"January, 1987.""This manuscript is based on results obtained in IPC Project 3288 and is to be presented at the International Symposium on Wood and Pulping Chemistry in Paris, France on April 26-30, 1987.

Atalla, Rajai H.

Vanderhart, David L.

Scholarly Materials And Research @ Georgia Tech

THE INSTITUTE OF PAPER CHEMISTRY, APPLETON, WISCONSINIPC TECHNICAL PAPER SERIESNUMBER 217STUDIES ON THE STRUCTURE OF CELLULOSE USINGRAMAN SPECTROSCOPY AND SOLID STATE 13C NMRRAJAI H. ATALLA AND DAVID L. VANDERHARTJANUARY, 1987Studies on the Structure of Cellulose Using Raman Spectroscopyand Solid State 13C NMRRajai H. Atalla and David L. VanderHartThis manuscript is based on results obtained in IPC Project 3288 and is tobe presented at the International Symposium on Wood and Pulping Chemistryin Paris, France on April 26-30, 1987Copyright, 1987, by The Institute of Paper ChemistryFor Members OnlyNOTICE & DISCLAIMERThe Institute of Paper Chemistry (IPC) has provided a high standard of professional service and has exerted its best effortswithin the time and funds available for this project. The information and conclusions are advisory and are intended only forthe internal use by any company who may receive this report. Each company must decide for itself the best approach to solv-ing any problems it may have and how, or whether, this reported information should be considered in its approach.IPC does not recommend particular products, procedures, materials, or services. These are included only in the interest ofcompleteness within a laboratory context and budgetary constraint. Actual products, procedures, materials, and services usedmay differ and are peculiar to the operations of each company.In no event shall IPC or its employees and agents have any obligation or liability for damages, including, but not limited to,consequential damages, arising out of or in connection with any company's use of, or inability to use, the reported informa-tion. IPC provides no warranty or guaranty of results.STUDIES ON THE STRUCTURE OF CELLULOSE USINGRAMAN SPECTROSCOPY AND SOLID STATE 1 3C NMRRajai H. AtallaThe Institute of Paper ChemistryAppleton, WI 54912 U.S.A.andDavid L. VanderHartNational Bureau of StandardsGaithersburg, MD 20899 U.S.A.ABSTRACTStudies of cellulose using Raman and 13 C NMRspectroscopy reveal new dimensions to the com-plexity of its structures and their relativestabilities. We present further evidence insupport of our hypothesis that native cellulosesare composites of two crystalline modifications,Ia and IS, which coexist in most native forms,and we elaborate on our understanding of thesestructures and their relative stabilities. Wealso present studies of the transformation ofalgal cellulose to cellulose III, and itsreversion to cellulose I, which are relevant tothe broader question of stability of the nativeforms of cellulose.Summary of Prior ResultsIn our past studies (1,2) the resonancemultiplicities in the solid state 1 3C NMRspectra of native celluloses were examined athigh resolution for a variety of native forms aswell as samples of regenerated celluloses I andII. These spectra are an important source ofnew structural information that is complementaryto that from other sources.The spectra of pure samples of cellulose IIcan be rationalized in terms of nonequivalentsites within a unique unit cell. The spectra ofthe native celluloses, however, reveal multipli-cities that cannot be ascribed to a unique unitcell. In particular, the sharper components ofthe C-4 and C-6 resonances, as well as the C-1resonance, possess multiplicities that suggestthat they arise from magnetically nonequivalentsites within crystalline domains. The narrowlines observed differ among the samples ofnative forms; the relative intensities are notconstant and they are not in the ratios of smallwhole numbers as would be expected if they arosefrom different sites within a single unit cell.We have found the proposal that native cel-luloses are composites of two distinct crystal-line forms, identified as Ia and 18, the mostplausible basis for interpretation of thespectra. A decomposition of the spectra basedon such a model has been described in our earlierreports. The spectra were obtained by taking ap-propriate linear combinations of the spectra ofthe regenerated cellulose I and of the cellulosefrom Acetobacter xylinum; these two celluloseswere judged to be the closest to the two extremeson the basis of a two component model.In studies of the Raman spectra one of us (3)addressed the question of conformational statesin the two forms of native cellulose. The ques-tion was whether the two forms Ia and I8 repre-sent true solid state allomorphs, with moleculespossessing identical conformations, or whetherlike celluloses I and II they contain moleculeswith different conformations as well as differentlattices.Comparisons of the Raman spectra in the con-formation sensitive region revealed a great dealof similarity in the spectra of the native cellu-loses included in the previous studies using 13CNMR. In the OH stretching region, on the otherhand, there were significant differences. Theseobservations have been described in detail in