C.M. Hansen and A. Björkman: Ultrastructure of Wood 335

Holzforschung52 (1998) 335-344

The Ultrastructure of Wood from a Solubility ParameterPoint of ViewBy C.M. Hansen1 and A. Björkman2

1 FORCE Institute, Denmark2 Technical University of Denmark, Denmark

KeywordsSolubility parametersWood ultrastructureCelluloseHemicellulosesLigninWood polymer solventsHydrophobie bondingHydrophilic bonding

SummaryIn later years scientists have taken increased interest in understanding wood ultrastructure, but withlimited emphasis on close molecular relationships of its polymers. This aspect is examined from theHansen solubility parameter concept and from the requirement that contacts between wood polymersstrive to reduce the free energy. Polymer segments with matching solubility parameters tend to locatenear each other. Wood polymers have larger solubility parameters than the test solvents used tocharacterize them, but the use of model compounds and group contribution methods yields consistentsolubility parameters for the key wood components. Lignin and cellulose are not compatible. Celluloseregions and backbones of hemicelluloses are compatible to a degree. Some hemicellulose segments withhydroxylated side groups are expected to orient themselves towards cellulosic regions, whereassegments, e.g. with acetylated side groups, orient themselves towards lignin regions, in both cases byvirtue of similarity of their local solubility parameters. The diverse nature of the side groups is expectedto determine the movements of water and monomer species through regions of similar solubilityparameters in the fibre wall. Hemicellulose molecules may possibly be contained within celluloseregions with the backbones and hydroxylated side groups towards the cellulose and the acetylated sidegroups inwardly. Hemicelluloses are generally likely to act as "surfactants", binding the high energycellulose regions to the lower energy lignin regions. Though wood cannot be dissolved, some "strong"solvents may alter its ultrastructure, while others do not affect the structure in spite of extreme softening.Short introductory recapitulations are given of the general knowledge of ultrastructure and of the Hansensolubility parameter concept.

The fibre wall

In recent years considerable progress has been made in thedetailed knowledge of the fiber wall and its organization,i.e. the ultrastructure, but it is still not revealed. In order toamplify this understanding an attempt is made to augmentour insight by an account of the implications, which can begathered from the Hansen solubility parameter concept.This concept is recapitulated with recent advances belowbut at first a short review is given of relevant knowledge ofthe fibre wall. The properties of the bulk of the stem mustbe organized to give the wood a sufficient strength, but alsoa structure which allows it to participate in the life of thetree, particularly as a transport medium for the rising sap.

The anatomy of wood species, though being an import-ant issue, does not explain several of the basic properties ofstem and branches by which the wood gets the physical andphysicochemical features needed for functioning and survi-val. It is fundamentaly a matter of the ultrastructural details,which are still far from understood, even labelled a mystery,but the ultrastructure is indeed a very useful combinationof its polymers, cellulose, hemicelluloses (polysacharides)and (native) protolignins, giving wood everywhere proper-ties optimal for the purpose.

A first approach in the disclosure of the ultrastructurewas studies of the constitution (configuration) and physico-chemical properties of the polymer constituents. (The minorcompounds may exhibit influence on the ultrastructuralproperties but are not explicitly discussed in this treatise.)The large cellulose molecule as such is a fairly simple affairwith only one monomer in a homopolymer without branch-ing. Clarification of the configurations of the several hemi-celluloses with different monomers and side groups re-quired new tools (at first paper chromatography) andconsiderable effort. Protolignins, on the other hand, havepresented great configurational difficulties. The monomers(monolignols) - though not much different - combine inseveral ways to form a network of unknown dimensions.Thus lignins do vary in their configurations and have aheterogeneous character. The availability (Björkman 1956)of milled wood lignin (MWL) offered definite possibilitiesto disclose the various ways of polymerization and thestructural elements. Yet MWL represents only part of thelignin (from the secondary wall) and the structure may varywithin and between wood fibres (Terashima et al. 1992,1993). This is less of a problem today, since modemspectroscopic methods (NMR, atomic force microscopy,microautoradiography) do reveal local details of the proto-

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336 CM. Hansen and A. Björkman: Ultrastructure of Wood

lignin within the wood. Still we say "lignin molecule",when we have in reality only a lignin fragment. Further-more lignin monomers may polymerize as a spiral (Shev-chenko and Bailey 1996).

A second approach to disclose the ultrastructure has beenstudies of the polymeric properties of wood constituents.Isolated specimens do not represent their natural state in thewood structure, but can give indications on possible confor-mations. Lignin and hemiccllulose molecules can form com-plexes labelled lignincarbohydrate complexes, LCC (Björk-man 1957I}). The two components in LCC may even beinseparable in the native state without having formed coval-ent bonds (inclusion LCC; Shevchenko and Bailey 1996).Lignin "molecules" as M WL or degraded lignin in pulps mayneed months to diffuse out of finely divided wood and fibresat room temperature. We are close to "seeing" molecules orat least agglomerates, as has been the case with cellulose,forming fibrils, but it does not necessarily tell us aboutmolecular relationships. Observations, calculations andspeculations indicate that lignins may have some kind ofordered conformation, being of fractal nature (Gravitis 1992;Gravitis et . 1995). It is also proposed (Erins et al. 1976;Terashima et al. 1997) that lignin is formed as globuli whichget connected to a network, being fairly loose and thusflexible (Gravitis and Erins 1983), but it is also maintainedthat lignin-lignin and lignin-hemicellulose connections areconformationally firm (Shevchenko 1994), being a maincontribution to the rigidity of wood.

