previously been reported.

8.2 Improving the solubility of softwood kraft pulps in LiCl/DMAc (paper II) 8.2.1. Derivatization of cellulose A study was carried out in order to increase the solubility of softwood kraft pulps in the solvent LiCl/DMAc. The aim was to increase the solubility by simultaneous derivatization and dissolution of the cellulose, thus allowing characterizations of the MMD of softwood kraft pulps by SEC in LiCl/DMAc. Figure 12 shows the MMD of an underivatized softwood kraft pulp with an intrinsic viscosity of 1080 ml/g and a kappa number of 18 dissolved to an extent of 92% in 0.5% LiCl/DMAc.

39 RI UV dw/dlogM

345678 log M (relative to pullulan)

Figure 12. MMD of an unbleached softwood kraft pulp dissolved without derivatization in LiCl/DMAc. RI and UV denote refractive index and ultraviolet detection, respectively. MMD of derivatized sample are shown in figure 13.

The MMD in figure 12 differs substantially from those of birch kraft pulps shown in figures 10 and 11. The MMD of bleached and unbleached softwood kraft pulps has been thoroughly studied (Sjöholm et al. 2000a) and it has been shown to be distributed differently in comparison with that of a hardwood kraft pulp. The carbohydrate composition in the eluting MMD changed with decreasing molar mass of the sample. This result, in combination with studies of the non-dissolvable part of softwood kraft pulp (Sjöholm et al. 1997), suggested that glucomannan is associated with cellulose during the dissolution and characterization of underivatized softwood samples in the LiCl/DMAc solvent system. This association makes it difficult to elucidate the correct molar mass of the cellulose and hemicellulose fractions. The same situation seems to be valid in the sample shown in figure 12. Seven derivatization reagents were tested with regard to increase the solubility of an unbleached softwood kraft pulp. The derivatives tested were: acetyl chloride, propionyl chloride, butionyl chloride, methyl isocyanate, ethyl isocyanate, propyl isocyanate, phenyl isocyanate. Acylations increased the solubility of the softwood kraft fibers in LiCl/DMAc, but the treatments did not always lead to full dissolution. The reason for the poor reproducibility may be the lack of acid acceptor, or the fact that the samples in the present study had a considerably larger molar mass than those used by McCormick et al. (McCormick and Lichatowich 1979; McCormick and Callais 1987), resulting in a lower accessibility towards the reagents. The formations of carbamates increased the solubility of the softwood pulp sample. Phenyl isocyanate worked well as derivatizing reagent, but the strong UV absorption of the phenyl

40 carbamate made the detection of lignin impossible and the use of this reagent was because of this not further studied. Methyl isocyanate was omitted due to its high vapor pressure and to handling restrictions because of its toxicity. Ethyl isocyanate (EIC) and propyl isocyanate (PIC) worked equally well. EIC is a smaller molecule than PIC and was assumed to affect the hydrodynamic volume of the samples less than PIC, and derivatization of pulps with EIC was therefore chosen for further study.

8.2.2. Influence of derivatization on the MMD and molar mass The derivatization of the softwood kraft pulp introduced major changes in the MMD (figure 13) compared with that of the underivatized softwood kraft pulp (figure 12). Figure 13. MMD of a softwood kraft pulp derivatized with ethyl isocyanate. RI and UV

RI UV dw/dlogM

345678 log M (relative to pullulan) denote refractive index and ultraviolet detection, respectively. MMD of underivatized sample are shown in figure 12. The typical bimodal MMD of hardwood pulps was also observed for the derivatized softwood pulp, although the resolution between the low molar mass (LM) distribution and the high molar mass (HM) distribution was somewhat lower than in the case of the hardwood kraft pulp. The Mw of the HM distribution in underivatized softwood kraft pulp samples were surprisingly high, especially considering the low intrinsic viscosity of the unbleached pulp (table 1).

41 Table 1. Solubility, weight average molar masses (Mw, relative to pullulan) and relative amounts of low molecular mass (LM) and high molecular mass (HM) distributions as calculated from the chromatograms (UV and RI) of derivatized and underivatized softwood and hardwood kraft pulps. The Mw was calculated from the RI- distributions.

Mw Area% (RI) Area% (UV) Sample Solubility (%) LM HM LM HM LM HM Softwood1 92 32000 2338000 4 96 19 81

Softwood1, derivatized 100 35400 1765000 20 80 75 25

Hardwood2 100 25000 2081000 28 72 84 16 Hardwood2, derivatized 100 29000 2226000 28 72 84 16

1) intrinsic viscosity 1080 ml/g and kappa number 18 (underivatized sample) 2) intrinsic viscosity 1310 ml/g and kappa number 17 (underivatized sample) Derivatization decreased the average molar mass to a more expected value. The relative areas of the LM and HM distributions in the RI-chromatograms of derivatized softwood kraft pulps were also different from those of their underivatized counterparts (table 1). The relative areas of the LM and HM distributions of the derivatized softwood pulps (20%: 80%) reflected the ratio between hemicellulose and cellulose in the sample better than the MMD of the underivatized sample. A hardwood kraft pulp was derivatized with EIC in the same manner as the softwood kraft pulp. The SEC-characterization revealed that the derivatization resulted in no redistribution of the sample from the HM distribution to the LM distribution. In addition, the increase in Mw of the LM and HM distribution was small, implying that the introduced EIC-group did not influence the hydrodynamic volume of the sample to any great extent (table 1). It has been suggested that glucomannan is associated with cellulose in spruce fibers (Salmén and Olsson 1998). Adsorption of glucomannan was observed when cotton linters were treated with softwood glucomannan fractions isolated from holocellulose (Hansson 1970). MMD measurements showed that high molecular mass material formed when hardwood pulp was treated with softwood glucomannan (Karlsson and Westermark 1997), which was suggested as evidence of strong interactions between cellulose and glucomannan. Further, it has been reported that the carbohydrates in the HM distribution isolated from underivatized softwood pulp contained mannose (Sjöholm et al. 2000a). All these observations strongly suggest that there are associations between cellulose and glucomannan in softwood kraft pulp, as is also indicated in figure 11 and table 1. The redistribution of material from the HM range to the LM range of the distribution seen after EIC-derivatization of softwood kraft pulps (figures 12 and 13) clearly indicates that strong

