Solid-State 13C NMR and Raman Studies of Cellulose Triacetate: Oligomers, Polymorphism, and Inferences about Chain Polarity

Abstract

13C CPMAS NMR and Raman spectra have been taken for oligomers (DP = 2−9) and several preparations of cellulose triacetate (CTA). CTA exhibits polymorphism and the NMR spectra of the CTA(I) and CTA(II) allomorphs along with the spectrum of noncrystalline CTA have been isolated. Magnetic 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 attempt is made to correlate some of the NMR resonances with proposed X-ray crystal structures for each of these allomorphs. From both the Raman and the NMR spectra of the oligomers, in comparison with the different CTA preparations, it is determined that the pentamer and higher oligomers crystallize into the CTA(I) lattice. On the basis of the Raman spectra, it is argued that CTA(I) and CTA(II) are distinguished by backbone 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) la...