The molecular arrangements in the ultrastructure, factsand speculations are presented as different types of acopious mix. The problems are mainly of four types:(1) Where and how are the three polymers synthesized?(2) When (in overlapping orders) do the polymers appear?(3) How are the locations of the polymers accomplished?(4) How do the polymers interact in these processes?

Cellulose is formed from enzymes at the cell membrane(cell plate), where parallel molecular strings are "extruded"forming fibrils. A cellulose molecule as such has a hydro-phobic character due to intramolecular hydrogen bonds, butthe molecules normally form crystalline regions. The nativecellulose, Cellulose I, is not in a thermodynamically stableform, but produced by "biomolecular engineering". (Thisexpression indicates that biochemistry has refined means toarrange molecules.) The true structure of the crystallineregions has recently been disclosed (Heiner et öl. 1995;Kuutti et al. 1995) by molecular dynamics simulation incompliance with up-to-date measurements. However - as afeature of the fibril - some hemicellulose could be asso-ciated with the cellulose.

The hemicelluloses are produced by the Golgi apparatus.Some of these polysaccharides are present early on in the"arena" and are supplemented in later stages. The moleculesassociate with a lot of water to form a gel. Thereby thehemicelluloses may even control the size and direction(s)of the cellulose fibrils. The influence of hemicelluloses may

!)The acronym LCC was, like MWL, coined by Björkman in 1957.

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be still more powerful in the growth of the protoligninnetwork (Atalla 1995; Terashima and Atalla 1995). Thepolymerisation (unlike the events with cellulose and hemi-celluloses) is accomplished by different lignol radicals anda variety of covalent bonds is produced? resulting in some-what different protolignins depending on location (cellwalls) and order (age) of formation (Barnett 1995). Themechanisms of formation and thereby structure can begoverned by polysaccharides, and LCC with covalent bondsmay be formed. The conditions of lignification are indeedvery difficult to specify.

As the three polymers form separate phases, one maydescribe the ultrastructure as an interpenetrating network.This concept and its details (e.g. transition regions) areindeterminate. Yet the combination of the three polymersare to the greatest avail for a tree. The ultrastructure is likelyto be a compulsory compatible state (Gravitis et al. 1991),which is why separation of the polymers may shift theconditions towards a thermodynamically more stable state.The incompatibility of isolated polymers does not allow alaboratory production of the wood fibre wall, being aproduct of "biomolecular engineering". Thus the state of thewood polymers is irreversible.

In any case the molecular arrangements of the woodpolymers determine the physical properties of wood. Abasic problem is that our comprehension of this statementis incomplete. Notably, we have a confused perception ofthe spectrum of affinities which exist within and betweenthe wood polymers. This is a main incentive for the presentreport. Words like adhesion and aggregation are quite oftenused. Cohesion has perhaps been somewhat disregarded. Ina solid like wood the presence of water is critically impor-tant, and hydrophilic conditions must prevail. While the"free" cellulose molecule has a hydrophobic character, thisis not what we have in wood. Cellulose (fibrils) has theability to differ in orientation depending on the location inthe cell wall. Lignin and even LCC are often maintained tobe hydrophobic. This is not true. Dry MWL absorbs waterrapidly, though only to a limited amount. Also "saturated"lignin is not necessarily hydrophobic, even if the ligninstructure contains hydrophobic segments.

What kinds of bonds can we have in wood? Covalencyis not a problem though types and amounts are not easy toascertain. Ionic bonds may not be frequent although disso-ciated acid groups have ionic character. Theoretical essaysdeal also with electrostatic or Coulomb interactions. An-other concept is the Lewis acidity/basicity (Larsson andJohns 1988). All wood polymers as well as water areamphoteric. Donor/acceptor numbers may throw light onthe constitution of many compounds and bonded systems.

