Polymerdynamics of cellulose and other polysaccharides in solid state-secondary dielectric relaxation processes

Prog. Polym. Sci. 26 (2001) 1419±1472

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Polymerdynamics of cellulose and other polysaccharides in solid state-secondary dielectric relaxation processes J. Einfeldt*, D. Meiûner, A. Kwasniewski University of Rostock, Department of Physics/Polymerphysics, UniversitaÈtsplatz 3, D-18051 Rostock, Germany Received 9 November 2000; revised 12 June 2001; accepted 18 June 2001

Abstract Dielectric relaxation spectroscopy (DRS) separates different molecular groups of a repeating unit of a polymer with respect to the rate of its orientational dynamics. In the case of dry solid polysaccharides, four modes of relaxation processes can be observed in the sub-Tg range, which we interpret in the following way. The local main chain motion forms the b-relaxation, and the side groups motion in the repeating unit generates the g-relaxation. Additionally, the so-called d-relaxation can be observed in the low frequency side of the b-relaxation for well dried samples and a further bwet-relaxation occurs only in wet samples in the room temperature range, but the origins of this last process are not clear up to now. In the high temperature range (T . 808C), the s -relaxation can be measured which is associated with the hopping motion of ions in the disordered structure of the biopolymeric material. For all these processes, we give experimental evidence. In addition, further relaxation processes are detected in the electrical inhomogeneous polysaccharide samples, which are associated with internal interfaces and the interface to the electrode and are well known as Maxwell±Wagner±Sillars and the electrode polarisation. The in¯uence of the type of the glucosidic linkage to the b-relaxation is discussed by comparing the dynamic dielectric behaviour of different polysaccharides. Small amounts of water or other swelling solvents in the sample modify the relaxation processes in a characteristic manner and increase the activation energy and the cooperativity of the local chain motion. The morphological structure of the cellulose affects the dielectric spectra in the low frequency range below the b-loss peak. This spectral range in the DR spectra correlates with the chemical accessibility and the water retention capacity of chemical pulps. In the case of derivatives of cellulose or starch, we can show that the relaxation of side groups can be separated depending on the type of the side group and its position in the anhydroglucose unit (AGU). Results are presented both in the form of dielectric spectra and as ®t parameters calculated with the help of the Havriliak±Negami function and also in the form of the activation energies and the preexponential factors resulting from the Arrhenius representation. Essential literature concerning relaxation processes in polysaccharides is reviewed and the results given are compared with our ®ndings. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Dielectric relaxation; Polysaccharides; Perivatives; Water effects * Corresponding author. Tel.: 149-381-4981621; fax: 149-381498-1626. E-mail address: [email protected] (J. Einfeldt). 0079-6700/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0 0 7 9 - 6 7 0 0 ( 0 1 ) 0 0 02 0 - X

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Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1420 2. Experimental aspects of dielectric spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1423 3. Theoretical aspects of dielectric spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1427 3.1. Phenomenological theory of dielectric relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1427 3.2. Parameterisations of the dielectric spectra using a relaxation model function . . . . . . . . . . . . . . .1428 3.3. Molecular interpretation of the dielectric spectra of polysaccharides . . . . . . . . . . . . . . . . . . . . .1430 4. Experimental data and their interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1433 4.1. Assignment of the single secondary relaxations to their corresponding molecular motions . . . . .1433 4.2. Native celluloses, pulps and regenerative celluloses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1436 4.3. Cellulose in comparison with other polysaccharides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1439 4.4. Effect of low water content in polysaccharides on the molecular polymer dynamics . . . . . . . . . .1444 4.5. Relation between the chemical accessibility of cellulose pulps and dielectric spectra . . . . . . . . .1450 4.6. Cellulose derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1452 4.6.1. Derivatives with statistical substitution pattern on the AGU . . . . . . . . . . . . . . . . . . . . .1452 4.6.2. Derivatives with regioselective substitution pattern at the AGU . . . . . . . . . . . . . . . . . . .1458 4.7. Starch derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1462 5. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1467 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1468 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1468

1. Introduction Dielectric relaxation time spectroscopy (DRS) is a well-established tool in the investigation of synthetic polymer material. Many review articles [5±32] deal with this subject, presenting theoretical and experimental aspects and summarise many experimental data of this substance class. However, in all these reviews, the biopolymers only play a subordinate role. The books of Pething [17] or Takashima [19] on the (di-)electric properties of biopolymers concentrate on proteins and DNAs, whereas the chapters on polysaccharides are very small and their content is not representative. On the other hand, there are spare discussions about the dielectric and dynamic properties in modern books and review articles about cellulose or polysaccharides [33±42]. In general, the dielectric spectroscopy of polysaccharides has been considered controversial by many scientists, up to now. Cellulose and other polysaccharides, which are the subject of this review, consist of anhydroglucose units (AGU) (Fig. 1) carrying two hydroxyl groups (-OH) and one methylol group (-CH2 ±OH). The AGUs are linked in the polymer chain via acetal oxygens in the equatorial- (b-form) or in the axial directions (a-form). This O-bridge forms the so-called glucosidic linkage. The dielectric relaxation studies of cellulose, which date back to the measurements made by von Schweidler [43] and Wagner [1,162], show a marked progress in the last 15 years [44±116] caused by the commercial availability of dielectric broadband spectrometers [117] in the frequency range of 1 mHz±1 GHz [29] and, additionally, by the new research activities in the ®eld of polysaccharide chemistry and physics, in general.

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Fig. 1. Xd-anhydroglucoseÐthe constitutive unit (AGU) of the cellulose molecule Ðand the dielectric site model of the repeating unit of polysaccharides.

Cellulose and other polysaccharides are technically very interesting, renewable, biocompatible and owning excellent biodegradable resources and versatile chemical and physical properties. The technical application and technological processes in the cellulose industry are not without problems, which are related to the complex structure of these types of biopolymers. For an extended commercial application and a better understanding of these biopolymers, the investigation of their dynamics at the molecular level is a useful new contribution to complete structural and energetic investigations. DRS is a method which can investigate molecular motions in the extended time scale of 0.1 ns±100 s if the moving sites of the repeating unit and the attached side groups own a permanent dipolar moment. In each case, the morphology of cellulose and other polysaccharides strongly depends on the preparation conditions of the sample before the measurements and on the kind of chemical functionalisation [118±122]. The properties of polysaccharides, on the one hand, are very sensitive to low water content and its distribution in the sample and, on the other hand, to the complex supramolecular structure of polysaccharides (often characterised as secondary, tertiary or quaternary structure). This structure is characterised, for instance, in the case of cellulose by elementary ®brils, which associate by hydrogen bonds to micro- and macro®brils forming a morphology with a speci®c system of holes, pores, micro crevices and capillaries. These hydrogen bonds are a dominant aspect of the structure of cellulose and also the other polysaccharides. Because of the polarity of the hydroxyl groups on the cellulose chain, strong hydrogen bonds are not only found inside the cellulose chain, but also between chains and between larger agglomerates (see Fig. 2). Hydrogen bonds are abundant in the native structure of cellulose and some are formed additionally in pulps when water is removed from the ®bres and from the interfaces between the cellulose chains. The formation of some bonds is irreversible, of other ones it is reversible. The physical result reveals itself in a densi®cation of the micro- and macrostructure, an irreversible closing of pores, lower re-swelling ability and changes in the properties (mechanical, chemical and also electrical) [41]. The accessibility of cellulose for a reactant or a solvent molecule is strongly dependent on the preparation conditions of the pulp and especially on the drying procedure of the pulps [123,124]. The irreversible structural changes in swollen, microporous pulp after drying are also designated in an unspeci®c way as `horni®cation'. The details of this phenomenon are not yet elucidated. It is known, however, that the mechanisms of collapse of pores, slight recrystallisation of domains adjacent to micro-crystallites and the formation of additional H bonds within the less ordered regions will contribute to it. Some authors have already investigated this effect of horni®cation and its in¯uence on the chemical accessibility and reactivity of pulps [122±125]. All these processes also have an effect on the dielectric properties of these materials.

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Fig. 2. Intra- and inter-catenar H bonds in cellulose.

In the case of starch, which is a blend of amylose and amylopectine with a helical polymer chain structure, the same dominance of supermolecular structure effects can be observed. Starch is characterised by a granular supramolecular structure depending on the origin and the treatment of the sample. The granules are formed by layers and within them the amylopectine forms a branched polymer with a very high molecular weight and the shorter amylose molecules are embedded in the amylopectine matrix forming crystalline or amorphous regions. In starches, a similar effect as in cellulose can be observed. The starch is restructured irreversibly in comparison with the native starch by drying and regeneration. These complex structure elements (different types of ®brils or granules, crystalline or amorphous regions in a ®bre forming noncrystalline spheres in which also a preorientation exists and no complete disorder) exist in different length scales and are typical for each special polysaccharide. This structure forming complicates the interpretation of dielectric spectra in a signi®cant manner. In this context, the description of polysaccharides by a two phase model with a crystalline and an amorphous phase is an inadequate simpli®cation to interpret the results of dynamic measurements. That also means, polysaccharide samples in most cases are not homogeneous dielectric solids and their characterisation by electrical material parameters such as the dielectric function or the electrical conductivity is a fundamental problem. Considering all these aspects, the preparation of the samples for dielectric spectrometric measurements is an important point for the successful application of this analytical technique. Many controversial results and discussions in literature can be put down to the problem of the insuf®cient physico-chemical characterisation of the polysaccharides tested. Therefore, we hope to show in this review, that, today, practical DRS of this class of biopolymers has reached such a level that it can work as new and useful analytical method to investigate different problems in present polysaccharide research. This paper is structured in the following way. Firstly, we give a short overview of a few experimental and preparatory aspects of practical DRS. Next, we present a short introduction into the phenomenological theory and the description of the dielectric spectra with the help of well-tried model functions and

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relaxation parameters. Furthermore, we create the prerequisite for the evaluation of dielectric spectra. After that, we summarise and discuss the basic molecular models for the interpretation of the relaxation modes measured in dielectric spectroscopy of polysaccharides and present new experimental proofs of the assignment of the different dielectric loss processes to molecular motions. The main part of this review focuses on experimental data con®ned to the sub-Tg region and their interpretation. The so-called primary or a-relaxation, which is associated to the glass transition temperature (Tg), should not be discussed within this review, because in all our experiments we have not found any evidence for a dynamics with a Vogel±Fulcher temperature dependence which is typical for the glass-transition dynamics [126±128]. We subdivide this main chapter into the following classes of substances: (1) celluloses in native form and regenerative cellulose and pulps; (2) other pure polysaccharides in comparison with cellulose; (3) water in¯uence on the dielectric properties of cellulose; (4) cellulose derivatives; and (5) starch derivatives. Concluding remarks give an outlook for investigations under preparation and to speci®c basic problems which have to be solved in the future to use all possibilities of this analytical tool for polysaccharide research. 2. Experimental aspects of dielectric spectroscopy The dielectric spectra presented here were measured in the frequency range from 10 mHz to 2 MHz and in the temperature range of 2135 to 11808C using the Novocontrol Broadband Dielectric Spectrometer System BDS 4000 with the active sample cell BDC-S. Normally, the frequency range up to 1 GHz is routinely applied for experimental dynamic studies [13,29,32]. All samples measured in our laboratory were prepared in an identical manner. First, the material was dried at 110 or 1308C under vacuum for 20 h and then pressed under vacuum in a hydraulic press with a pressure of 1900 bar into thin sheets with a diameter of 30 mm and a thickness of 0.07±0.3 mm. These material sheets were transferred into the sample cell. Before inserting the capacitor into the dielectric spectrometer the sample cell was pressed again under vacuum with the pressure reduced to 500 bar to realise a good electrical contact with the gold electrodes and to ®ll the measuring cell volume completely. Samples prepared in this way and well dried have a water content less than 0.3% w/w. After inserting the sample in the measuring capacitor into the spectrometer, the sample was heated to 1308C with a dry nitrogen stream controlled by the thermostat system of the BDS-4000 to drive out residues of water adsorbed in the capacitor while ®lling it. Then, the measurement of the permittivity was started with the lowest temperature (in general, 21358C). For samples with a de®ned content of water or other swelling solvents, the drying temperature was reduced to reach a certain water concentration in the sample. Because of good cooperation, we have got many sources for different cellulosic materials. Several cellulose samples were delivered by Buckeye Cellulose Corp. in the form of prehydrolysed sulphate pulp (DP 2000), prehydrolysed wood pulp (DP 800) and linters (DP 1470, 2000, 7000). Other pulps used were sulphite pulps from Norwegian spruce from Borregaard. Regenerated cellulose ®bres (viscose, DP 250) and lyocell ®bres were provided by TITK Rudolstadt, Schwarza (Germany) and Lenzing AG (Austria). Lenzing AG (Austria) also kindly sent samples of their modal ®bres. Furthermore, we investigated commercially available cellulose powders from FLUKA (Avicel PH 101, cellulose microcrystalline) and from Carbomer, Inc. (high amorphous cellulose). Most of the persubstituted cellulose derivatives

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Fig. 3. Low temperature store coef®cient spectra of well-dried linters-pulp.