42 associations, mainly between glucomannan/cellulose, are ruptured by the derivatization of the hydroxyl groups of the polysaccharides. This redistribution means that it is possible to make more correct estimations of the MMD and molar masses of softwood kraft pulps after derivatization, as the low molecular hemicellulose elutes at the expected elution time, and does not seem to contaminate the cellulose fraction of the MMD. However, the carbohydrate composition of the eluting MMD has not been determined within the scope of this study. The concomitant shift of the lignin from the HM to the LM part of the distribution suggests that lignin is associated with the glucomannan rather than directly with the cellulose. However, the UV-absorbance in the high molar mass fraction after derivatization indicates that considerable amounts of lignin remain associated with the HM distribution, both in hardwood and softwood kraft pulps (table 1).

8.2.3. Reproducibility, degree of dissolution and substitution It is desirable to dissolve softwood pulp samples, which provide information about the earlier stages of a kraft cook, but unfortunately the solubility of underivatized kraft pulp fibers in LiCl/DMAc is reduced at higher kappa numbers. In order to study the effect of increasing lignin content during EIC-derivatization, five kraft pulps with kappa numbers ranging from 13 to 53 (corresponding to lignin contents of 2 % to 8 %) were dissolved using the EIC/LiCl/DMAc method. A gravimetric determination of the non-dissolved residue showed that 90% dissolution is possible for kraft pulps with kappa numbers around 50 (figure 14). Thus, kraft pulps containing fairly large amounts of lignin have a high solubility when derivatized with EIC.

100

% dissolved 85

80 13 18 31 41 53

Kappa number

Figure 14. Percentage dissolved for unbleached kraft pulps with different lignin content. Samples dissolved through derivatization according to the EIC/LiCl/DMAc-method.

43 The maximum theoretical degree of substitution is three, since the glucosidic units in cellulose contains three hydroxyl groups. The degree of substitution of derivatized softwood pulp samples was determined by nitrogen analysis to be 1.9, with a standard deviation of 0.13 (n=4). This is slightly lower than the previously reported values of 2.1 (McCormick and Callais 1987), but it was nevertheless considered to be high enough to provide complete of dissolution of the sample.

The reproducibility of the EIC/LiCl/DMAc-method was evaluated in five separate MMD determinations of the same softwood kraft pulp. The level of reproducibility of the determination of the molar mass of the HM distribution was high, with a relative standard deviation (RSD) of 2.7 % (n=5). These numbers are similar to the reproducibility for hardwood pulps where a RSD of 2.2 % (n=3) has been reported for the HM distribution (Sjöholm et al. 2000b).

8.3 Degradation patterns observed during alkaline degradation of softwood and hardwood (paper III) The purpose of chemical pulping is to degrade and dissolve the lignin in wood. In kraft pulping processes, lignin is degraded in a solution containing hydrogen sulfide and hydroxide ions at an elevated temperature. Unfortunately, the cellulose and hemicelluloses are also partly degraded and dissolved under these conditions. This decreases the yield and the molecular mass of both cellulose and hemicelluloses. A decrease in molecular mass is undesirable, since the mechanical strength of wood pulps, cotton and cellulose derivates decreases with decreasing molecular mass (Staudinger and Reinecke 1938; Sookne and Harris 1945; Ifju 1964; Gurnagul et al. 1992). The aim of the work reported in paper III was to study the pattern of the alkaline degradation of cellulose and hemicelluloses during kraft pulping and how the MMD and fiber strength were influenced by different alkaline conditions. The degradation of Norway spruce (Picea abies) and birch (Betula sp.) was studied in two series: by varying the pulping time using the same initial alkali concentration, or by varying the initial alkali concentration and maintaining a constant cooking time. Experimental conditions are given in appendix II and the sample designations are listed in table 2.

44 Table 2. Designations of the samples in paper III. Sample abbreviation Wood Specie Degradation strategy BA Birch Increased initial Alkali level BT Birch Increased pulping Time SA Spruce Increased initial Alkali level ST Spruce Increased pulping Time

Figures 15 and 16 show the MMD of birch pulps produced at different alkali concentrations and at different degradation times, respectively. The typical bimodal MMD is clearly seen for pulps with high molecular masses. The bimodality of the MMD for the more severely degraded pulps is less clear, since the degraded cellulose fraction in these cases overlaps the hemicellulose fraction.

Increasing alkali concentration dw/dlogM

34567 log M (relative to pullulan)

Figure 15. MMD of birch pulps produced with different alkali concentrations (BA-series).

Increased pulping time dw/dlogM

34567 log M (relative to pulllan)

Figure 16. MMD of birch pulps produced with different pulping times (BT-series).

