THE STRUCTURES OF CELLULOSE

Transcription

1 In: Caulfield, D. F.; Passaretti, J. D.; Sobczynski, S. F., eds. Materials interactions relevant to the pulp, paper, and wood industries: Proceedings, Materials Research Society symposium; 1990 April 18 20; San Francisco, CA. Pittsburgh, PA: Materials Research Society; 1990: Vol THE STRUCTURES OF CELLULOSE RAJAI H. ATALLA USDA Forest Service, 1 Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI ABSTRACT This report presents an overview of studies on the structures of cellulose. After presenting a brief historical perspective, the report reviews diffractometrically based structural models and then describes recent developments based on models that are consistent with both diffractometric and spectroscopic observations. The primary impetus for development of these models was provided by Raman and 13 C NMR (CP MAS) spectral results that could not be rationalized on the basis of classical structural models, which are constrained by diffractometric data alone. The structures derived from integrating the spectral information into the data base, which constrain the models, represent relatively small but very significant departures from those structures derived on the basis of diffractometry alone. In addition to rationalizing all the structurally sensitive information, the new models provide a basis for complementary use of spectroscopic and diffractometric methods to monitor variations of the states of aggregation of celluloses with source and history. Thus, it is now possible to investigate the effects of different processing variables, which are important in industrial practice, on both secondary and tertiary levels of structure in celluloses. These levels of structure have a major influence on the material properties of commercial celluloses, yet adequate characterization of these levels has been quite elusive. INTRODUCTION The major constituents of higher plant cell walls area group of polymers with backbones made up of ß- 1,4-linked monosaccharides. The dominant one among them is cellulose, which is the ß -1,4-homopolymer of anhydroglucose (Fig. 1). It is the primary structural component responsible for much of the mechanical strength of the cell wall. Its structural properties derive from its ability to retain a semicrystalline state of aggregation even in an aqueous environment; this is unusual for a polysaccharide. When cellulose is isolated from woody tissue, as in most industrial pulping processes, most other constituents of the cell walls are broken down or solubilized and removed. Thus, cellulose and its distinctive phenomenology define the character of pulp fibers. To understand the properties of these materials, it is necessary to understand the properties of cellulose and the relationship of properties to structure. A prerequisite then is a clear definition of the structures of cellulose and its states of aggregation. In this report, we present an overview of the current understanding of the structures of cellulose and describe some sources of uncertainty that are frequently reflected in the seemingly contradictory reports encountered in the literature. We begin with the conceptual frameworks needed to rationalize those aspects of cellulose phenomenology that are sensitive to structure. We then present a brief historical perspective and summarize some key observations upon which recent structural models are based. We show how some of these concepts have been used when cellulosic materials that are used in practical applications need to be characterized from a structural point of view. 1 The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. Mat. Res. Soc. Symp. Proc. Vol Materials Research Society

2 Figure 1 Structures of cellulose. CELLULOSE STRUCTURE AND STATES OF AGGREGATION Cellulose is known to aggregate in many different forms that possess distinctive properties. Its states of aggregation have generally been described in terms of two conceptual frameworks: the first is the one used for semicrystalline polymers, the other is that used forpolymorphic crystalline solids, To the extent that the aggregation of cellulose molecules is characterized by the occurrence of microcrystalline domains, cellulose is typical of the general class of linear homopolymers that can aggregate to form such domains and are usually described as semicrystalline. On the other hand, the crystalline domains in cellulose can occur in more than one crystal lattice form, hence its classification as polymorphic. A central problem in characterizing the physical structure of cellulose is the analysis of its polymorphic variation and its relationship to specimen history. In addition, as with all other semicrystalline polymers, any specification of structure needs to address the balance between crystalline and amomhous microdomains. As structual information has been developed from new methodologies sensitive to different aspects of structure, it has been useful to consider three levels of structure that define a particular state of aggregation. The first, the primary level of structure, is the chemical structure that reflects the pattern of covalent bonding in cellulose molecules; it is fairly well established and is usually not in question. The next level, the secondary structure, is that of the conformations of individual molecules. It defines the relative organization in space of the repeat units of an individual molecule. This level of structure is important in spectroscopic studies where the energy levels between which transitions are observed are determined by the values of the internal coordinates that define molecular conformations. The final level, the tertiary structure, reflects the arrangement of the molecules relative to each other in a particular state of aggregation whether it be amorphous