(cellulose acetate DS 2.45, hydroxypropylcellulose, hydroxyethylcellulose, cyanoethylcellulose) and other pure polysaccharides (xanthan, curdlan, pullulan, dextran, chitin, chitosan) were delivered by Carbomer, Inc. Special regioselectively substituted cellulose and starches derivatives were provided by the work group of Professor Klemm, Institute of Organic and Macromolecular Chemistry, University of Jena (Germany). In addition, we applied d(1)-glucose, d(1)-cellobiose and d(1)-gentiobiose delivered by Fluka and d(1)-cellopentaose by Sigma. All our measurement curves show a good reproducibility in the dynamic magnitudes and their overall con®dence interval in the real and imaginary part is lower than 5%. In general, the precision of the intensity of the relaxation (relaxation strength) is less good, especially for celluloses. On the one hand, it means that this parameter is very sensitive to slight differences of the sample preparation conditions (e.g. relative humidity of air, homogenity, anisotropy effect of the sample) and the porosity of the sample sheet. On the other hand, the measuring of the mean thickness of the sample has an uncertainty of 10%. The direct output of the dielectric spectrometer is the complex impedance Z p( f, T ) of the measuring capacitor ®lled with the sample represented in the form of the equivalent elements R( f, T ) and C( f, T ) connected parallely: …Zp … f ; T††21 ˆ R21 … f ; T† 1 jvC… f ; T† ˆ jwCo …e 0 … f ; T† 2 je 00 … f ; T†† ˆ A=d…s 0 … f ; T† 1 js 00 … f ; T††: The complex permittivity e *( f, T ) ˆ e 0 ( f, T ) 2 je 00 ( f, T ) or, equivalently, the complex conductivity s *( f, T ) ˆ s 0 ( f, T ) 1 js 00 ( f, T ) with the help of the value of the empty capacitance of the measuring capacitor Co ˆ e oA/d (area A and distance d of the plan parallel electrode system; e o, permittivity of the vacuum; imaginary unit, j 2 ˆ 21) results in the dielectric store coef®cient: e 0 … f; T† ˆ C… f; T†d=eo A; and the dielectric loss coef®cient: e 00 … f; T† ˆ R21 … f; T†d=eo vA: Fig. 3 shows the dielectric relaxation spectra of cellulose linters in the low temperature range in the form of the real part of the complex dielectric function e 0 … f; T† (dielectric store coef®cient) and Fig. 4

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Fig. 4. Low temperature loss coef®cient spectra of well-dried linters-pulp.

represents the imaginary part e 00 … f; T† (dielectric loss coef®cient) of the same cellulose to illustrate the primary experimental data which have to be interpreted by ®t evaluation. Figs. 5 and 6 show the experimental dielectric spectral results of well dried regenerative cellulose (viscose ®laments) in the high temperature range. For the high temperature case, or, in general, if the sample has a noticeable electrical conductivity, it is necessary to discuss the experimental results in connection with the conductivity spectra s *( f, T ) (exactly the real part of the complex conductivity s 0 ( f, T ) ˆ R 21( f, T )D/A). Fig. 7 represents these high temperature conductivity spectra of the same cellulose sample as in Figs. 5 and 6. Another common graphical representation of dielectric data is the Cole±Cole plot, which is a parametrical plot of e 00 (vt ) versus e 0 (vt ) (parameter vt ) giving a semicircle for an ideal Debye process. Fig. 8 shows the data for well dried cellulose linters pulp over the whole temperature range. The different relaxation processes, which can be identi®ed in the case of cellulose, will be later discussed in detail.

Fig. 5. High temperature dielectric store coef®cient spectra of well dried viscose ®bres.

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Fig. 6. High temperature dielectric loss coef®cient spectra of well dried viscose ®bres.

In many papers and other literature, the dielectric data are obtained by dielectric thermoanalysis (DETA). DETA spectra represent the experimental results in the form e 0 (T; fi) or e 00 (T; fi) versus the temperature (T ) for ®xed frequencies fi. Fig. 9 shows the DETA spectra for well dried cellulose linters. We prefer to present our data in the rest of this paper as isothermal function of frequency, because this form has advantages according to the quantitative evaluation (all spectroscopic parameters such as t (T ), De (T ), a (T ) and b (T ) are constants for one isothermal curve).

Fig. 7. Conductivity spectra of well-dried viscose ®bres at high temperatures. Window inserted: temperature dependence on the dc conductivity extracted from the s 0 (f, T) curves.

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Fig. 8. Cole±Cole presentation of the dielectric function of cellulose linters in the whole temperature range.

3. Theoretical aspects of dielectric spectroscopy 3.1. Phenomenological theory of dielectric relaxation The phenomenological or empirical theory of dielectric relaxation is presented in many books and review papers [2,5,16,23±28,32,163] in an excellent way. Therefore we can only repeat a few ®nal relations which are important for a physical understanding of the dielectric spectroscopic results. In electrodynamics, the phenomenological value of the complex permittivity e *exp( f, T ) contains the

Fig. 9. Dielectric thermo-analytic (DETA) spectra of well dried cellulose linters pulp.

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dielectric polarisation (e *( f, T )) and the conductivity response (s ( f, T )) of the material:

epexp … f ; T† ˆ ep … f ; T† 2 is… f ; T†=eo v:

…1†

This means that the permittivity measured tends to in®nity for low frequencies if the sample shows a noticeable electrical conductivity (especially in the case of high temperatures or wet samples). For the correct evaluation, we have to separate the conductivity effect from the e *exp( f, T ) data using Eq. (1) and s ( f, T ) values extracted from the s 0 ( f, T ) curves (Fig. 7Ðwindow). To understand the physical meaning of the complex dielectric material function e *( f, T ), we shortly discuss its relation to dielectrical polarisation by an acting external electrical ®eld. A small macroscopic sphere with the volume V in a polymeric material is electrically characterised by the dipole moment M(t 0 ) in the time-moment t 0 , which is stochastically ¯uctuating with time. The mean intensity of ¯uctuation is given by the time-autocorrelation function kM(t 0 )´M(t 0 1 t)l, which is independent of the absolute time moment and, therefore, a pure function of the time difference t (we set t 0 ˆ 0): kM…0†´M…t†l ˆ kM…0†´M…0l´F…t†:

…2†

After removal of an external electric ®eld, the mean square of the dipolar moment returns with time from a ®nite value kM(0)´M(0)l in the moment t ˆ 0 to zero at t ) 1 described by the decay function F (t). In the case of equilibrium, the dielectric decay function F (t) ˆ 0. The dielectric function e *( f, T ) is the Fourier transform (F{´´ ´}) of the time autocorrelation function of the macroscopic dipolar moment reduced to the intensity of the internal acting electrical ®eld:

ep … f ; T† ˆ es 1 …es 2 e1 †F { 2 dF…t†=dt}

…3†

(e s, static or low frequency dielectric constant; e 1, high frequency dielectric constant representing the spontaneous polarisability of the material). The dielectric spectroscopy also measures the decay of a macroscopic polarisation caused by orientational ¯uctuations, which are scaled by the mean square of the static dipolar moment kM(0)´ M(0)l and reduced to the intensity of the internal acting ®eld in the small volume inside the sample. The internal acting electrical ®eld is calculated from the external ®eld with the help of the Onsager ®eld factor: Eint ˆ Eextp(e *) [32,50]. In the simplest case, the decay function F (t) has an exponential form characterised by a material time t (T ), this case is often called Debye relaxation:

F…t† ˆ exp{ 2 t=t}:

…4†

This case means that the velocity of the reorientation is proportional to the deviation of the actual orientation from its equilibrium value (®rst-order kinetics). The representation of this ideal Debye-like relaxation in the Fourier space measured in our spectroscopic experiment is

ep … f ; T† ˆ es 1 …es 2 e1 †=…1 1 jvt†:

…5†

3.2. Parameterisations of the dielectric spectra using a relaxation model function In comparison to the ideal Debye-like relaxation behaviour, which is typical for small rigid dipoles in a viscous apolar liquid medium, real polymers are characterised by more complex and broader decay,

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which is often described by an asymmetrical relaxation function. A common approach to represent these dielectric relaxation processes is to express the real relaxation as superposition of continuously distributed Debye relaxations. In this case the result of the evaluation is a distribution function of the relaxation strength G(t ) in the t -scale. dG ˆ G(t )dt means the intensity of relaxation in the small relaxation time interval of t to (t 1 dt ). Another way is to use model relaxation functions in the frequency space (Fourier space) and to interpret the total polarisation as superposition of different discrete orientational processes: X ep … f ; t† ˆ e1 1 Depi …6† i

In this presentation, e 1 is the real part extrapolated to the high frequency end of our measuring window. A very common model for such individual relaxation processes is the relaxation model function of Havriliak and Negami (Refs. [8,9]). For the complex contribution of the ith relaxation process the HNexpression is: Depi ˆ Dei =‰1 1 …ivti †ai Šbi :

…7†

This general relaxation equation includes the special cases of the relaxation expressions of Debye, Cole± Cole and Davidson±Cole [5,16]. The application of this procedure allows the description of each relaxation process with four relaxation parameters. The relaxation time t i(T ) represents the central position of the process in the time scale and is approximately the inverse of the maximum frequency: t i(T ) < 1/(2pfmax,i) (exactly only if b i ˆ 1, that corresponds to the case of a symmetrical relaxation process). The relaxation strength De i(T ) means the step in the real part of the complex permittivity or, equivalently, the area below the loss peak e 00 (log f ) (compare Figs. 3 and 4). The a - and the b -parameters describe the shape of the relaxation process in the frequency mode and thereby the form of distribution of relaxation times. The a (T )-parameter characterises the width and b the asymmetry of the relaxation process in relation to the Debye process with a ˆ b ˆ 1. Broad distributions of the relaxation times are described by low a -values, and low b -values represent an asymmetrical distribution in the time scale and in the frequency scale, too. For better imagination (without interpretation here), Fig. 10 represents the HN parameters in their temperature dependence, which were obtained by nonlinear ®ts of the dielectric spectra given in Figs. 3 and 4 for the well-dried cellulose linters sample. The temperature dependence on the relaxation time t (T ) follows an Arrhenius behaviour characterised by a constant molar activation energy Ea (Fig. 11):

t…T† ˆ to exp{Ea =RT}:

…8†

In the thermodynamic framework of the Eyring rate theory [129], the orientation dynamics of dipoles is expressed in a similar form:

t…T† T=To ˆ tD exp{ 2 DS =R}exp{DH=RT}

…9† 213

with the so-called Debye relaxation time t D ˆ h/kBTo ˆ 1.7 £ 10 s. The deviation of the Arrhenius prefactor t o from the Debye time t D can be interpreted as an entropy effect DS ˆ R ln{ t o/t D} during the orientational polarisation. In literature, this entropy term DS is interpreted as cooperativity of an orientational motion within the dipolar groups [130,131] (compare Fig. 11).