The depiction of the ultrastructure of the wood fibre wallmust involve quantitative comprehensions of the locationsof hemicellulose and lignin segments. The ipurpose of thispaper is to attract attention to the hitherto largely neglectedpotential use of solubility parameters to reveal the mole-cular interaction between polymers of different nature andto the gains this approach offers in understanding theultrastructure. v *

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CM. Hansen and A. Bj rkman: Infrastructure of Wood 337

Recapitulation of the Hansen solubility parameters

The essence of the Hansen concept is that each solvent isrepresented by a 3-dimensional solubility parameter vectorand that a polymer, soluble in a solvent, encompasses inthe system of coordinates a volume in which the solventvector terminates. The parameters have also been used tocharacterize many types of polymers. They have also beenused to characterize many surfaces including those ofpigments, fillers, fibres, polymers, etc. (Hansen 1997 a,1997b). These characterizations are based on correlationsof different physical behaviour on contact with differentsolvents including wetting and/or dewetting, suspension offine particles for extended times, prolonged sedimentationrates, zeta potential measurements, etc. These solubilityparameter (or better cohesion parameter) correlations de-monstrate that the interactions being encompassed are ona molecular scale. The adage often used is that "likedissolves like". This has by these methods been extendedfor polymers to "like seeks like". The overall principle forthe solubility parameter predictions is that those polymersor segments of thereof, whose solubility parameters areclosest, will have the highest mutual affinity. Such affinitymay result in a true solution of a polymer in goodsolvents. However if a polymer has a very large molecularmass (say, 2 million) even the best match in solubilityparameters between solvent and polymer will not lead toits true solution. There are too many entanglements toovercome. Still high affinities for such high molecularmass polymers can be observed in terms of greaterswelling in the better solvents. The presence of crystal-unity has an effect which may be compared in somerespects with that of cross-linking between molecules orvery large molecular mass. An additional result of affinityto be discussed is the preferential clustering of segmentsof polymers into domains of similar solubility parameter.Some polymer segment domains in wood will attractwater, going to high solubility parameter (hydrophilic)domains, whereas a lignol monomer which will seek outa lower solubility parameter (hydrophobic) domain. Thekey to the preferential proximity is a close match insolubility parameter.

The most widely used solubility parameters for estimationof affinities among materials are developed by HanseiL Thetotal cohesion energy, E, is divided into three parts (eq. (1)).These are attributable to nonpolar (London) interactions, ED,permanent dipole to permanent dipole interactions, Ep, andhydrogen bonding (electron interchange) interactions, EH.The nonpolar interactions are common to all atoms becauseof cohesion derived from electrons, while the polar andhydrogen bonding interactions involve molecules whichorient themselves mutually.

It has been possible to attain this division of the totalcohesion energy for a large number of materials includinggases, solvents, polymers, surfaces, biological materials,etc. The term cohesion energy parameters has come to beused interchangeably with solubility parameters, because

these parameters reflect energy relations in general. Theiruse is not limited to solubility relations only, as alreadyindicated. Dividing equation (1) by the molar volume, V,gives

E/V = EC/V + Ep/V +

and furthermore

δ = δ[> + op -H OH

(2)

(3)

where δ is the Hildebrand solubility parameter (Hildebrandand Scott 1950, 1962) and δο, δρ, and δΗ are the Hansenparameters (Hansen 1967; Hansen and Beerbower 1971),being the square roots of the cohesion energy densities forthe respective energy types. Thus, the solubility parameterof a solvent is the length of a vector in a 3-dimensionalsystem with the three Hansen parameters as coordinates.Additional subscripts p and s are used below with theparameters to indicate values for solute and solvent, respec-tively.

The total cohesion energy, E, for a liquid is equal to theenergy of vaporization. This value can usually be found inhandbooks at the normal boiling point for most commonliquids. The energy quantitatively accounts for the breakageof all the bonds which make a liquid a liquid and must befound or estimated at 25 °C. The nonpolar cohesion energyED can usually be calculated by methods described in theliterature. This involves determining the cohesion energyfor look-alike hydrocarbon counterparts (homomorphs)and/or generalized corresponding state correlations basedon the critical temperature and molecular volume. The polarparameter EP can be found with help of the dipole moment.When it is not available, group contributions are usuallyemployed. The hydrogen bonding parameter EH has beenfound according to equation (1) as the remaining energyafter the others were determined. Group contributions arefrequently more reliable than this procedure, which in-volves subtracting large numbers from each other (Hansen1995). The Hansen characterization of a solute is usuallytransformed to a sphere (cf. below). A sketch for lignin isshown in Figure 1. The cohesion energy parameters ofliquids with the highest affinities are located within thesphere. The center of the sphere has the values of the δορ.δρρ, and δΗρ parameters, taken as being characteristic for thesolute. The magnitude of the radius of the sphere. R0. isdetermined by the type of interaction being correlated. Thiswill be smallest for true solubility, larger for swelling to say25 %, and still larger for swelling to say 3 %.

The difference in solubility parameters between a solute(center of the sphere) and a solvent is expressed by thequantity RA. RA is given by the relation

(RA) = (δΗρ - (4)

The radius of the sphere, RO, defines how large a differencein the solubility parameters can be tolerated for solubility.The boundary of the spherical characterization is based onthe requirement that the good solvents have a distance fromthe center of the sphere, RA. less than R0. The coefficient 4in equation (4) has been confirmed by a large number of

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C.M. Hansen and A. Bj rkman: Ultrastructure of Wood

Hansen Solubility Parameters

5D δΡ δΗ R0

Lignin 21.9 14.1 16.9 13.7

Polar ISSolubility joParameter, δΡ

15 Hydrogen10 Bonding

SolubilityParameter, δΗ

0 0

Fig. 1. Sketch of spherical Hansen type correlation for the solu-bility of lignin, whose parameters must be lower than those ofcellulose but perhaps not too different.

