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Hydration Mechanism of Polysaccharides:A Comparative Study

S. DESPOND,1,2 E. ESPUCHE,1 N. CARTIER,2 A. DOMARD1

1Laboratoire des Materiaux Polymeres et des Biomateriaux, Unite Mixte de Recherche 5627, Batiment Institut Sciences etTechniques de I’Ingenieur de Lyon, 15 Bd A. Latarjet, 69622 Villeurbanne Cedex, France

2Ahlstrom Research, Zone Industrielle de l’Abbaye, Impasse Louis Champin, 38780 Pont-Eveque, France

Received 18 March 2004; revised 23 July 2004; accepted 25 July 2004DOI: 10.1002/polb.20277Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Water sorption was studied at 20 °C on films composed of different naturalpolymers. Three polysaccharides were investigated: chitosan, cellulose, and alginate.The major differences between these polymers, from a structural point of view, lay inthe substitution of an OH group by an NH2 function for chitosan and by an ionicCOO�Na� group for alginate. An analysis of the experimental water sorption iso-therms, expressed as the number of water molecules sorbed per repeating unit in theamorphous phase, associated with an analysis of the enthalpy profile related to thewater sorption allowed us to propose a water sorption mechanism in two steps for allthe polymers: water sorption on polymer-specific sites in the first step and waterclustering around the first sorbed water molecules in the second step. It was deter-mined that two water molecules interacted with the polymer chains for cellulose andchitosan, whereas four water molecules were bonded to alginate chains. The specificsorption sites were identified as OH groups for cellulose, OH and NH2 groups forchitosan, and ionic and OH groups for alginate. A systematic reduction of the half-sorption time was observed in the activity range corresponding to this first sorptionstep, and it was explained by a water plasticization effect. On the other hand, anincrease in the half-sorption time was observed in the second sorption step, at a highactivity (�0.8), for chitosan and alginate. A modelization associating the Guggenheim–Anderson–de Boer model and the clustering theory, applied to our systems, allowed usto relate the occurrence of this last phenomenon to the formation of water clusterscontaining more than two water molecules. © 2004 Wiley Periodicals, Inc. J Polym Sci PartB: Polym Phys 43: 48–58, 2005Keywords: alginate; cellulose; chitosan; water sorption; hydrophilic polymers; biode-gradable polymers

INTRODUCTION

Biopolymers are in strong demand for applica-tions such as edible packaging,1,2 membranes,3,4

and aqueous effluent treatments.5,6 Their biode-

gradable character, associated with the presenceof specific interaction sites in their structure,make them very attractive. Indeed, the polargroups and even in some cases the ionic groups7

present in these polymers are the reason for thehigh cohesive energy density, which represents adetermining parameter for some applications, es-pecially packaging. All these functional groupscan also be used to modify the polymers and in-crease their interaction capacity toward well-de-

Correspondence to: E. Espuche (E-mail: [email protected])Journal of Polymer Science: Part B: Polymer Physics, Vol. 43, 48–58 (2005)© 2004 Wiley Periodicals, Inc.

48

fined species. This possibility is particularly inter-esting for water depollution, for example.8,9 How-ever, these functions are also the reason for thewell-known hydrophilic character of many naturalpolymers. In most applications, the ability of a poly-mer to swell in water and the hydration kinetics areof great importance. Fringant et al.10 reported thatthe water uptake greatly depends on the polymerstructure, but no data were reported concerning thewater sorption kinetics. It seems, therefore, essen-tial to have a better understanding of the watersorption phenomenon from thermodynamic and ki-netic points of view.

In this work, we have chosen to compare thewater sorption behavior of polysaccharide sam-ples differing in their chemical structures. Cellu-lose, chitosan, and alginate were all investigatedin the form of films. The sorption kinetics and thesorption isotherms were determined and ana-lyzed as a function of the polymer structure.

Water sorption in hydrophilic polymers is usu-ally a nonideal process leading to plasticizationand/or clustering phenomena and, as a result,to complex Brunauer–Emmett–Teller (BET)type III11,12 or sigmoidal isotherms.13,14 Somemodels have already shown their ability to de-scribe such experimental isotherms.15–20 Amongthem, we have chosen the Guggenheim–Ander-son–de Boer equation18–20 because it has beenalready used for natural polymers21 and can pro-vide information about clustering.11

EXPERIMENTAL

Materials and Film Processing

Table 1 shows the structures of the different poly-mers used in this study: cellulose, chitosan with alow degree of acetylation (DA � 1.6%), and algi-nate with a mannuronic/guluronic residue ratio of0.8. Cellulose and chitosan belong to the family oflinked �,1,4-polysaccharides, whereas alginate iscomposed of both �,1,4- and �,1,4-linkages. Themajor differences between these polymers, from astructural point of view, lie in the substitution ofan OH group by an NH2 function for chitosan andby a COO�Na� group for alginate.