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Fig. 10. HN ®t results for the dielectric spectra of the well-dried cellulose linters pulp in Figs. 3 and 4.

3.3. Molecular interpretation of the dielectric spectra of polysaccharides For the molecular interpretation of the phenomenological model, the physical meaning of Eq. (2) is shortly summarised. The macroscopic dipolar moment M of the small volume V is assumed as vector molecular groups with a permanent dipole moment and the sum of the dipolar moments mik of all PP possibility to move separately: M ˆ i imik (i, j, type of dipolar site in the repeating unit and k, l, index of the different repeating units in the same chain and in all other chains).

Fig. 11. Activation plot (Arrhenius plot: log t (T) versus 1/T) and Eyring plot (T/To log t (T) versus 1/T) of the temperature dependence on the dielectric relaxation time t .

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In Fig. 1 we suggest a site model of the repeating unit of cellulose, the anhydroglucose unit (AGU), wherein all relevant dipolar moments and all possible reorientation motions are labelled. There are three types of dipolar and movable sites: (i) the pyranose ring with a dipolar moment of approximately 0.7 D [132], which is movable by orientational motions around the glucosidic bonds; (ii) the side groups in the positions C2 and C3, with a rotation mobility around the C±O linkage; and (iii) the side group in the C6 position, which owns one mobility around the C5±C6 linkage and an additional rotation mobility around the C(6) ±O linkage. For pure polysaccharides, where all functional residues of the side groups are Ri ˆ H, this -C(5) ±C(6) ±OH- group often is called methylol-group (-C(6)H2 ±OH). In the case of derivatives, the substituents Ri change the dipolar moments of the side groups and the intra- and intermolecular interactions by hydrogen bonds, too. Branched polysaccharides such as dextran or amylopectine own further ¯exibilities because their side chains can produce additional relaxations. In the case of wet polysaccharides with a low water content, water molecules are bound to the hydrophilic groups of the AGU resulting in an increase of the dipolar moment of the side groups and affecting the polymer main chain dynamics. Furthermore, water can also produce an additional relaxation process [56] in addition to the pure polymer relaxation. The time correlation function in Eq. (2) is, therefore, transformed to kM…0†´M…t†l ˆ

XX i

k

‰kmi …0†´mi …t†l 1

X j±i

kmki …0†mkj …t†l 1

X l±k

kmki …0†mli …t†l 1

XX i±k j±i

kmki …0†´mlj …t†lŠ

…10† by the model in Fig. 1 and for kM(0)´M(0)l an analogous form exists with t ˆ 0. The macroscopic moment and the P complex permittivity function, respectively, are a sumPover all types of relaxating molecular sites ( i) and proportional to the number of repeating units (N ˆ k) in the volume V, supporting the expression in Eq. (6). The terms of the right side of Eq. (10) represent the molecular autocorrelation function of the dipolar site type i at all repeating units, the cross-correlation function of the different dipolar groups i ± j at the same repeating unit, the cross-correlation function of identical dipolar groups in neighbouring repeating units k ± l of the same and of other chains and the cross-correlation function of different dipolar groups i ± j at adjacent repeating units of the same and other chains k ± l, respectively. These cross-correlations produce a cooperativity of the different relaxation processes and are responsible for the scope of relaxation processes observed. In the case of polysaccharides, characterised by a complex H-bridge bond system within the same chain and between different chains, these cross-correlation terms play an especially role. In general, a quantitative molecular interpretation of the static polarisation kM(0)´M(0)l and the relaxation strength De i of the individual relaxations, respectively, is dif®cult in the case of polysaccharides because of their complex biopolymeric structure. In the simple model case of rigid dipoles without cross-correlation between the dipolar groups, the relaxation strength can be expressed as [5,16,28,32] Dei < ‰Ni …T†´m2i =TŠ´p…ep †´g;

…11†

where Ni(T ) is the number of dipoles of the type i per volume unit, mi is the mean effective

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Fig. 12. Principal structure of the dielectric loss spectra for all polysaccharides (arbitrary units). (a) Dry pure polysaccharides without side group relaxations. Window inserted: dry derivatives with side group processes distinguishable for the positions C2C3 and C6 at the repeating unit. (b) Wet polysaccharides with an additional bwet-process.

dipole moment of this polar group i and p(e *) is the Onsager screening factor to take into account the difference between the external and the internal electrical ®eld. In the case of polysaccharides, Ni(T ) describes only that number of dipolar groups which can participate in the orientation process, regarding the general opinion in literature that dipolar groups in crystalline regions are immovable. The structural Kirkwood factor g [5,16] in Eq. (11) means a function which describes the orientational effect in the space of the dipole moments and is strongly dependent on the real morphology of the sample under test. Therefore, Eq. (11) represents only a qualitative heuristic formula for De in the molecular interpretation of the dielectric results of this type of biopolymers. Nevertheless, it is useful for qualitative discussions. The activation energy (Eq. (8)) can be interpreted as the height of the potential barrier between two orientational conformation states, which can be reached during the molecular motion. The pre-exponential factor t o in Eq. (8) represents the inverse oscillation frequency of the molecular group in a conformational potential minimum and is related to the curvature of the potential curve in the minimum. Both parameters are affected by the type of glucosidic linkage in the main chain backbone, the local structural environment of the dipolar side groups and the water content of the sample. These facts are important for the application of the dielectric spectroscopy as analytical tool in the polysaccharide research. Because of the strong cross-correlation in the dynamics of the polysaccharides, it must be realised that the different modes of molecular motions are not independent of each other.

J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472

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Fig. 13. The Arrhenius plots of the dielectric relaxation times for all relaxation processes found in pure polysaccharides.

4. Experimental data and their interpretation 4.1. Assignment of the single secondary relaxations to their corresponding molecular motions Fig. 12 represents in a qualitative form the four types of molecular relaxation processes which we found in different polysaccharides in the form of a loss spectrum. Fig. 13 summarises the dielectric results for different substances in the form of the Arrhenius plots of the relaxation times. The different relaxation processes are denoted as g-, b-, d-, bwet-, and s-relaxation in this review with regard to their occurring in the spectrum beginning with high frequencies. A fundamental and, in literature, controversially discussed problem of the dielectric spectroscopy of cellulose and other polysaccharides is to assign the single relaxation processes found in the experiment to the corresponding orientational processes in the biopolymers on the molecular level. It is quite common in the dielectric spectroscopy of synthetic polymers to distinguish between the primary relaxation (or a-relaxation) and several secondary relaxations. The a-relaxation is related to the glass transition and observed in the temperature range above the glass-transition temperature Tg. The secondary processes observed here in the sub-Tg range are related either to the local main chain dynamic (b-relaxation), to side-group motions or to an orientational motion of associated groups (clusters) of molecules. The side-group processes are called g-relaxations and speci®ed with respect to the type of the substituent and its position in the AGU (e.g. g2(OH) for the OH side group relaxation in position C2). The main secondary relaxation found in all polysaccharides in the low temperature range (see Figs. 3 and 4) is associated with molecular orientations, but discussed in a controversial way in literature. It should be mentioned here that the results of dynamic mechanical investigations and dielectric results are interpreted in an analogue way followed by comparing the activation-energy values calculated, which were obtained by these different methods. In some cases, it seems to be quite hypothetical if the results of both methods are really comparable in a physically correct way. However, ®ve different assignments are

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Fig. 14. Dielectric loss spectra of monomer, dimer, oligomers and polymers of (1±4)b-glucanes at low temperatures.

proposed. (i) The low temperature relaxation represents the rotation mobility of the methylol side-group [46,50,81,83,97,100]. This model of the methylol group motion was ®rstly discussed experimentally by Mikhailov et al. [133] and supported theoretically by Zhabankov [134]. (ii) This relaxation is associated with side group motions, both methylol and hydroxyl groups in the AGU [52,53,82]. (These two interpretations lead to a g type for the main relaxation at low temperatures in our convention.) (iii) The low temperature relaxation is the local chain motion. Therefore ,it is a pure b-relaxation in the general language of the polymer dynamics [72,91,101,114,135]. (iv) This relaxation is related to the orientation of bound water molecules in the sample [77,150]; and (v) this low temperature process is a boat±chair interconversion of the pyranose ring. The latter possibility has been ruled out by energetic reasons in the solid phase, since a boat±chair inversion requires both a high activation energy and the cooperative motion of several glucose units [50,135]. If the low temperature relaxation is associated to a side-group motion, this motion should also be observed in the monomers of the polysaccharides. In Ref. [79] we have investigated this problem in detail (see Fig. 14). The relaxation observed in glucose (monomer of cellulose) shows a Debye-like behaviour and its position differs clearly from the polymer-dynamic peak. We calculated an activation energy of 19 kJ/mol for the signal described. From the shape and the high mobility of this process, we must conclude that this relaxation belongs to the motion of the methylol or the small hydroxyl side groups. In the dimer compound cellobiose, which contains one glucosidic linkage, a small relaxation is observed at high frequencies (with EA ˆ 19.2 kJ/mol) beside another relaxation (EA ˆ 32.8 kJ/mol), which is very similar in its shape to those found in cellulose and other polysaccharides. Obviously, the broader peak must be assigned to small amplitude oscillations of the sugar rings via the glucosidic linkage. The side-group motion in the cellobiose spectra is broadened intensively by cross-correlations with the main-chain motion. The methylol peak is also found in b-cyclodextrine apart from the motion via the glucosidic bound. The spectra measured for cellopentaose show only one broad relaxation process (EA ˆ 51.5 kJ/mol) with an intensity about 10 times higher than the signals of cellobiose. We assume that the small peak belonging to the side groups is totally overlapped here by the intensive local chain dynamic mobility or outside of our frequency range. So, we have deduced that, in general, the low

J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472

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Fig. 15. DP dependence on the activation energy of the b-relaxation for (1±4)b-linked saccharides.

temperature relaxation in pure polysaccharides represents a local main chain motion and is, therefore, a b-type relaxation. It is interesting that the activation energy for this local backbone motion of the polymer in the solid state shows a dependence on the chain length measured as degree of polymerisation (DP). This correlation has a maximum at around DP ˆ 7±12 as shown in Fig. 15 [136]. The relation of the activation energies: dimer , polymer , pentamer can be a hint that, on average, more than two, but fewer than ®ve, repeating units take part in the local chain mobility. This would correspond to Montes et al., who proposed an attendance of four glucosidic linkages at the chain mobility as a result of computer simulations [83]. Further evidence for the validity of our interpretation is that in dextran, which owns no methylol sidegroups, this b-relaxation mode was also found, but the b-process of dextran shows a distinct frequency position and shape of the loss peak (see Section 4.3). We interpret this result in such a way that the chain mobility in the case of a (1 ! 4) glucosidic bonded backbone (-O-bridge) is different to the faster motion of a (1 ! 6) bonded system of pyranose rings(-O±CH2-bridge). Investigations with cellulose and starch derivatives, which are regioselectively substituted in position C6 and, therefore, do not contain methylol groups, have shown that this type of secondary b-relaxation also exists in these materials, additionally to the side group relaxation of the substituent in position C6 in the AGU. We present more experimental evidence for this association of the low temperature relaxation to the local chain motion in the following section. We will call d-relaxation the relaxation observed in many well dried polysaccharidic materials in the low frequency side of the b-relaxation. The molecular origin of this process has not been made clear up to now. In some cases, there is a hint that this relaxation is associated to end groups and it is especially observed for branched polymers such as amylopectine or dextran and for polymers with short sidechains such as xanthan. A twist motion of side chains around the main chain is well known for SCLC polymers [137,138] (see also Section 4.3). From recent measurements of cellulose samples with step-wise reduced water contents (from 13 to 0% w/w), it can be deduced that the d-relaxation is only the bwet-relaxation shifted, if we assume that in cellulose heated to 1208C and more residues of water also exist.