45 Differences in MMD of the fibers were observed between the BT and BA series. In the BT series, a fraction of higher molecular mass cellulose distributed at log M>6, was seen throughout the entire series. At high alkali concentrations, all the high molecular mass cellulose was degraded and no such high molecular mass fraction was evident (figure 15). The difference in degradation pattern is also observed when the polydispersity of the MMD of the pulps is plotted versus the intrinsic viscosity of the pulps, figure 17.

25 ) n

/M 20

w BA 15 BT SA 10 ST 5 Polydispersity (M 0 0 500 1000 1500 Intrinsic viscosity (ml/g)

Figure 17. Polydipersity (Mw/Mn) for the birch (B*) and spruce (S*) pulps produced with a) different pulping times (*T) and b) different alkali concentrations (*A). Molar masses calculated from the entire MMD. In addition to the fraction of high molar mass cellulose, the fibers in the BT series contained a larger fraction of xylose than those in the BA series. It is known that more xylan is dissolved in cooks with higher alkali concentration (Aurell and Hartler 1965; Gustavsson and Al-Dajani 2000). In the case of the BT series, a distinct distribution of xylan was observed in the MMD of all the pulps, regardless of degradation level (figure 16). However, in the MMD of the BA series, no distinct xylan distribution could be discerned in the more degraded pulps produced with high initial alkali concentration (figure 15). During this work, it was assumed that if the wood polymers had been degraded homogeneously, the entire MMD would have been shifted towards lower molar mass. This was the case for the MMD observed in the BA-series. The remaining high molar mass cellulose fraction in the MMD of the BT-series resulted in the suggestion that the cellulose degradation in the BT series was interpreted to occur less homogeneously than the degradation in the BA series. The pulps in the BT-series had higher strengths compared to the BA-series pulps, evaluated as dry zero-span tensile index (figure 18) at comparable viscosity levels. It is

46 suggested that the reason for the higher strength is related to the residue of the high molecular mass cellulose found in the MMD of these fibers, despite severe alkaline degradation. The plot of dry zero-span tensile indexes versus Mz-values versus exemplifies this. A single relation between fiber strength and Mz is observed, as Mz is more affected by larger molecules than Mw or than the intrinsic viscosity. Plotting zero-span tensile index versus the other estimates of the molar mass revealed two different relations for the two birch series. However, when the fiber strength was measured as rewetted zero-span tensile index, the same relationships were found for the birch pulps, irrespective of degradation strategy (data not shown). Rewetted zero-span tensile index is more sensitive to local defects than dry zero-span tensile index (Mohlin and Alfredsson 1990). It seems that these defects were more important for the rewetted zero-span tensile index than the presence of higher molecular mass material.

A 220

180 BA BT 140 SA ST

index (Nm/g) index 100

Dry zero-span tensile 60 0 500 1000 1500 Intrinsic viscosity (ml/g)

220 B 180 BA BT 140 SA 100 index (Nm/g) ST

Dry zero-span tensile 60 0 500 1000 1500 2000

Mw (kg/mol)

C 220

180 BA BT 140 SA ST

index (Nm/g) 100

Dry zero-span tensile 60 0 1000 2000 3000 4000

Mz (kg/mol)

Figure 18. Dry zero-span tensile index plotted versus: A) intrinsic viscosity, B) Mw and C)

Mz for birch and spruce pulps.

48 The development of the derivatization method (paper II) made it possible to study the degradation pattern of lignin-containing softwood kraft pulps. Figure 19 shows the MMD of the softwood series subjected to different alkali concentrations.

Increasing alkali concentration dw/dlogM

34567 log M (relative to pullulan)

Figure 19. MMD of spruce pulps produced with different alkali concentrations (SA- series). Degradation of spruce chips, independent of degradation conditions, resulted in degradation patterns similar to those observed in the BA-series: a high molecular mass cellulose fraction was not evident in the MMD of the wood polymers in any of the spruce- series. The same results was also suggested when the polydispersity was plotted versus intrinsic viscosity (figure 17). Thus, it was concluded that there were no differences in homogeneity of the cellulose degradation pattern of the spruce chips. The relations between fiber strength and molar masses in the spruce series were also the same, irrespective of degradation strategy (figure 18). Alkaline degradation of birch and spruce seems to proceed differently, depending both on the wood specie and degradation type, when degradation is evaluated as changes in MMD and decrease in fiber strength.

8.4 Improving the accuracy in the determination of molar mass of cellulose in kraft pulps (paper IV) The objective of paper IV was to implement a multi-angular laser light scattering (MALLS) detector in the SEC-characterizations of kraft pulps. This was performed because the separation system used during SEC needs a calibration. The direct-standard calibration normally performed is based on a number of assumptions, where the assumption of chemical similarity between standards and samples is the most important

49 and probably erroneous. The advantage of using a MALLS-detector is that calibration with external standards is avoided. The most common method of calibrating the chromatographic system used for the characterization of pulps is by using a series of well-characterized pullulan standards (Westermark and Gustafsson 1994; Strlic et al. 1998; Sjöholm et al. 2000a; Sjöholm et al. 2000c). Pullulan is a polymer consisting of (1→4) linked α-D-glucopyranose units with every third (1→4)-linkage replaced by a (1→6)-linkage (Popa and Spiridon 1998), while cellulose consists solely of (1→4)-linked β-D-glucopyranose units. This difference in linkage structure between standards and samples has been found to cause an overestimation of the molecular mass and its distribution in water-soluble cellulose derivatives (Poché et al. 1998) as well as in chitosans (Ottöy et al. 1996), and there is a suspicion that a similar situation may exist in the case of cellulose and pullulan (Bikova and Treimanis 2002). The MMDs of the same sample of unbleached birch kraft pulp determined by two different sets of detectors, SEC/RI and SEC/MALLS/RI, are shown in figure 20.