The main characteristics of the different poly-mers and the process used to prepare the filmsare described next.

Cellulose

Cellulose was studied in the form of parchmentpaper. The paper was composed of pure cellulosic

wood fibers without any additive included in thefinal fibrous dispersion (i.e., 100% cellulose modelpaper). Blotting paper was first realized, and thenparchment paper was obtained by the immersionof this initial paper in a concentrated sulfuric acidsolution. Fibers were then partially dissolved,and this formed a cellulose gel that precipitatedand partially filled the paper pores. Figure 1 pre-sents scanning electron microscopy pictures of theinitial blotting paper and the correspondingparchment paper. The thickness of the papersheet was 40 �m. We focused our study on theparchment paper because it was less porous thanthe blotting paper and, therefore, correspondedbetter to the films obtained with the other poly-mers.

Chitosan

Chitosan was provided by France Chitin. Theflakes were purified through dissolution in a stoi-chiometric amount of acetic acid, and the solution(1%) was filtered through 0.22-�m-pore mem-branes (Millipore). Aqueous ammonia was thenadded to precipitate the polymer. After severalwashings with deionized water up to a neutralpH, the product was dried overnight under re-duced pressure. DA was determined by 1H NMRspectroscopy according to the method proposed byHirai et al.22 and was found to be 1.6%.

The average molecular weights were deter-mined by means of size exclusion chromatography(SEC) with an online multi-angle laser light scat-tering detector. The weight-average molecularweight (Mw) was 200,000 g mol�1, and thenumber-average molecular weight (Mn) was165,000 g mol�1 (with a precision of 5%).

Chitosan films were prepared via the casting ofa 1% (w/w) aqueous acetic acid solution contain-ing chitosan onto a polystyrene plate. After dry-ing at room temperature, the obtained 20-�m-thick films were immersed in an ammonia/meth-anol solution for 15 min for neutralization. Theywere then rinsed with water and dried. Chitosanfilms in the free amine form were thus obtained.The crystallinity of the films was determined byX-ray diffraction with filtered Cu K� radiationgenerated at 30 kV and 5 mA. The degree ofcrystallinity was 40 � 5%.

Alginate

This polysaccharide was composed of polymerblocks of 1,4-poly(�-D-mannuronic acid), 1,4-

HYDRATION MECHANISM OF POLYSACCHARIDES 49

poly(�-L-guluronic acid), and segments of alter-nating D-mannuronic and L-guluronic acid resi-dues.

The sodium alginate was provided by FMCBiopolymer AS. The composition of the copolymer[80% mannuronic sequences (M)] was determinedby 1H NMR. The guluronic acid units (G) wereessentially located in sequenced MGMGMGblocks. Mw and Mn were determined by SEC: Mw� 182,000 g mol�1 and Mn � 85,500 g mol�1.

Alginate films were prepared via the casting ofa 1% (w/w) aqueous sodium alginate solution ontoa polystyrene plate. After drying at room temper-ature, the films were analyzed by X-ray diffrac-tion. The degree of crystallinity was 35 � 5%.

Water Sorption Apparatus

The apparatus used for the water sorption studiesconsisted of a Setaram B92 microbalance and a

microcalorimeter. The balance precision was�5 �g.

Two samples of exactly the same weight wereused. One sample was added to the microbalance,and the other was added to the microcalorimeter.After desorption in vacuo (2.10�6 mbar) at a con-stant temperature (20 � 1 °C), a partial pressureof water was established within the apparatus bymeans of an evaporator placed at temperature T.The water uptake at time t (Mt) was followed untilthe equilibrium sorption (M�) was attained, andthe thermal changes associated with the wateruptake were recorded. The exothermic peak wasintegrated and corrected with values of the en-ergy obtained under the same conditions withempty boats. The ratio of the obtained energy tothe water uptake gave the interaction enthalpy(kJ mol�1). It was representative of the internalenthalpy change of one molecule from the gaseousstate to the sorbed state.