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Fig. 16. Correlation between the relaxation time of the s-relaxation and the dc conductivity for various polysaccharides.

This process measured in the room temperature range is always observed in wet samples and, therefore, called bwet-relaxation (see Section 4.3) as proposed by Crofton and Pethrick [52,53]. For its interpretation, we favour a model that this process represents an orientational motion of a mixed phase of both polysaccharide and water (or other swelling solvents), which is formed in wet systems by a swelling process. In ionic conducting systems, a relaxation process is discussed, which is associated with the electrical conductivity of the water in the sample. Its relation to the bwet-process is an open question at present. The s-process found at high temperatures presents a proton migration process occurring in all amorphous solid systems and also in polysaccharides [139] and other polymers. The problem of charge carrier hopping between localised sites in amorphous solids was theoretically treated by Mott [140], Pike [141] for instance, summarised by Jonsher [142] and investigated by dielectric relaxation measurements by several work groups for various substances [143±145]. The main argument for this interpretation is the strong correlation between the dielectric relaxation time of the s-relaxation and the dc conductivity measured for all samples (Fig. 16). The values of the activation energies for the dielectric s-relaxation and the ion migration process in the polymer matrix determined by the electrical conductivity are the same within the experimental accuracy for all polysaccharides investigated. This relaxation represents no special effect only for biopolymers or synthetic polymers and is, therefore, not be discussed further in this review. 4.2. Native celluloses, pulps and regenerative celluloses It is known that the physico-chemical properties of cellulose are mainly dependent on the amorphous regions of cellulose. The amorphous (or non-crystalline) content of different natural celluloses differs between ca. 30% (bleached cotton linters) and 46% (soft wood pulp) and even increases for regenerated celluloses: 66% for viscose rayon [146]. Moreover, cellulose occurs in different molecular lengths equal

J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472

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Fig. 17. Arrhenius plots (log t (T) versus 1/T) of the low temperature relaxation (b-relaxation) in different pure cellulosic materials.

to 1000 up to 15,000, depending on its origin and the extent of possible degradation during its isolation [41,147]. Native cellulose crystallises monoclinic spheroidic and is called cellulose I. Furthermore, there are at least ®ve other modi®cations of cellulose [148]. In addition, the preparation treatments of the cellulose from the native to the ®nal product under investigation play a dominant role for the physical properties (see Section 1). As a consequence, from the practical point of view, the following question played an important role in the research: Are there differences in the dielectric spectra of several native celluloses, different types of pulps and different regenerative celluloses (viscose ®laments)? What in¯uences do the different morphologies of these materials produced by very distinct technological processes have on their polymer dynamics detected with the dielectric spectroscopy? The result found by our investigations and in literature of many pure and well dried celluloses is that all dielectric b-spectra have nearly the same position in the frequency scale, the same shape and the same activation energy as shown in Fig. 10 for cellulose linters (see also Figs. 17 and 18). The spectra shown in Figs. 3±9 are, therefore, representative for all pure cellulose samples. Figs. 19 and 20 compare with its dynamic chain properties, native celluloses (bacterial cellulose and cotton Linters) and various cellulose pulps (sulphite and sulphate pulps from different origins) all with a cellulose I crystalline structure and secondly various regenerative celluloses (normal viscose, modal ®bres and lyocell ®bres) all with a cellulose II crystalline structure. Other pure polysaccharides are also integrated into these ®gures. The results for this morphologic different celluloses are summarised by representing the activation energy Ea (Fig. 19) and the HN-relaxation time for 1/T ˆ 0: t o (preexponential factor of the Arrhenius Eq. (8)) (Fig. 20). While, in general, the morphology of cellulose has a big in¯uence on its physical properties, the morphologic effect on the secondary polymer dynamics of well dried cellulosic material is not important. This experimental result is not surprising from the point of view that the dielectric structure of the

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Fig. 18. The HN a (T) shape parameter of the b-relaxation for various cellulosic materials and its temperature dependence (a (T) means the width of the relaxation process in the frequency scale).

Fig. 19. Comparison of the activation energy (in kJ/mol) for the b-relaxation in various cellulosic materials and other pure polysaccharides. Cellulosic pulps: 1. sulphate pulp (Buckeye); 2. sulphite pulp (Filtrac); 3. sulphite pulp (Borregaard); 4. sulphite P (Gruber A, 558C dried); 5. sulphite P. (Gruber B, 1108C dried); 6. sulphite P. (Gruber C, machine dried); 7 tree sulphite P. (Lenzing 3); 8. tree sulphite P. (Lenzing 13); 9. tree sulphite P. (Lenzing 23); 10. tree sulphite P. (Lenzing 33); 11. tree sulphite P. (Lenzing 43); 12. tree sulphite P. (Lenzing 53); 13. tree sulphite P. (Lenzing 63); 14. tree sulphite P. (Lenzing 73); 15. tree sulphite P. (Lenzing 83). NaOH swelled pulps (Lenzing): 16. tree sulphites P. (0%NaOH); 17. tree sulphite P. (7%NaOH); 18. tree sulphite P. (13%NaOH); 19. tree sulphite P. (18%NaOH). Cellulosic ®bres: 20. lyocell (TITK); 21. lyocell (Lenzing); 22. modal viscose (Lenzing); 23. Regular Viscose Lenzing); 24. Viscose 250 (Schwarza). Cellulose powder: 25. CarboMer m100; 26. CarboMer m50; 27. CarboMer m35. Microcrystalline cellulose: 28. avicel (Fluka). Native cellulose: 29. bact. cell. (aceto bact. xylin.); 30. linters. Starches: 31. starch, degra. (Merck); 32. starch (95% Amyl./Serva); 33. starch (AMS 70% Amyl.); 34. starch (WMS ˆ amylopectine). Other polysaccharides: 35. xanathan; 36. curtlan; 37. pullulan; 38. dextran.

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Fig. 20. Comparison of the pre-exponential factor (in 10 216 s) in the form of the Arrhenius presentation of the relaxation time t (T) for the b-relaxation in various cellulosic materials and other pure polysaccharides (the same substances as in Fig. 19).

AGU and the glucosidic linkage in all these cellulose types are the same. This also means that after an intensive drying treatment of the cellulose sample, the great differences in the crystalline structure of native and regenerated cellulose, the differences in the degree of crystallisation and polymerisation and the differences in sub-molecular structures between all cellulosic materials investigated, have no in¯uence on the local chain or segmental dynamic. Therefore, we can show that the great differences observed in many macroscopic material parameters of these cellulose samples do not have their origin in the length scale of few AGUs or the dimensions of the crystal elementary cell. These ®ndings restrict the analytical application of DRS for morphologic investigations in well dried celluloses (see also Sections 4.3 and 4.4). Great differences were found for the relaxation strength De between the different cellulosic samples. However, this measuring parameter seems to be not well reproducible and its quantitative interpretation provides many theoretical problems at present (see Section 3.2). Table 1 illustrates that our experimental results for dry cellulose are in good correspondence with literature results found for the dielectric relaxation energy Ea (measured in kJ/mol). Note that these values are evaluated on the basis of different molecular interpretations (in the table a means the relaxation is interpreted as a methylol side-group motion and b means that the authors have interpreted the low temperature relaxation as a local chain motion.) and on the basis of different types and levels of experiments. Another experimental fact seems to be important in the discussion of dielectric low temperature spectra of celluloses: In the low frequency side of the b-main peak in all cellulose samples measured, a little structured undergroundÐor d-processÐis observed. This part of the loss spectrum obviously presents a slow molecular motion of dipolar sites or cooperative clusters in very disordered states. In the following section we will discuss this information in detail, because this spectral information seems to correlate with other physico-chemical properties of the cellulose system (see Sections 4.3 and 4.4) 4.3. Cellulose in comparison with other polysaccharides The primary difference in the chemical structure between the distinct polyglucanes is the type of the

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Table 1 Dynamical HN-parameter of the secondary relaxation (b-process) of different well dried polysaccharides (error speci®cations are estimated considering the accidental scattering of the measuring data and systematical uncertainty from the HN evaluation) Substance

Ea (kJ/mol) t o (10 216 )s a (2608C) De ^ 1.5% ^ 10% ^ 1.5% (2608C) ^ 15%

Linters pulp Bacterial cellulose Avicell Sulphate pulp Sulphite pulp Normal viscose ®laments Modal viscose ®laments Lyocell ®laments Viscose (Schwarza 250) Amorph. cellulose powder

45.4 45.1

8.6 8.8

0.35 0.35

0.87 0.17

50.0

Montes and Cavaille [82] a

48.0 45.0 45.6 45.4

1.0 4.2 0.9 8.0

0.33 0.32 0.35 0.33

0.32 0.91 0.86 0.48

46.0

Crofton and Pethrick [53] a

28.2 31.6

Saad and Furuhata [97] a Saad et al. [98] a

45.7

11.0

0.335

0.41

40.8

Schartel et al. [149]

46.5

8.0

0.345

0.57

46.9

4.1

0.33

0.42

45.9

4.6

0.33

0.76

51.0

3.1

0.29

1.00

62.8

Kakisaki and Hideshima [72] b

50.4

1.5

0.34

0.87

39.8

Nishinari and Fukada [73]

53.2

0.75

0.97

62.8

45.8

6.6

0.30

0.16

Kakisaki and Hideshima [72] b Butler and Cameron [135] b

15 16

AMS (70% amylose: Hylon VII) WMS (100% amylopectin: amioca powder) Amylose (95% amylose: Serva) Merck starch (degraded) Curdlan Dextran

52.3 38.6

0.23 4.7

0.32 0.28

0.74 0.38

17 18 19

Pullulan 50.6 Xanthan 36.2 b-Cyclodextrine 47.9

0.32 0.41 0.33

0.74 0.12 0.23

No.