System II System I dw/dlogM

34567 log M

Figure 20. MMD of a birch kraft pulp characterized by size-exclusion chromatography calibrated with I) pullulan standards (SEC/RI), and II) light scattering detection (SEC/MALLS/RI). The results from system I were obtained by calibration with nominal peak molar masses of pullulan standards, and the results from system II were obtained by MALLS-detection in combination with RI-detection (dn/dc of a fully bleached kraft pulp dissolved in 0.5% LiCl/DMAc determined at 488 nm and 40°C to 0.108). Clearly, there are differences in the reported MMD, and consequently, in the calculated molar masses of the samples, depending on how the detection is performed and calibrated.

50 The difference in the MMDs in figure 20 is probably due to structural differences between standards and samples. The (1→6)-linkages in pullulan lead to conformational freedom of the pullulan (Brant and Burton 1981) compared to structures containing only (1→4)- linkages. At a given hydrodynamic volume, or elution time, the calculated molecular mass of the cellulose is smaller than the molecular mass of pullulan. This leads to an overestimation of the molecular mass of cellulose. A similar situation was found when a water-soluble cellulose derivative was studied (Poché et al. 1998), and two methods of obtaining more correct molar mass were presented. Similar methods, based on the findings by Poché et al. (1998) were thus derived for the cellulose/LiCl/DMAc system.

8.4.1. Method I: Correction of the molar mass of cellulose The differences in molar mass determined by the two detector techniques are shown for cellulose samples with a large range of molar masses in figure 21.

6.2 Mw 5.8 R2 = 0.963 Mn R2 = 0.995 MALLS

x 5.4

log M 5

4.6 4.6 5.1 5.6 6.1 6.6

log Mx relative pullulan

Figure 21. Relation between average molar masses (Mw and Mn) determined relative to pullulan standards (SEC/RI) and by light scattering detection (SEC/MALLS/RI).

The resulting correlations for the Mn and Mw values were found to be

= ± × (0.85±0.02) Mw,Correlated (4.00 1.08) Mw,Pull (27) (28) = ± × (0.69±0.04) Mn,Correlated (43.8 25.3) Mn,Pull where Mw,Pull and Mn,Pull is the weight average and number average molar mass, respectively of the cellulose fraction estimated relative to pullulan standards. The confidence limits were calculated according to a t-test at the 95% confidence level. (Miller and Miller 1993).

51 The relations shown can be used to relate erroneous molar masses of the cellulose fraction estimated relative to pullulan standards to more correct molar masses, measured by light scattering detection. These relationships are useful when determining the average molar masses of cellulose by RI-detection when no light scattering detector is available.

8.4.2. Method II: Cellulose-equivalent molar masses of pullulan standards A second method was developed in order to calculate cellulose equivalent molar masses of the standards. The advantage of this method is that not only the average molar masses but the entire MMD is taken into account. The purpose of calibrating a SEC-system with a set of standards is to obtain a relationship between the logarithm of the molar mass of the standards and the elution time (equation 29). A similar calibration line is normally obtained from the SEC/MALLS/RI measurements (equation 30). The equations hold the calibration constants a and b from the corresponding detection system.

= + (29) log M pull b pull a pull t = + log M MALLS b MALLS a MALLS t (30) Figure 22 shows that the regression lines from the two systems differ, but this difference may be used to express new, cellulose-equivalent molar masses of the standards used.

5 log M

Pullulan, log M according to supplier 4 Kraft pulps, log M according to system II

3 20 22 24 26 28 30 32 34 Elution time (min)

Figure 22. Calibration curve obtained by direct-standard calibration with pullulan standards (system I) and calibration curves for SEC/MALLS/RI-detection (system II). The ellipse indicates the region corresponding to elution time of cellulose. At the time t where the calibration curve for the pullulan standards and the elution curve for the pulp samples intersect, the following expression may be derived.

= = p M celleq M MALLS qM pull (31) where q and p are functions of the slopes (apull and aMALLS) and intercepts on the ordinate

(bMALLS and bpull) of the pullulan and MALLS calibration lines, respectively. If the molar mass of an arbitrary pullulan standard is inserted in equation 31, the equivalent molar mass of the cellulose eluting at any time t from the MALLS detection is obtained. It is thus possible to calculate the cellulose-equivalent molar mass of the narrow standards,

Mcelleq, by using equation 31.

The calibration constants bMALLS and aMALLS differ slightly from sample to sample and also from injection to injection, which is indicated in figure 22. In order to minimize the errors in the functions q and p in equation 31, thirty injections were performed with high molar mass birch kraft pulps and wood polymer beads (WPB) with lower molar mass. Only the cellulose fraction was taken into account in the estimation of the functions q and p, as the low S/N ratio of the hemicellulose fraction would increase the error in the functions. The resulting cellulose-equivalent molar masses of the pullulan standards are shown in table 3, with standard deviations

53 Table 3. Nominal and cellulose-equivalent molar masses of pullulan standards based on method II, together with standard deviation of the cellulose-equivalent molar masses (n=30). Nominal molecular Cellulose-equivalent Standard mass* molecular mass deviation 738 2300 980 5800 11200 3600 12200 19800 5500 23700 33100 8400 48000 57300 12700 100000 101000 19100 186000 164000 26800 380000 287000 39300 853000 540000 62300 1660000 912000 98000

*As stated by the supplier.