or represents one or another of the crystalline allomorphs that occur because of the polymorphy characteristic of the crystallinity of cellulose. This is the level of structure probed by diffractometric measurements, which are inherently most sensitive to the three-dimensional organization represented by a particular state of aggregation. It is important to keep in mind that the different levels of structure represent a heirarchy of structures nested within each other. Thus, a specification of the primary structure is implicit in the specification of a secondary structure. Similarly, a precise definition of both primary and secondary structures is implicit in a precise definition of the tertiary structure. For low molecular weight compounds that can be made to form single crystals, the tertiary structure can be determined with sufficient precision through diffractometric studies that it can become a vehicle for determining both primary and secondary structures as well. Indeed, the distinction between the different levels of structure does not often arise in the discussion of the structures of low molecular weight compounds. For polymeric materials, in contrast, diffractometric data are usually not adequate for a solution of the structure in the manner possible for lower molecular weight compounds. The diffractometric data from polymeric materials are usually much more limited in content, and

3 they must be complemented with structural information derived from studies carried out on monomers or oligomers as well as with information derived from investigative techniques that provide additional independent information about the structure [1 5]. The distinction between the levels of structure is particularly helpful in this connection; it facilitates analysis of the relationship of different sources of structural information to levels of structure. Thus, the data adopted from structural information on monomeric systems are usually at the primary level of structure, whereas data from studies of oligomeric systems are often at both primary and secondary levels. The ultimate objective of structural investigations is not so much an acceptable fit to the diffractometric data but rather development of a model that provides a valid basis for organizing, explaining, and predicting experimental observations of phenomena expected to be sensitive to structure. With respect to these criteria, the models of cellulose developed solely on the basis of diffractometric studies leave much to be desired. Their capacity to integrate and unify the vast array of information about cellulose is limited indeed. A central objective of the author s research using spectroscopic methods has been the development of structural models that are not only consistent with diffractometric data but also provide a basis for rationalizing spectroscopic observations and, perhaps more import ant, for understanding the great diversity in the states of aggregation of cellulose. HISTORICAL OVERVIEW The evolution of ideas concerning the primary structure of cellulose has been described in an excellent overview by Purves [6]. Another valuable historical perspective is presented by Flory [7] in his general review of the evolution of the polymeric hypothesis, which highlights investigations of the three common natural homopolymers: starch, cellulose, and natural rubber. Finally, Hermans [8] focuses on the physical chemical aapects of early structural studies; this account is an excellent complement to the Purves review, which dwells primarily on the claasical organic chemical phase in structural studies. The most comprehensive of more recent reviews are those by Jones [9], and Ellefsen and Tonessen [10,11]. Preston [12] and Frey-Wyssling [13] also touch upon the problem of the structure of cellulose. The author discussed the historic perspective in a previous publication [14]. The reader is referred to all these sources for a comprehensive view of how the structures of cellulose have been described in recent decades. The polymorphic nature of the crystallinity of cellulose was recognized fairly early in x-ray diffractometric studies. The studies established that native cellulose, on the one hand, and both regenerated and mercerized celluloses, on the other, represent two distinct crystallographic allomorphs [15], which are identified as celluloses I and II, respectively. In addition, two other allomorphs, III and IV, have been recognized, the first derived from treatments with anhydrous ammonia, the second from high temperature treatment or regeneration from solution at elevated temperatures. However, considerable controversy has centered on the secondary structures of cellulose. For example, on the basis of extensive analyses of electron-density distributions from x- ray diffractometric measurements, Petitpas et al. [16] suggested that chain conformations are different in celluloses I and II. In cent rast, Norman [17] interpreted the results of his equally comprehensive x-ray diffractometric studies in terms of similar conformations for these allomorphs. At a more basic level than the comparison of celluloses I and II, the structure of the native form itself has remained in question. For example, Gardner and Blackwell [18], in their analysis of the structure of cellulose from Valonia ventricosa, assumed a lattice belonging to the P2 1 space group, with the twofold screw axis coincident with the molecular chain axis. On the other hand, Hebert and Muller [19], in an electron diffractometric study of several celluloses including that from Valonia, confirmed the findings of earlier investigators who found no systematic absences of the odd-order reflections forbidden by the selection rules of P2 1. Hebert and Muller conclude that the cellulose unit cells do not belong to that space group.