Table 1. Solubility parameter data relevant to the behaviour ofwater soluble polymers and wood polymers. Data sources for thecorrelations are indicated in parantheses.

Material FIT G/T(cf. text)

DextranC 24.3 19.9 22.5 17.4 Good 5/50(Hansen 1969)

Sucrose 23.4 18.4 20.8 16.0 Good 6/50(Hansen 1969)

Urea 22.9 14.9 21.3 16.2 Good 14/50(Hansen 1969)

PVP 21.4 11.6 21.6 17.3 Good 23/50(Hansen 1969)Polyethylene- 21.5 10.9 13.1 15.9 Fair 43/56

oxide 4000(Barton 1983)

PVA 15.3 13.2 13.5 8.8 FairBreakthrough< l hr (Hansenand Hansen 1988)

Solvent solubility 15.1 20.4 16.5 18.1 Fair 88/167> 1 % in watercf. text)(Hansen andAndersen 1988)

Complete ' 18.1 37.1 16.9 13.0 Fair 47/166solubility ofsolvent in water

Water single 15.5 16.0 42.3 -molecule

experiments including over 1000 correlations and takesaccount of "specific interactions" as differentiated from thenonpolar interactions. Specific interactions involve polarand hydrogen bonding components and opening of solvent

to solvent bonds and polymer to polymer bonds, formingsolvent to polymer bonds. The coefficient 4 is found for thesame purpose in the Prigogine Corresponding States Theoryof polymer solutions (Prigogine 1957; Delmas et al 1962;Patterson 1968; Biros et al. 1971; Hansen 1997). Thus thetheories of Prigogine and Hansen are mutually confirmed(Hansen 1997). The coefficient being 4 (=2x2) indicatesthat differences in polar and hydrogen bonding parameterscan be as much as twice the difference in dispersionparameters with solubility still being possible, i.e. keepingwithin the boundary of the polymer sphere.

Wherever solubility plays a key role, the solubilityparameter will help to interpret physical affinities andphenomena. Many of these applications have been summedup in Danish (Hansen 1992) and more recently in English(Hansen 1995). In addition to these, Barton has discussedother applications in extensive handbooks (Barton 1983,1990). This report specifically concerns the use of solubilityparameters in connection with systems where water is orhas been an important component. Water is an anomalousmaterial. It gives strong hydrogen bonds, forming specialstructures with itself and with other compounds, and caneven cause polymers to rearrange their physical structurewith primarily hydrophobic surfaces becoming hydfophilicwhen contacted with water. The solubility parameters do notnecessarily explain all these effects as such, but a betterunderstanding of similarities and differences in energy,expected from a given molecular structures in each case, iscertainly a help.

Water soluble polymers

A selection of solubility parameter data from varioussources for some water soluble polymers and relatedmaterials of interest in connection with water as thecontinuous phase are included in Table 1. The discussionpertains also to solubility parameters as an aid in elucidat-ing the ultrastructure of wood. Tables in this report includethe 5D, δρ, 5H, R0 parameters and, where relevant, molarvolumes for the material in question as well as someindication on the quality of the given characterization.Data sources are indicated but some data are original withthis publication. A number (G/T) for "good" solvents isindicated as a ratio to the total number of solvents studied.The data refer to solubility unless otherwise indicated.

Dextran has somewhat larger values than sucrose,whereas there is very little difference between the solubilityparameters for the different sugars. Most polymers havelarger solubility parameters than those of the monomer fromwhich they are made. The parameters for sugar polymersdepend of course on which groups disappear and whichgroups are formed in polymerization. For present purposesthe parameters for dextran are assumed to be similar tothose of (amorphous) cellulose. However,' crystallinity incellulose presents a differentiating factor.

The solubility of polyethyleneoxidei as with otherpolymers such as glucose oligomers and cellulose, dependsto a great extent on molecular mass. The polymer deservesspecial attention in this respect since it forms, similar to

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C.M. Hansen and A. Bj rkman: Ultrastructure of Wood 339

cellulose, a special structure with itself (Curme andJohnston 1952). This is illustrated in Figure 2. It has beennoted that polyethyleneoxide appears to dissolve truly innitromethane, whereas we are dealing with a special struc-ture in most if not all of the other solvents including water.This special structure is the reason why polyethyleneoxideis not soluble in salt water, the structure formed not allow-ing solubility.

The remaining entries in Table 1 indicate some types ofcharacterization possible.

