The facile preparation of macroporous, super water absorbing, biocompatible hydrogels of chitosan involving the hydrothermal reaction of a mixture of chitosan (CH), succinic acid (SA) and urea (UR), all of which are sustainable materials, is reported. The structure of the dry CHSAUR was ascertained by CP MAS-SS NMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, powder x-ray diffraction analysis (PXRD), and thermogravimetric analysis (TGA). The principle role of UR in the synthesis was identified as the source of ammonia, which increased the pH of the acidic chitosan solution with reaction time, leading to the formation of the insoluble hydrogel of chitosan accompanied by the formation of pores of different sizes and volumes. In addition, a small fraction of urea participated in chemical reaction with the primary hydroxyl groups in the sixth position of the glucosamine repeat units of chitosan resulting in carbamate linkages. The as-prepared hydrogel, following workup and methanol extraction, was found to be chitosan crosslinked with succinic acid through electrostatic interaction. It was macroporous with percentage porosity varying between 49.4% to 64.2%. It also exhibited different extents of water uptake with the maximum of 760 ± 20 g/g being for the one prepared with the weight ratio of 1: 4: 4 of chitosan: succinic acid: urea. The absorption of water is found to arise out of the porosity as well as presence of water attracting chitosan ammonium cation-succinate electrovalent bonds that are formed by the reaction between SA and ammonium cation of the chitosan backbone. The absorption of saline water was relatively poor suggesting that the saline water absorption might be arising largely due to the presence of micropores and specific interaction. The hydrogels exhibited Herschel-Bulkley rheological behavior. The extraction of CHSAUR with 0.1 N NaOH in methanol resulted in the removal of the physical crosslinks, consisting of succinate anions; the presence of chitosan with porous morphology was confirmed additionally by copper (+2) adsorption. In contrast to the widely reported method of preparing microporous chitosan scaffold of cylindrical shape that takes several days to a week, the present method offers a simple means of preparing macroporous chitosan of any shape and size in very large scale with soft foam-like morphology. With its biocompatibility towards mouse fibroblast cells it could find applications in drug delivery, biodegradable super water absorbency and haemostatic applications.
Keywords: Copper (+2) adsorption; Herschel-Bulkley gel; Hydrogel; Macroporous chitosan; Super water absorption.
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