4 Several new structure-sensitive techniques have been developed. These techniques have been applied to studies of cellulose. They include Raman spectroscopy and solid state 13 C nuclear magnetic resonance (NMR) in the experimental arena and confirmational energy calculations in the theoretical domain. These techniques have been used to complement the information available from diffractometric measurements in analyses of structure-related phenomena in cellulose. DIFFRACTOMETRIC STUDIES As noted by Kakudo and Kasai [1], the primary difficulty in structural studies on polymeric fibers is that the number of reflections usually observed in diffractometric studies are quite limited. In the case of cellulose, it is generally difficult to obtain more than 50 reflections. Consequently, the number of structural coordinates to be determined from the data must be minimized by adopting plausible assumptions concerning the structure of the monomeric entity. The limited scattering data are then used to determine the organization of the monomer units with respect to each other, and their packing in the lattice. In the majority of diffractometric studies of cellulose published so far, the monomeric entity has been chosen as the anhydroglucose unit. Thus, structural information from single crystals of glucose is implicitly incorporated in the analyses of the structure of cellulose. The coordinates that are adjusted in search of a fit to the diffractometric data include those of the primary alcohol group at C6, those of the glycosidic linkage, and those defining the positions of the chains relative to each other. In terms of the different levels of structure, the process is one of generating a primary structure on the basis of the crystal structures of glucose and using the cellulose diffraction data to define the secondary and tertiary levels of structure simultaneously. In addition to selecting the structure of the monomer as the basis for defining the internal coordinates of the repeat unit, the possible structures are usually further constrained by taking advantage of any symmetry possessed by the unit cell. The symmetry is derived from the systematic absence of reflections that are forbidden by the selection rules for a particular space group. In the case of cellulose, the simplification usually introduced is the application of the symmetry of space group P2 1, which includes a twofold screw axis parallel to the direction of the chains. The validity of this simplification remains the subject of controversy, however, because the reflections that are disallowed under the selection rules of the space group are in fact frequently observed. In most studies, these reflections, which are usually weak relative to the other main reflections, are assumed to be negligible. However, some controversy continues because the relative intensities can be influenced by experimental conditions such as the exposure periods of the diffractometric plates. Furthermore, the disallowed reflections tend to be more intense in electron diffractometric measurements than in x-ray diffraction measurements. Thus, more often than not, investigators using electron diffraction challenge the validity of the assumption of twofold screw axis symmetry. If the twofold screw axis is acknowledged as an element of unit cell symmetry, there is usually the additional assumption that this axis coincides with the molecular chain axis of cellulose. This additional assumption implies a number of additional constraints on the possible structures that can be derived from the data. It requires that adjacent anhydroglucose units are related to each other by a rot at ion of 180 about the axis, accompanied by a translation equivalent to half the length of the unit cell in that direction. The assumpt ion therefore implies that adjacent anhydroglucose units are symmetrically equivalent and, correspondingly, that alternating glycosidic linkages along the chain are symmetrically equivalent. In terms of the levels of structure, this assumption is equivalent to imposing on the secondary structure a symmetry that is demonstrable only at the tertiary level. If the assumption concerning coincidence of the twofold screw axis and the molecular chain axis were excluded, for example, by locating the twofold screw axis between the molecular chains though still parallel to the chain axes, the diffractometric patterns would admit nonequivalence of alternate glycosidic linkages along the molecular chain, as well as the nonequivalence of adjacent anhydroglucose units. This possibility has been largely ignored