Celluloses 1 2 3 4 5 6 7 8 9 10 Starches 11 12 13 14

a b

6.3 14.1 5.6

0,35

Literature Reference Ea (kJ/mol)

50±65 32 33.5

Montes and Cavaille [82] a Kakisaki et al. [72] b

This relaxation is interpreted as a methylol side-group motion. The authors have interpreted the low temperature relaxation as a local chain motion.

glucosidic linkage between the pyranose rings in the polymer chain. Cellulose forms a (1±4)(b-bonded semi¯exible linear chain. Starches are blends of the helical amylose with a (1±4)a linkage between the AGUs and the branched amylopectine with (1±4)a-linkage, too, containing about 5% of (1±6)-linked branch points. These side chains are linear and two main populations are observed: short chains with DP~15; and long side chains with DP~45 [149]. The chains form a helical structure. Curdlan is

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Fig. 21. Types of the glycosidic linkage (chemical or primary structure) in different polysaccharide chains: dextrane ((1±6)a(poly)glucane), cellulose ((1±4)b-(poly)glucane), chitosan (2-O-derivative of cellulose); curdlan ((1±3)b-(poly)glucane) and starch ((1±4)a-(poly)glucane).

a (1±3)b-linked homogeneous polyglucane chain. Dextran is a (1±6)a-linked high molecular polyglucan with 5% branching. The dextran main chain is, thus, linked via an -O±CH2- bridge, while in all other polyglucanes discussed a pure glucosidic linkage (-O- structure) exists (see Fig. 21). Pullulan has a block structure with three cellulose-like (1±4)b-linked pyranose rings which are (1±6)a-connected in the polymer chain. Xanthan is an anionic polysaccharide from Xantomonas camprestis with (1±4)-b-d-glucose backbone and a trisaccharide in the side group consisting of two mannose residues and a glucuronic acid residue [41]. The local main chain motion detected within the low temperature spectra is similar for all of these polysaccharides in a well dried state as shown in Figs. 22±24. We found an intensive relaxation in dextran, as did Montes and Cavaille [83], and it is not clear why

Fig. 22. Comparison of the local main chain motion (b-relaxation) for well-dried polysaccharides with different types of the glycosidic linkage: dielectric loss spectra.

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Fig. 23. Comparison of the local main chain motion (b-relaxation) for various well dried polysaccharides with different types of the glycosidic linkage: Arrhenius plot for the relaxation time of the b-relaxation.

Bradley and Carr [50] did not ®nd any low temperature relaxation for dry dextran in their dynamical mechanical experiments. However, the main chain dynamic in the -O±CH2- linked dextran is noticeably different from other -O-linked polysaccharides: Fig. 22 compares the dielectric loss spectra of various pure polysaccharides at the temperature of 2 758C. Fig. 23 shows the temperature dependence of the relaxation times determined by an HN ®t procedure and Fig. 24 shows the shape parameters a (relaxation width) of the b-relaxation of different pure and well dried biopolymers. The activation energies Ea and the prefactors t o, both calculated with the help of the Arrhenius Eq. (8), are summarised in Table 1. (Compare the presentations in Figs. 19 and 20.) It does suggest that the stereochemical con®guration of

Fig. 24. Comparison of the local main chain motion (b-relaxation) for various well-dried polysaccharides with different types of the glycosidic linkage: HN-width parameter a (T).

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Fig. 25. The d-relaxation in various well dried polysaccharides: dielectric loss spectra of celluloses, starches, dextran and pullulan at 2158C.

the glucosidic linkage ((1±4)b, (1±4)a; (1±3)b or (1±6)a) has an in¯uence on the polymer dynamics. The local chain motion of starches is slightly shifted to lower frequencies in relation to celluloses. The b-relaxation process in dextran occurs at higher frequencies caused by the higher ¯exibility of the 1±6 linkage (a -CH2 ±O- bridge between the repeating units). In accordance with that, the activation energy of dextran is lower compared with cellulose. The curdlan motion is more cooperative (t o @ t D), but its activation energy is higher than for cellulose. Pullulan has a similar activation energy to curdlan, but its pre-exponential factor is similar to the values for cellulose and dextran. As can be seen from Figs. 19 and 20, this general characteristic is markedly greater than the variations caused by the preparation conditions of the cellulose samples. In summary, the type of linkage in the polysaccharide chain produces characteristic differences in the local chain motion detected with dielectric spectroscopy. The differences in the local polymer dynamics of different types of starches (Figs. 23 and 24): amylomaize starch (HylonVII) with 70% amylose, waxy-maize starch (100% amylopectine), Serva starch with 95% amylose or Merck starch, a product decomposed in the chains rami®ed are insigni®cant. This must be interpreted in such a way that the local chain motion in the ®rst order is in¯uenced by the local primary or chemical structure of the polymer chain and that morphologic effects (supramolecular structures) play a secondary role for the local dynamic properties. In Fig. 25, the dielectric spectra for different polysaccharides are compared. All samples were carefully dried under vacuum at 908C and were also in a ªdryº state. In substances with side chains (xanthan) or with chains rami®ed (dextran (5%), amylopectine) the spectra show an additional slow relaxation process (named d-relaxation). In cellopentaose and cellobiose we have found this process, too. In the case of pullulan, this relaxation could be interpreted as motion of the cellulose-like chain segments around the (1±6)-linkage in the polymer chain, but this interpretation is not proved at present. This relaxation in the middle temperature range has been observed by other writers, too, but they have often assigned this process to a glass transition of bound water in the biopolymer [149]. In contrast to this

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Fig. 26. The low frequency side of the b-process in the dielectric loss spectra (spectral region of the d-process in polysaccharides) of various well dried cellulosic materials.

explanation, we have not found a hint of a WLF behaviour of any relaxation process in all our investigations, which is in accordance with ®ndings of Bizot et al. [150] in the glass transition of polysaccharides. For all celluloses in the low frequency region, only a less structured loss underground is found (Fig. 26) and it is likely that it corresponds with the d-process in the other polysaccharides. In Section 4.4 we discuss the in¯uence of the drying treatment for cellulose pulps on this spectral region. 4.4. Effect of low water content in polysaccharides on the molecular polymer dynamics Cellulose is a hydrophilic, hence, water-insoluble, polysaccharide. In general, three forms of water adsorbed on cellulosic materials are distinguished: non-freezing bound water; freezing bound water (producing an own glass transition); and free (or volume-like) water [151±153]. The physical and chemical properties of cellulose and other polysaccharides can be changed drastically by adding only small amounts of water [154±156]. The increase of the electrical conductivity of the sample is the major effect, which superposes the dielectric processes in the loss spectra and demands a conductivity correction of the dielectric loss spectra (see Eq. (1)). Therefore, the dc conductivity (s o) must simultaneously be determined from the conductivity spectra (log s 0 versus f ) (especially in the case of dielectric measurements of wet polysaccharides) by ®tting the low frequency range with the help of Eq. (12)

s 0 … f † ˆ so ´f n :

…12†

The problem is that the frequency dependence of the conductivity is also masked by the dielectric relaxation effects and also by the electrode polarisation (leading to a Debye-like relaxation) in the very low frequency range. Because of these electrode processes, the dc conductivity must be determined by extrapolation to in®nity in the frequency range (!) (see also Refs. [3,4,164]). Fig. 27 shows the effect of the water content on the dc conductivity in the form of the activation plot in an experiment with a temperature sweep. On the one hand, the step-by-step increase of the temperature

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Fig. 27. Temperature dependence of the dc conductivity for cellulose (never dried linters sulphate pulp: Buckeye) with different water contents in a temperature sweep experiment. The high temperature values present a `dry' sample, residues of water are in a strongly bonded state. Window: dc conductivity for starch (amylo-maize starch: Hylon VII) measured in a similar experiment.

drives out the water at temperatures higher than 508C. On the other hand, the log s o(T )-curves show a gradient depending on the water content in the sample. This means that the activation energy for the ion migration increases when reducing the amount of water in the sample. Compare the curves in the middle temperature range (150 to 11208C) for wet samples in Fig. 27. Note: these data do not exactly present equilibrium states, because the time between different frequency points during the measurement is too short for driving out all surplus water from the material in the measuring capacitor. Nevertheless, the transition from the wet state to the well dried state is clearly noticeable at temperatures over 1008C. In addition, the fact that the high temperature conductivity for wet samples lie on the same straight line as the points for samples dried under vacuum conditions con®rms the conclusion that both data re¯ect the same state of the polymer containing only very low residues of strongly bound water. The wetness in¯uence of the sample on its dielectric properties can be interpreted by different arguments: On the one hand, the hydrophilic groups at the AGU are solvated and these bond water molecules increase the dipolar moment and the moment of inertia of the movable groups. Water molecules bonded additionally produce a bridge parallel to the glucosidic linkage along the chain and between two adjacent chains, which increases the stiffness of the polymer chain. On the other hand, the macroscopic ¯exibility of the cellulosic ®brous material strongly increases by the swelling power of water. The `dielectrically active' phase of the material increases during the swelling process. The dielectric spectroscopy principally can investigate the changes within the molecular dynamics of polysaccharides caused by low water contents [1,8,53,56,162]. Unfortunately, this method is limited to water contents lower than 12±15 wt%, because water increases the electrical conductivity in a drastic way and masks the real dielectric effects. Fig. 28 shows the dielectric spectra of never dried cellulose linters sulphate pulp with 3.4 wt% water content in the form of the Cole±Cole plot. In comparison with the well dried samples (Figs. 3, 4 and 8), the low-frequency part of the spectra (b-relaxation) is almost similar, but in the room temperature range

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Fig. 28. Dielectric properties of never dried cellulose linters sulphate pulp in the form of the Cole±Cole diagram (e 00 (f) versus e 0 (f)).

a new type of relaxation is observed, called bwet-relaxation. This relaxation is absent in dry samples and must be distinguished from the s(-relaxation observed in the high temperature range (Fig. 8). Fig. 29 shows the low temperature spectra of never dried, well dried and rewetted cellulose pulps. A slight increase of the frequency position of the loss maximum ( fmax) for the b process can by observed for the wet samples in comparison with the dry samples. Additionally, a drastic increase of the losses in the low frequency side of the b peak is measured (compare with Figs. 25 and 26). Fig. 30 presents the effect of water content on the HN-relaxation parameter for the same samples. These ®ndings are typical for all cellulose samples investigated. The activation energy of the local main chain dynamics increases to values between 52.5 and 54.1 kJ/mol in comparison to dry celluloses with values of (45.0 ^ 2.5) kJ/mol (see Fig. 19 and Table 1). In contrast, the pre-exponential factor t o is

Fig. 29. Dielectric loss spectra of never dried, well dried and rewetted cellulose pulps at the temperature of 2908C.

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Fig. 30. The dielectric b process in wet cellulose pulps. Effect of the water content on the HN-relaxation parameters t (T), De (T) and a (T).

reduced (from 10 216 to 10 215) s for dry celluloses to values between (0.2 and 10) £ 10 219 s for wet celluloses. This points out that the chain motion detected in the main b-peak is more cooperative in wet celluloses than in dry cellulose (see Eq. (9)) caused by the swelling power of water and the forming of water bridges between polysaccharide chains. Measurements in wet and dry starch and dextran samples show a similar characteristic [56]. Note that the b-loss peak can shift by water addition to higher (e.g. cellulose, starch) or lower (e.g. dextran) frequencies. This means that the frequency position of the relaxation peak obviously is a superposition of two contradictory effects: the increase of the activation energy Ea (energetic effect) and the decrease of its limiting value t o (entropic effect) by water sorbtion by the polysaccharide sample. Although there are some problems with a good reproducibility of the relaxation strength De for different samples, it is evident that the relaxation strength De of the b-relaxation for all polysaccharides is more intensive for the wet samples. We assume, on the one hand,

Fig. 31. The bwet-relaxation in the dielectric loss spectra for wet cellulose (never dried linters sulphate pulp with DP ˆ 1400: Buckeye) for different water contents at 358C.

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Fig. 32. The bwet-relaxation in the dielectric loss spectra for wet cellulose powder (amorphous cellulose powder, 100 mm: CarboMer) for different temperatures with 4% w/w water content.

that more dipoles can take part at the reorientation motions in the polymer structure after swelling, because the sterical hindrance of some rotation motions decreases and, on theother hand, that water molecules bonded on the repeating units increase its dipolar moment. Despite the swelling effect of water, it is reported [157] that the higher ordered structures of biopolymers are more stable in the wet state. So, particularly the increase of Ea could also correspond to this general stabilisation effect. The bwet-loss peak in the room temperature range (0±608C) can be observed in all wet polysaccharides investigated (Figs. 31 and 32). This dielectric effect is markedly masked by the Maxwell±Wagner± Sillars relaxation [1±3,158,162±164] of the internal interfaces and in many cases this bwet relaxation appears only as a little shoulder in the spectra after the conductivity correction.