The standard deviations in the low molar mass range are large, and this is due to the low S/N-ratio of the light scattering in that range. The standards of low molar masses are however located well outside the elution range of cellulose. The deviations in the high molar mass range are considered low as they only have small effects on the calibration curve (data not shown).

8.4.3. The applicability of the improved evaluation methods The applicability of the two methods was evaluated by the characterization of the molar mass and the MMDs of WPBs subjected to acid hydrolysis on a second column set. Figure 23 shows the MMD of the undegraded WPB as obtained by three different methods:

a) SEC/RI relative to nominal molar masses of standards (MMDrel.pull.) by direct- standard calibration

b) SEC/RI relative cellulose-equivalent molar masses of standards (MMDrel.cell.eq.) by using method II

c) SEC/MALLS/RI (MMDMALLS)

MMDMALLS MMD rel.cell.eq.

MMDrel.pull. dw/dlogM

34567 log M

Figure 23. MMD of a birch kraft pulp determined by three calibration techniques using: a) the nominal molar mass of the pullulan standard (MMDrel.pull.), b) the cellulose- equivalent molar masses of the pullan standards (MMDrel.cell.eq.), c) light scattering detection (MMDMALLS). The SEC/RI method relative to nominal molar masses of standards provided a distribution of cellulose with higher molar mass than the SEC/MALLS/RI technique, as was expected. The method of calibrating the chromatographic system with cellulose-equivalent molar masses of pullulan standards resulted in an MMD (MMDrel.cell.eq) that agreed much better with the MMD characterized by the MALLS detector (MMDMALLS). Consequently, method II is preferred to characterize the MMD of cellulosic samples rather than by using the direct-standard calibration method. The relative errors of the weight average molar mass from the MMDs of the degraded WPBs are shown in table 4, assuming that the SEC/MALLS/RI method provides the correct value.

Table 4. Relative error of the estimated Mw values of acid-hydrolyzed WPB shown in figure 23. Mw estimated: a) relative to the nominal molar masses of standards, b) by method I, and c) by method II. All the values are compared to values from SEC/MALLS/RI. The signs denote overestimation (+) and underestimation (-). Hydrolysis t (h) a) Nominal b) Method I c) Method II 0 +55 % -20 % -16 % 1 +70 % -12 % -6 % 3 +70 % -9 % -1 % 23 +90 % +6 % +20 %

55 The values were estimated relative to the nominal molar masses of standards, by correcting the molar mass according to method I, and by using cellulose equivalent molar masses of the standards (method II). The errors introduced by using the nominal molar masses of standards was reduced by using either method I or II. It is however recommended that method II should be used when a light scattering detector is not available, as an estimation of the entire distribution is obtained. By using method I, only the average molar masses of the distribution is obtained.

8.5 Simulation of cellulose degradation (paper V) A study was undertaken to simulate the way in which the shape of the MMD of wood polymers would develop during degradation, provided the degradation occurred according to pre-defined patterns of cellulose chain scission in the fiber. The study focused on random degradation and end-wise degradation, so-called peeling, of the molecules within the pulp fiber.

8.5.1. Validation of the simulation model In order to validate the simulation model, both fictive and real distributions were subjected to simulated random degradation. The results were found to be consistent with those of earlier studies (Ballauff and Wolf 1981; Guiata et al. 1990) indicating that the polydispersity (Mw/Mn) of a single distribution, irrespective of the initial polydispersity, approaches the value of 2 during the simulation of random degradation (data not shown). Figure 24 shows an example of the simulation of random degradation. The MMD measured by RI-detection while performing SEC of an unbleached birch kraft pulp, was used as initial data.

56 dw/dlogM

34567 log M

Figure 24. MMD obtained by simulating homogeneous random degradation of the molecules in a birch kraft pulp.

The distribution was divided into subunits having a size corresponding to 20 glucosidic units. The smaller the subunits used, the more exact will the calculated results be. However, as the subunits became smaller, more and more computer-memory was required, and longer simulation times were necessary. Supplementary calculations made with larger and smaller subunits indicated that the results did not depend to any considerable extent upon the size of the subunits.

8.5.2. Random degradation The assumption made to simulate the MMD (figure 24) was that each linkage in all polymer chains had the same degradation rate, and that all the linkages were equally susceptible. The degradation pattern shown in figure 24 is thus not only interpreted as representing degradation according to a random pattern, it is also interpreted as representing degradation according to a homogeneous pattern, since all the material was subjected to random degradation. The simulated MMD in figure 24 reveals several interesting features. In the case of the high molar mass distribution, it is observed that the entire distribution is shifted towards lower mass range. The entire distribution is degraded, as no cellulose initially found to have a molar mass higher than logM = 6 retained its initial molar mass. This has previously been observed during the exaggerated kraft pulping of birch and pine chips in paper II, where only empirical conclusions could be drawn regarding the homogeneity of the degradation. It can thus be confirmed that some of the assumptions made in that study were correct; homogeneous degradation does result in a shift of the entire MMD, including the high molar mass region, towards lower molar masses, as exemplified in figure 24.