5 because it requires expanding the number of internal coordinates that need to be determined from the diffractometric data. Furthermore, it excludes the possibility of antiparallel alignment of chains in the unit cell. The assumptions that the unit cell possesses the symmetry of space group P2 1 and that the twofold screw axis is coincident with the molecular chain axis have formed the basis of recent refinements of the structure of cellulose I. In one such refinement [18], the forbidden reflections were simply assumed negligible, and the intensity data from Valonia cellulose were used to arrive at a final structure. In another study [20], the inadequate informational content of the diffractometric data was complemented with analyses of lattice packing energies; the final structures were constrained to minimize the packing energy as well as to optimize the fit to the diffractometric data. Here, the assumptions implicit in the weighting of the potential functions used in the energy calculations further complicate the interpretations. As noted by French et al. [21], the structures derived in the studies by Gardner and Blackwell [18] and Sarko and Muggli [20], though based on parallel chain arrangements, are nevertheless very different tryst al structures. When the same convention is used to define the axes of the crystal lattice, the structure most favored in one analysis is strongly rejected in the other. Furthermore, neither of these is strongly favored over yet a third, antiparallel structure [22]. The structures of oligomers are another important source of relevant information cited by Kakudo and Kasai [1]. The implications of disaccharide structures were considered by the author [23] and were the basis for reassessing the assumption of coincidence noted previously; that is, the imposition of the constraint of twofold helical structure at the secondary level. Structures with alternating nonequivalent glycosidic linkages were found to be more consistent with spectroscopic data [24]. SPECTROSCOPIC STUDIES Spectroscopic studies are useful in structural investigations because they provide information that is complementary to that derived from diffractometric data. The information derived from spectra is not directly related to the coordinates of molecules in the unit cell. However, the spectra are sensitive to the values of the internal coordinates that define secondary structure. Thus, they provide a basis for testing the degrees of equivalence of structures at the molecular level. In addition, specific spectral features can often be identified with Particular functional groups defined by distinctive sets of internal coordinates. Two types of spectroscopies techniques have recently been applied to the study of the structures of cellulose: Raman spectroscopy and solid-state 13 C NMR using the CP MAS (cross polarization-magic angle spinning) technique. Both techniques have raised questions concerning the assumptions about symmetry incorporated in the diffractometric studies. Although these techniques cannot provide direct information concerning the structures, they establish criteria that any structure must meet to be regarded as an adequate model. The information from spectroscopic studies represents a major portion of the phenomenology that any acceptable structural model must rationalize. Although the new spectroscopic methods have also been used in investigations of structural changes induced by mechanical treatments or treatments with swelling agents, the following discussion will be limited to studies that have focused on questions related to structure itself. The results of such studies have to be rationalized by any model derived from crystallographic investigations and thus can test the consistency of diffractometric data. Raman Spectroscopy Raman spectroscopy is the common alternative to infrared spectroscopy for investigating molecular vibrational states and vibrational spectra. It has enjoyed a significant revival since the development of laser sources for excitation of the spectra. Its key advantage in the present context is that it is primarily sensitive to the skeletal vibrations of the cellulose molecule, with the mode of packing in the lattice having only secondary effects. Again,

6 in terms of the levels of structure, this is equivalent to a high sensitivity to differences in secondary structure with a more limited influence of tertiary structure. This feature is a consequence of the dependence of Raman spectral activity of molecular vibrations on changes in the polarizability of vibrating bond systems, rather than changes in associated molecular dipoles. The most intense contributions to the spectra result from bond systems that are predominantly covalent in character, with the more polar systems resulting in much weaker bands. In the first detailed comparison of Raman spectra of celluloses I and II, the author concluded that the differences between the spectra, particularly in the low frequency region, could not be accounted for in terms of chains that have the same conformation but are packed differently in the different lattices; that is, chains that differ in structure only at the tertiary level [25]. As noted earlier in this report, this had been the general interpretation of diffractometric studies of the two most common allomorphs. The studies of the Raman spectra led to the proposal that two different stable conformations of the cellulose chains occur in the different allomorphs. That is, differences occur at both secondary and tertiary levels. To establish the differences between the conformations, information from other sources was considered. The results of published conformational energy calculations suggest two stable conformations for the glycosidic linkages [26,27]. These represent relatively small left-handed and right-handed departures from the conformation of the glycosidic linkage in a twofold helical structure. These