Fig. 33. The effect of ammonia as swelling solvents on the bwet-relaxation for a cellulose sulphate pulp: Low temperature dielectric loss spectra of well dried and then NH3-activated pulps.

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Fig. 34. The bwet-relaxation in cellulose: The effect of the swelling solvent content (water and ammonia) on the intensity of the bwet relaxation.

The activation energy of this bwet-relaxation has values between 63 and 70 kJ/mol. The pre-exponential factor t o (10 210 ±10 212 s) and the shape parameter (a ˆ 0.7±1.0) of this relaxation characterise the motion as a more Debye-like process. Measurements in ammonia activated cellulose pulps have shown a similar bwet-relaxation process. The NH3 in¯uence on the low temperature b-relaxation is analogous to that of water (see Figs. 33 and 34) [52,56]. Wet dextran samples (Fig. 35) or wet starches show this bwet process, too. It seems that the bwetrelaxation is a general phenomenon in solvent-swollen polysaccharides. The molecular origin of this bwet process is not clear at present. A few characteristics point out that this relaxation could be a motion of the swollen polymer±solvent mixing region. The relaxation strength De bwet increases and the peak position shifts to higher frequencies with increasing water content (see Figs. 32 and 35). This dielectric loss process disappears after drying the sample. Several papers [97±99] deal with wet polysaccharides samples. Montes and co-workers [81,82] found

Fig. 35. The bwet-relaxation of wet dextran (DP ˆ 150,000): effect of the temperature on the dielectric loss spectra at 8% w/w water content. Upper window: comparison of wet and well dried dextran spectra at 408C.

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a relaxation in wet cellulose with an activation energy from 60 to 85 kJ/mol which corresponds with our values for the bwet-relaxation. McBriety et al. [50] and Scandola ans Ceccorulli [100] discussed the water effect in cellulose samples in a similar way. Crofton and Pethrick [52,53] pointed out that the effect of water on the activation energy is qualitatively different for low water contents and for higher contents. A special relaxation process associated with the reorientation of single bond water molecules in the biopolymers is only expected at higher frequencies ( f . 100 MHz) [17,19,154] and we did not observe it in the low frequency range ( f , 10 MHz) reviewed in this paper. Therefore, we will not explain these dynamical processes of bound water in more detail. 4.5. Relation between the chemical accessibility of cellulose pulps and dielectric spectra The molecular interpretation of dielectric spectra is one aspect in the dielectric spectroscopy, the other one is to ®nd relations between the polymer motion detected with DRS and physical and chemical properties of cellulose. A central problem in the chemical cellulose technology is the enhancement of the chemical reactivity or chemical accessibility of pulps for derivatisation reactions. We have correlated the dielectric relaxation parameters with the reaction velocity of sulphite pulp, which could be increased by a special careful drying procedure of never-dried pulp [57]. The acetylation velocity determined by measuring the degree of acetylation DSAc reached after 10 min correlates in an excellent manner with the relaxation strength in the low-frequency side of the b-loss peak (Fig. 36 and insert windows of Fig. 37). The relaxation strength of the b peak correlates with the acetylation velocity in the inverse direction as expected (Fig. 37). Annealing of the specially dried samples at 1708C reduced the relaxation intensity in the low frequency range and increased the intensity of the b-relaxation in the dielectric spectra (Fig. 38).

Fig. 36. Comparing the dielectric relaxation intensity from the b-process for chemical pulps (sulphite pulps from Borregaard) prepared by different drying treatments: sample A is carefully dried at 958C under laboratory conditions, sample B is dried at 1558C and sample C is a machine dried sample (open symbols). Full dots are the results for the same samples after annealing at 11708C. Window: the dielectric loss spectra for this three pulps dried at different conditions in comparison at the temperature of 2758C

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Fig. 37. Correlation between the intensity of the b- and d-relaxation and the acetylation velocity determined by the degree of acetylation reached after 10 min in the derivatisation reaction at 458C. Windows inserted: the relative intensity of the loss peaks (Eq. (13)). De rel versus the acetylation velocity.

These differences can be interpreted as the morphologic effect, which is related with the chemical reactivity. Only the relative changes of the intensities in the two different spectral regions Derel ˆ ‰…De…after anneal:† 2 De…before anneal:†Š=De…after anneal:†

…13†

correlate in a manner expected with the reactivity (window inserted into Fig. 37). Hence, the disordered

Fig. 38. Annealing effect on the dielectric loss spectra for a chemical pulp specially dried (the sample A from Fig. 36)

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Fig. 39. Relation between the dielectric relaxation strength and the water retention capacity (WRV) for the same chemical pulps discussed in Figs. 35 and 36. Windows inserted: correlation between the relative intensity changes after annealing the sample and the WRV.

underground in the low frequency side of the low temperature spectra (Figs. 4 and 26) represents the motion of chain segments with a loosed disordered environment, which is very sensitive to morphologic changes and swelling solvent contents. These morphologic regions seem to contain the starting points for a chemical reaction. In the same investigation, it is found that the carboxymethylation velocity does not correlate with the dielectric spectral parameters. This can possibly be explained in such a way that the carboxymethylation reaction is carried out in a basic medium, which autoproduces a swelling of the cellulose. However, the acetylation reaction is carried out in an acidic medium with a lower swelling power. The water-retention capacity (expressed by the water-retention value: WRV) of these cellulosic pulps investigated also shows a good correlation with the disordered underground relaxation (Fig. 39) and, again, only the relative intensities show the expected positive trends with the water-retention power. Similar results for the polymeric dynamics are found in the dielectric studies of chemical pulps soda lye treated. The sensitive magnitude here is the difference between the parameters of the wet and the dry sample. Fig. 40 presents the relative change of the activation energy and the pre-exponential factor for the local chain motion with the concentration of the soda lye used for the activation of the sulphite pulps. All these experimental ®ndings support that it is necessary for morphologic investigations with the help of the DRS to use a swelling medium (for instance water) as a probe and then to measure relatively to the pure sample unaffected the response of these swollen or activated substances. 4.6. Cellulose derivatives 4.6.1. Derivatives with statistical substitution pattern on the AGU Substitution of hydroxyl groups at the AGU causes: (i) a variation of the dipolar moment of the side

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Fig. 40. Activation effect of soda lye with different concentrations on the dielectric spectra: Relative activation energy ([Ea(wet)/Ea(dry)]/Ea(dry)) and the relative pre-exponential factor (log{t a(wet)/t a(dry)}) for sulphite pulps as function of the activating NaOH concentration (window inserted).

groups and the whole pyranose ring (electron drag); (ii) a changing of the volume and the mass of the side groups and, thereby, results in a change of the moment of inertia and an increase of the spatial hindrance for reorientation; and (iii) a destruction of the hydrogen bridge system in the polysaccharide. All these processes can shift the relaxation times, the shape factors and the relaxation intensity of the molecular orientation motions. In general, the intensity of the main relaxation at low temperatures decreases by derivatisation as Figs. 41±43 show, an exception is the cyanoethyl cellulose caused by the great dipolar moment of the cyano group in comparison with the moments of a hydroxyl or other side groups [38,96,104]. Especially the large thexyl dimetylsilyl side-group (-Si(CH3)2 ±C(CH3)2 ±CH(CH3)2) used as a protective group in the synthesis chemistry of polysaccharides [158]) with its low dipolar moment and great

Fig. 41. Dielectric loss spectra of different cellulose derivatives at the temperature of 2758C in comparison: The dielectric intensity decreases from cyanoethyl to pure and to 6-O-thexyl dimethylsilyl cellulose.

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Fig. 42. Dielectric loss spectra at 2608C of cellulose formiate with DS ˆ 0.4 (®lm). The two peaks are associated with the relaxations of the -O±formiate side group and the main chain, respectively. Window inserted: activation plots of the chain (bprocess) and the side group relaxation (g-process).

volume reduces the dielectric effect. We assume that any kind of derivatisation produces a higher entangling of the macromolecules because of the missing H-bridge spacers between the chains and destroys the orientation effect by the hydrogen bonds between the polymer chains. All these factors cause a reduction in the mean value of the dipolar moment (vector sum of all dipolar sites) in the volume unit. Besides, a statistical substitution pattern along the chain results in a number of different substituted chain segments and leads to a broader distribution of the relaxation times. The reduction of H-bridge linkages along the chain increases the number of the dynamic chain modes which acts in the same direction as the inhomogeneous substitution pattern. Therefore, in most cases, a lower a -parameter for the b- or local main chain relaxation of cellulose substituted has been determined.

Fig. 43. Dielectric loss spectra of hydroxyethyl (left side) and hydroxypropyl cellulose (right side) at 2608C. Both spectra show two polymer processesÐone for the b-relaxation and the other for the g-side group motion.

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Side groups with more atoms, a larger volume and a separate mobility can show their special relaxations in cellulose (or also in starch) derivatives within the measuring region [60,65,99,111]. In some cases, e.g. cyanoethyl (Fig. 41), cellulose acetates (Fig. 45) or thexyl dimethylsilyl celluloses (Figs. 49 and 50), the substituent side group motion and the local chain mobility overlap very strongly, in other cases, e.g. tosyl celluloses (Figs. 46 and 47), cellulose formiate (Fig. 41) or hydroxyalkyl celluloses (Fig. 43) both processes are well separable. There are hints that the degree of substitutions has in¯uence on the intensity and the overlapping of the different modes of relaxations. In general, the dielectric spectra are better separated in different relaxation peaks for low substituted derivatives. Fig. 42 shows the dielectric low temperature spectra of a cellulose formiate ®lm with DS ˆ 0.4. The loss peaks clearly separated can be identi®ed with the local main chain (b-relaxation) and the formiate side group motion (g-process). The cross correlation between the different modes of the polymer motion produces a cooperativity in the relaxation processes and the slower process determines the frequency position in the spectra. The coupling of the side group motion to the local dynamics of the main chain also causes the great distribution of the relaxation times of this g-process in many cases. Furthermore, the variation of the polar interactions of the side group also affects the chain motion. Comparing the different derivatives it is obvious that high dipolar moments of the side group produce a very intensive g-peak, as to be seen for instance for cyanoethyl cellulose, and the superposition of all polarisation effects makes a uniform broad relaxation process. In contrast, a small dipolar moment results in a small g-signal as thexyl dimethylsilyl cellulose and different molecular orientational processes can be separated in the relaxation spectra (see Figs. 42, 43, 46 or 47). To compare morphological effects on the dielectric signals, not only powders, but also ®lms and ®brous samples, have been measured. Films of several cellulose derivatives were prepared by slowly evaporating the solvent from a solution cast on a glass sheet. The ®lms (thickness 0.03±0.17 mm) were dried as described in the experimental section. In general, it can be said that the dielectric activity in ®lm materials is lower than in powders. This experimental result can possibly be explained with a parallel orientation preferable of the polymer chains in the ®lm plane. For hydroxyethyl (HEC) and hydroxypropyl celluloses (HPC), we have also selected two processes in the dielectric spectra (Fig. 43). The faster process can be associated with the local backbone motion by comparing the activation energy and its position in the frequency scale. The slower process presents the hydroxyalkyl side group motion. As can be seen from the loss spectra the b-relaxation is shifted to higher frequencies, but has a similar shape (distribution of relaxation times) in comparison with the pure cellulose. HEC was also investigated by Montes and co-workers [81,82], Liedermann and Lapcik [159] and Kim et al. [160] and the dielectric spectra found are similar to our results, but the interpretation of the dielectric analysis is different. The hydroxyethyl side group affects the chain dynamics to a lesser extent than the hydroxypropyl substituent. The dipole in the hydroxypropyl side group reorientates faster than the dipoles in the hydroxyethyl group as shown in the activation plot in Fig. 44. In the case of commercial cellulose acetate (DSAc ˆ 2.45, heterogeneous synthesis and statistical distribution pattern of acetyl groups in the AGU), three different relaxation processes in the dielectric spectra are found (Fig. 45). The dielectric spectrum experimentally determined is the superposition of three different processes. The b-relaxation is strongly broadened in comparison with cellulose and shifted to low frequencies. The activation energy of the chain dynamics is nearly the same as in cellulose, but the chain motion is little cooperative as in cellulose (t o(CA) . t o(Cell)). Both acetyl side group processes are smaller and more Debye-like.