Parameters for materials related to the ultrastructureof wood

Tables 2-4 include data relevant to interpreting the ultra-structure of wood. Table 2 reports solubility parametercorrelations useful for some relevant comparisons. Table 3gives the solubility parameters for some wood polymermonomers and a cellulose solvent (MMNO). Table 4includes some additional solubility parameters for modelsubstances used to generate the data in Table 3. It shouldbe remembered that the solubility parameter is the squareroot of the cohesive energy density. Comparisons betweenthe numbers in the tables and in the discussion concernmore generally cohesion energy. Thus energy comparisonsusing cohesion energy parameters can often be directlyapplied to relations involving surface phenomena (Beer-bower 1971). Yet many steric effects cannot be accountedfor in this simple energy-only approach. Dow Corporationhas reported Hansen solubility parameters for several oftheir methyl cellulose and ethyl cellulose derivatives(Archer 1991). A single value for Methocel 311 from thiswork is included in Table 2. Theoretical estimates forthese parameters have been made also for hydroxyethylcellulose and hydroxypropyl cellulose by molecular simu-lation (Choi et al. 1994). The characterizations regardingtensile strength of paper presumably reflect some aspectof cellulose affinity. It is likely to be affinities of thesurfaces of the fibers rather than those of the bulkpolymer, however. The tensile strength correlation basedon data reported by Robertson (Robertson 1964) is verysimilar to the cellulose accessibility (swelling) correlation.

The data reported in Table 3 are for calculations basedon molecular -structure and physical properties whichcould be found in standard handbooks. In several casescomparisons were made with other closely related ma-terials for which additional physical data were found. Thisthen allowed greater accuracy in the estimations reportedin Table 3. Some of the more pertinent materials of thistype are included in Table 4.

Additional factors pertinent to wood ultrastructure

When dealing with surfaces, a lower energy material willspread onto a higher energy material if its viscosity allowsthis. Thus an oil will spread onto and wet a higher energymetal surface, for example, but water will not spread ontoa lower energy wax. Nature lowers or maintains low energylevels in this way. In the ultrastructure of wood lower

energy materials will tend to be excluded from higherenergy materials but may sometimes cover them up. Thechange in orientation of a polymer molecule to match theenergy of a liquid in contact with it is another example ofNature matching energies wherever this is dictated by freeenergy considerations.

There are additional problems than matching solubilityparameters in characterizing materials such as cellulose,

H2 H2 H2 H2

I I I Iο c c ο c cΑ. / Ν / Χ / Ν / Ν / Χ / Χ /

C Ο C C Ο CI I I I

H2 H2 H2 H2

Shorter Chains

Ο Ο Ο/ \ / \ / \

CH2 CH2 CH2 CH2

B. I I I ICH2 CH2 CH2 Cn2

\ / \ /Ο Ο

Longer Chains

Fig. 2: Structures A and Β of the polyethyleneoxide polymer(Curme and Johnston 1952). The structures were confirmed byviscosity measurements as well as x-ray studies and conformationalcalculations. The Β structure appears to break down with additionsof salt and an increase in temperature.

Table 2. Solubility parameter correlations for materials related tothe structure of wood. Data sources for the correlations all givenin parentheses.

Material FIT G/T(cf. texte)

DextranC 24.3 19.9 22.5 17.4 Exc. .999 5/50solubility(Hansen 1969)

Methyl Cellulose 17.1 9.8 13.2 10.2 - —(Dow Corp.)Methocel 311(Archer 1991)

Sucrose solubility 23.4 18.4 20.8 16.0 Exc. .981 6/50(Hansen 1969)

Lignin solubility 21.9 14.1 16.9 13.7 Exc. .981 16/82(Hansen 1969,1992)

Paper tensile 20.3 16.3 18.7 11.7 Fair .894 6/31strength(calculated fromRobertson 1964)

Chem.accessibility 18.8 15.8 15.2 11.7 Good .991 6/19of cellulose(Larsson andJohns 1988)

Cellophane 16.1 18.5 14.5 9.3 Good .955 4/22swelling

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340 C.M. Hansen and A. Bj rkman: Ultrastructure of Wood

Table 3. Solubility parameters for materials related to the struc-ture of wood - calculations based on molecular structure andproperties. V is the molar volume, cmVmole.

Material δρ δΗ

Water single moleculeSinapyl AlcoholConiferyl Alcoholp-Coumaryl AlcoholFerulic Acid4-Hydroxy Cinnamic Acid

15.519.219.019.119.019.1

16.07.37.07.06.66.7

42.316.116.317.315.115.9

18.0210.2171.6136.5155.5128,3

N-Methyl Morpholine-N-Oxide(MMNO; approximate) 19.0 16.1 10.2 97.6

Table 4. Solubility parameters for selected reference materials

Material δρ δΗ

Cinnamyl Alcohol2,6-Dimethoxy PhenolEugenolcis-Cinnamic Acid %*trans-Cinnamaldehyde

19.119.319.019.119.4

6.07.67.53.9

12.4

13.013.713.010.66.2

129.1136.4154.0115.4125.9

which will be illustrated by a few examples. Crystallinityor partial crystallinity generally means the solvents requiredto dissolve a polymer should have somewhat larger solu-bility parameters than expected from considerations of thestructure of the polymer only. Dimethyl formamide is atroom temperature a sole solvent for the highly crystallinepolyvinylfluoride, whereas cyclohexanone, dimethyl aceta-mide, and dimethyl sulfoxide require a higher temperaturefor solvation. When and why does a "polar" solvent dis-solve a "non-polar" polymer? The answers to such ques-tions are found in the change in the entropy term of the freeenergy equation being zero or negative. Only solvents suchas gamma-butyrolactone, N-methyl-2-pyrrolidone, and di-methyl acetamide can dissolve polyvinylidinefluoride atroom temperature while elevated temperatures allows manycommon solvents to do this.