departures are well approximated by the experimentally observed conformations of the glycosidic linkages in the crystal structures of the model disaccharides cellobiose [28] and methyl-ß-cellobioside [29], respectively. An analysis of the vibrational spectra in the OH stretching region for both the model disaccharides and for celluloses I and II suggested that nonequivalent glycosidic linkages alternate along the molecular chains [23]. The solid state 13 C NMR spectra were found to be consistent with this model [30], although alternative interpretations are also possible. Finally, the Raman spectra in the methylene bending region indicated that the C6 carbons occur in two nonequivalent environments in cellulose I but appear merged into a single set in cellulose II [31]. The results of the spectroscopic studies were interpreted in terms of nonequivalence of adjscent anhydroglucose units in the molecular chains, requiring the basic repeat unit of secondary structure to be taken as the dimeric anhydrocellobiose unit. The difference between cellulose I and II was associated with the locus of the nonequivalence. In cellulose II, the locus was thought to be at the glycosidic linkages; in cellulose I, it was taken to be centered at C6 and the adjacent segment of the pyranose rings. To reconcile the conclusions from spectroscopic studies with the requirements of chain packing, the proposal was made that cellulose chains possess alternate left-handed and right-handed glycosidic linkages in sequence along the chain axes [32,33]. The left-handed and right-handed linkages were envisioned as representing relatively small departures of the dihedral angles from those prevailing for a twofold helix. The degree of departure from the parameters of a twofold helix was seen as somewhat greater for cellulose II than for cellulose I. This model was described previously. Solid-State 13 C NMR Spectroscopy High-resolution solid-state 13 C NMR based on the CP MAS technique has been applied to the study of the structures of cellulose primarily during the past decade. In this technique, cross polarization (CP) is used to enhance the 13 C signal, high power proton decoupling is used to eliminate dipolar couplings with protons, and magic angle spinning (MAS) of the sample about a particular axis relative to the field is used to eliminate chemical shift anisotropy. The spectra acquired by this method are of sufficiently high resolution that chemically equivalent carbons that occur in magnetically nonequivalent sites can be distinguished.

7 Although 13 C NMR spectroscopy has been used by several different investigators [30,34-37], we focus here on the studies by VanderHart and Atalla [38,39] as representative of the structural questions addressed. Some resonance multiplicities for chemically equivalent carbons occurred in the spectra of all the celluloses investigated. The spectra of high crystallinity samples of cellulose II showed clear splittings of the resonances associated with C4 and Cl. These have been interpreted as evidence of nonequivalent glycosidic linkages along the molecular chains [30], though one study suggests that the splittings may be evidence for nonequivalent chains in the unit cell [37]. The latter argument leaves open the question of why the resonances for carbons 2, 3, 5, and 6 do not display similar splittings. Perhaps the most significant new information derived from the CP-MAS spectra is that relating to the native celluloses. The spectra reveal multiplicities that cannot be interpreted in terms of a unique unit cell, even though they arise from magnetically nonequivalent sites in crystalline domains. The narrow lines observed have relative intensities that are neither constant among the samples of different native celluloses nor in the ratios of small whole numbers that would be expected if the spectra arose from different sites within a relatively small unit cell. VanderHart and Atalla proposed that native celluloses are composites of two distinct crystalline forms [38,39]. Spectra of the two forms were resolved through linear combination of the spectra of native celluloses possessing the two forms in different proportions. The two types were designated celluloses I α and I β. The I α form was found to be dominant in celluloses from lower plant forms and bacterial celluloses, and the I β form was found to be dominant in celluloses from higher plants. In studies of the Raman spectra of different native celluloses, Atalla [24] concluded that the two forms I α and I β consist of molecular chains that have the same molecular conformation. Wiley and Atalla [32] presented evidence that suggests that although the molecular conformations of the two forms are the same, the hydrogen-bonding patterns differ. That is, although the secondary structures are the same, there are subtle differences between the tertiary structures. Additional studies by VanderHart and Atalla [40], based on observations of spin diffusion 13 and relaxation in the C NMR CP MAS experiments, confirmed the existence of the I α and I β forms in native celluloses. More recent studies showed that the particular tertiary structure occurring in a native cellulose is correlated with the architecture of the cellulosesynthesizing complexes on