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Fig. 44. The HN-®t parameter for the dielectric relaxation of HPC and HEC calculated by an HN ®t at different temperatures (activation plot of the relaxation times, the relaxation strength and the width of the relaxation process).

The general problem of assigning the different loss peaks to individual molecular motions in the case of derivatives is still more dif®cult than for the pure polysaccharides and the acetyl cellulose is an excellent example of this task. It is necessary and successful in many cases to investigate regioselectively substituted polysaccharides, too. The spectral results obtained from the dielectric relaxation analysis for various cellulose acetates are displayed in Fig. 46. The cellulose acetates are also a good example for systems, which have a dielectric response without a clear structured spectrum. It is very complicated in such cases to separate different processes and assign

Fig. 45. Dielectric loss spectrum of commercial cellulose acetate (powder, DS ˆ 2.45, a CarboMer product). The spectrum is a superposition of three processes, which can be associated with the local motion of the polymer chain (b-relaxation) and with the side group motion in the positions C2 or C3 (-O±Ac) and in the C6 position (-CH2 ±O±Ac) on the AGU, the so-called grelaxation.

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Fig. 46. Dielectric loss spectra of various regioselectively derivatisated cellulose acetates (6-O±Ac and 2,6-diO±Ac) and acetates with a statistical substitution pattern at 2608C in comparison.

these to molecular orientational processes. In general, the evaluation with the help of the HN equation of dielectric spectra requires to determine four parameters for each relaxation process, which means 12 parameters as a minimum for the acetates of cellulose. In a mathematical context, the system is badly conditioned and the result of the ®t procedure is ambiguous. To solve this fundamental problem in the dielectric spectroscopy of such complex systems, it is common to ®x the b -parameter to the value b ˆ 1 (symmetrical relaxation peaks) and use the additional demand that all parameters have a monotonous development with the temperature. In addition, the value of a special HN parameter shows a `physically reasonable' behaviour within a system of structurally similar substances. That means it is always useful to measure a complete system of similar substances for a good interpretation of DRS data. The spectra found for the different cellulose acetates with a DSAc $ 1 (Fig. 46) cannot be completely evaluated in a quantitative and consistent manner. However, all acetates, also the 6-O-derivatives, show a b-process and additional processes, which we associate with the motion of the acetyl side groups in the positions C6 (g(-6-O±Ac relaxation) and C2 or C3 and g(-2-O±Ac)- or g(-3-O±Ac)-relaxation). The reorientational motion of the side group in position C2 and C3 can not be distinguished in the dielectric spectra because of the similarity of the linkage and the steric conditions for these groups. From Fig. 46 it can be seen that the g(-2,3-O±Ac)-relaxation is absent in the case of the 6-Oderivative and the g(-6-O±Ac)-process leads to a higher intensity in the frequency range in the right side of the b-process. This -CH2 ±O±CO±CH3 side group motion is faster than the motion of the g(-2,3O±Ac) group caused by the greater mobility of the dipolar C6-end-group. A higher degree of substitution (DSAc) reduces the total intensity of the dielectric response, in general. The dielectric analysis results of cellulose tosylates (tosyl group: -SO2 ±phenyl±CH3) with a different degree of substitution (DS) and a statistical substitution pattern on the anhydroglucose unit presented in Fig. 47 show an interesting result. Three relaxation processes can be distinguished for samples with a

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Fig. 47. Dielectric loss spectra for cellulose tosylate with a statistical substitution pattern on the AGU and different degree of substitution (DS ˆ 0.5±2.3) at the temperature of 2458C.

low degree of substitution (DS , 1.0): The faster relaxation can be associated with the sequential chain motion (b-relaxation). Both other processes are the total side group motions and a third very slow process, whose molecular origin is unknown. For higher degrees of substitution, the intensity of the dielectric response is strongly reduced and it is dif®cult to separate individual processes. Obviously, at high DS values the voluminous and stiff tosyl side groups coat the glucose chain in such a way that the polymer chain is strongly hindered in its reorientation in the electrical ®eld. The side groups are also impeded in their mobility by cross-correlation between the side group and the chain motion. In summary, it can be said that the intensity of the dielectric relaxation (relaxation strength De in the HN framework) reduces with the increase of the degree of substitution and the shape of the dielectric spectra can be qualitatively changed. The in¯uence of the position of the substituent at the repeating unit (AGU) on the dielectric response of the substance is apparent and can be clearly found in all systems regioselectively substituted under investigation. In the following section we review these results and discuss the dynamical properties of the different orientational motions. 4.6.2. Derivatives with regioselective substitution pattern at the AGU The question is how to determine the chain motion by the distribution of substituents along the chain. In Fig. 48 one cellulose tosylate with a statistical distribution along the chain is compared with a tosylate with a more block-like substitution pattern along the backbone Both derivatives have nearly the same degree of substitution (0.89 and 0.90, respectively). The dielectric spectra for these derivatives with different substitution patterns along the chain are clearly different. The activation plots in Fig. 49 and also directly the spectra in Fig. 48 show that the side-group motion is independent of its distribution along the chain, while the backbone dynamics is more cellulose-like in the case of a block-like derivatisation (more exactly: no pure statistical distribution of substituents along the chain) as in the case of statistical distribution. The form parameter a (relaxation width) also shows a systematic change in the line cellulose . block-like . statistical derivative (window in Fig. 49).

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Fig. 48. Dielectric loss spectra of cellulose tosylates with a different type of substitution along the chain: statistical or block-like distribution along the backbone. The degree of substitution is nearly equal in both cases (0.89 and 0.90, respectively).

The thexyl-substituent (Cell±O±Thx DMSi ˆ Cell±O±Si(CH3)2 ±C(CH3)2 ±CH(CH3)2) is often used as a protective group in the polysaccharide synthesis. Dielectric results for regioselective thexyl dimethylsilyl celluloses shown in Figs. 50±52 present a typical dynamical behaviour in relation to all samples investigated. The dielectric effects from these ethers are small because of the low dielectric moment and the large volume of the apolar side group. Only in position C6 at the AGU can the spectra with substitution be presented as superposition of two relaxation processes. The faster, and in its intensity dominant, relaxation can be associated with the chain dynamics well found by both its frequency position, on the one hand, and, on the other hand, its graph in the Arrhenius plot and by both dynamical parameters: activation energy; and pre-exponential factor.

Fig. 49. Arrhenius plot of the relaxation times t (T) and the relaxation width a (T) (in the window) for the tosylates in Fig. 48.

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Fig. 50. Dielectric loss spectra of regioselective 6-O-thexyl dimethylsilyl cellulose with and without permethylation compared at the temperature of 2908C.

Derivatives with substitution in position C6 and C2 at the AGU show three relaxation processes: the large b-process and the two weak g-processes of the side groups for the two different positions at the repeating unit. For all these regioselective thexyl derivatives, the relaxation time of the b-process is nearly the same both at all temperatures and for all types of thexyl celluloses. Chemical functionalisation only in position C6 or in both positions C2 and C6 affects the local chain dynamics to a small degree. The intensity of the dielectric response decreases only if the repeating unit owns thexyl groups in two positions. The exchange of the remaining hydroxyl groups by alkyl groups (per-methylation or perpropanoylation) does not strongly change the dielectric spectra. The g2-relaxation shows less intensity than the g6-relaxation.

Fig. 51. Dielectric loss spectra of regioselective 2,6-diO-thexyl dimethylsilyl cellulose both with and without peralkylation compared at the temperature of 2908C.

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Fig. 52. The activation plot for the 6-O- and 2,6-diO-thexyl dimethylsilyl celluloses from Figs. 50 and 51.

Note: a presentation of the dielectric spectra with another set of HN functions is possible and shows the same deviation with the experimental points, but the set presented here is the only one with physically reasonable HN parameters. Apart from cellulose, chitin is the most abundant natural polysaccharide and chitosan, though less prevalent in nature, is an easily accessible derivative of chitin. Chitin is chemically 2-deoxy-2-acetoamino cellulose (shorter: 2-NH±Ac±Cell) and also a regioselective cellulose acetate with an NH-spacer between the position C2 of the AGU and acetyl group. Chitosan is a 2-deoxy-2-amono cellulose (shorter: 2-NH2 ±Cell) produced by hydrolysis of the acetyl group from chitin.

Fig. 53. Dielectric loss spectra of chitin, chitosan and cellulose at 2908C. Window inserted: activation plot of the relaxation times for these substances.

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Chitosan shows similar spectra to cellulose (Fig. 53) as reported in different papers [72]. The derivative with the NH2 group in position C2 at the AGU produces also a similar effect on the chain motion similar to the OH group in the original polymer. The spectra are characterised by one broad relaxation process (b-relaxation). The spectra of chitin additionally present a special side group process (g-relaxation) produced by the -NH±Ac group in position C2 at the AGU. In summary, it is obvious that the local chain process exists in all derivatives and is only slightly modi®ed in comparison with pure cellulose. The side-group dynamics is strongly dependent on the type and the position of the side group at the repeating unit. The relaxation strength and the activation energies for all cellulose derivatives are summarised in Table 2. The results from literature are integrated in this table. 4.7. Starch derivatives Recently we have presented dynamical investigations of regioselective acetyl starches [58]. The regioselective starch derivatives like 2-O±acetyl±amylo-maize starch with a degree of substitution of DS ˆ 1 show two relaxation processes in the low temperature range. This can be associated, in the light of the argumentation in this article, with the broad local main chain relaxation (b-process) and the acetyl-side group motion (g-process of the Ac(2) side group) (Fig. 54). (The argumentation in this review article takes new results into consideration and does differ from the original paper [58] in the assignment of the relaxation peaks to molecular motions) The intensity of the acetyl group peak in position C2 increases linearly with increase of the acetylation degree (DSAc) in this position (Fig. 55) as measurements at substances with different DSAc in position C2 have shown. Besides, the strength of the b-relaxation decreases with the total acetylation degree in the typical way, which could be interpreted as the reduction of the dipolar moment of the repeating unit and the enhancement of the local chain stiffness by decoration of the polymer chain by functional side groups. (Note: an identi®cation of this loss peak with the methylol side-group motion cannot explain this decrease). Because of the coupling between the chain motion and the side group motion, a decrease of

Fig. 54. Dielectric loss spectra of 2-O±acetyl±amylo-maize starch (DSAc ˆ 1.0) with the clearly separated local backbone and the acetyl C2 side group motion.