57 Although there are differences in reactivity between linkages in hemicellulose and cellulose, no consideration was here given to the kind of molecule existing at a given chain length (Lai 2001). The low molar mass distribution, representing the hemicellulose fraction, was clearly visible as a low molecular shoulder throughout the simulation. Some degradation of the hemicellulose distribution is indicated in figure 24, but the distribution is still visible throughout the simulation. The reason for this is found in equation 14. The rate of degradation of a given hemicellulose molecule is lower than that of a cellulose molecule, as there are fewer hydrolysable linkages in a hemicellulose molecule. The simulated MMDs differ from the MMDs reported for random degradation of linear polymers (Emsley and Heywood 1995). In that study it was found that random degradation did not change the position of the MMD. This is contradicted by the results in figure 24. The explanation of the differences may be related to how random degradation is defined and how the calculations are performed. Although our model is based on an algorithm, the approach used by Emsley and Heywood (1995) was a Monte Carlo simulation in which random degradation was defined differently. Random degradation may be defined in terms of the reactivity of different glucosidic linkages within a cellulose. This definition is the same in both studies. It may also be expressed in terms of the size of the fraction of the material that is subject to any kind of degradation, and it is this parameter that probably explains the differences in the MMD obtained by simulating random degradation. It is here proposed that if the polymers in pulp fibers are assumed to be subjected to homogeneous degradation, it implies that the entire material, i.e. 100% of the material constituting the pulp fiber, has the same reactivity according to a random pattern within the polymer. The resulting MMD of birch kraft pulps is exemplified in figure 24.

8.5.3. End-wise depolymerization Figure 25 shows the effect of pure end-wise degradation with rate constants according to equations 23 and 24 applied to the MMD of a birch kraft pulp. The position of the high molar mass cellulose distribution and, consequently, the molar mass is only slightly affected by peeling. This is expected, as extensive peeling is required to have an effect on these large molecules. The effect on the low molar mass distribution is however more obvious. Here, the material is degraded during the course of simulation, and finally removed from the system after it reaches a DP of 2 (which translates to 40 glucosidic units since the smallest unit in the simulation has the size of 20 units).

dw/dlogM

34567 log M

Figure 25. MMD obtained after theoretical simulation of end-wise depolymerization of the molecules in a birch kraft pulp.

8.5.4. Linear combinations of chain scission cases Several modes of degradation are active simultaneously under process-like conditions. It is possible to assign different rates of reaction and different chain scission cases to different relative amounts of the MMD in the simulation model. By doing this, it was possible to simulate experimental results by linear combinations of peeling and random chain scissions. Figure 26 shows experimental MMDs from the RI-detection during SEC-characterization of softwood, kraft pulped to different lignin contents. The shift in the position and the change in shape of the MMD during pulping was not the same as in the simulation in figure 24, and this is interpreted to indicate that the cellulose degradation during kraft pulping is of a more heterogeneous character than is depicted in figure 24.

59 dw/dlogM

34567 log M

Figure 26. MMD of spruce kraft pulps, digested to different lignin contents. Efforts were made to simulate the indicated heterogeneity of the degradation in the experimental MMD in figure 26, by using the MMD of the pulps with highest lignin content as initial data when simulating the degradation. The assumptions made prior to these efforts were: • The wood fibers contained two different fractions of polymers; one more accessible fraction giving a higher rate of degradation and one less accessible giving a lower rate of degradation, • The two fractions contained polymers from the entire MMD, • Both fractions were susceptible to random degradation with different rate constants depending on to which fraction the material belonged, and end-wise depolymerization with the same rate constant, regardless of fraction. The molecules are still degraded into chains of random lengths but with different rates, i.e. the degradation is no longer homogeneous, but still random. In order to find a minimum in the difference between the experimental and simulated MMDs, the relationship between the two random rate constants and the composition of the system was varied. The ratio between the reaction rates was varied from 0.05 to 0.5 and the composition of the system was varied in 10 %-increments, from 0-100%. The resulting simulated MMDs with the best fit to the experimental MMDs are presented in figure 27. The conditions found to best describe the experimental MMDs were: • 20 % of the sample was more accessible, while 80% was less accessible. • The relation between the rate constants was 0.1, i.e. the rate constant of the less accessible fraction was an order of magnitude lower than the rate constant of the more accessible fraction.

dw/dlogM

34567 log M

Figure 27. MMD obtained by simulation of heterogeneous degradation of wood polymers during softwood kraft pulping. The material was assumed to be composed of two fractions of different relative amounts with different reactivities: 20% with higher reactivity and 80% with a lower reactivity. These parameters resulted in simulated MMDs very similar to the experimental MMDs. However, one reservation is necessary. The molar masses of the cellulose in figures 24 and 27 all greatly exceed the molar masses of hydrocelluloses previously studied (Machell and Richards 1957; Haas et al. 1967; Johansson and Samuelsson 1975). It is not possible to extend the interpretation of the degradation pattern to cellulose having a molar mass lower than that depicted in figure 24, as the rate of degradation is assumed to be affected as the size of the molecules approaches the size of the molecules in cellulose crystallites (Battista et al. 1956; Gentile et al. 1987). It nevertheless seems probable that the alkaline degradation depicted in figure 26 is due to a random degradation of the linkages in the cellulose chain. A more careful analysis using simulation in combination with experimental data can be used to better understand and describe the molecular degradation of the cell wall during conditions similar to those prevailing in industrial processes. The analysis provides information that is neither observed by intrinsic viscosity measurement nor by the study of the development of the MMDs per se. This difference in reactivity is very interesting. Although it is not difficult to envisage e.g. that cellulose on the surface of fibrils or fibrillar aggregates is more susceptible to degradation than chains in the interior, this is seldom discussed in the literature. Instead, most studies suggest that the fiber cell wall should be considered to be a swollen gel in which active species can move quite easily (Malcolm and Grace 1989). The experimental

61 results given here actually suggest that the alkaline degradation process proceeds heterogeneously.