When the mutual properties of wood polymers are con-sidered one may also examine the properties of polymersegments in relation to water. In general hydrophobia bondingis caused by the (low energy) solubility parameters of seg-ments of polymer molecules, being too small to allow watersolubility, in spite of other segments of the molecule havingwater solubility. The water insoluble segments associate hy-drophobically. Side groups of hemicellulose with relativelysmall solubility parameters can hydrophobically bond to or"dissolve" in lower energy material including lignin. Thebackbones and side groups with hydroxyls are almost com-patible with the still higher energy cellulose molecules or theirhydrophilic fibril surfaces. This latter affinity is called hydro-philic bonding. Thus hydrophilic bonding and hydrophobic-bonding are in principle equivalent.

Ultrastructure of wood

The orientation of a hemicellulose molecule in wood isthought to have been determined partly by side groups

with small solubility parameters. These groups find anenergetically fitting place of residence by orientation,either towards the lower energy lignin or towards groupsof another hemicellulose molecule with similar par-ameters. Such groups can be substituted e.g. with acetyl-or methylgroups having parameter values not too differentfrom those of lignin. Conversely, hemicellulose backbonesenergetically resemble cellulose and are expected to orientthemselves towards cellulose (fibril) regions. Also hemi-cellulose groups, which have retained their hydroxyls,have parameters closer to those of cellulose and seekorientation accordingly. Thus hemicellulose regions havea dual nature in that these may accommodate both tohigher and lower energy domains. Estimates indicate thatwith an increase in acetyl content of a hemicellulose, thedegree of compatibility increases for hemicellulose-ligninand decreases for hemicellulose-cellulose (Gravitis andErins 1983). Solubility parameter calculations point alsoto the possibility that regions with adjacent side groupswith higher and lower energy levels may allow passage oflow molecular species which have a large solubilityparameter on one end of their molecules and a small oneat the other end, the ends orienting themselves towards theadjacent regions. Consideration of solubility parametersalso implies interpenetration of hemicelluloses and ligninto come forward as an issue. The hemicelluloses generallywould not be particularly compatible with ligninmonomers and much less with lignin molecules. This iswhy hemicelluloses as polymers (except for some sidegroups) tend to exclude lignin monomers and the growinglignin polymer. This topic needs to be examined moreclosely.

The view emerging from the above is that lignin andcellulose are in no way compatible with each other (Gravitisand Erins 1983; Andersons et al. 1991). This fact does leavethe hemicelluloses as a kind of surface active intermediatematerial, stabilizing the wood structure. Portions of thehemicelluloses (backbone and larger solubility parameterside groups) have solubility parameters close to those ofcellulose, and other portions (e.g. the acetyl and methylsubstituted side groups) have solubility parameters closer tothose of both the lignin monomers and the lignin polymer.Cellulose is formed from its monomer in a reasonably highenergy environment of a hemicellulose gel with a consider-able amount of water. Sucrose and other related sugars havehigh solubility parameters (see Table 2). They are fullysoluble in water at lower concentrations. The non-crystal-line Dextran C also has a rather large solubility parameter,slightly larger than sucrose. The characterizations for tensilestrength and accessibility (swelling) of cellulose yieldedslightly lower values for the solubility parameters thanthose found for the solubility of Dextran C (cf Table 2). Thereasons for such deviations are not immediately apparent,but may depend on the method of characterization ratherthan on any real difference.

The data in Table 3 show that the lignin monomers(sinapyl, coniferyl and p-coumaryl alcohols) have pro-gressively lower molar volume as the number of methoxygroups are reduced from 2 to 1 to 0. The substituents

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CM. Hansen and A. Bj rkman: infrastructure of Wood 341

influence the mobilities of a monomer/radical to reactivesites. The solubility parameters of the three compounds areessentially equal and are considerably lower than the corre-sponding values for sugars or particularly the estimates ofthe cellulose solubility parameters, deduced from the datafor Dextran. The solubility parameters for the ligninmonomers place them in the boundary region of solubilityfor the Dextran C studied. With the precision of thesecharacterizations and calculations it is possible that theycould be either just inside or just outside the Dextransphere, but they are excluded from a dextran/cellulose typepolymer as it grows in molecular mass (and particularly incrystallinity). On the other hand the highmolecular ligninmay stay dispersed through stabilization by the dual natureof the hemicelluloses.

The solubility parameters for the lignin monomers arelower than those derived for lignin itself but all threemonomers are clearly good solvents for the lignin, whichhas parameters lower than those found for the Dextran.

