Carbon-13 NMR distinction between categories of molecular order and disorder in cellulose

Abstract

Differences between values of proton rotating-frame spin relaxation time constants can be exploited to separate a solid-state13C NMR spectrum of cellulose into subspectra of crystalline and noncrystalline regions. Variations in chemical shifts and13C spin-lattice relaxation time constants can then be used to study variations in molecular order and disorder within each of the two broader categories. Mechanical damage during Wiley milling increases the content of noncrystalline cellulose and changes the nature of molecular disorder within that category. Resolution enhancement of the subspectrum assigned to crystalline cellulose reveals pairs of signals at 83.9 and 84.9 ppm (cellulose I) or 86.8 and 88.3 ppm (cellulose II) assigned to C-4 on well-ordered crystal surfaces. A broader peak in the subspectrum of crystalline cellulose I is assigned to poorly-ordered surfaces. Relative proportions in Avicel microcrystalline cellulose were estimated as: 54% in crystal interiors, 22% on well-ordered surfaces, 8% on poorly-ordered surfaces, 16% in domains of disorder extending more than a few nanometres.