8.5.5. Peeling of cotton linters In order to study the effect of the peeling reaction on cellulosic material and the utilization of the simulation model, cotton linters were subjected to alkali at rather low temperatures (130°C) compared to normal kraft pulping temperatures. At this temperature, the degradation is dominated by peeling and only minor alkaline hydrolysis occurs (Haas et al. 1967; Lai and Sarkanen 1967). In addition, the ratio between the rate constants of peeling and stopping reaction is approximately 250 (Haas et al. 1967). The stopping reaction thus has only a minor influence on the results. The resulting weight average molar masses are plotted vs. treatment time in figure 28.

The changes in weight average molar mass show that the initial degradation rate is high. This rapid degradation is interpreted as being a degradation of the more accessible parts of the structure, or more probably, as alkaline cleavage of oxidized cellulose structures that are more sensitive towards alkali (Richards 1971). After the initial depolymerization stage, the rate of degradation decreases, and the weight average molar mass even increases during the intermediate stages of degradation. Attempts were also made to simulate the experimental MMDs. The original MMD from the cotton linters was used as raw data, and simulated MMDs were obtained by assuming that there were two different degradation phases. During the first phase, it was assumed that only random, homogeneous, chain scission took place. The initial stage of hydrolysis was assumed to last until the Mw-value of the simulated MMD reached the same Mw-value as the cotton linters subjected to 60 min of alkaline degradation. Thereafter, only end-wise depolymerization of the cellulose was simulated. The resulting weight average molar masses from the experimental MMDs and after such simulation attempts are also showed in figure 28.

1.2E+06 Simulated Experimental 1.0E+06 w M 8.0E+05

6.0E+05 0 100 200 300 400 500 t (min)

Figure 28. Weight average molar masses (Mw) of experimental MMD and Mw of MMD obtained while simulating peeling of cotton linters.

The development of the Mw-values of the simulated MMD is similar to that of the experimental Mw-values. After the initial stage of hydrolysis an increase is observed, that passes over to a decrease in the final stages of hydrolysis. The simulation model may thus be used to study the development of the molar mass during end-wise depolymerization. However, the theoretical yield from the simulated peeling of cotton linters in figure 28 is low (Johansson and Samuelsson 1975), especially after longer degradation times, implying that the model may be too simple to describe the peeling reaction in terms of yield with a high level of accuracy (table 5).

Table 5. Weight average molar mass (Mw), turnover (equation 25) and yield (equation 26) from the simulation of alkaline peeling of cotton linters.

Simulation time t0 t1 t2 t3 t4 t5 t6 t7 t8

Mw (kDa) 1148 963 829 827 786 786 789 784 771 Turnover, X 0.0 0.3 0.6 3.0 60.6 85.0 94.3 97.8 99.2 Yield, Y 100.0 100.0 100.0 97.6 39.8 15.1 5.7 2.2 0.8

A tentative explanation of this deficiency is that not only one unit at a time is peeled off from the reducing end during experimental peeling. Instead, a number of units are removed simultaneously before a stopping reaction occurs (Franzon and Samuelsson 1957; Lai and Sarkanen 1967). Hence, further development of the simulation model is needed in order to describe the development of the yield and simultaneous development of the MMD during the end-wise depolymerization of cellulose.

63 8.5.6. Limitations of the simulation study The construction of simulation models always involves simplifications that introduce errors and drawbacks when studying complex materials, and paper V is no exception. The model used in this study is a bulk model, meaning that no consideration is given to differences in reactivity caused by topochemical differences. The model could nevertheless simulate gross differences in reactivity by assuming different reactivities to mass fractions of an MMD. For more complex modes of degradation, however, such as acid hydrolysis, this approach is not feasible. One of the objectives of this study was to simulate acid hydrolysis, which has been proposed to occur in disordered regions of cellulose (Sharples 1957; Durawalla and Shet 1962; Gurnagul et al. 1992). That objective was not fulfilled, mainly because of the rather complex character of the acid hydrolysis, which means that several parameters of the cellulose in the cell wall need to be taken into account to simulate acid hydrolysis properly. Some examples of these parameters are; the rates of the two phases of degradation (Durawalla and Shet 1962), the leveling off degree of polymerization (Battista et al. 1956) and the fibrillar structure of cellulose (Wickholm et al. 2001). Instead of using the model of Ballauff and Wolf, which is a versatile, but nevertheless a rather simplified model, it would be useful to develop a model based on a Monte Carlo- technique, where it would be possible to achieve an assessment of cellulose with different reaction rates and different crystallinities. There are other limitations in the work presented in paper V. The peeling reaction is accompanied by a stopping reaction (Franzon and Samuelsson 1957; Machell and Richards 1957), and a correct implementation of the stopping and peeling reaction has not yet been achieved. The different polysaccharides present in the pulp fiber all have different reactivities during alkaline degradation (Lai 2001), and a more exact treatment of these reactivities is preferred.

