Evaluation of chitosan crystallinity: A high-resolution solid-state NMR spectroscopy approach

Abstract

We propose a novel approach relied on high-resolution solid-state ¹³C NMR spectroscopy to quantify the crystallinity index of chitosans (Ch) prepared with variable average degrees of acetylation (DA¯) from 5% to 60 % and average weight molecular weight (M¯_w) ranged in 0.15 × 10⁶ g mol^-1-1.2 × 10⁶ g mol^-1. The Dipolar Chemical Shift Correlation (DIPSHIFT) curve of the C(6)OH segment revealed increased mobility dynamic, which induced different distribution from trans-to-gauche conformations in relation to C(4). Indeed, ¹H-¹³C Heteronuclear Correlation (2D HETCOR) showed that distinguished C4 chemical shifts correlates with the same aliphatic protons. The short-range ordering can be assigned to C4/C6 signals on ¹³C CPMAS and, for our case, the deconvolution procedure between disordered and ordered phases revealed increasing crystallinity with DA¯, as confirmed by SVD multivariate analysis. This work extended the knowledge regarding the use of ¹³C CPMAS technique to predict the crystallinity of chitosans without the use of amorphous standards.

Keywords: Chitosan; Crystallinity; High-resolution SSNMR spectroscopy.