Table 1. Chemical Structures of the Polymers

50 DESPOND ET AL.

The partial pressure was then increased in suc-cessive steps through discrete changes in temper-ature T from �12 to 20 °C, and so over the entirerange of activity, the kinetics of water sorption,the sorption isotherm, and the interaction en-thalpy profile were obtained.

Methodology Used To Analyze the Water SorptionData

The water sorption results were examined as afunction of the activity and the considered poly-mer in three main ways.

First, the sorption data were fitted to an em-pirical equation:

Mt

M�� ktn (1)

The n value indicates the type of diffusion mech-anism; there are three possible cases. For the first

one, corresponding to Fickian transport, the rateof diffusion is much lower than the rate of relax-ation, and n is equal to 0.5. For the second one,the diffusion is very fast, contrary to the rate ofrelaxation, and n is 1. The third case correspondsto anomalous diffusion with n values lying be-tween 0.5 and 1.

Second, the sorption rates were estimated inthe range of all partial pressures via the half-sorption time (t1/2).

Third, the water isotherms were determinedand analyzed with respect to the enthalpy profile.Considering, as generally assumed, that the crys-talline parts were impermeable to small mole-cules, we also presented the isotherms for eachpolymer as the number of water molecules sorbedper repeating unit in the amorphous phase. Thisrepresentation allowed us to discuss the role ofthe polymer chemical composition in the watersorption mechanism. At last, the experimentalisotherms were modeled with the Guggenheim-Anderson-de Boer (GAB) equation.18–20 Thisequation has been widely used to describe watersorption in foods and natural polymers21,23,24:

c �aGAB cP,GAB kx

�1 � kx��

11 � �cP,GAB � 1�kx (2)

According to this equation, the water concentra-tion (c) is related to the Guggenheim constant(cP,GAB), to the water concentration correspondingto the saturation of all primary adsorption sitesby one water molecule (formerly called the mono-layer in BET theory; aGAB), and to a factor cor-recting the properties of the multilayer moleculeswith respect to the bulk liquid k value. Bizot’smethod25 was used for calculating the parametersof the model. The curve-fitting efficiency was es-timated from the residual sum of squares (RSS):

RSS��(yexp� ycalc)2 (3)

where yexp and ycalc are the reported experimentalvalues and the corresponding calculated values,respectively.

The GAB model is also interesting because itcan provide more information about water clus-tering. A positive deviation of M� from Henry’slaw sorption is generally interpreted by a cluster-ing tendency of the penetrant in the polymer ma-terial. The cluster integral G11 can be calculatedwith the following equation26:

Figure 1. Scanning electron microscopy photographsof (a) blotting paper and (b) parchment paper.


G11

�1� � �1 � 1��a1

1�

a1�

P,T

� 1 (4)

where �1, 1, and a1 are the molecular volume,volume fraction, and activity of component 1, re-spectively, and P and T are the pressure andtemperature, respectively.

The average number of solvent molecules in acluster (Nc) is defined as follows:

Nc � 1

G11

�1� 1 (5)

For an ideal solution, there is no clustering, andNc is equal to 1.

According to Zhang et al.’s study,11 Nc valuescan be deduced from the GAB model parameterswith the following equation:

Nc � � �1 � 1� � � 1

aGAB cP,GAB(�2 cP,GAB kx

� 2kx � cP,GAB�2)�1� (6)

Nc values were determined and examined for allthe polymers.

RESULTS

Diffusion Mechanism and Sorption Rates

The n values representative of the sorption mech-anism were determined at various activities forchitosan and alginate films (Table 2).

For chitosan, n values close to 0.5 were ob-tained for activities lower than 0.3. In this do-main, the diffusion mechanism can be consideredFickian. For higher activity, n increased progres-sively from 0.5 to 0.85, and the diffusion mecha-nism became anomalous.

For alginate, n values were higher than 0.5even at very low activities. As for chitosan, nincreased with the activity and reached a plateauaround 0.85 for activities higher than 0.43. Thus,a Fickian diffusion domain could not be deter-mined for alginate.

In conclusion, anomalous sorption mechanismswere observed over a wide range of activities forthese natural polymers.

As we demonstrated that the diffusion mecha-nism was not Fickian over the entire activityrange, we did not calculate the diffusion coeffi-cients but instead chose to compare t1/2. For thecomparison of the results, and because the thick-ness of the two films (chitosan and alginate) wasequal to 20 �m, all the data were expressed forthis thickness. The results are presented inFigures 2(a) and 3(a) for chitosan and alginateand in Figure 4(a) for paper.