9. Conclusions This thesis has shown some of the benefits achieved by using size-exclusion chromatography in combination with more traditional analytical techniques to understand the effect of cellulose degradation during pulping and bleaching processes. The thesis also describes the development of dissolution procedures and detection techniques to be used for characterizing the molar mass distribution of chemically produced wood pulp fibers. It was found that: • SEC revealed that ozone degradation and acid hydrolysis resulted in two completely different degradation patterns, not observed in intrinsic viscosity measurements. The differences in zero-span tensile strength between ozone- degraded and acid-hydrolyzed pulp could not be explained by differences in shape factor. The type of degradation influenced the fiber strength more than the change in shape factor. • The solubility of the polymers in softwood kraft pulp fibers was enhanced by derivatization of the samples during simultaneous dissolution in 8 % LiCl/DMAc. This makes it possible to characterize the MMD by SEC using 0.5% LiCl/DMAc as solvent system, allowing a more correct determination of the mass averages of the pulp polymers than by the characterization of underivatized softwood samples. • Alkaline degradation of cellulose in birch and spruce wood showed significant differences between different degradation conditions and different wood species. Differences in fiber strength observed during alkaline degradation were related to changes in molar mass distributions. • The comparison between the molar masses of the samples estimated by direct- standard calibration and by MALLS showed that the direct-standard calibration resulted in an overestimation of the molar masses of the samples. Two methods to correct for this overestimation were developed. • The usefulness of applying theoretical considerations and models to better understand complex degradation phenomena occurring during kraft pulping of wood fibers was demonstrated. Alkaline hydrolysis has been interpreted to occur as a random, not necessarily homogeneous, degradation of the polymer chains. A hypothesis for the development of the MMD of wood fibers under kraft pulping conditions was proposed.

10. Acknowledgements First, I would like to thank my supervisor Mikael Lindström. This work would not have been accomplished without his support, belief and mentorship from the beginning to the end. Special thanks to Fredrik Berthold and Elisabeth Sjöholm for all their inspiration, encouragement and ideas. Many thanks are also due to my other co-authors (Kristina Gustafsson, Ulrika Molin and Helena Lennholm) for a very enjoyable time. Parts of this work were carried out within the framework of the Wood Ultrastructure Research Centre (WURC), and financial support from WURC and STFI is gratefully acknowledged. Many people have been involved, directly or indirectly, during the course of this work, and I thank

• Eva-Lisa, Rickard, Anders, Maria, Anna, Sofia, Jessica, John, Sara and Lina, for all the laughter, • Per Persson for all the discussions of the important and not so very important aspects of life and for valuable criticism of this manuscript, • Maria Nordström for valuable criticism of this manuscript, • Anthony Bristow for the linguistic revision, • Therese Sundquist, Erica Wedin and Manne Kallin for the exceptional support of books, journal articles and computers, • the guys at KTH: Ragnar, Patrik, Martin L, Mikael and Gunnar • all my colleagues at STFI and KTH for the past five years. Last, but certainly not least: a thought to my family, particularly Maria who has been very tolerant and loving to the absent-minded person hanging around the house during the last months.

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78 Appendix I. Data of pulps used in this work.

Hardwood pulps M / L 1) Paper Bleaching Intrinsic Kappa no sequence viscosity (ml/g) no L I, IV Unbleached 1160 15 M I O D EOP DD 820 NA L II Unbleached 1310 17 L IV Unbleached 1300 17 L IV Unbleached 1100 11 L IV Unbleached 1260 13 M IV Unbleached, 1180 16 M IV Unbleached 1090 15 M IV O 930 10 L IV DEDED 1150 <1 L V Unbleached NA 23 Softwood pulps L II Unbleached 950 13 L II Unbleached 1080 18 L II Unbleached 1230 31 L II Unbleached 1270 41 L II Unbleached 1330 53 Cotton linters M I - 920 NA M V - 2080 NA

1) M and L denote pulps produced in a mill or in the laboratory, respectively. 2) NA: not analyzed

Appendix II Pulping conditions for experiments in paper III.

Cooking conditions for birch Pulp Cooking time at [HS-] [OH-] Residual [OH-] 170οC (hours) (mol/l) (mol/l) (mol/l) BT1 0.85 0.3 1.2 0.56

BT2 2.1 0.3 1.2 0.47 BT3 3.6 0.3 1.2 0.42 BT4 5.2 0.3 1.2 0.37 BT5 7.3 0.3 1.2 0.33 BT6 10 0.3 1.2 0.30 BA1 1.0 0.3 0.8 0.20 BA2 1.0 0.3 1.2 0.52 BA3 1.0 0.3 1.8 1.04 BA4 1.0 0.3 2.5 1.59 BA5 1.0 0.3 3.2 2.19 BA6 1.0 0.3 4.0 2.89

Cooking conditions for spruce. Pulp Cooking time at [HS-] [OH-] Residual [OH-] 170οC (hours) (mol/l) (mol/l) (mol/l) ST1 1.5 0.3 1.0 0.46 ST2 2.5 0.3 1.0 0.42 ST3 3.75 0.3 1.0 0.39 ST4 5.25 0.3 1.0 0.36 ST5 8.0 0.3 1.0 0.30 ST6 21.5 0.3 1.0 0.18 SA1 2.0 0.3 0.7 0.18 SA2 2.0 0.3 1.0 0.44 SA3 2.0 0.3 1.4 0.75 SA4 2.0 0.3 1.9 1.12 SA5 2.0 0.3 2.6 1.69 SA6 2.0 0.3 3.5 2.21