the cell membranes [33]. All of these studies indicated that the I α form of tertiary structure is dominant in celluloses from algae or bacteria, and the I β form is dominant in celluloses from higher plants. Characterization of Celluloses by Spectroscopy The studies and considerations reviewed in this report place the problem of the structures of cellulose in a fresh perspective. Prior to the development of spectroscopic methods, characterization of the states of aggregation of cellulose, in both fundamental and applied investigations, was limited to qualitative characterization of x-ray diffractograms. The much more laborious measurement of the leveling off degree of polymerization, used in some instances as a measure of crystallinity, is of little meaning when the states of aggregation correspond to multiple secondary and tertiary structures, as is often the case. Thus, it was difficult to develop quantitative measures of the variation in patterns of aggregation with sample origin and history. With spectroscopic methods, more complete and quantitative descriptions of the states of aggregation can be developed and can be correlated with the properties oft he cellulose of interest. Although space limitations do not allow a comprehensive discussion, we will describe some recent studies that illustrate how Raman spectroscopy can be used to characterize celluloses. A frequently encountered problem in characterizing celluloses is the quantification of the degree to which celluloses that have been subjected to different swelling treatments are converted from the native form (I) to the mercerized form (II). Given the knowledge that the

8 two forms possess different secondary structures, it was possible to develop a quantitative method based on resolution of the Raman spectra into linear combinations of three different standard components [41]; these components correspond to the secondary structures associated with celluloses I and II, together with a third disordered structure. This technique was then used to assess the susceptibility of different commercial pulps to different stages of commercial processing. One such study explored the difference between kraft and sulfite-dissolving pulps [42]. The kraft pulp was significantly less swollen and converted when treated with 11 percent sodium hydroxide than was sulfite pulp. These results indicate that the tight tertiary structure of the kraft pulp places a greater constraint on the capacity of sodium hydroxide to transform the secondary structure. This transformation is a key step in some industrial chemical derivatization processes. Another study explored the effect of mechanical refining [43]. Such refining is an essential step in most papermaking operations. Moderate refining of some pulps had little effect on the secondary structure, whereas more extensive refining disrupted the tertiary structure sufficiently to result in transformations at both secondary and tertiary levels. Perhaps more important, the results showed that differences in the responses of different wood pulps to mechanical refining are associated with structure transformations at the secondary level. In other studies, spectroscopic methods were used together with x-ray diffractometry to characterize samples where complete characterization depended on the definition of both secondary and tertiary structures. In a study of the effects of heat treatments on the states of aggregation of amorphous celluloses, x-ray diffractometry and Raman spectroscopy were found necessary for full characterization of the evolving states of aggregation [44]. In a study on the formation of cellulose III upon treatment by anhydrous ammonia, Raman spectroscopy and 13 C NMR were used together with x-ray diffractometry to develop a more complete characterization of structural transformations [33]. These studies demonstrate the potential of both the new conceptual frameworks and the spectroscopic methodologies, in conjunction with diffractometric methods, for the characterization of celluloses when structural variations are central to the variation of material properties. These methods will undoubtedly be applied much more extensively in the future. ACKNOWLEDGMENTS This study was supported by the DOE Division of Energy Biosciences, the DOE University Research Instrumentation Program, the USDA Forest Service Forest Products Laboratory, and the Institute of Paper Chemistry; all these sources of support are gratefully acknowledged. REFERENCES 1. M. Kakudo and N. Kasai, X-ray Diffraction by Polymers, (Elsevier, New York, 1972, p. 285). 2. S. Arnott, in Fiber Diffraction Methods, ACS Symposium Series No. 141, (American Chemical Society, Washington, DC, 1980, p. 1). 3. E.D.T. Atkins, in Fiber Diffraction Methods, ACS Symposium Series No. 141, (American Chemical Society, Washington, DC, 1980, p. 31). 4. H. Tadokoro, in Fiber Diffraction Methods, ACS Symposium Series No. 141, (American Chemical Society, Washington, DC, 1980, p. 43). 5. H. Tadokoro, Structure of Crystalline Polymers, (Wiley, New York, 1979, p. 6). 6. C.B. Purves, in Cellulose and Cellulose Derivatives, Part I, E. Ott, H.M. Spurlin, and M.W. Graffline, Eds., (Interscience, New York, 1954, p. 29).

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