re-cent reports by Wiley and Atalla (4,5), and leadto the conclusion that the Ia had I8 forms ofcellulose represent lattices with nearly identi-cal conformations of the heavy atom molecularskeletons, but with different hydrogen bondingpatterns.Recent Spectral StudiesThe more recent studies undertaken in ourlaboratories have been directed at understandingthe nature of the differences between the Ia andI8 forms. They have included additional Ramanspectral studies, NMR relaxation studies, andstudies on susceptibility to chemical reactions.The Raman spectral studies have focused onthe OH stretching region of the spectra. Asnoted earlier, Wiley and Atalla had found signi-ficant differences in this region between thespectra of celluloses in which the Ia and IBforms were dominant. In an extension of thiswork, the question arose as to whether the dif-ferent spectra could be resolved into those ofthe Ia and IB components.The spectra used in the resolution were thoseof the regenerated cellulose I which was known tobe essentially of the pure I8 form, and that ofCladophera glomerata, an alga which produces cel-luloses which are very similar to those fromValonia ventricosa. Beginning with the propor-tions of the two forms determined from the SolidState 13C NMR spectra, it was possible to resolvethe Raman spectra in the OH stretching regioninto two component spectra corresponding to thetwo pure forms.1The two component spectra are shown in Fig.1, where it is seen that the Ia spectrum posses-ses bands which do not occur in the IB spectrumand vice versa. Though in the studies by Wileyand Atalla it was not possible to distinguish thebands seen in Fig. 1, it was clear that the lowfrequency band in the spectrum of the Ia compo-nent does not occur in the spectrum of the IBcomponent. Similarly, the high frequencyshoulder that is pronounced in the spectrum ofthe IB component does not occur in the spectrumof the Ia component.I/3I . , I , . I ,3100 3200 3300 3400 3500WAVENUMBERS, cm -1Figure 1. The resolved spectra of the Ia and I Bcomponents of native celluloses inthe OH stretching region of the Ramanspectra.In the investigations of Solid State 13 C NMRspectra, relaxation studies on both protons andcarbon nuclei were used to confirm that unit cellinequivalences rather than crystal surface chainresonances determine the profiles of the sharpmultiplets. It was also found that the higherplant celluloses contain a smaller fraction ofthe Ia component than had previously been pro-posed.Experiments based on weak 13 C-13 C spin ex-change were also conducted to probe the spatialenvironment, within a 0.7-1.0 nm radius, aroundcarbons identified with individual multiplet com-ponents, which are assumed to belong exclusivelyto the IQ or IB forms. The spectra of "nearestneighbors" were isolated for three differentmultiplet lines in an algal cellulose and for twolines in a higher plant cellulose. The resultsrule out the possibility that tertiary morphol-ogy can give rise to any multiplicity in thesespectra. The results also provide strong supportfor the hypothesis of multiple crystalline formsin the algal celluloses; however, no clear evi-dence for multiple crystalline forms in the high-er plant cellulose is found by this method.In the studies of susceptibility to chemicalreaction, samples of Cladophera glomerata cellu-lose were subjected to hydrolysis in boilingacid, and the resulting changes in the 1 3 C NMRspectra investigated. The hydrolysis was carriedout in boiling 4N HC1 for 44 hours. The massyield was between 12 and 22%. The Solid State1 3C NMR spectra revealed a significant declinein the content of the I component. In one in-stance, shown in Fig. 2, the resulting spectrumsuggested proportions of the two components simi-lar to those in higher plants, though the resolu-tion remained typical of the spectra of the algalcelluloses.I I I I I I I I I I I I I ,100 80 60 PPMFigure 2. Solid state CP MAS 13C NMR spectra ofCladophera glomerata cellulose beforeand after acd hydrolysis.The results of the hydrolysis studies leavevery little question that the susceptibility ofthe I, form to acid hydrolysis is greater thanthat of the IB form. It seems to us quite likelythat differences in reactivity will be observedin other contexts as this matter becomes the sub-ject of further studies.2Stability of Native CellulosesIn much of the early literature on polymorphyin cellulose it has been assumed that cellulose Iis a less stable form of cellulose than celluloseII because the latter is the form most commonlyrecovered from solution. Although such an argu-ment is not sound from a thermodynamic view, ithas gained wide acceptance. One of us has ques-tioned this on the basis of experimental resultsof regeneration studies (6,7). For example, itwas found that celluloses regenerated at elevatedtemperatures into the cellulose IV lattice pos-sessed Raman spectra indicating the presence ofsome cellulose in the conformation correspondingto cellulose I. That work also led to the prep-aration of regenerated cellulose I under morecarefully controlled conditions (8).In the context of this presentation it isinteresting that the regenerated cellulose I hasalways been the pure IB form as identified by 13CNMR spectra. We believe this is not unrelated torecent observations of the effects of conversionof cellulose I to cellulose III, and its rever-sion to cellulose I upon boiling in water, whichseem to alter the balance between the Ia and theI8 forms.In this new series of observations samples ofcellulose from Cladophera glomerata were con-verted to the cellulose III form according to aprocedure suggested by Hayashi (9). The x-raydiffractogram of the converted form showed it tobe of the highest crystallinity, approaching thesame order of crystallinity as that of the nativeform. The solid state 13C NMR spectra, shown inFig. 