Two main observations can be drawn from theexperimental curves with respect to chitosan andalginate:

1. For each of these polymers, the variation oft1/2 as a function of the activity is not mo-notonous, and three domains can be deter-mined. In the first domain, correspondingto a low activity (domain I), a decrease int1/2 can be observed. For an intermediatevalue of the water activity aw (domain II),no great variations of t1/2 can be observed,and at a high activity (domain III), an in-crease in t1/2 can be observed.

2. In domains I and II, the t1/2 values mea-sured for chitosan and alginate are quitesimilar. On the other hand, in domain III,the t1/2 values measured for alginate filmare higher than those determined for chi-tosan.

A particular behavior has been noticed for pa-per. The kinetics are very fast, and only a smalldecrease in t1/2 can be observed as the activityincreases [Fig. 4(a)].

Sorption Isotherm and Interaction Energy

The water sorption isotherms are BET type II forall the studied polymers. The isotherm is curvedtoward the x axis at a low activity, and a positivedeviation from linearity can be observed at a highactivity (Fig. 5). At a given value of aw, the wateruptake can be classified as a function of the poly-mer, and the following order can be obtained:

Table 2. n Values Determined at DifferentActivities for Dense Chitosan and Alginate Films

aw 0.21 0.25 0.43 0.5 0.8 0.97n

Chitosan 0.49 0.54 0.57 0.65 0.85 —a

Alginate —a 0.69 0.85 0.84 —a 0.85

a Not determined.

52 DESPOND ET AL.

paper chitosan alginate. The hydrophiliccharacter of all the studied polymers is very im-portant: the water uptakes measured for activi-ties close to 1 are equal to about 20% for thepaper, 55% for chitosan, and 160% for the algi-

nate. As the crystallinity is not identical for allthe polymers, the isotherms are presented as thenumber of water moles sorbed per repeating unitin the amorphous phase [Figs. 2(b), 3(b), and4(b)]. The curve for paper corresponds quite well

Figure 3. Water sorption results obtained for algi-nate: (a) the variation of t1/2 as a function of the activ-ity, (b) the isotherm represented as the number of wa-ter moles sorbed per monomer repeating unit in theamorphous phase, and (c) the variation of the enthalpyrelated to the sorption mechanism as a function of theactivity.

Figure 2. Water sorption results obtained for chi-tosan: (a) the variation of t1/2 as a function of theactivity, (b) the isotherm represented as the number ofwater moles sorbed per monomer repeating unit in theamorphous phase, and (c) the variation of the enthalpyrelated to the sorption mechanism as a function of theactivity.


to the isotherm obtained by Joly27 for model cel-lulose. The number of water molecules sorbed perrepeating unit in the amorphous phase can beclassified as follows as a function of the polymerstructure: paper chitosan alginate.

To determine the water sorption mechanism,we analyzed the isotherms in parallel to the meaninteraction energy developed per mole of sorbedwater. Figures 2(c), 3(c), and 4(c) show that theabsolute enthalpy values are high at a low activ-ity and that they progressively decrease as theactivity increases to reach a plateau value nearthe water condensation energy value. Thus, for allthe studied polymers, the first sorbed water mol-ecules interact with specific sites of the polymerchains, whereas water–water interactions be-come predominant at a high activity, when theenergy plateau is reached. Despite this globaltrend, which can be applied to all the studiedpolymers, some specificities can be observed con-cerning the energy involved in the sorption mech-anism, the number of water molecules interactingwith the polymer, and the range of activity forthis sorption step.

For cellulose, two water molecules interactwith each repeating unit in the amorphous phase,and this sorption step concerns the activity rangeof 0–0.85.

More precisely, for activities below 0.4, the wa-ter sorption enthalpy is constant and equal to�60 kJ/mol, whereas the water uptake increasesup to a value corresponding to one water moleculesorbed per repeating unit. It can be concludedthat hydrogen double bonds are formed betweenthis molecule and the polar OH groups of cellu-lose. Indeed, a hydrogen bond with an OH groupinvolves �29 kJ/mol.27

For activities between 0.4 and 0.85, the abso-lute value of the enthalpy decreases progressivelyfrom 60 to about 50 kJ/mol, and the water uptakeincreases up to two water molecules per repeatingunit. This decrease in the enthalpy can be ex-plained by the presence of some single-bondedwater molecules.

