Diffractometric studies cellulose

The primary sources of information concerning the molecular structure of cellulose have been x-ray and electron diffractometric studies, conformational analyses, and vibrational spectroscopy. The work up to 1971 was very ably reviewed by Jones (10), and by T0nnesen and Ellefsen (II, 12). They generally concluded that although much evidence can be interpreted in terms of cellulose chains possessing a two-fold axis of symmetry, in both Celluloses I and II, none of the structures proposed... 

The considerations involving comparisons of the structures of cellobiose and / -methylcellobioside with the structures of mercerized and native cellulose, respectively, when taken together with the additional observation that the basic repeat unit derived from the diffractometric studies is 10.3 A rather than 5.15 A, require that data relating to the structure of cellulose be reexamined with the constraint that the anhydro-cellobiose unit, rather than the anhydroglucose unit, is the basic repeat unit. To the author s knowledge, no efforts have been made to interpret... 

In summary then, the Raman and infrared spectral studies undertaken after the discovery of the composite nature of native celluloses point to the conclusion that the only difference between the two forms is in the pattern of hydrogen bonding between chains that possess identical conformations. Yet electron diffractometric studies have been interpreted to indicate that the two forms represent two crystalline phases with different crystal habits." More recently, diffractometric studies by Nishiyama have also been interpreted along... 

One of the discoveries growing out of the early diffractometric studies of cellulose was that it can occur in a number of allomorphic forms in the solid state, each producing distinctive X-ray diffractometric patterns. In addition to the cellulose II form, which has been discussed extensively, two other forms have been recognized these are cellulose III and cellulose IV. It is of interest to consider them briefly because they reflect the capacity of cellulose to aggregate in a wide variety of secondary and tertiary structures and because some of the higher plant celluloses produce diffraction patterns that are not unlike those of cellulose IV. Furthermore, they reflect the tendency for some of the celluloses to retain some memory of their earlier states of aggregation in a manner not yet understood. 

Quite early in the x-ray diffractometric studies of cellulose it was recognized that its crystallinity is polymorphic. It was established that native cellulose, on the one hand, and both regenrated and mercerized celluloses, on the other, represent two distinct crystallographic allomorphs (14). Little has transpired... 

Two classes of spectral studies have been applied for the first time during the past decade as the basis of structural studies of cellulose. These are Raman spectroscopy, and solid state NMR using the CP/MAS technique. Both have raised questions concerning the assumptions about symmetry incorporated in the diffractometric studies. And while they 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 one of the major portions of the phenomenology that any acceptable structural model must rationalize. 

In addition to their diffractometric studies reported in prior publications, they add in their contribution to the present symposium analyses of the infrared spectra as well as analyses of the CP-MAS 13c NMR spectra. Their thesis is not inconsistent with the proposals of Atalla and coworkers concerning differences between the conformations of celluloses I and II. However, Hayashi and coworkers go beyond this by proposing that the differences in conformation can be preserved in the course of heterogeneous derivati-zation reactions, and also in the process of generating the other allomorphs of cellulose, namely celluloses III and IV, from the two primary allomorphs 1 and II. 

It is clear that the new information developed from spectroscopic and multidisciplinary studies provides a basis for initiating diffractometric studies with a different set of constraints than those used in the past. The refinements are likely to be more complex, but the expectation is that the structures thus derived will more closely approximate the molecular structure of cellulose. Such models may then provide more comprehensive rationalizations of the phenomenology of cellulose. 

With the wide acceptance of the proposal of the two crystalline forms (I and I/3) came the challenge of understanding the differences between them and their relationship to each other within the morphology of native cellulosic tissues. A number of complementary approaches were pursued by different investigators in the search for answers. Some were based on further application of solid-state C NMR to the study of different celluloses as well as to celluloses that had been subjected to different modifying treatments. Others were based on the application of Raman and infrared spectroscopy to new classes of cellulosic samples. Others were still based on the refinement of electron microscopic and diffractometric methods. The results of these investigations will be presented in summary. 

Thus, they provide information complementary to the diffractometric data in that it serves to constrain the acceptable structural models to a smaller subset than that otherwise admissible on the basis of diffractometric observations alone. In this respect, the spectroscopic information complements the diffractometric data in the same way as the assumptions concerning the symmetry of the unit cell. Furthermore, it appears that the structures suggested by the spectroscopic studies represent relatively small although significant departures from those derived on the basis of diffractometry alone. In anticipation of future directions in studies of celluloses, it is noted that multidisciplinary approaches, similar to some described in later chapters, hold great promise for future progress in understanding the structural diversity that is characteristic of cellulose. 

The procedures for structural studies on cellulose have much in common with investigations of structure in polymers in general. In most instances diffractometric data are not sufficient for a solution of the structure in a manner analogous to that possible for lower molecular weight compounds which can be made to form single crystals. It becomes necessary, therefore, to complement diffractometric data with structural information derived from studies carried out on the monomers or oligomers. 

An acceptable fit to the diffractometric data is not the ultimate objective, however. Rather it is the development of a model that possesses a significant measure of validity as the basis for organization, explanation and prediction of experimental observations. With respect to this criterion, the models of cellulose which have been developed so far leave much to be desired, for their capacity to integrate and unify the vast array of information concerning cellulose is limited indeed. One of the objectives of this symposium is to facilitate identification of points of departure for further studies in search of models which are more useful. 

The assumptions concerning the symmetry of the unit cell noted above have been the basis of recent refinements of the structure of cellulose I. In one such refinement (17) the forbidden reflections were simply assimed negligible, and the intensity data from Valonia cellulose were used to arrive at a final structure. In another study, the inadequate informational content of the diffractometric data was complemented with analyses of lattice packing energies (29) the final structures were constrained to minimize the packing energy as well as optimizing the fit to the diffractometric data. 

---