The exact parameter values vary with composition. Thelowering of energy for the presence of several groupscompared to the presence of an alcohol group can be seenfrom the data in Table 5, which involves group contribu-tions for estimating the Hansen δρ and δΗ solubility par-ameters. The solubility parameters of lower molecularweight liquids and solids have been estimated by adding upthe contributions for the groups present, dividing by themolar volume, and taking the square root of this result.

Based on the data in Table 5 the hemicelluloses are esti-mated to have solubility parameters intermediate betweenthose of cellulose and lignin. The group contributions listedin Table 5 confirm that replacing an alcohol group(4650cal./mole for 6H) in a hemicellulose side group by anyother of the groups listed in Table 5 will reduce both the localpolar and the local hydrogen bonding solubility parameters.The lignin monomers are therefore expected to be com-patible with such hemicellulose side groups. The ligninmonomers and lignin can therefore be conceived to prefer tobe in the immediate vicinity of hemicelluloses, and morespecifically the acetylated and methylated substituted sidegroups. Low molecular volume esters, ketones and ethershave solubility parameters, which have an energy level toolow to dissolve lignin. This implies that the hydrpphobicbonding of these hemicellulose side groups to lignin may bemore of a surface adsorption effect rather than true solubility.

TableS. Group contribution values for estimating Hansen solu-bility parameters (Hansen 1995). Units are (cal/mole) and applyto cyclic compounds. Similar values exist for aliphatic and aro-matic compounds. Removal of the hydroxyl group and replace-ment of it by any of the other groups results in a lowered solubilityparameter.

GroupContribution (cal./mo!e) toδρ δΗ

-OH (alcohol)=O (ketone)-O- (ether)-COOH (acid)

11001000600300

4650800450

2750

The net result would be the same configuration, but a surfacebond is weaker than a side group penetrating the lignin. It isnot yet clear when these interactions are more of a surfacenature or there is an actual local interpenetration. It calls formore advanced studies.

When cellulose is generated, the formation of anordered (fibril) structure must accompany polymerization.The branched hemicellulose polymers do not fit wellwithin this ordered structure and have a lower energy aswell. The hemicellulose side groups with higher solubilityparameter will tend to be compatible with cellulose andthe hemicellulose backbone. The ability for suchpolymeric chains to change the polymer orientation is wellknown. From the discussion above, it appears that at leastone wood component (hemicelluloses) has the ability tochange orientation from hydrophobic to hydrophilic andthen to return again when the external condition, thepresence of water, is removed. Cellulose is normallyhydrophobic, but can change orientation to some extent.Water or other swelling agent can enhance mobility in thistype of motion. (However, cellulose "crystals" may havea layer of water molecules attached to its surface (Teleman1996).)

Insolubility of wood

Table 3 includes parameters for the cellulose solventMMNO. These values are approximate and should beconfirmed by solubility experiments, testing polymers withknown data, and/or making additional comparisons withother solvents of the N-oxide type. Using the approximatevalues for MMNO, it can be confirmed that this solventis predicted to dissolve Dextran C and lignin at roomtemperature. The three partial solubility parameters aresmaller than those of Dextran C. It is not surprisingtherefore that small additions of water with its largerhydrogen bonding parameter do improve solubility. Theaverage molecular volume of a water/solvent mixture isalso presumed to be lower than that of the pure MMNO,depending on how the water associates with the solvent.Lower molecular volume does enhance solubility.

Solvents which are able to dissolve cellulose (possiblywith moderate heating) should generally be good solventsfor hemicelluloses and lignin. Still wood itself remainsinsoluble under these conditions, though liquids like waterswell wood considerably. Liquid ammonia (at -40°C) isknown to soften wood by opening hydrogen bonds. It issignificant to note the effects at room temperature of threesolvents commonly discussed in connection with wood,dimethyl sulfoxide (DMSO), MMNO, and N,N-Dimethylacetamide containing 8 % lithium chloride (DMAA), andcompare the results with the effect of the non-solventammonia, having a small dispersion solubility parameter.The comparison has been done with spruce by Bj rkman(Bj rkman 1994) by measuring stiffness, axial compression,creep and dimensions of small bone-dry "axial" sample bars(6x8 mm cross section) before and after the treatment, thesolvent being removed carefully.

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342 C.M. Hansen and A. Björkman: infrastructure of Wood

In the ammonia treatment, the wood swells somewhatat first but then shrinks considerably in less than 3 days.The ammonia-free sample has less than 3/4 of the originalcross section with an insignificant weight loss and amarginal reduction of the axial dimension. Amazingly, thestiffness and the compression strength, normalized to thehigher density, are decreased, while the creep is increased.Thus an ammonia treatment results in a contracted samplewith reduced strength properties. It is difficult to under-stand that a volume reduction has such a negative effect,although it is known that liquid ammonia may change thenative Cellulose I into Cellulose III. Generally a thermo-dynamically more stable structure may be reached, naturalwood being in a "compulsory" stable nonequilibrium state(Andersons el al 1991). The effect of DMA A is to swellwood slowly, the maximum volume increase beingreached after a couple of months with the sample sizechosen. After the solvent and the lithium chloride havebeen carefully removed, the dry wood shrinkage is aboutthe same as for the ammonia treatment. However, thedensity normalized values of stiffness and compressionstrength did increase while the creep deteriorated as withammonia.