For chitosan, as for cellulose, two water mole-cules interact with the repeating unit in theamorphous phase, but this mechanism operatesin a narrower range of activity (0–0.6). The en-thalpy profile is similar to that observed for cel-lulose. Thus, hydrogen double bonds are formedbetween the first sorbed water molecule and thepolar groups of chitosan, and then some single-bonded water molecules are sorbed.

As a hydrogen bond with an NH2 group in-volves �33 kJ/mol,27 both OH and NH2 groupscan be considered specific sorption sites for thewater molecules for chitosan.

For alginate, four molecules are bound per re-peating unit, and the activity range is 0–0.7.

Figure 4. Water sorption results obtained for parch-ment paper: (a) the variation of t1/2 as a function of theactivity, (b) (�) the isotherm represented as the num-ber of water moles sorbed per monomer repeating unitin the amorphous phase and (�) a comparison withJoly’s results,27 and (c) the variation of the enthalpyrelated to the sorption mechanism as a function of theactivity.

54 DESPOND ET AL.

These four molecules could correspond to the hy-dration sphere of COONa determined by Berri-aud et al.28 or more likely to sorption on both theCOONa group and unsubstituted OH groups.

Indeed, the absolute enthalpy value measuredfor alginate for an activity range of 0–0.3, inwhich two water molecules are sorbed per resi-due, is greater than the values measured for chi-tosan and paper. We can thus presume that thesetwo first sorbed molecules are tightly bound to theCOONa ionic group.

For higher activities, the enthalpy values be-come similar to those measured for the other poly-mers, and this shows that the two additionallysorbed water molecules are probably interactingwith the alginate polar groups.

At last, we can observe that the deviation fromlinearity observed at a high activity is most im-portant for alginate and is less pronounced forpaper. This behavior is discussed with the GABequation and a water clustering analysis.

Isotherm Modelization and Water ClusteringAnalysis

The experimental isotherms were modeled withthe GAB equation. A previous study on chitosan14

showed that of different other models (BET, mod-ified BET, etc.), this model was the best adaptedto represent the experimental data. The values ofthe three model parameters determined for eachpolymer are listed in Table 3, and RSS valuesindicative of the agreement between the modeland experimental curves are also reported. Thevery low values of RSS obtained for chitosan andpaper indicate that the GAB model describes withgood accuracy the sorption isotherms of these twomaterials (Fig. 5). Concerning alginate, the exper-imental data can be well modeled at activitieslower than 0.9, but the model fails to representthe high increase in the water uptake at a highwater partial pressure (Fig. 5). Nevertheless,aGAB values have been compared for all the films

Figure 5. Water experimental sorption isotherms with respect to (�) chitosan, (Œ)alginate, and (■) paper. The dotted line represents, in each case, the isotherm modeledwith the GAB equation.


because this parameter is related to the watercontent corresponding to the saturation of all theprimary adsorption sites by one water molecule,and it is thus representative of the sorption mech-anism at a low activity. The aGAB values are sim-ilar for chitosan and paper and are two-fold lowerthan for alginate. Here again, we find the factor 2,which we previously determined between thenumber of water molecules interacting with thealginate monomer repeating unit, on the onehand, and the paper and chitosan monomer re-peating units, on the other hand.

In Table 3 are also reported the values of theactivity for which Nc becomes higher than 1(awNc�1). For activities higher than awNc�1, waterclusters are formed, and for activities belowawNc�1, water is distributed in the polymer with-out association phenomena. This last activityrange corresponds, for each polymer, to the do-main in which t1/2 decreases as a function of thewater content (i.e., domain I in the kinetic study).We have further demonstrated that in this do-main the sorbed water molecules are interactingwith the polymer. Thus, the plasticization effectexpressed by the decrease in t1/2 can be explainedby the decrease in the chain cohesion as the watermolecules are bound to the polymer chains.

For chitosan, we have also studied the phenom-enon occurring for activities higher than awNc�1,at which water clustering becomes predominant.We have determined the activity at which Ncreaches the value of 2: it is equal to 0.8. In theactivity range of 0.6–0.8, corresponding to thesorption of an additional water molecule nearthe first sorbed molecules interacting with thepolymer, thus to the formation of a second waterlayer, no great modification of the kinetics can beobserved. t1/2 does not vary to a significant extent.At last, at higher activities, Nc greatly increases,and because of the formation of big water clus-ters, the apparent diffusion rate becomes lower.

