Super water-absorbing hydrogel based on chitosan, itaconic acid and urea: preparation, characterization and reversible water absorption

Abstract

The preparation of super water-absorbing hydrogel through the hydrothermal reaction of a mixture of chitosan (CH), itaconic acid (IT) and urea (UR), all of which are sustainable materials, is reported. The structure of the new material, CHITUR, is established by 13C solid-state NMR spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, powder X-ray diffraction analysis (PXRD) and thermogravimetric analysis (TGA). It consists predominantly of unmodified CH and a small fraction of ionotropically cross-linked CH involving ITUR salt as cross-links. The structure of the CHITUR was independent of the composition of IT, UR as well as water content used in the preparation, while the morphology differed significantly. CHITUR exhibited high water and aqueous sodium chloride (0.01 w/v%) absorption. Compared to all the CHITUR preparations, the one with CH:IT:UR of 1:3:3 (all by weight) and 7.5 mL of water per gram of chitosan provided high water absorption, but when the CH:water ratio is increased for this composition of CH:IT:UR, water absorption did not increase significantly. The best water uptake of about 100 g/g (10,000%) was obtained with CHITUR prepared with the following weigh ratio of reactants, namely CH:IT:UR of 1:3:3 with the chitosan:water weight ratio being 1:7.5. In preliminary studies, CHITUR was observed to be suitable for hydroponic growth of seeds.