3, differed from any that we had previouslyrecorded for cellulose, in that none of theresonances of the carbons in the anhydroglucoseunits showed any multiplicities. Thus, the lat-tice and the molecular conformation were thoughtlikely to be distinct.Raman spectra confirmed our expectations onthe basis of the NMR spectra. As shown in Fig.4, the conformation sensitive region between 200and 700 cm- 1 showed some dramatic differences be-tween the spectra of the cellulose III and thatof the Cladophera cellulose from which it wasprepared, as well as distinct differences betweenthe cellulose III spectrum from that of celluloseII. These differences point to a change in theconformation of the cellulose skeleton, thoughnot necessarily a very large one.When the sample of cellulose III was boiledin water, it reverted to cellulose I. However,the Solid State 13 C NMR spectra, also shown inFig. 3, revealed it to have been transformed froma cellulose in which the I, form was dominant toa cellulose in which the IT form is dominant.Indeed, the spectra seemed to be quite similar tothose expected from a highly crystalline higherplant cellulose. As seen in Fig. 5, the Ramanspectrum also reverted, in the conformation sen-sitive region, to one characteristic of a higherplant crystalline cellulose similar, for example,to that of hydrocellulose from ramie. In the OHstretching region the spectrum of the recoveredcellulose I was typical also of the celluloses inwhich the Ig form is dominant.inm 2m a;fi " At r eI,~i i~ v s^110 100 90 80 70 60PPMFigure 3. Solid state CP MAS 1 3C NMR spectra ofcellulose III prepared from Cladopheraglomerata, and after recovery ofcellulose I by boiling in water.These observations suggest to us that theIg form may be the more stable of the two nativeforms of cellulose. But they also suggest thatthe general assumption that cellulose I is aless stable lattice needs to be reexamined. Ifcellulose II were the more stable form, onewould have expected the treatment in water toresult in conversion of the cellulose III tocellulose II, particularly since both the Ramanand 13C NMR spectra leave little doubt that theoriginal cellulose I lattice had been completelyconverted.ACKNOWLEDGMENTSThe authors wish to acknowledge the help ofMs. Rebecca Whitmore and Mr. Clark Woitkovich inthe preparation of some of the samples, andacquisition of the Raman spectra.3wIJm1mlI I I I. . . . .I I I200 400 600 800 1000 1200 1400 1600WAVENUMBERS. cm-1Figure 4. Raman spectra of celluloses I, II,and III, in the conformation sen-sitive region.REFERENCES1. ATALLA, R. H. AND VANDERHART, D. L. Nativecellulose: a composite of two distinctcrystalline forms. Science 223: 283-5(Jan. 20, 1984).2. VANDERHART, D. L. AND ATALLA, R. H. Studiesof microstructure in native celluloses usingsolid state C NMR. Macromolecules 17:1465 (1984).3. ATALLA, R. H. Polymorphy in native cellu-lose: recent developments. Structure,Function and Biosynthesis of Plant CellWalls, W. M. Dugger and S. Bartnicki-Garcia,eds., American Society of Plant Physiolo-gists, Rockville, MD, 1984. p. 381.4. WILEY, J. H. AND ATALLA, R. H. Ramanspectra of celluloses, in "Solid StateCharacterization of Cellulose." R. H.Atalla, ed., ACS Symposium Series, in press.5. WILEY, J. H. AND ATALLA, R. H. Ramanmicroprobe studies on cellulose fibers.Carbohydrate Research, special issue onphysical chemical studies on macromolecularcarbohydrates, in press.6. ATALLA, R. H. AND NAGEL, S. C. Cellulose:its regeneration in the native lattice.Science 185: 522 (1974).7. ATALLA, R. H., DIMICK, B. E., AND NAGEL, S.C. Studies of polymorphy in cellulose.Cellulose IV and some effects of tem-perature. ACS Symposium Series 48: 40-31(1977). Macromol. 7: 182 (1985).8. WHITMORE, R. E. AND ATALLA, R. H. Factorsinfluencing regeneration of cellulose I fromphosphoric acid solutions. Int. J. Biol.Macromol. 7: 182 (1985).9. Hayashi, J., personal communication.m,I'. I I I I I I I I a I I I . I I I I I I I I I I200 400 000 800 10 100 1 400 100 ' 2600 2800 3000 3200 3400 3800WAVENUMBERS. cm'Figure 5. Raman spectra of Cladophera cellulose, in the native form, after conversion to celluloseIII, and after reversion to the cellulose I lattice upon boiling in water..

Studies on the structure of cellulose using Raman spectroscopy and solid state 13C NMR

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Studies on the structure of cellulose using Raman spectroscopy and solid state 13C NMR

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