SOLID-STATE C-13 NMR AND RAMAN STUDIES OF CELLULOSE TRIACETATE - OLIGOMERS, POLYMORPHISM, AND INFERENCES ABOUT CHAIN POLARITY

C-13 CPMAS NMR and Raman spectra have been taken for oligomers (DP = 2 -9) and several preparations of cellulose triacetate (CTA). CTA exhibi ts polymorphism and the NMR spectra of the CTA(I) and CTA(II) allomorp hs along with the spectrum of noncrystalline CTA have been isolated. M agnetic inequivalence within the unit cell of CTA(II) is greater than for CTA(I), indicating a lower symmetry in the CTA(II) unit cell. An a ttempt is made to correlate some of the NMR resonances with proposed X -ray crystal structures for each of these allomorphs. From both the Ra man and the NMR spectra of the oligomers, in comparison with the diffe rent CTA preparations, it is determined that the pentamer and higher o ligomers crystallize into the CTA(I) lattice. On the basis of the Rama n spectra, it is argued that CTA(I) and CTA(II) are distinguished by b ackbone conformational differences. In a single attempt to explore the role of the solvent in determining the crystal habit of the oligomers , the pentamer persisted in the CTA(I) lattice even when crystallizati on took place in dibenzyl ether, the solvent out of which the CTA poly mer forms CTA(II) crystallites. Based on these findings, an approach t o obtaining a definitive crystal structure of CTA(I) would be to grow and perform a structural determination on a single crystal of an oligo mer of DP > 4. Spectra of these oligomers indicated partial crystallin ity; hence, care must be taken in growing single crystals. Since it is known that there is a correlation between the allomorphs of CTA forme d by heterogeneous acetylation and the allomorphs of the precursor, ce llulose, and since there are still points of ambiguity regarding the c rystal structures of the allomorphs of cellulose and CTA, the topic of chain polarity (directionality) is addressed herein. As a contributio n to the dialogue on the structure of these materials, we present plau sibility arguments, i.e., not definitive arguments, that CTA(I) has pa rallel and CTA(II) antiparallel chain character.