For alginate, good agreement can be observedbetween the GAB model and the experimentalisotherm for activities lower than 0.9. Nc has beencalculated at aw � 0.88 to be 1.61. In the range ofactivity of 0.7–0.88, t1/2 seems to be stable; itincreases at very high activities, as the wateruptake greatly increases and big water clustersare certainly formed. As the GAB equation couldnot describe the experimental isotherm in thisdomain, it was not possible to determine the valueof Nc around an activity of 1.

At last, Nc remains below 2.3 for the entirerange of activity for paper (Table 3). This couldexplain why no increase in t1/2 was observed forthe paper at a high activity, contrary to alginateand chitosan.

DISCUSSION

From a chemical structure point of view, the maindifferences between the different polymers thatwe have studied lie in the partial substitution ofan OH group by an NH2 group for chitosan and byan ionic COONa group for alginate.

In agreement with Fringant et al.,10 the watersorption mechanism has been found to be com-posed of two main steps: water sorption on poly-mer sites and water clustering surrounding thefirst sorbed water molecules.

Thus, at a low activity, water is tightly boundto the polar groups of paper and chitosan and tothe ionic and polar groups of alginate. Replacingan OH group with NH2 does not lead to a greatchange in the first step of sorption mechanism.Indeed, the number of water molecules interact-ing with the monomer repeating unit is equal to 2for the cellulose paper and chitosan film, and thevalues of the energy associated with this sorptionmechanism are equivalent for these both mate-rials.

Table 3. Values of the Three GAB Model Parameters (aGAB, cP,GAB, and k)Obtained for the Natural Polymers, awNc

�1, and Nc Determined near Saturation

Polymer aGAB cP,GAB k RSS awNc�1

Nc ataw 1

Paper 0.058 16.75 0.753 8.04 10�6 0.85 2.3Chitosan 0.059 18.23 0.879 4.05 10�4 0.6 3.6Alginate 0.116 17.68 0.832 6.436 10�1 0.7 —a

a This value could not be calculated because of the bad agreement observed between the GABmodel and the experimental isotherm in this domain.

56 DESPOND ET AL.

On the other hand, the substitution of an OHgroup by the highly hydrophilic COONa ionicgroup increases the total number of interactingwater molecules. The two first water moleculesare located on the ionic site, and an importantexothermic peak is associated with this wateruptake.

In all cases, as water is bound to specific poly-mer sites, the polymer cohesion decreases, and aplasticization effect can be noted: t1/2 decreases.

The activity range for this first step of sorptiondepends on the polymer: 0–0.6 for chitosan,0–0.7, for alginate and 0–0.85 for cellulose in thepaper form. For each polymer, in this activityrange, Nc does not exceed 1.

The second step of the sorption mechanism,occurring at a high activity, concerns water clus-tering and can be divided into two parts.

The first one consists of the sorption of an ad-ditional water molecule on the initially boundwater molecules. Nc increases progressively up to2, and t1/2 remains quite constant. This step hasbeen observed for all the studied polymers.

The second part consists of the formation ofbigger water associations, and an increase in t1/2can be observed. This last step only occurs foralginate and chitosan.

The occurrence of this last sorption step doesnot seem to depend on the number of sorbed wa-ter molecules interacting with the polymer chainbut depends more on the range of this first do-main. Indeed, chitosan and alginate are the twopolymers for which the first sorption mechanism,that is, sorption on specific sites, occurs in a nar-rower range.

CONCLUSIONS

Water sorption was studied for cellulose-, chi-tosan-, and alginate-based films, and three do-mains were identified.

At a low activity, for all the films, the watersorption mechanism consisted of sorption on spe-cific sites: OH groups for cellulose, OH and NH2groups for chitosan, and COONa and OH groupsfor alginate. For cellulose and chitosan in the freeamine form, two water molecules were bound perrepeating unit in the amorphous phase. For algi-nate, four water molecules interacted with therepeating unit, and among them, two were tightlybound to the ionic COONa group. The activityrange for this first sorption step depended on thepolymer, but in this domain, a systematic reduc-

tion of t1/2 was observed because of a plasticiza-tion effect.

A second domain concerned the sorption of asupplementary water molecule surrounding thewater molecules that were bound to the polymerchains. In this domain, no great variation in thewater sorption kinetics was observed.

A third domain was at last observed for chi-tosan and alginate and consisted of the formationof big water cluster, which led to a decrease in theapparent water diffusion.

The authors acknowledge the financial support of Ahl-strom and the Ministry of Research and Technology.

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