Sulfation of poplar wood xylan using a deep eutectic solvent: effects of the degree of substitution on the properties of biobased hydrogels

Abstract

Xylan sulfation using a Deep Eutectic Solvent (DES), formed by sulfamic acid and urea (1:2 molar ratio), was investigated with different reagent dosages (xylan/sulfamic acid molar ratio 1:10 and 1:5) and time (30 and 60 min). The effect of xylan/reagent ratio on the degree of substitution (DS) was higher than time, varying from 0.4 to 1.1. NMR spectra confirmed that the sulfate group was introduced into the C-2 and C-3 of the xylose moieties. Polyelectrolyte complex suspensions were prepared using the different sulfated xylan (SXyl) and chitosan (Ch) (mass ratio 55 SXyl/45 Ch) and deeply characterized. It was found that a precipitate was obtained for moderate conditions of sulfation; however for the others, stable suspensions were formed. Hydrogels with acceptable dry and wet mechanical properties (up to 45 MPa and 6 MPa of resistance, and 10% and 90% of elongation, respectively), and thermal properties were produced. Differences in crystallinity as well as in water uptake at 75% RH, depending on the SXyl used, were found. Promising results for potential use in drug delivery were the swelling capacity, which reached a maximum of 384% and was directly related to the degree of substitution of xylan, and the high water-resistant.