Synthesis and spectroscopic analysis of cellulose sulfates with regulable total degrees of substitution and sulfation patterns via 13C NMR and FT Raman spectroscopy

Abstract

Synthesis and spectroscopic characterisation of cellulose sulfate (CS) were reported. Various CS exhibiting diverse degrees of sulfation (DSS) were prepared through acetosulfation or direct sulfation of cellulose. During the acetosulfation, intermediate product – cellulose acetate sulfate (CAS) – was formed after the comparative esterification and subsequent deacetylation of CAS led to CS. The direct sulfation proceeded quasi-homogeneously and heterogeneously in N,N-dimethylformamide (DMF) or homogeneously in N,N-dimethylacetamide (DMAc)/LiCl mixture. The total DSS between 0.21–2.59 and partial DSS6 as well as DSS2 of up to 1 were determined via elemental analysis and 13C NMR spectroscopy. Besides, solid-state CP/MAS 13C NMR could characterise CS regarding the sulfation. Subsequently, FT Raman investigation of obtained CS was conducted with the aim to establish analysis methods quantifying the total DSS. The intensities of Raman bands ascribed to the vibrations of OSO and C–O–S groups were used as analysis parameters, yielding calibration curves with high correlation coefficients of more than 0.96.