Substance

substitution 50.2 42.6 42.0 41.9 48.1 47.8 44.7 38.7 38.6 Ea ˆ 57.5

Cellulose esters with regioselective 7 2,6-diO-ThxDMSi 8 2,6-diO-ThxDMSi-3-O-Pr 9 2,6-diO-ThxDMSi-3-O-Me 10 6-O-ThxDMSi 11 6-O-ThxDMSi -2,3-diO-Me 12 Chitosan 13 Chitin

Cellulose ethers 14 Hydroxyethyl-Cell 15 Hydroxypropyl-Cell 16 Cyanoethyl-Cell

22.0 16.3 t a ˆ 0.2 £ 10 217

pattern on the AG 0.003 0.7 0.3 0.7 0.003 0.8 3.9

44.8

60.0 58.4

4.0 £ 10 217 2.5 £ 10 217

3.4 £ 10 215 1.8 £ 10 214 2.5 £ 10 215 5.5 £ 10 215 1.8 £ 10 214 ± ±

2.3 £ 10 215

Ea to (kJ/mol) (s) ^ 10% ^ 30%

thexyl-dimethylsilyl celluloses 45.5 4.8 £ 10 217 36.4 38.8 9.7 £ 10 214 35.3 40.2 1.1 £ 10 214 38.1 ± ± 36.6 ± ± 34.4 ± ± ± 42.3 1.7 £ 10 214 ±

3.1 £ 10 214 3.0 £ 10 213 1.2 £ 10 215 0.4 £ 10 215

Ea to (kJ/mol) (10 2 16 s) ^ 5% ^ 30%

Other cellulose esters with statistical substitution pattern on the AGU 3 Cell-formiate (DS ˆ 0.4) 53.3 48 31.6 4 Cell-tosylate (DS ˆ 0.55) 38.2 25.8 51.9 5 Cell-tosylate (DS ˆ 0.89) 38.7 22.0 60.5 6 Cell-tosylate (DS ˆ 0.90) 44.8 2.0 53.6 block-like

t o ˆ 2.0 £ 10 215 26.4

to (10 216 s) ^ 20%

Liederman [161] Saad et al. [98]

(49.4) CH2-NH(CH2)2-CN

Saad et al. [98] Crofton and Pethrick [52] McBrierty et al. [77]

Reference

33.2

46

49.2 46

Ea (kJ/mol)

g(2,3)-relaxation

g(6)-relaxation

Values in literature

g-relaxation

3.0 £ 10 216

Ea ˆ 54.2 46.8

Ea (kJ/mol) ^ 2%

b-relaxation

50.1

Cellulose esters Acetyl cellulose 1 CeAc (DS ˆ 1.7) 2 CeAc (DS ˆ 2,45)

No.

Table 2 Dynamical parameters of cellulose derivatives (the side group relaxation (g-relaxation) is divided into the processes of the C2 and C3 substituents (g(2,3)) and in the C6 substituents (g(6)), if it is possible. (Error speci®cations are estimated as in Table 1)

J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472 1463

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J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472

Fig. 55. In¯uence of the degree of substitution on the intensity of the relaxation processes in several acetyl starches: 2-O± acetyl±amylo-maize starch (DSAc ˆ 0.34 to 1.1), triacetyl±amylo-maize starch and triacetyl±waxy-maize starch (De tot is the sum of the intensities of the main chain- and the side group motion).

the intensity of the acetyl group relaxation can also be observed at higher degrees of substitution (DSAC . 1). Starch acetates with nearly the same degree of substitution (DSAc ˆ 1.0 and 1.05, respectively), but one with regioselective derivatisation in position C2 an the AGU and the other with a statistical substitution pattern at the AGU show different dielectric spectra. There is an additional relaxation peak in the acetate statistically functionalised caused by the acetyl side group existing here in position C6 (Fig. 56). The acetyl groups in positions C2 and C3 cannot be dynamically distinguished by DRS. Triacetates of amylo-maize starch (AMS) and waxy-maize starch (WMS) also show the acetyl side group relaxation in position C6 (Fig. 57).

Fig. 56. Comparison of the dielectric loss spectra of starch acetates with nearly the same degree of substitution (DSAc ˆ 0.10 and 1.05), but one with a regioselective in C2 position and the other with a statistical substitution pattern on the AGU.

J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472

1465

Fig. 57. Dielectric loss spectra from tri-acetates of amylo-maize starch and waxy-maize starch in comparison with 2-O±acatylAMS and pure AMS. Window: Arrhenius plots of the relaxation times of all relaxation processes detected with the help of DRS.

Table 3 Dynamical parameters of starch derivatives (the side group relaxation (g-relaxation) is divided into the processes of the C2 and C3 substituents (g(2,3) and in the C6 substituents (g(6)), if it is possible. (Error speci®cations are estimated as in Table 1) No.

Substance

b-relaxation Ea (kJ/mol) ^ 1.5%

g-relaxation

t o (s) ^ 20%

g(2,3)±relaxation

g(6)-relaxation

Ea (kJ/mol) ^ 10%

t o (s) ^ 30%

Ea (kJ/mol) ^ 10%

t o (s) ^ 30% 1.1 £ 10 212 1.6 £ 10 212

Acetyl starches 1 triAcetyl-WMS 2 triAcetyl-AMS 3 2-O±Ac±AMS (DS ˆ 1.0) 4 2-O±Ac-AMS (DS ˆ 0.78) 5 2-O-Ac-AMS (DS ˆ 0.34) 6 6-O±Ac±2,3-O±Me-AMS 7 S stat. O±Ac-AMS `(DS ˆ 1.05)

51.2 48.0 46.3 48.2 49.9 53.5 46.9

2.6 3.4 7.6 1.6 2.7 1.0 5.2

54.0 57.8 58.4 60.8 56.1

1.1 £ 10 214 1.8 £ 10 216 3.4 £ 10 216 5.4 £ 10 216 2.3 £ 10 213

56.0

6.1 £ 10 215

36.4 33.7 ± ± ± 35.4 29.6

Other 8 9 10 11

45.5 43.0 49.2 52.2

3.6 14.1 0.9 2.7

63.0 54.7 53.2 66.6

4.4 £ 10 217 6.3 £ 10 213 6.9 £ 10 214 1.0 £ 10 217

± ± ± ±

2-O±ester starches 2-O±propanoyl-AMS 2-O±butanoyl-AMS 2-O±crotonoyl-AMS 2-O±benzoyl-AMS

1.8 £ 10 212 3.4 £ 10 214

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J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472

Fig. 58. Effect of the substituent type on the polymeric dynamics of regioselective starch derivatives. Comparison of the dielectric loss spectra for different regioselective 2-O-esters of amylo-maize starch: 2-O-acetyl-, 2-O±propanoyl-, 2-O±butanoyl-, 2-O±crotonoyl- and 2-O±benzoyl-AMS.

The type of the starting starch (waxy- or amylo-maize starch) has no in¯uence on the dynamics of the ®nal acetate. The branches in the polymers are not detectable in the local chain motion. The results of the HN-®t procedure for all starch derivatives are also included in Table 3. Dielectric investigations of other regioselective starch esters in the form of 2-O±propanoyl-, 2-O± butanoyl-, 2-O±crotonoyl- and 2-O±benzoyl-amylo-maize starches (Figs. 58 and 59) show qualitatively the same results like starch acetates. However, in all changes of the dynamic parameters with the variation of the alkyl chain in the ester group hints of a systematic effect can be observed. Three ®ndings are interesting. (i) Crotonoyl and butanoyl starches show similar relaxation intensities,

Fig. 59. Arrhenius plots of the relaxation times for the same substances as in Fig. 58.

J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472

1467

and acetyl and propanoyl starch form another group in their dynamical properties. The benzoyl ester of starch owns a lower dielectric relaxation strength than the rest. (ii) All starch esters shift the main-chain motion to higher frequencies by reducing the pre-exponential factor in comparison with pure starch with the exception of the benzoyl ester. These ®ndings could be interpreted in such a way that the cooperativity of the dynamics for the benzoyl starch is lower than the other ethers. (iii) The frequency position of the 2-O-side group motion forms groups like the intensity characteristics. It is interesting that the double bond in the side chain of the crotonoyl derivative has only a slight effect on the dynamics of these polymers in comparison with the butanoyl starch without double linkage. The large and stiff benzoyl side group decreases the dielectric intensity more than the more linear-like alkyl esters. In summary, the derivatisation of starches effects the polymeric dynamics in a similar manner to the substitution in celluloses. The type of starting starch for chemical functionalisation has only a slight effect on the molecular dynamics of the derivatives. 5. Concluding remarks Many results concerning the molecular dynamics of pure polysaccharides and their derivatives have been acquired recently and now a consistent ®rst picture can be presented concerning the assignment of the distinct molecular motions to the single loss peaks observed in dielectric spectroscopy. Unfortunately, theoretical calculations of the activation energies for the different dynamic modes are still missing on the basis of realistic structural models for polysaccharide materials. Model calculations presented in literature [81,134] have used monomers or isolated small particle systems and their validity is limited for the very complex polysaccharide systems. Therefore, it is questionable to compare experimental results with these calculations. Modern modelling of cellulose describes a time interval shorter than 1 ns and the processes discussed in this review are in the region between 100 s and 0.1 ms. Especially, the effect of cross correlation in the polymer dynamics and the different structural levels complicate the problem. Another issue to be settled in future is the development of a well-founded theoretical correlation between De and the polysaccharide structure. This static problem includes the correct consideration of the morphologic structure of polysaccharides in the calculation of the vector sum of all dipole moments in the solid sample. Progress in this ®eld will broaden the application of DRS as an analytical tool. The dielectric function e *(v ) is a macroscopic property of the whole volume of the sample. In literature the opinion dominates that ®rst of all the amorphous regions in the polymer sample are dielectrically active and detected in the dielectric spectra. Here, quantitative experimental evidence is pending at present. The relaxation strength, found in our measurements of different pure cellulosic materials, seems not to correlate in a simple way with the degree of crystallinity of our cellulose samples, which vary only in the interval from 0.5 to 0.63 in their degree of crystallisation. Other morphological effects mask the crystallinity effect in many cases. Pursuing this direction, an answer to the following question must be found: is the dielectric activity of cellulose related to the amorphous regions in the bulk of the sample or is there a correlation between the internal surface of the ®brillar system on the dielectric intensity? For the investigation of water and other swelling solvent effects on the polymer dynamics in polysaccharides and its derivatives, investigations in a frequency range above 1 MHz and with samples containing more than 10% water are necessary. The bwet-relaxation found in wet samples in the room

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J. Einfeldt et al. / Prog. Polym. Sci. 26 (2001) 1419±1472

temperature range must be investigated with different swelling solvents and correlated with other chemical and physical properties of these samples to explain the molecular origin of this relaxation process. Systematic dielectric measurements of polymer solutions (including the observation of the normal mode of the general polymer dynamics), DRS studies of solvent effects in the polymer dynamics and dynamic investigations of the structures forming in polysaccharide solutions are rare in literature [44,162] caused by the low dielectric effects in diluted solutions and the very high viscosity in the medium concentration range. Our own experiments in this direction also have not been successful so far. Independent of these necessary tasks for the future, the great progress in dielectric spectroscopy of this substance class recently is considerable, but not yet suf®cient to understand the molecular dynamics of these complex-structured biopolymers. However, the integration of dielectric relaxation spectroscopy into the system of other analytical methods, mostly directed on structural and energetic problems, has been a great step forward in the last few years. Acknowledgements Our investigations are supported with different projects by the German Research Foundation (DFG);. We are deeply indebted and grateful for many discussions and assistance in providing us with necessary substances to colleagues from the DFG-Schwerpunktsprogramm `Cellulose and Cellulose Derivatives: Professor D. Klemm (U. Jena), Professor W. Burchard (U. Freiburg), Professor P. Zugenmayer (U. Clausthal-Zellerfeld), Professor E. Gruber (U. Darmstadt), Professor W. Mormann (U. Siegen). We also thank Dr H. Sixta from Lenzing AG (Austria) for good cooperation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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