The actions of DMSO and MMNO are entirely differentfrom that of DMAA. DMSO can only swell amorphouscellulose while MMNO dissolves this polymer. Neverthelessthe two solvents influence wood in almost the same way. Astrong swelling takes place rapidly, but changes proceed andafter a couple of weeks the spruce bar is quite soft. Theremarkable thing is, however, that the dry wood after com-plete removal of the solvent does regain very closely theproperties of the original bar. Thus, the two solvents actsimilarly to water but considerably stronger and wood canaccomodate the "strong" fluids, weakening the strengthproperties to "zero" without changing the ultrastructure, asmust be the case with ammonia and DMAA. It is particularlyremarkable that DMAA and MMNO, both being solvents forall three wood polymers, act very differently.

A main conclusion from the above is that solubilityparameters alone cannot explain the results of the solventtreatments in this comparison of effects on wood and woodpolymers. Insolubility of wood may of course be explainedby the existence of covalent "bridges" between polymers inwood (Tamminen et al 1995). Such bonding has by one ofthe authors (Björkman) been likened to a sparceiy "spot-welding" of a mechanical fitting (Baldwin and Goring1968). (Cf. the difference in the solubility of natural rubberbefore and after vulcanization.) The strong action of DMAAand ammonia, changing the ultrastructure, may be of acompletely different nature, due to the formation of saltswith acid groups (hemicelluloses) and phenolic groups(lignin). These ionic bonds can be expected to lead to highcohesive energy densities and to large electrostatic forces *of repulsion. The ionic bonds may cause local swelling,with forces that are able to break covalent bonds. Ammoniahas a smaller molecular volume than the organic solventsand penetrates wood easily, which is why it acts rapidly.The use of solubility parameters to account for electrostaticenergy interactions or to describe ionic bonding has been

considered in the past (Hansen 1969) but there is obviouslya need to develop this field by considerations and investi-gations.

•Conclusions

So far the solubility parametric data of the wood polymersare incomplete and the figures and inferences presentedhere are in a first stage of development. Energetic con-siderations based on comparisons of solubility parametersconfirm that the semicrystalline cellulose exists largely asan entity for itself. It will exclude hemicelluloses andlignin for energetic as well as structural reasons. Hemicel-lulose side groups with alcohol groups have relativelylarge solubility parameters suggesting compatibility withcellulose (surfaces). These large solubility parameter re-gions will also preferentially absorb more water thanregions with smaller solubility parameters. Hemicellulosesmay surround the cellulose regions (fibrils) as interfacialboundary regions. On the other hand side groups of hemi-celluloses (e.g. alkylated) with small solubility parametersare not compatible with cellulose and they are energeti-cally and physically assigned to be at the exterior of thecellulose/hemicellulose interface. This interface may notbe precise, but can hardly be large in terms of moleculardimensions. The outside of this interface is directedtoward the lignin.

Hemicellulose regions within a cellulose region mayexist. In such a case the hemicellulose side groups willorient inwardly towards side chains from other hemicellu-lose molecules while backbones are oriented towards thecellulose, forming an interpenetrating polymer networkwith hemicelluloses within a cellulose fibril. Hemicellu-lose side groups with methyl and acetyl side groups arecompatible with the lignin monomers and lignin itself andthe hemicelluloses and their side groups will be orientedin energetically favourable manners. This may constitutean interpenetrating polymer network of hemicelluloseswithin a matrix of lignin (or as the above boundaries)giving regions of diffuse character. The hemicellulosemolecules may form an interior phase, resembling reversemicelles, since the lower energy side chains are noworiented outwardly. Whether or not hemicellulose regionsexclude the lignin monomers in the same way expectedfor cellulose remains to be confirmed, but it may occur.

Further work with mixtures of solvents and woodpolymers can hopefully clear up some uncertainties in theassigned values for the solubility parameters of all thesematerials. The action of the "strong" solvents liquidammonia (-40°C), DMAA (with LiCl), DMSO andMMNO on the insoluble wood indicates that physico-chemical phenomena other than solubility parameters haveto be incorporated in the ultrastructural considerations toexplain the properties and behaviour of wood. Still,solubility parameters are an excellent "tool" from whichsystematic courses of action can be devised. It appearsthat Nature has used the basic concepts behind this "tool"to arrive at an ultrastructure for WQQd, which allows treesin various forms to exist. Our understanding of Nature's

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C.M. Hansen and A. Björkrnan: Ultrastructure of Wood 343

methods is far from complete, but it can be stated that thecontrol of segmental interactions is central to the results.

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344 C.M. Hansen and A. Bj rkman: infrastructure of Wood

Terashima, N., K. Fukushima and T. Imai. 1992. Morphological Received April 2nd 1997origin of milled wood lignin studied by radiotracer method.Holzforschung 46(4)\ 271-275. Dr. Charles M. Hansen Prof. Anders Bj rkman

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